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AN INTRODUCTION TO OCEANOGRAPHY
AN INTRODUCTION TO
OCEANOGRAPHY
WITH SPECIAL REFERENCE TO GEOGRAPHY AND GEOPHYSICS
JAMES JOHNSTONE, D.Sc.
Professor oj Oceanography in the University oj Liverpool
THE UNIVERSITY PRESS OF LIVERPOOL LIMITED HODDER AND STOUGHTON LIMITED, LONDON
1923
Made and Printed in Great Britain by C. Tinling & Co. Ltd. 53, Victoria Street, Liverpool, and at London and Prescot.
CONTENTS
Preface Chapter I. Chapter II.
IX.
The World- Ocean
page
Chapter III.
The Origin of the Oceans
The origin of the earth, 8 ; the gaseous- molten hypotheses, 8 ; the tetrahedral earth hypothesis, 10 ; spherical harmonio deformation, 11 ; failure of these hypotheses, 14 ; planetesimal hypothesis, 15 ; growth by accretion, 20 ; rotational changes, field-tracts, 21 ; oceanic and continental regions, 29 ; permanence of the oceans, 3 1 ; smallness of these effects, 32.
The Depths of the Ocean
Estimation of depth, 34 ; sounding machines, 36 ; the bottom gradients, 39 ; continental shelf, 40 ; continental slope, 41 ; form of the ocean floor, 44 ; contours, 46 ; the Antarctic Ocean, 47 ; the Arctic Ocean, 48 ; the Atlantic Ocean, 60 ; the Pacific Ocean, 66 ; the Indian Ocean, 58.
90; 92; 96; 97.
Chapter IV. The Sea Bottom
Sea bottom deposits, 62 ; original nature of the deposits, 63 ; classification, 65 ; categories of deposits, 66 ; httoral deposits, 68 ; shallow-water deposits, 68 ; terri- genous muds, 71 ; neritic deposits, 74 ; oceanic neritic deposits, 78 ; coral forma- tions, 79 ; origin of coral reefs, 82 : pelagic deposits, 88 ; plankton, pteropod ooze, 92 ; globigerina ooze, radiolarian ooze, 94 ; diatom ooze, red clay, 96 ; pelagic oozes in general.
Chapter V. The Oceanic Margins
The foreshore, 103 ; foreshore gradients, 104 ; shallow-water bottom gradient, 106 ; the marginal seas, 107 ; the epi-continental seas, 108 ; the Mediterranean and relict seas, 109 ; marginal sea-bottom deposits, 111; foreshore neritic deposits, 112; shallow-water neritic deposits, 112; the continental margins, 113; interior of the earth, 114; isostasy, 117 ; earth-movements and isostasy, 121 ; the shield-lands, 122 ; the oceanic abysses, 122 ; the great features of the continental margins, 124 ; the Continental displacement hypothesis, 127.
34
61
100
viii CONTENTS
Chapter VI. The Chemistry of Sea Water .
Chemistry of sea-water, 133 ; composition of sea-salts, 134 ; salinity, 137 ; variations of salinity, 138 ; salinity charts, 140 ; the gases of sea-water, 146 ; alkalinity, 148 photosynthesis and carbon dioxide, 151 distribution of gases in sea -water, 154 food substances in the sea, 156 ; origin of the salts of sea water, 160.
Chapter VII. The Physical Characters of Sea Water ....
Pressure, 165 ; temperature, 167 ; tempera- ture variations, 170 ; oceanic isotherms, 174 ; ice, 184 ; vertical temperature vari- ations, 188 ; seasonal temperature changes, 192 ; density of sea-water, 192 ; osmotic pressure, 195; optical characters, 196.
Chapter VIII. The Tides ....
The tides from ordinary observation, 199 ; rise and fall, 199 ; tide gauges, 200 ; sea level, 201 ; range, 202 ; periodicity, 202 ; springs and neaps, 202 ; types of tides, 204 ; tidal streams, 206 ; tide-generating force, 210 ; aperiodic tidal variations, 217 ; theories of the tides, 219 ; the equilibrium theory, 220; the progressive wave theory, 220 ; stationary wave theory, 222 ; tidal predictions, 225 ; empirical prediction methods, 226 ; establishment of a port, 230 ; prediction by harmonic methods, 239 ; the " tidal satellites," 241 ; tidal constituents, 242 ; harmonic analysis, 245 ; harmonic synthesis, 249 ; tides as marine agencies, 250 ; long period tidal variations, 251.
Chapter IX. The Oceanic Circulation
The general scheme, 255 ; causes, 257 ; Ferrell's law, 260 ; effect of the earth's rotation, 263 ; currents as they are, 263 ; equatorial currents, 264 ; the effect of the land, 265 ; depth, 266 ; salinity, 267 ; the great oceanic currents, 270 ; Atlantic currents, 270; Gulf Stream, 271 ; Labrador stream, 271 ; Sargasso Sea and the Atlantic stream, 271 ; South Atlantic currents, 273; Pacific oceanic circulation, 276 ; the Indian Ocean, 279 ; the Polar seas, 283 ; the marginal seas, 285 ; the North Sea, 286 ; seasonal variations, 287 ; mid-Atlantic circulation, 288 ; the Atlantic gjnral, 289 ; the vertical circulation, 293 ; methods of investigation, 296.
131
165
199
CONTENTS ix
Chapter X. Secular Changes in the Ocean 301
The shield-lands, 303 ; oceanic depressions, 304 ; oceanic -continental margins, 304 ; the Pacific Ocean, 309 ; the Atlantic Ocean, 314 ; the Indian Ocean, 319 ; History of the oceans, 321 ; Evidence available, 321 ; paleontological evidence, 323 ; ancient land and sea, 325.
Appendix. Literature Consulted . . 337
Index ....... 343
PREFACE
The modem science of oceanography has been developed to such an extent during the last twenty-five years that it is now impossible to deal comprehensively with its results in a book of any reasonable size. There is hardly any limit to the amount of descriptive detail that might be given with respect to such subjects as the currents of the ocean and its tributary seas ; the distribution of the various kinds of bottom deposits ; or the chemistry of sea water in all parts of the world. Further, the science blends into marine biology in such an intimate manner that two lines of treatment are now quite necessary : Physical Oceanography on the one hand and Hydrobiology on the other. It has also its mathematical sides in the modem theories of the tides and in hydrodynamics, and for the study of these subjects a certain discipline and technique are necessary that have been acquired by only very few scientific men. Lastly the study of the ocean can be approached from the aspect of geography and geophysics.
Although an attempt has been made here to deal in a general way with the science of oceanography, it is rather with the outlook of the student of geography and geology that the book has been written. Even then the method of treatment is as general as possible, so that it is hoped that a summary account of marine science from the physical side has been provided, and that it will be useful not only to those whose main interest is in geography and geology but also to biologists — who, it may be noted, have mainly been responsible for the development of our science of oceanography.
A very great difficulty in such a book as this is the illustrations. A multitude of charts and maps is quite
xii PREFACE
necessary in order that the presentation of the results may be helpful in the highest degree. This wealth of illustration is, of course, quite impossible at the present time, and so the best kind of reader will have to make very frequent use of a good modern atlas of physical geography, and he will also have to consult the small scale Admiralty charts ior details of ocean depths, currents, tidal streams, and the like. The figures contained in the text are, of course, in the nature of sketches, and the amount of reduction and simplification that has been involved in their prepara- tion precludes the statement of much desirable detail. For this the reader must seek elsewhere, but he will •certainly find the task of doing so interesting to a remark- able extent.
No references are given in the body of the book, but I have prepared a short Appendix which gives most of the authorities consulted, and which will also be a guide to the reader who cares to go further with the subject. The books and journals mentioned are mostly such as are ■easily obtained in any good library.
J.J.
University of Liverpool, October, 1923.
CHAPTER I
THE WORLD-OCEAN
When we look at the Ocean as it is represented on a globe certain regularities become apparent : that is, the distribution of land and water does not seem to be purely arbitrary. Figures 1 to 6, which follow hereafter, are made from photographs of a globe, and they may be taken to represent telescopic views of the earth, seen, it is true, under rather favourable conditions. These appearances we must now discuss. Fig. 1 is a view of the Antarctic hemisphere. Here we see a circumpolar land
Fig. 1. Telescopic view of the earth : the Antarctic aspect.
mass, the Antarctic continent. Although the land boundaries of this are not yet precisely known, the depths of the surrounding ocean indicate very clearly that it is
B 1
2 AN INTRODUCTION TO OCEANOGRAPHY
a truly continental earth feature. About it there is the great southern ocean and this is nowhere less than 600 miles in width. Three meridians may be taken as dividing it into as many sectors. These are :
(1) About 75° West longitude. This meridian passes through Graham Land in Antarctica, and Tierra del Fuego, in South America. Here the Southern Ocean is most constricted, and it is also more shallow than elsewhere. At its narrowest part it is about 600 miles wide.
(2) About 20° East longitude. This meridian passes through the South pole and the Cape of Good Hope. The Southern ocean is here rather over 2,000 miles in width.
(3) About 140° East longitude. This passes through the pole and the Australian Continent in the neighbourhood of the Gulf of Carpentaria. In its vicinity the Southern Ocean is about 1,600 miles in width.
Thus there is a nearly symmetrical arrangement of land and sea round about the South pole. There is a polar land- mass of considerable elevation, and round it there is a circumpolar oceanic region. Opening out from this, between America and Africa is the South Atlantic Ocean, and this covers a sector of 90°. Between Africa and Australia, and covering a sector of 120°, is the Indian Ocean, while between Australia and South America, a sector of about 150°, is the Pacific. If the arrangement were strictly symmetrical these three sectors would each be 120°.
Fig. 2 represents the view seen where the North Pole is in the centre of the field. Here there is very roughly the opposite kind of distribution of land and water from that which Fig. 1 represents. In the centre, round about the North Pole, there is an oceanic area of considerable dimensions and depths, and round this again is a circum- polar land-mass, the principal boundaries of which are
THE WORLD-OCEAN 3
the Eur- Asian and North American Continents. The land ring is broken through in three places : between Siberia and Alaska, in the narrow Behring Straits, is the entrance into the Pacific ; between Europe and the eastern coast of Greenland is the Norwegian Sea (and the entrance to the North Atlantic), while between Greenland and North America is Davis Straits. These communications between the Arctic, and the Pacific and Atlantic Oceans are, as we shall see later, rather shallow and not truly " oceanic " in character. They do not obscure our general picture.
Fig. 2. Telescopic view of the earth : the Arctic aspect.
which is that of a Polar oceanic area surrounded by continental land masses with fringes of islands.
Three Oceanic " Gulfs " open out from the great Southern Ocean.
Fig. 3 represents the Pacific water hemisphere.
Here we see the greatest oceanic area of all. The Pacific Ocean is bounded by the coasts of America and Asia and there is only the very small interruption formed by Behring Strait. Then the Asian land boundary is continued across to Australia by an archipelago round which the ocean is
4 AN INTRODUCTION TO OCEANOGRAPHY
relatively shallow. Below Australia the Pacific passes into the Southern Ocean.
Fig. 3. Telescopic view of the earth : the Pacific aspect.
Fig. 4. Telescopic view of the earth : the Indian Ocean.
This is our second Oceanic Gulf. The third Oceanic Gulf is the Atlantic one and this, like the Pacific Ocean, joins together the Antarctic and Arctic
THE WORLD-OCEAN 5
Oceans. It is convenient, for reasons that will appear in the next chapter, to regard it as consisting of two oceanic areas, North and South Atlantics. To the south the communication between Atlantic and Antarctic is very wide and deep, while to the north the opening into the Arctic is constricted and shallow. Also each of the continents of Africa and South America presents towards the other a bold promontory, Cape Verde and Cape San Roque respectively, and these suggest the division of the whole Atlantic into North and South regions.
Fig. 5. Telescopic view of the earth : the South Atlantic aspect.
Fig. 6 represents the North Atlantic Basin, directly continuous on the south with the South Atlantic and bounded on the north in the way that we have noticed.
Now it is obvious that there are certain features in this distribution of land and sea on the surface of the earth that call for explanation. These are :
(1) The north polar oceanic region placed antipodally to the south polar continental region.
(2) The northerly, roughly annular, continental land region which we set over against the southerly, annular, oceanic region.
AN INTRODUCTION TO OCEANOGRAPHY
(3) The condition that the great oceans are (roughly) antipodal to the great continental regions.
(4) The general forms of the continents — these diminish in breadth from north to south (North and South America and Africa) and, as a consequence, the great land margins run obliquely towards and across the equator.
Fig. 6. Telescopic view of the earth : the Xorth Atlantic aspect.
(5) A rough tri-radiate arrangement of land and ocean round the poles. This is shown best by the ways in which S. America, Africa and Australia are directed towards the Antarctic Continent, but it is also suggested in the contours of North America, Green- land and Asia. These land masses are broad to the north, where they form the boundaries of the Arctic Ocean and they taper away towards the equator.
This arrangement is the reverse of that which we see in
the southern hemisphere.
CHAPTER II THE ORIGIN OF THE OCEANS
The precise distribution of land and sea on the face of the earth is, to a great extent, " accidental " : that is, it is due to the operation of a great number of causes, no one of which is much more important than any other. Round the coasts of all the continents there is an extensive region where the sea is not more than about 1,000 fathoms in depth, and here the changes in the extent and form of the land have been very frequent in geological, and even in historic time. There is continual erosion of the coast and the extent and rate of this depend on the nature of the materials of the land. Thus shallow, submarine fiats are formed on the one hand while, on the other, the eroded materials may be carried away by the sea to form shoal^ elsewhere. Then there are movements of elevation and depression, both of the dry land and of the sea-floor, and the result is that the ocean transgresses on the land and vice versa. So the contours of the continents fluctuate t6 a considerable degree and large tracts of the surface of the earth are alternately land and water.
In spite of this flux certain broad, general features of the earth are relatively permanent : they are the deep depressions forming the oceanic basins and the great continental plateaus and mountain ranges. These are the characteristic and noticeable markings of the earth as a whole. Even if they are not absolutely permanent, they still maintain a recognisable form in spite of extensive changes in the precise outlines of land and sea. They are geological rather than geographical earth- features. They are not " accidental " in the sense in which we have employed the term above, but they are due to the operation
7
8 AN INTRODUCTION TO OCEANOGRAPHY
of several large causes which are predominant among the numerous ones to which the fluctuations of land and oceanic contours are due.
The origin of the earth. — These larger causes to which continental and oceanic forms are due are cosmic ones, but they must have terrestrial peculiarities. It has been noted that the surface markings of the moon and the planet Mars difEer notably from those of the earth. If those markings indicate areas of elevation and depression (as in the case of the earth), then the evolution of superficial planetary features must have taken dif?erent directions in each case, and we must seek for causes which have been in operation on the earth, but not at all, or not to the same extent, in its nearest neighbours in the solar system. Differences may easily be found, as for instance, in the different masses of the three bodies ; that of the moon being only l/80th, and that of Mars 1/lOth, of the mass of the earth. We have to deal, then, with factors which are both cosmic and terrestrial ones. Hypotheses of the origin of the continents and oceans must, therefore, depend, to some extent, on the hypothesis of the origin of the earth itself that we adopt.
The gaseous-molten hypotheses. — The classical hypothesis of the origin of the earth is, of course, the Kant-Laplace one, and its main outlines must be familiar to the reader. It leads us to consider an earth which was originally gaseous, and then molten, and very much larger than it now is. As this fluid globe cooled it contracted and, therefore, its rotatory speed increased so that centrifugal forces developed. Modern mathematical research has greatly modified the crudities of the original hypothesis and has shown that the rotating molten earth must have become pear-shaped — the stalk of the pear being a protuberance on the equator. This knob became detached when the rotatory velocity exceeded a certain limit, and so the moon originated, repeating the process by which the earth itself became detached from the parent, solar
THE ORIGIN OF THE OCEANS 9
mass. Tidal evolution followed : the moon, in its primitive orbit, set up enormous tidal waves in the body of the fluid earth, and these set up friction which had the threefold effect of reducing the rotatory speed of the earth, reducing the rotatory speed of the moon and increasing the distance of the latter from the earth. Moon and earth cooled down still further and skinned over, so to speak, by the formation of solid, rocky crusts. For a time these crusts must have been highly unstable, so that they became broken up again and again, but by and by they became permanent and the water vapour previously held in the earth's atmosphere condensed to form the primitive ocean. The crater-like elevations on the surface of the moon may be taken to represent the results of the original instability of the crust and the " boiling- out " of the original gases held in the fluid body. The mass of the moon is, of course, too small to generate a gravitational field intense enough to hold water vapour and the other gases that now form the earth's atmosphere. Sub-aerial or aqueous erosion did not therefore, occur, nor was there chemical action by an atmosphere, and so the original markings have remained.
The consequence of this hypothesis is the idea that the earth was originally intensely hot and has been losing heat throughout its geological history. That it is still hot inside is shown by observations of subterranean temperatures. This rise of temperature as we descend into the interior of the earth is very variable : in Great Britain it may increase at the rate of 1° F. for every 30 feet, or 1° F. for every 130 feet. Taking an average, however, and adopting the very risky method of extrapolating from the curve showing the gradient, we can find what is the expected temperature at the earth's centre, and so get some quite improbable value, such as 350,000°F. There is, of course, no reason why we should not conclude, with just as much probability, that the heated regions beneath the earth's surface are strictly local, not continuous with each other, and at no great depth. That there are localised regions
10 AN INTRODUCTION TO OCEANOGRAPHY
where the temperature is high enough to hold rock in the fluid condition is quite certain : volcanic phenomena demonstrate this, but that the whole earth at a depth of 50 to 100 miles is so intensely hot as to be potentially fluid is really a deduction from the gaseous-molten hypothesis of origin. If — as is undoubtedly the case — the assumption involves us in serious difiiculties we are quite at liberty to reject it on the nature of the evidence in its favour. Now the parent hypothesis, and the observed temperature increase with increasing depth, have left us with the very generally accepted belief that the earth as a whole, was originally intensely hot, is still very hot inside and is, and has been, cooling rather slowly. The superficial layer is, however, cold although the internal mass is hot and is cooling. Therefore the core must still be contracting while the crust has ceased to contract merely because the core is losing heat. It follows, then, that the crust must fall in, so to speak, upon the shrinking interior and, therefore, become crumpled, folded, buckled or otherwise deformed. Here we have a basis for an hypothesis of the origin of the oceanic depressions and the continental elevations.
The tetrahedral earth hypothesis. — Admitting, then, that the earth as a whole is, and has been losing heat, we have an hypothesis which, merely as a consequence of that assumption, professes to account for the existing series of large elevations and depressions of the crust. A closed tube collapsing under external pressure sometimes assumes a shape which is roughly triangular in section and a hollow globe similarly collapsing is said to cave in so as to approximate in shape to a regular tetrahedron. In such a solid there are four faces each of them an equilateral triangle in shape, there are six edges of equal length and four solid angles. If the earth tended to contract, on loss of heat, in such a way we should expect to find that the central parts of the tetrahedral faces would be nearer to the centre than are the solid angles, or coigns, and so the
THE ORIGIN OF THE OCEANS 11
water of the ocean would gravitate towards the central faces and the land masses would be placed round the coigns. One such prominent land-mass is the Antarctic Continent, and so we place a coign of the tetrahedron there. Then the three meridians diverging from the centre of this are approximately fixed in position, for evidently one must pass through the southern apex of South America, and the other two must be at angles of 120° distant on either side. The other three coigns will then be situated on these meridans and roughly in N. latitude 33°. If they are joined by great circles the trace of the enclosed tetrahedron on the surface of the earth is complete.
By no possibility can a regular tetrahedron be traced on the earth's surface so as to approximate to this hypothetical distribution of continents and oceans. The proof of this will be shown by trying. In order, even to make a rough approximation, the tetrahedron must be deformed, and with this sacrifice of its attractive simplicity the hypothesis loses much of its interest. But, apart altogether from this practical test we are not certain that its basal assumption — that the earth as a whole is losing heat and contracting round its centre — is justified. And, that being so, the lack of approximation of theory and test seems to be fatal.
The hypothesis of spherical harmonic deformation. — Starting with the hypothesis of the molten earth the present distribution of land and water may be explained in another way (which appears to be exclusive of the tetrahedral hypothesis). In the molten condition the earth body was, of course, far from being rigid. It protruded equatorially in a very oblate, spheroidal shape and then calved off the moon from a part of its equatorial bulge. That left it asymmetrical along both the polar diameter and along another at right angles to this. As it cooled and contracted it would become more rigid, but until its material became as unyielding as if it were solid granite, the centre of gravity of the earth could not have been its
12 AN INTRODUCTION TO OCEANOGRAPHY
centre of rotation. This is because the material must have been crushed under its own gravity. As the earth rotated it must, therefore, have wobbled.
Assuming, then, that its centre of gravity was not the same as the centre of figure, certain consequences may be traced. If the centre of gravity were nearer to the North than to the South Pole, water would tend to accumulate on the former region, while at the latter, land would protrude. If it were slightly pear-shaped so that the axis of rotation passed through the long diameter of the pear, water would accumulate round the " waist " of the figure. The equatorial bulge would not be a simple swelling but a series of low, annular ridges with furrows between them. Further, it may be assumed that the substance of the primitive earth was heterogeneous and, that being so, the heavier materials would tend to fly out towards the periphery by reason of centrifugal force. Thus the figure of the earth, as it progressively stiffened would take a certain form such that some regions on its surface Avould be nearer to the centre of gravity than some other ones. So there would be areas of depression and elevation, and the water of the ocean would tend to accumulate on the former. The mathematical investigation of the origin of these elevations and depressions, as a consequence of the assumptions made, cannot be reproduced here, and it may be sufficient to indicate that it leads to a distribution rather like that which now exists. We must, however, take some liberties with geographical features : there should be land at the North Pole instead of an obvious oceanic depression and the boundary between the continental and oceanic areas has to be taken at a depth given by the 1,400-fathom contour line. There must be a considerable degree of " smoothing " of the shapes of the continents. When these qualifications are considered along with the original assumption of an earth body, solidifying from the molten state, the hypothesis loses much of its appeal. Nevertheless it has some plausibility.
THE ORIGIN OF THE OCEANS 13
at all events, and it may be that it does not depend absolutely upon a juvenile, molten earth. Like the tetrahedral theory it presents us with an earth in which the oceanic depressions are permanent surface features : that is, they owe their origin to the causes that have produced the solid earth-body.
Now it is certainly the case that the centre of figure of the earth is not the centre of rotation. Very careful observations made during several decades have shown that the latitude of a place is not constant in its value. Latitude is a measurement from the zenith, that is the point in the celestial sphere cut by a prolongation of the earth's axis, and so it follows that the latter is not quite fixed. In other words the poles are points that shift about on the earth's surface.
This shifting of the pole is, of course, a very small motion : its effect is that the latitude of a place varies by about 0*27 second of arc, which means a shifting of the pole by about 30 feet, or so, periodically. The shifting follows a path on the earth's surface which is a sort of complicated spiral, so that it is not cumulative. But another motion enters into the total effect. There is a small periodic change in the latitude which affects all stations simultaneously, and this is to be explained by supposing that the centre of rotation shifts up and down and along the axis of rotation by about 10 feet. As it shifts, so the axis sways upon the shifting point, and so we get the total effect.
It was known that there would be such a precession of the earth's axis, and (before it was actually observed) it was predicted by Euler and given a period of 10 months : that was on the assumption that the earth-body was absolutely unyielding. Every 10 months, then, the north and south poles would have rotated in small circles of about 30 feet in diameter, always pursuing a re-entrant path. The actual period is, however, about 14 months, and this is because the earth is not perfectly rigid, but
14 AN INTRODUCTION TO OCEANOGRAPHY
really yields a little to the tide-generating force set up by the combined gravitational field of the sun and moon. Such earth-body tides have actually been observed.
Failure of the gaseous-molten hypothesis. — There are, of course, huge difficulties in accounting for an original, intensely hot, immensely attenuated nebula with a move- ment of slow rotation. But even when we concede the existence of this nebula further difl&culties arise. If it is given certain provisional dimensions, a mass, a rotational speed and a physical state (a temperature, etc.) its further development may be deduced. The original crudities of the hypothesis have been removed. The nebular matter that formed the planets could not have been cast off in the form of rings, on the Saturnian model, but was calved off in the way suggested above : this follows from Sir George Darwin's mathematical investigation. But even so the process must have had certain dynamical consequences, and these ought to be traceable to-day in the distances of the planets from the sun and the distance of the satelUtes from the planets ; in the masses and rotational speeds of the various bodies ; in their revolutionary paths, etc. Observation is now fastidious enough, and mathematical analysis sufficiently penetrating, to show that the consequences to be expected are not apparent to observation. Therefore the nebular hypothesis, even its later modified form, cannot be sustained. It leaves us also with th^ notion of an earth body which was originally intensely hot and fluid, and which has long been cooling and contracting. If that were so, certain consequences are to be expected : the amount of contraction ought to be calculable within certain limits. There should be limiting values to the amount of contraction of the earth's crust which results from the shrinking away from it of the contracting core. Somewhere in the materials accessible to us there ought to be relics of the original crust that finally solidified. The hypothesis, in fact, must
THE ORIGIN OF THE OCEANS 15
come to such a state that it becomes the subject of physico- mathematical investigation, and so far as that has proceeded it does not support the gaseous- molten conception. It must be again noted that there is no really convincing evidence of an earth with an intensely hot interior, now or in the past, and there is no good evidence that the earth body is cooling and not actually getting warmer.
The planetesimal hypothesis of the earth's origin. — The above hypothesis, as it is elaborated by Chamberlin and Moulton, makes a new start. Given the discrete universe* we have the picture of a very large number of massive bodies pursuing paths which are Uke those of the molecules in an imperfect gas. Originally the rather crude idea of stellar collisions between these bodies was entertained. But the distribution of the stars is such that there are prodigious distances between them on the average — that is the density of the visible universe is exceedingly small. Therefore, the probabihty of colUsions can be calculated, since we know a certain amount as to the velocities of the stars and their spacing apart from each other. This probability of colhsion is very small.
But the probability of a near approach of two stars is much greater. By " approach " we mean that they may come so near as to perturb each other. It is permissible to assume a certain physical state of the perturbing stars : such a state as that of the sun. Here we have an intensely hot body from which eruptions (as shown in the solar prominences and sunspots) occur with, considerable force. The heated and expansive material of the sun is held by its own gravitation, so that erupted substance mostly returns to its surface. If, however, another cosmic body of sufl&cient mass were to approach the sun near enough it would set up a tide in the solar mass and cause an eruption.
* Some hundreds of millions of cosmic bodies of rather small dimensions instead of one large body. Why ? There is some reason for it, Lodge says. To ask for the reason leads on into philosophical, rather than cosmogonical speculation.
16 AN INTRODUCTION TO OCEANOGRAPHY
With a close enough approach disruption of the solar body- would occur.
There is, in fact, a limit within which two cosmic bodies could not approach each other and still hold together by their own gravities. The moon is far outside this limit, with respect to the earth, yet quite significant tides are set up by it in the yielding ocean and there are even very small tides in the solid earth. Given, then, a very close approach on the part of the sun and another cosmic body of much the same order of mass, and fragmentation of one or both bodies must ensue. Such fragmentation might be the first stage in the evolution of a new planetary system.
Just as the model for the Kant-Laplace nebular hypothesis was given in the ring system of Saturn, so the model for the planetesimal hypothesis is afforded by the spiral nebulae which are such conspicuous features of the heavens. It is not to be thought that these spiral nebulae are really stages in, or actual examples of the evolution of planetary systems. Their distances and dimensions appear to be very great so that we see in them something incomparably greater than a planetary system of the order of magnitude of our one. Possibly they may represent stellar galaxies, or universes similar to the system that we see grouped about the Milky Way. In any case, however, they have suggested the possible mechanisms of a planetary evolution.
That mechanism is fairly simple (at this stage of the hypothesis, at all events).
One of the two cosmic bodies (which are of nearly equal magnitude) we may call the " eruptive sun," and the other the " projectile." They are assumed to approach each other, not " dead on " but nearly so. As they come within each other's gravitational field they will take hyperbolic paths about each other, approaching from, and then receding to, very great distances. If they are approximately in the same physical state the same series
THE ORIGIN OF THE OCEANS
17
of events must occur in each. Here, however, we trace only the events that occur in the eruptive sun, and it is assumed that the latter is in rotation. When the projectile approaches to within a certain distance, which will depend on the masses of the two bodies, the attraction will become
Fig. 7. Stages in the disruptive approach of two stellar bodies.
SO great that the eruptive sun will become partially disintegrated. As in the case of an earthly moon-tide, there will be two protuberances, one on either side, and in the line joining the two stars, and at a certain moment each of these will be projected as an explosive " bolt." This is the condition represented by Fig. 7, D, I.
i
18 AN INTRODUCTION TO OCEANOGRAPHY
Then, as the projectile and eruptive sun swing round each other into positions II, the process will be repeated and a further pair of bolts will be projected. In the meantime the eruptive sun will have rotated on its own axis and the first pair of bolts will have swung out of line with that joining the two bodies. The result will be the disposition shown in Fig. 7, D, II. Passing on the same thing will again occur and, because of the rotation of the disruptive body, the bolts will have swung still further out of line so that, as Case III represents, the two trains of ejected matter will now have assumed the characteristic spiral arrangement. We may suppose that there were initially as many distinct bolts as there are planetary bodies in the evolving system. The first embryo form of the juvenile system will then be as Fig. 7, D,III represents. Here we have two trains of ejected matter partially wrapped round the parent sun as the two curved arms characteristic of the spiral nebular form.
Of course there will be great irregularities in the process. In general it may be expected that the nearer are the two bodies to each other the greater will be the quantities of matter ejected. While the latter are being belched out and are still within the strongly developed double gravitational field set up by the projectile and the eruptive sun, they will have gravitational tendency due to their mass. They will have a proper motion of their own due to the impetus given to them on eruption. They are sure to have some rotatory motion about their centres of gravity because of the irregularity of their ejection and the attrac- tions of the parent stars. As a rule a considerable degree of dispersion of the ejected material may be expected so that a true nebula, in which there is the large central body and the knots, or nuclei, of condensed matter would result.
The bolts, on ejection, would have a temperature approximating to that of the eruptive sun, and they might reasonably be expected to be vaporous. But there would be such a vast expansion of this gaseous material that
THE ORIGIN OF THE OCEANS 19
cooling would be a verv rapid process, and condensation to the liquid or solid condition would soon occur except in the central parts of the nuclei. The system that would come into existence, therefore, would be (1) the central, partially disrupted sun, still in its original physical state and having lost only a rather small fraction of its mass ; (2) a number of bolts arranged at first in two spiral arms but gradually settling down into regular orbital revolutionary paths round the central body ; (3) a vast, widely extended cloud of wholly condensed matter in large or small fragments or particles, and also following orbital paths round the central body, or the individual knots, or the entire system. These are the planetesimal bodies. They would be cold. The nuclei or knots of the bolts would, for a time, retain much of their heat — how much, and for how long, would depend on their masses.
Such a process, if it were accessible to our observation, would perhaps be represented by the phenomena seen in a " new star." Here we can observe a very sudden increase in the luminosity of a star that was previously very faint. The latter we may suppose to have been in the physical condition of our own sun, perhaps even colder and undergoing condensation : perhaps even " skinned over " and with a solid, glowing crust. The approach of another similar star, or even of one of the cold and dark bodies that we know to exist, would initiate the series of huge eruptions suggested above and so there would be an enormous increase in the luminosity. It is characteristic of new stars that the light emitted rapidly decreases and is variable in the course of the decrease, and this is easily explained by supposing that the ejected matter quickly cools so that it forms a rather dense, nebulous mass of absorbent matter surrounding the glow- ing central, eruptive body and the incandescent bolts. This would be in slow rotation ; it would not be uniformly wrapped about the central body and bolts, and so the absorption of Ught would not be uniform.
20 AN INTRODUCTION TO OCEANOGRAPHY
The process of accretion. — Such may reasonably be supposed to be the genesis of the evolving planetary system. This would come to consist of a glowing central sun having a mass which would be (as in our own case) very much greater than that of all the other bodies budded off from it. Round this there would be a greatly extended nebular cloud consisting mostly of cold and dark cosmic " dust," that is, discrete solid bodies of magnitudes varying from that of molecules perhaps up to or even greater than those of the largest meteorites that fall to the surface of the earth. In this nebula there would be a number of " nuclei," each representing a separate bolt. These planetary nuclei would be in orbital revolution ■ about the central sun, being held by the gravitational field of force of the latter, and they would also be in rotation about their own axes.
Each of them would set up its own gravitational field and draw to itself the cosmic dust, or planetesimal bodies, that were at first in the condition of orbital revolution about the parent body. Thus each nucleus would sweep space about itself, gathering up the planetesimals and growing in mass and volume with this process. A certain condition of the evolving planet may be imagined in this preliminary phase : it would be much smaller than in the final one, but would be growing all the time. At first the infalling planetesimals would form a light, porous stratum or shell, but this would soon settle down by its own gravitation ; pack together closely, and finally harden by compression as it underwent burial under the further infalls of the surrounding planetesimal matter. At first the growing planet would not be able to set up a gravitational field strong enough to hold the lighter gases : this is the stage attained by the moon, while it is perhaps the case with Mars, which appears to have an atmosphere consisting of carbonic acid gas and water vapour. The gases that now form the earth's atmosphere, or are combined with mineral matter in its crust, as well as the
THE ORIGIN OF THE OCEANS 21
water of the oceans, must have originally been part of the nebular mass, held there by the gravitational field of the central body. But as the mass of the earth increased in consequence of the continued infall of planetesimal bodies it would, by and by, become great enough to collect the gases that how surround it. Before that, however, it would capture water vapour and this, at first, would be largely incorporated in the porous, growing, planetary substance.
Changes in rotational speed. — Given that the ejected bolt of solar matter had a certain initial, rotational speed it appears that we must postulate changes in this. Planetesimals would fall in from all sides, some overtaking the young earth and falling into it so as to communicate momentum to it, while others would meet it and so fall as to arrest its rotational speed. There might be such a distribution of the planetesimals that these effects would cancel each other, but it seems most reasonable to expect such irregularities in the nebula that the infall of matter would sometimes accelerate the terrestrial, rotational speed and sometimes retard the latter. At all events this effect may be assumed in the provisional stage of the hypothesis. It must have had momentous consequences, if it occurred.
Changes in rotational speed must, then, have affected the figure of the earth. At present the equatorial semi- diameter of the earth is about 13"4 miles greater than the polar semi-diameter. Even now an increase of rotational speed would tend to increase the equatorial protuberance and accentuate the polar flattening — axidiviceversa. Given, then, that the primitive nebula consisted of planetesimal materials irregularly scattered along the great tracts diverging spirally from the central body, it seems likely that the accretions that the earth received would at one time increase its rotational speed and, at other times,' retard it. Therefore the figure of the earth (which must also be assumed to have been more yielding in its earlier
22 AN INTRODUCTION TO OCEANOGRAPHY
stages than it now is) must have oscillated and taken on greater or less oblateness from time to time.
Yield-tracts in the growing earth. — Next a " mechanism " must be assumed such as to allow the earth body to change its shape as it also changed its rate of rotation. Stresses would be set up in it as the rate changed and these would, on the whole, be greatest near the centre and least at points near the poles. They would, in general, diminish from centre to surface. Given, then, a change in the rate of rotation and a stress will be set up. In order that this stress may be relieved material must be shifted from the polar to the equatorial regions, or vice versa. If the rotational speed increased there would be a tendency to further flattening of the polar regions and so the crust there would be compressed from above downwards. At the same time the equatorial crust would tend to become more protuberant, and so it would be in a state of tension from above downward. If the rotation slackened the reverse would be the case. Somewhere between the poles and the equator there would be zones of no stress (of this kind) and the semi-diameter there would remain the same.
If the earth body were liquid, or even somewhat viscous, these stresses would be relieved by an actual transport of material, the molecules of the earth substance literally slipping on each other, or flowing in the same way that the ocean yields to the tidal generating force. We must assume, however, an earth-body that is solid and rigid in a high degree and incapable of viscous flow. Material must, therefore, be shifted in some other way. Under great mechanical pressure, assisted, it may be, by heat, tracts of " rock-flow," schistosity, or other similar conditions are estabhshed. In that kind of metamorphism seen in schistose, gneissose, or slaty rocks, planes of cleavage are , set up and the rock splits easily along these planes (which are initially perpendicular to the direction along which the pressure is exerted). Such cleavage planes are formed
THE ORIGIN OF THE OCEANS 23
by the mechanical re- arrangement of the particles of the rock in thin sheets, or laminae, or by the re-crystalhsation of the rock substance in such a way that the long axes of the platy or columnar crystals set themselves at right angle to the pressure. Then the rock yields and great masses become displaced, folded, ruptured, faulted, etc., in such ways as to relieve the stresses by actual dislocation and fracture and consequent re-arrangements of the material of the crust.
Directions of the yield-tracts. In such speculations as this some analogy suggests the form of the hypothesis. Assuming an increase of rotational speed the crust at the polar regions becomes compressed, or it may be thought about as being stretched everywhere from the pole along the meridians. A common response to stresses distributed in this way is a cracking along three lines radiating from a point at angles of 120° : this gives rise to such rock forms as the familiar six-sided basaltic columns, which are due to contraction of a slowly cooling rock mass. A tentative explanation of the earth body accommodating itself to a change of rotational speed supposes, then, that a number of great yield-tracts will tend to radiate out from the poles meridionally towards the zone of no stress — the " fulcrum- zone," in Chamberlin's terminology — and this we may take to be somewhere about 30° in N. and S. latitudes. First we may consider a symmetrical arrangement of the yield-tracts and then consider how this is to be modified. There are, then, three meridianal lines radiating out from each pole at angles of 120° apart, and terminating at 30° N. and S. latitude. There will be a difference of 60° in longitude between the positions of these lines in the north and south hemispheres. Oblique hues are next drawn from the extremities of the northern meridianal ones to meet the southern meridianal lines. Thus the earth is divided up into six quadrilateral segments, each of which is bounded by parts of great circles. The reader should trace these lines on a globe.
24 AN INTRODUCTION TO OCEANOGRAPHY
It has been noted that the stresses set up by changes in the rotational speed of the earth, or changes in the intensity of the tide-generating force are greatest towards the centre of the earth-body and least at the surface. Therefore, we have to consider, not only the surface of the earth, but its whole body down to the centre. The three pairs of quadrilateral areas represent, therefore, the bases of three pairs of four-sided pyramids, or wedges, having their apices touching each other at the earth's centre, their sides formed by great planes passing through the centre and their bases on the surface. These pyramids will sway on their centres, moving against each other, and it will be seen, on reflection, that this kind of segmentation lends itself to the changes in the figure of the earth that must occur as the results of the change in rotational velocity. We assume now that such a segmentation existed during the period of earth-growth by accretion and that oscillations in rotatory speed occurred, in the course of which the earth-body yielded to the changes in stress. Along the planes which bounded the quadrilateral segments, therefore, there were regions or tracts of fracture and readjustment. The yield-tracts on the surface would, then, be somewhere near the meridianal lines radiating out from the poles, and near the oblique lines crossing the equator.
Now it is clear that very many factors must have influenced the precise course of the surface yield-tracts. The latter would not be lines, so much as rather broad regions of fracture and disturbance, and it is not to be expected that they would take the symmetrical courses indicated above. They would not be straight, nor would the angles of the poles be 120°, but something rather more or less. What we have to expect, in general, is a rough correspondence to the ideal scheme, and also yield- tracts subsidiary to those indicated. Further, as the earth- body became more rigid, in the course of its growth and consolidation other yield-tracts must have developed. It
THE ORIGIN OF THE OCEANS 25
will, however, be sufficient to show that there is really a rather striking correspondence between the postulated theory and the results of actual observation. This correspondence is shown in the following Figures, 8 (1) to (9). These are modified from Chamberlin's figures : the angles between the meridianal lines, and so the directions of the oblique ones depart from the theoretical scheme, but not to such an extent as might be thought necessary. The heavy continuous lines, then, represent these modified yield-tract directions ; the heavy, broken lines continue them under the ocean floor, and the dotted lines are subsidiary yield-tracts not contemplated in the symmetrical scheme.
The actual earth yield-tracts.
(1) South polar region. Fig. 8 (1). — There is little doubt about our choice of the directions the tracts ought to take here. The Antarctic Continent is placed nearly symmetrically above the pole and three great land masses, S. America, Africa and Australia are placed round it in positions that approximate to equal spacing about a centre. There are indications of land connections (see Fig. 11) between the Antarctic Continent and the land regions that bound the great Southern Ocean, and so we have little difficulty in drawing the first three, basal, meridianal yield-tracts. They radiate out to Cape Horn, Cape of Good Hope and Tasmania. But obviously a subsidiary yield-tract is indicated in the position of New Zealand : this is represented by the dotted line,
(2) South Atlantic aspect. Fig. 8 (2).— The Antarctic yield-tract directed towards S. America is seen to bifurcate not far from the fulcrum-zone in about S. Lat. 30°. One of the oblique lines connecting it with the corresponding northern yield-tract runs along the eastern side of S. America, and its course underneath the tropical Atlantic is indicated by the heavy broken line. The other obhque line runs along the western borders of S. and N. America (see Fig. 8 (3)). The second meridianal line that joins
26 AN INTRODUCTION TO OCEANOGRAPHY
Antarctica with Africa bifurcates into its two oblique prolongations : which embrace Africa along its eastern and western coastal regions. On the north the oblique connecting line running up the western side of Africa is
Fig. 8. The Chamberlin yield -tracts. Various views of the earth.
seen to join a radial one running down from the N. Pole. Antillean region. Fig. 8 (3). — Here again two of the postulated meridianal tracts in the high N. and S. latitudes are shown, as well as the oblique one that bounds the Pacific coasts of the Americas. But two subsidiary yield-
THE ORIGIN OF THE OCEANS 27
tracts are indicated by dotted lines : one of these is raeridianal and runs down from the North Pole through the Gulf of St. Lawrence and the great American lakes towards the Floridan coast, while another is obUque and, together with the postulated line, bounds the north-east coast of S. America and the Antillean region — an area, as is well known, of great earth-disturbance.
The Eur-African Quadrilateral. Fig. 8 (4).— All the yield-tracts represented here are postulated ones, but the angles formed between them diverge (though not greatly) from the theoretical values. The Antarctic-African meridianal, the Arctic meridians embracing the W. side of Europe on the one hand, and striking down through Asia, on the other, and the two African oblique yield- tracts are shown.
East Indian yield-tracts. Fig. 8 (5). — Here we see the counterpart to the Antillean region. There are shown (a) the postulated N. and S. meridianal and oblique lines, but the figure also represents the subsidiary meridianal tract striking from Antarctica through New Zealand, and a subsidiary northerly tract descending through China. These are joined together, in a regular way, by a subsidiary oblique tract. ■ This together with the postulated one in this region include between them the East Indian area — one of evident disturbance — in the same way as postulated and subsidiary tracts embrace the Antillean area.
The western borders of the Atlantic and Pacific. Figs. 8 (5) and (6). — These figures represent an obvious symmetry which is suggestive in a high degree. The East Indian region with its pair of parallel yield-tracts bounds the western side of the coalesced N. and S. Pacific Oceans. The West Indian region, also with its pair of parallel yield- tracts similarly bounds the western side of the coalesced N. and S. Atlantic Oceans.
The Indian Ocean region. Fig. 8 (7). — Here we again have the postulated yield-tracts : on the south, two meridianal ones joining Antarctica with Africa and
28 AN INTRODUCTION TO OCEANOGRAPHY
Australia ; on the north, the meridianal tract striking down through Asia and, running obliquely across the Equator, the two connecting lines that frame the Indian Ocean.
The South Pacific region. Fig. 8 (8).— Both postulated and subsidiary tracts are required here. On the S. W. Antarctica and New Zealand are joined by the subsidiary south meridianal tract, and on the N. W. the broken line represents the oblique connecting line that goes under the Pacific Ocean : this is the approximate line of coalescence of N. and S. Pacific. On the east the yield-tracts are all postulated ones.
The two Pacifies. Fig. 8 (9). — Here the junction of the N. and S. Pacific Oceans is represented by the drowned oblique yield-tract shown by the broken heavy line. On the S. W. the border of the N. Pacific is formed by the southerly, subsidiary, meridianal yield-tract.
The reader is strongly recommended to trace these lines, as they are shown in Fig. 8, on a small globe : then he will easily see how very helpful is the Chamberhn hypothesis. He must again note what are the assumptions made :
(1) An earth growing by the accretion of solid planetesimals and with a rotational speed which oscillates because of the collisions of the infaUing bodies with it ;
(2) The consequences of this — stresses set up by the tendency of the earth-body to assume spheroidal shapes corresponding with its changing rotational speeds ;
(3) The further consequences — " rock-flows " and tracts of yielding of the earth body in its adaptive changes of figure ;
(4) A possible " mechanism " permitting of this adaptive change of figure — the segmentation of the earth body by the establishment of three pairs of quadrilateral wedges extending down to the centre.
Thus a certain series of zones, meridianal and oblique (with reference to the equator) are assumed. Along these
THE ORIGIN OF THE OCEANS 29
zones the earth-body yields to stresses (in so far as the yielding can be traced on the surface). Assume now that yielding takes place most frequently along the margins of the continents. This is indeed fairly certain, for there we find the greatest inequalities of level, the continental plateaus descending fairly steeply, to the oceanic abysses. There also we actually find regions of obvious earth disturbance, volcanic and earthquake areas and great mountain folds.
Continental and oceanic areas are, therefore, bounded by a series of zones which are mostly parts of great circles. These zones *are representable by a theoretical scheme. The actual zones, in so far as they can be traced on the earth's surface, diverge somewhat from the theory, but the extent of divergence is not very great when we consider that the causes assumed are certainly far less complex than the actual ones. And quite unimportant changes in the angles assumed, with the recognition of subsidiary yield- tracts, lead to a fairly satisfactory correspondence of theory and observation.
Oceanic and Continental regions. — We find, therefore, that a series of meridianal and oblique yield-tracts divide up the surface of the earth into six great, roughly quadrilateral areas. Some of these are continental and others are oceanic. The oceanic areas are arranged in pairs. North and South Atlantics, North and South Pacifies and the Indian Ocean which is southerly to a corresponding area of predominant depression occupied by the Mediterranean, Baltic, Black Sea, Caspian, Red Sea and Persian Gulf. North and South Atlantics have coalesced just as North and South Pacifies have done. There is a further symmetry with regard to the situations of each pair of oceans : the North Atlantic lies obliquely to the west with respect to the South Atlantic, and this is also the case with the North Pacific in respect to the South Pacific. The Mediterranean-Black Sea basins lie obliquely to the west with respect to the Indian Ocean. There is
30 AN INTRODUCTION TO OCEANOGRAPHY
little geological evidence of the former existence of the two Pacifies, but the forms of the Atlantics, and the markedly volcanic and seismic nature of the ocean floor along the region between the Straits of Gibraltar and the Antilles, suggest strongly that this has been a zone of yield of the earth's crust.
Some of the great areas delimited by the yield-tracts are depressions beneath the mean earth level, and are, therefore, oceanic, while others are elevations, and, therefore, are the loci of continental plateaus. These differences are to be explained, and two new processes are invoked for this purpose : (1) the prevalent atmospheric circulation and (2) the disintegrative effect of water upon the elevated land regions. The general form of the atmospheric circulation is that of an imperfect eddy over each of the great oceans, just as in the case of the oceanic circulations themselves. -Such circulations are modified to an enormous extent by the distribution of land and sea, but their prime motive forces are independent of this distribution. In general, equatorial regions must be those of high tempera- ture and ascending movements of ocean water and atmosphere, while sub-polar regions are characterised by low temperatures and descending motions. Further, the rotation of the earth on its axis has a deflecting effect such that currents of water and air tend always to turn to the right-hand side in the northern, and the left-hand side in the southern hemispheres. We must assume that tendencies to these gyratory movements of both the ocean and the atmosphere existed in the juvenile earth during its phase of growth by the accretion of planetesimal particles.
If that is so there would be a tendency for the heavier particles to settle in the central parts of the great atmospheric eddies and, therefore, these regions of the earth's surface would be likely to come to consist of material of higher specific gravity than the surrounding ones. As soon as this became the case water vapour
THE ORIGIN OF THE OCEANS 'SI
drawn to the surface of the growing earth would gather over the places where the force of gravity was greatest. Thus the loci of the primitive oceans ought to have been roughly marked out in the earlier phases of the earth's growth.
Whenever rain began to fall on the surface of the early contmental plateaus, a process of disintegration of the primitive land would begin. Weathering and solution of the rock surface would occur, and materials would be transported from the land to the seas. It is probable that this process was selective in the sense that heavier materials would be transported from the land and lighter materials left there, or on the shallow sea bottom fringing the land. Thus the difference between oceanic and continental regions — the former tending to become areas of greater, and the latter of lesser gravitation would tend to become accentuated. By some such cumulative process as this, starting from a slight initial difference that favoured the accumulation of w^ater on some of the original areas deUmited by the great yield-tracts, the oceanic and continental regions would become established. This part of the theory is, however, not very satisfactory.
Permanence of the great oceanic regions. — It is to be noted, at this stage, that the hypothesis of the origin of of the oceans outlined in the preceding pages involves the further hypothesis that the great oceanic depressions have always occupied the positions that they now do. This follows from the hypothesis of an earth-body contracting into a tetrahedral shape from a state in which it was intensely hot and also from the hypothesis of Love, according to which " harmonic deformation " occurred as the result of an earth-body rotating on a centre that was not its centre of gravity. There are other reasons for believing in the permanence of the oceanic basins, notably that of the great difference in nature of the materials forming the deep-sea deposits and those that are to be found in the series of sedimentary rocks. This we shall
32 AN INTRODUCTION TO OCEANOGRAPHY
deal with in the following chapter. There is also the theory of isostatic equilibrium between continental plateaus and ocean bottoms (see Chapter IV), and this seems incompatible with the idea that an earth area that is now an oceanic abyss may, in past geological periods, have been the locus of a continental plateau. The assumption that the South Atlantic region, for instance, was at one time a continental land mass is plainly an ad hoc hypothesis designed to explain certain facts of distribution of animals and plants, facts which may be susceptible of other explanations. There is, indeed, much evidence that sedimentary rocks situated in land areas far removed from the sea, are marine in origin, but this may merely mean that shallow seas transgressed, at one time, on former land areas. This does not imply that the sites of those strata were ever covered by sea of the depth of the present oceans. The question of the permanence of the oceanic basins will, however, be discussed more fully in the last chapter.
The smallness of the effects dealt with in this chapter. — It is very necessary to note that the deformations and inequalities of the earth considered in this chapter are very small indeed on the earth-scale. We shall see in the following chapters, that the oceanic depressions, as well as the continental elevations, are, in magnitude, only about 0"04 % of the earth's diameter. The " tetrahedral earth," with its coigns representing continental tablelands, and its faces the oceans would, if figured on this page, be indistinguishable from a perfect sphere. Even the equatorial protuberance and the polar flattenings, which are really very large features when compared with the land and sea elevations and depressions, cause the figure of the earth to deviate from true spherical form only to the extent of about 0'3%. Other deformations suspected to exist — the " pear-shape," and the "nipple" somewhere in Africa — are most difficult to detect, much more to measure precisely.
THE ORIGIN OF THE OCEANS 33
So also with the movements that we call " yielding " of the earth-body. Such phenomena as are seen in volcanic eruptions and earthquakes are indeed gigantic and destructive when judged by their effects on human constructions, , but they are the merest sweatings and shivers when they are regarded as terrestrial effects. Ocean tides have momentous consequences to sailors and fishermen, but they only produce infinitesimal differences of sea level when they are measured on the earth-scale. Tides in the body of the earth itself are only tremors. On the cosmic scale then, the causes that are competent to elevate continents or depress ocean basins are exceedingly small ones, and so the effects we have to explain do not require the operation of factors that are necessarily very great. The earth is, in fact, a body that is exceedingly stable, and however striking its instabiUty may be from the human point of view, it is infinitesimal from the cosmic standpoint.
CHAPTER III
THE DEPTHS OF THE OCEAN
, From the point of view of the sailor, fisherman, yachtsman or oceanographer great diversity of feature characterises the sea bottom. In respect of level tliey distinguish between abyssmal and shallow seas, flats, channels, gutters, deeps, shoals, ridges, bars, etc. In respect of the nature of the bottom there are equally marked differences : hard or soft, boulders and stones, gravel, sand, mud, sludge, ooze, coral, etc. These features are, however, inferred rather than seen, and the impression obtained when one looks at extensive areas of sea bottom, laid bare by, a very low spring tide, is that of monotony and sameness rather than diversity. Compared with the land there is a striking uniformity of level and the character of the materials laid down on the sea bottom may appear to differ very little over very great areas. That variety of depth which the experience of the fisherman or sailor enables him to visualise, depends rather on its utilitarian than on its absolute value. A few feet more or less in depth may make a very great difference from the point of view of the navigation of a vessel ; while the nature of the bottom itself, boulders, sand, v/eed, or mud, may be all important to the success of a fishing operation. Even the oceanographer usually exaggerates the '* bottom relief " as well as the differences in the nature of the bottom deposits. It is very important to have this influence of scale of value in mind when one is considering the morphology of the ocean floors.
The estimation of depth. — The practicable methods of finding the depth of the sea are :
(1) The traditional hand and deep-sea sounding lines,
34
THE DEPTHS OF THE OCEAN 35
Here the apparatus consists of a hard, closely made, well- stretched rope of small diameter marked at various intervals by pieces of leather, cloth, etc., worked into the strands of the rope. The sinker is a heavy piece of lead (10 to 14 lbs. for a hand line of 25 fathoms and 28 to 30 lbs. for the deep-sea line of 100 fathoms). The bottom of the lead is hollowed out and the recess is filled with tallow. The hand-line is simply kept in a loose coil, but the deep-sea line is wound on a wooden reel held in a man's hands when the sounding is being made. When the line is well stretched and carefully marked the soundings obtained are usually very accurate. The technique, however, of sounding by these means is complicated and not easy to acquire. :
(2) The pressure tube is used by large vessels that are compelled to make soundings while still under way. A steel rope consisting of a strand of a few wires is generally used, and this is coiled on a reel which forms part of a machine fixed to the deck of the ship. A glass tube of rather small bore, and with one end open and the other sealed up, is attached to the sounding line a little way up from the lead. The latter is thrown overboard and goes down to the bottom at an acute angle to the sea level, so that much more line is out than is represented by the depth of the water. When the sinker is felt to touch the bottom the apparatus is hauled in and the depth is estimated from the height to which the water has xisen in the sealed glass tube. At the surface of the sea the pressure on the column of air in the tube is, of course, one atmosphere. Roughly 30 feet of water exert the pressure of one atmosphere and from the relation, pressure x volume = constant, the depth is found. The inside wall of the tube is coated with a composition that is discoloured by contact with sea water, and when it is taken aboard the fraction, to which the original volume of air has been reduced by the pressure of the column of sea water standing on the sea bottom, is read off on a gauge calibrated for the
36 AN INTRODUCTION TO OCEANOGRAPHY
particular length of tube used. If, for instance, the volume is reduced to one half, the pressure has been doubled, and was exerted by about 30 feet of water (= 5 fathoms). If it is reduced to l/4th of the original volume the pressure has increased to 4 atmospheres, that is 30 X 3 feet due to the water column and one atmosphere, due to the air pressure. The depth is, therefore, 90 feet = 15 fathoms, and so on. The method is suitable for vessels that require only an approximate knowledge of the water depth but is not accurate enough for scientific investigations.
(3) Sounding Machines. Telegraph cable engineers and scientific workers use what is practically a deep sea line of very great length. This line is made of steel pianoforte wire of rather less than one millimetre in diameter but, nevertheless, exceedingly strong. There are various types of sounding machines, but those which are most compact and easily used are the Lucas forms. The larger machine is represented in its essentials in the figure on page 37.
The wire, which may be over six miles in length, is wound evenly on a very strong steel drum working in a triangular frame. It passes out over a leading-wheel which is carried by a moveable arm, to the lower part of which is attached a leather brake strap bearing on the rims of the drum. When the sinker is running out its weight is carried by the leading wheel, which is therefore pulled forward. This throws back the lower part of the arm and releases the brake strap. Whenever the sinker touches the sea bottom its weight is taken ofE the leading wheel, the two strong springs shown are then able to pull back the arm and the lower part of the latter is thrown forward and so pulls on the brake strap. Therefore the machine stops whenever the sinker touches bottom.
At this moment, a counter, geared on to the leading wheel, is read. The revolutions of the latter have been recorded and the circumference of the leading wheel being
THE DEPTHS OF THE OCEAN
37
STrab
Caver-
Jdi^s of
Kuj. 9. Hydrographic apparatus. (1) The Lucas deep-sea sounding machine ; (2) Detachable deep-sea lead ; (3) The Nansen- I'ettersson water-bottle ; (4) Deep-sea reversing thermometer ; (r») The Lucas bottom sampler ; (6) The Hudson collecting lead.
38 AN INTRODUCTION TO OCEANOGRAPHY
known the length of wire run out is found. As a matter of fact the revolution counter is directly calibrated in fathoms so that the depth is shown on it. The wire is then wound back again by a motor geared on to a pulley carried by the drum spindle, and as it comes in it is oiled to prevent rust and " laid " uniformly on the drum by a suitable mechanism.
The sinker is about 40 lbs. in weight, but in a deep sounding the weight of wire run out is also very great. In reeling-in there would, therefore, be a considerable strain on the wire — enough, perhaps, to break it — and so it is necessary to detach the sinker. Fig. 9 (2) represents the quadruple-tube sinker with its releasing mechanism. When the apparatus touches bottom the four short tubes plunge into the ooze and fill themselves, forcing up a weighted rubber washer that tends to close their upper, open ends. At the same time the steel spring shown throws off a ring from the hook attached to the wire and the apparatus turns over on its side. The sinker is now free to fall away from the sounding tube and the latter is hauled up to the surface. At the same time the rubber washer shuts down on the upper open ends of the tubes and this prevents the ooze from being removed. Sometimes a single steel sounding tube, of a foot or more in length, is used. Sometimes this contains a glass tube which is filled by the deposit. In this way any evidences of stratification of the latter can be observed. Fig. 9 (5) represents the Lucas grab. This goes down with its jaws open and when it touches bottom the pressure of the latter trips a Httle brass tumbling block within the jaws. The latter are thereupon closed by a powerful spring. Fig. 9 (6) represents another form of sounding tube.
In all cases the steel sounding wire itself is not directly attached to the sinker but to a fathom or two of " stray line " (strong, thin rope). The latter falls on the sea bottom but the wire is kept straight and thus kinking is avoided.
THE DEPTHS OF THE OCEAN 39
(4) Sounding by sound waves. Since the war of 1914, methods of finding the depth of the ocean by quite a different means have been developed. An apparatus attached to the bottom of the ship makes a small, sudden explosion, thus setting up a sound wave which travels outwards along concentric, spherical fronts. On reaching the sea bottom this wave is reflected and the echo is transmitted directly back to the surface, where it affects a receiving apparatus on the ship and makes a signal. The moments of the explosion and of the receipt of the reflected wave are recorded very precisely, and half of this interval of time is that required for a sound wave to travel through a column of water of the depth in situ. The rate of transmission of a sound wave through water depends on the temperature and salinity and so corrections have to be made.
(5) Sounding by an electrical oscillator. This mechanism is referred to in connection with tidal observations.
Methods (1) to (3) are obviously rather laborious. In the time of the Challenger expedition, hemp rope was used for the sounding line, and some hours were required in order to make a single sounding. The use of steel wire has shortened the time very greatly, but still the ship has to be stopped and so manoeuvred as to keep the sounding line vertical while it is running out. This makes sounding in deep water a very laborious operation, if the observations have to be made at close intervals, and the result is that there are everywhere great areas of ocean bottom which have not been sounded at all. Method (4) allows of the depth being determined while the ship is on her passage, and the last method gives a continuous record. So far, however, these latter means of deep sea soundings have not been completely developed.
The bottom gradients. — Ocean soundings have been made most frequently in rather close proximity to the land — in the neighbourhood of rivers, harbours, anchorages, bays, estuaries and, in general, in waters most used by fishing,
40 AN INTRODUCTION TO OCEANOGRAPHY
and other vessels. The result is that the depth of the sea is known in considerable detail, in many parts of the world at all events, where it is less than about 100 fathoms. Here there is much diversity of feature in the bottom, considerably more than Avhere the depth is greater than about 100 fathoms. There are shoals, channels, bars, gutters, ridges, etc., and the charts representing these are always more complex than they are for the truly oceanic regions. Just now, however, we neglect this variety of feature in shallow water and consider rather the average gradients.
The continental shelf. — Round the margins of all continents and continental islands there is a zone of sea where the bottom slopes dowm on the average very slowly to somewhere about 100 fathoms. If we " smooth out " the smaller inequalities (channels, shoals, etc.) we find that the inclination downwards is very small — usually less than about 1° (1 ft. in about 57 ft.).
The depth of 100 fathoms as the limit seawards of the continental shelf is rather arbitrary and is taken because it is a round, easily remembered number : in different continental regions it varies very greatly. The width of this zone of shallow water is also highly variable. Out from Achill Head (on the West Coast of Ireland), for instance, it is only about 30 miles wide, but due west from Land's End the depth of 100 fathoms is reached only at about 200 miles. All the North Sea and most of the Irish Sea are less than 100 fathoms deep, and only in a very few places round the British Islands is the sea deeper than this.
Beyond this rather vague boundary the inclination downward of the sea bottom becomes rather greater towards a depth of about 1,000 fathoms, and then it becomes greater still. If we join together all places where the water is 100 fathoms deep and then draw a smoothly running, irregular curve, passing as nearly as possible to all these points, we obtain the " 100-fathom contour line." Doing the same thing with regard to the 1,000, 2,000, and 3,000-
THE DEPTHS OF THE OCEAN 41
fathom soundings, we get a series of curved lines which run, very roughly, parallel to each other and to the general continental outlines. Measuring the average distances between these we then find that the 2,000 — 3,000-fathom contours are rather closer together than the 2,000 and 1,000 contours, and still more so than the 1,000 and 100 contour lines. On the whole, then, the gradient downwards of the sea bottom is greater just about the 1,000-fathom contour line than it is anywhere else. This part of the sea bottom is called the Continerdal Slope.
The continental slope. — It is usual to represent the continental shelf and slope by means of imaginary sections taken vertically to the surface of the sea. Generally the vertical scale (of depths) is made much greater in these sections than is the horizontal one, and when this is done the figures often represent both the shelf and the slope in a striking way. But the distortion of the two scales may convey rather misleading ideas both of the relative depth of water and the bottom gradient.
In Fig, 10 the sections are drawn to the same scale of distances both horizontal and vertical : in each case one millimetre represents a mile. Further, the figures represent the curvature of the earth's surface, on approximately the same scale. Now it is by no means easy to find data on the published charts that will show the continental shelf and slope, because there is generally a great paucity of soundings outside the 100-fathom contour line and so the inequalities of sea bottom, if they exist, must often be " averaged out " in the process of making the sections. But even when we do find data that enable us to construct a section, it is often the case that it is very difficult indeed to say where, precisely, the limits of the shelf should be, or where the steepest gradient, or continental slope, should be placed.
Fig. 10 (1) represents a section of the Atlantic Ocean taken from the West Coast of Ireland in Lat. 51° 30' N., at a place where there is a prolonged continental shelf.
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THE DEPTHS OF THE OCEAN 43
Out to 50 miles from the coast the sea does not exceed ]00 fathoms in depth, but it is difficult to see any decided change in gradient at about this depth and from there out to about 220 miles from land the bottom slopes down rather gradually to about 1,000 fathoms. Between about 220 and 240 miles out there is, hownever, a change in gradient and this is quite noticeable on the section. Here the average inclination downward is about one in ten (an angle of 5J°). This is the continental slope, and beyond it the depth and the average gradient do not change much. That is, at a distance of about 250 miles from land in this region the ocean " abyss " is reached.
Fig. 10 (2) represents a section taken along lat. 10° S., that is, out from the Peruvian coast, and nearly at right angles to the general trend of the western coast of S. America. It also includes the adjacent land (stippled part of the section), thus passing through the Andes, where the highest altitude on the line of the section is about 15,600 feet. The section thus represents the great " Pacific geo- syncline," and shows, at the best, the transition from an oceanic depression to an elevated continental region. At 20 miles seawards the depth is 50 fathoms, at 90 miles it is 1,000, and at 200 miles we attain the Pacific oceanic abyss at a depth of about 2,500 fathoms. Now we see here that the average inclination both of the " Pacific Slope " and of the adjacent ocean floor does not appear to be particularly striking. Neither is there any place where the sea bottom is flat enough to constitute an obvious shelf, nor so much steeper than the average as to give us a pronounced continental slope.
Fig. 10 (3) is taken east from the Japanese coast in lat. 30° 10' N., and here one of the greatest oceanic deeps is represented. At a distance of about 30 miles from the coast we find a depth of 100 fathoms, at 55 miles 1,000. at 80 miles 2,000, and at about 100 miles a depth of 4,000 fathoms. The inclination downwards is, so far as one can see, nearly uniform out to a depth of about 2,000 fathoms.
44 AN INTRODUCTION TO OCEANOGRAPHY
but from there the bottom descends more steeply to a depth of 4,000 fathoms. This is, however, not the typical Pacific Ocean floor but is the bottom of a " deep," a place where an exceptionally great depth, over a very limited area, has been observed. So we cannot, in this section, point to any obvious continental slope.
Fig. 10 (4) illustrates the slope and depths off an oceanic island, and represents a very different condition from that characteristic of the continental margins. The island is the Funafuti coral atoll in the Pacific Ocean, and the section is taken east and west (magnetic) across it. Going from west to east we find a depth of 500 fathoms at one mile from the rim of the atoll, 1,000 fathoms at 2^ miles, and the ocean floor is reached at a distance of about six miles. Here we find no indications of a shelf and, indeed, the gradient downwards appears to be steepest close up to the outside of the atoll. Within the latter the lagoon is, of course, only a few fathoms in depth.
In all these cases (Fig. 10), the section is made from the data given on the Admiralty charts. Contour lines are drawn and then the horizontal distances are taken from the latter. Because of the paucity of soundings there is always considerable difficulty in drawing the contours, and more than one way is usually possible.
The form of the ocean floor. — There is, therefore, a general slope downwards from the land to the ocean bottom. This slope is not uniform but varies in an .extraordinary way though, looking at it in a very general manner, there is, first of all, a very gradual descent down to a limited depth which varies between about 100 and 1,000 fathoms and, next, a rather steeper gradient down to about 3,000 fathoms. The fii-st, very gradual slope downwards may be called the continental shelf, and the second, steeper descent is the continental slope. Outside the latter the ocean bottom is, on the average, a rather flat plain presenting verv little relief. The gradients are, as a rule, exceedingly small, far less than the ordinary railway ones, and they
THE DEPTHS OF THE OCEAN 45
would, in general, be imperceptible to the e^'e if the ocean bed were dried up. On the great scale, however, these very small gradients give the ocean floor certain forms which we must now consider.
Terminology. — The various terms which are most
commonly used are :
Continental Shelf {Plateau ; Continental Schelf). The
bottom out to about 100 fathoms, or to any other
depth after which the gradient increases.
Plateau (Plateau). A flattened, extended, submarine
elevation. Shoal {Hautfotid ; Grund). The most elevated parts of
a plateau. Bank {Banc; Bank). A low non-rocky submarine
elevation. Ridge {Crete; Rucke). A submarine elevation of
elongated form. Rise {Seuil ; Schwelle). An extensive, gently sloping
elevation of the sea bottom. Reef {Riff; Reef). A rocky, dangerous, submarine
elevation of elongated form. Depression {Depression, Mulde). Any lowering of the
sea bottom in general. Basin {Bassin ; Becke). A depression of approximately
rounded form. Trench {Fosse; Grabe). Any marked, elongated
depression of the sea bottom.
Deep {Fosse ; Tiefe). The deepest part of a depression.
Generally all parts of the ocean floor greater than
3,000 fathoms in depth.
Contour lines. — The data for descriptions of the forms of
the ocean bottom are mainly the official charts published
by the various hydrographic departments (principally
Great Britain, Germany and the United States of America)
and the soundings made by deep-sea expeditions. Near
the land, in shallow water of less than 100 fathoms, the
soundings are generally " reduced " to what they would
46 AN INTRODUCTION TO OCEANOGRAPHY
be at low water of ordinary spring tides. In extreme eases, therefore, the depths of water may be some 5 or 6 fathoms greater than those marked on British Admiralty Charts. (See pp. 203-6).
Such charts are covered by a multitude of figures which are difficult to visualise as a whole. Therefore contours are drawn. On British charts these are usually drawn at 5, 10, 20, 50 and 100 fathoms. A contour line on a chart is usually defined as " a line joining all points where the depth is the same," but this is not quite the case. The contour passes as nearly as possible to all the points where depths of (say) 50 fathoms are marked but, as a rule, it would go near points where (say) 49 or 51 fathoms are recorded. In areas where there are few records the contour line will, so to speak, " split the differences " : thus it would pass nearly midway between positions where the depths are recorded as (say) 36 and 60 fathoms, but it would be nearer to 60 than to 36. It should be clear to the reader that contour lines are generally approximations and that two persons constructing them from the original data would not in general get quite the same results.
Further, the shape of a contour line will depend on the scale of the data : the more numerous the latter the more complex will be the shapes of the contours. In small- scale charts the contours are generally lines of sweeping curvature, and there is a tendency for the successive ones (1,000, 2,000, 3,000 fathoms, say) to " flow," so to speak, roughly parallel to each other. In large-scale charts as, for instance, those representing the sea bottom in the vicinity of an important harbour, such contours as the 5, 10, 20-fathom ones are often very complex. They may interdigitate, or dovetail, and there may be numerous *' islands " of shallower or deeper bottom.
Then the contours may be drawn for soundings in fathoms or metres. A fathom is nearly 2 metres and may be taken as such when we merely want a broad view of the features of the ocean bottom in depths of, say, 0 to 50
THE DEPTHS OF THE OCEAN
47
fathoms. But when an ocean is contoured, first by 100, 1,000, 2,000 and 3,000 fathom-lines and then by 200, 2,000, 4,000 and 6,000 metre-lines the results will not be quite the same.
The student should endeavour to think in terms both of fathoms and metres, and he should look on bathy metric contours merely as a general aid to visualising the forms of the sea bottom. He should not omit to study the uncontoured charts, trying to draw the contour lines for himself. Such work is most interesting.
We next consider small-scale charts of the various ocean bottoms.
Fig. 11. The region round the South Pole. The land outline, where known, is indicated by a heavy, continuous line ; the probable land outline is shown by heavy dots. The land (known and probable) is stippled. The zone between the land outline and the 1000-fathom contour is stippled. The soundings are in fathoms.
The Antarctic Ocean. — Fig. 11 is made from the latest Admiralty charts, but it will be seen that our knowledge of the immediate surroimdings of the South Pole is still very incomplete. Obviously there is an Antarctic Continent
48 AN INTRODUCTION TO OCEANOGRAPHY
which is deeply indented at two places, the AVeddell Sea and the Ross Sea. The boundary of the continent, for those places where land has not actually been seen, is taken to be near the great ice-barrier. The whole continent is covered by a permanent ice-cap which projects out into the Southern Ocean and breaks away as the great tabular icebergs.
The 1,000-fathom contour line round Antarctica is very imperfectly charted because of the paucity of soundings. Where it is not well known, it is represented by a broken contour line in Fig. 11. In some places even the 2,000- fathom contour is badly known. Clearly, however, our available data indicate two conditions that are very important: (1) the relatively shallow sea (between 1,000 and 2,000 fathoms) between Graham Land and Cape Horn, and (2) the relatively shallow sea (also 1,000 to 2,000 fathoms) between the Antarctic Continent and the Australian land.
The Arctic Ocean. (Fig. 12.) — The same conventional markings are employed. The land surface is left white and the continental sea margin (out to 2,000 metres in this case) is stippled. The shading represents, by its intensity, the depths. We see, then, a very extensive sea-area where the depth is everywhere less than 2,000 metres. On this are situated all the North American Islands, Greenland, Iceland, the Faeroes, the British Islands, the North Eurasian Islands, etc. Centrally to this archipelago is the North Polar Basni where depths exceeding 3,000 metres are to be found. The precise western boundaries of this basin are uncertain, for want of sufficient data, but its general form must be substantially as shown in the figure. It is clearly shut off from the Pacific by the shallow water in and near Behring Straits, both on the Arctic and Pacific sides : in the Straits themselves the sea is only 40 to 60 metres deep. It is shut off from the Norwegian Sea by water of about 800 metres in depth. The Norwegian Sea is itself a basin of at least 2,000 metres
THE DEPTHS OF THE OCEAN
49
in depth, and in it there are two large depressions where depths of over 3,000 metres are to be found.
Fig. 12. The Arctic Ocean.
Between Greenland and North America there is Baffin Bay, containing a trough where the depth varies between about 1,000 and 2,000 metres. The Norwegian Sea, which
50 AN INTRODUCTION TO OCEANOGRAPHY
interposes between Greenland and Norway is, itself, shut off from the Atlantic by the great rise which connects the British Islands with Iceland and Greenland. Along a narrow ridge joining the Shetlands, the Faeroes and Iceland the sea is only about 400-500 metres in depth. The existence of this Wyville- Thompson Ridge has important consequences for the general circulation of water in the North Atlantic and North Polar Oceans.
The general picture of the Arctic Ocean is, therefore, the reverse of that of the Antarctic. In the former we have a nearly land-locked ocean of considerable area and depth. It is shut off, in respect of the circulation of the bottom water, by the restricted Atlantic and Pacific openings, where the sea everywhere is much less than 1,000 fathoms in depth.
The Atlantic Ocean. — Fig. 13 represents in a simplified way, the main features of the Atlantic Ocean in respect of its depths. The contours that are drawn are 2,000, 4,000, 5,000 and 6,000 metres. The land surface is left white ; the region between this and the 2,000-metre contour (which we may regard as the Continental Shelf) is stippled ; the region between 2,000 and 4,000 metres is lightly shaded ; that between 4,000 and 6,000 metres is darkly shaded and the " deeps," that is the isolated patches where the depth is greater than 6,000 metres are cross hatched. All detail other than this is omitted because large scale charts are really necessary for its adequate representation.
Even the limited detail in Fig. 13 (which is only to be regarded as a sketch chart) shows that the Atlantic differs remarkably from the Pacific and Indian Oceans. This is due partly to the much greater amount of investigation that has been made in the Atlantic region and partly to certain structural features which we shall consider in Chapters V and X. Far too little oceanographic research has been made in the Pacific and Indian Oceans, and some speculations that are quite permissible with regard to
THE DEPTHS OF THE OCEAN
51
the Atlantic can hardly be elaborated in the cases of the two other oceans.
Fig. 13. The Atlantic Ocean.
The salient features of the Atlantic, with respect to its depth are as follows :
(1) The form of the Continental Shelf.
52 AN INTRODUCTION TO OCEANOGRAPHY
(2) The remarkable Central Atlantic Rise or Swell.
(3) The connection of S. America with the Antarctic Continent.
(1) The Continental Shelf. The region of sea adjoining the continental land and having a depth of less than 2,000 metres (approximately 1,000 fathoms) may, hereafter, be regarded as the continental shelf. It is an area of transition between truly oceanic and truly continental regions. It may have been sea or land, and undoubtedly it has been the locus of extensive geological changes. Transgressions of the land over the sea and vice versa have very frequently occurred in this region in the past. It is seen in characteristic form all round the coast of Africa and everywhere round the coast of South America, with the exception of Patagonia. In these regions the shelf is relatively narrow. Suppose that there is a similar wide zone of land on the margins of the Continents and call this the " continental rim " — then we see that both rim and shelf might be now sea and again land, but that the general form and extent of the continental elevations, and the adjacent oceanic depressions would still be much the same : that is in such regions as those that we have mentioned, where the shelf has the characteristic extension.
But off the coasts of North-East America and North- West Europe the case is quite different : here the shelf is greatly extended. Even between Florida and Newfound- land it is wider than the average, and from the northern shores of the Bay of Biscay on the one side, and from Labrador on the other, the shelf extends all the way across the North Atlantic Ocean. The two shallow inland seas, Hudson's Bay on the west of the Atlantic and the Baltic on the east, are situated on the shelf. The two deep inland seas, the Caribbean and Gulf of Mexico on the west and the Mediterranean on the east, on the other hand, contain depressions that are considerably over 1,000 fathoms : they are sea regions of quite another character.
THE DEPTHS OF THE OCEAN 53
(2) The Central Atlantic Rise or Ridge. This is quite the most remarkable feature of the Atlantic " bottom relief." South of the continental shelf, where it bounds Greenland, Iceland, the Faeroes and the British Islands, there is an extensive area of sea bottom on which the depth is between 2,000 and 4,000 metres : this has been called the Telegraphic Plateau because it was here that the first cables were laid. This now runs nearly south to about lat. 12° N. and then it turns to the E.S.E to about 2° S. lat. where it again turns nearly due south. The Rise is variable in width and tends to spread out laterally at the north and south extremities and contract very markedly at about the middle of its length. Further, it is nearly equidistant, at all latitudes, between Europe and N. America, and Africa and S. America. And, as the westward " nose " of Africa protrudes a little northerly of the easterly protruding nose of S. America, so the Rise alters the direction to remain equidistant between North- West Africa and Brazil.
Most of the Atlantic Islands are situated on the Rise. To the north there is the plateau which carries the Azores. Equatorially is a similar, but smaller plateau carrying St. Paul Rocks. To the south are Ascension, Tristan d'Acunha, Gough and Bouvet Islands. St. Helena is an exception in that it rises out of the West African Basin, while the Cape Verde Islands and the Canaries lie just outside the continental shelf.
Nothing like the Central Atlantic Rise exists in the Pacific or Indian Oceans. What it means, with regard to the origin and morphology of the Atlantic Ocean is not known, but a suggestion is made at the end of Chapter X.
(3) The American- Antarctic connection. The continental shelf of the extreme S.E. Coast of S. America is widely extended and projects down to the south-east : this is the " Patagonian Shelf " and on it are situated the Falkland Islands. Then the northerly directed peninsula of the Antarctic Continent, Graham Land, is also prolonged to
54 AN INTRODUCTION TO OCEANOGRAPHY
the north-east as an extended continental shelf : on this are the South Shetland, and South Orkney Islands, and beyond these, and rising out of water which is between 2,000 and 4,000 metres in depth, are the Sandwich Islands and those of South Georgia. It is not at all fanciful to see here much the same disposition as in the case of the Greater and Lesser Antilles in the West Indian region, that is a great arc of elevated sea bottom bent out from west to east. To this point, however, we return in Chapters V and X. Clearly there is evidence of a former actual, or at least, potential land connection between Antarctica and S. America. This, it will be remembered, was assumed in the Chamberlin hypothesis of the origin of the great oceans.
The connections of the Central Rise with the Continental Shdf. To the north the Central Rise expands out to form the Telegraphic Plateau, and then this bathymetric region (4,000 to 2,000 metres) thins out very greatly along the Eur-African and American Atlantic margins. There is really a very narrow zone of sea of this depth along the eastern and western borders of the xitlantic, although it is greatly extended to north and south. Now just as the central rise becomes continuous with the 2,000-4,000 zone off Labrador, on the west, and off the British Islands, on the east, so it tends to join up in a similar way in the south. OfE the Rio de la Plata there is a great extension to the east of the zone of 2,000-4,000 metre bottom, and this very nearly joins the Central Rise in about lat. 35° S. On the other side of the Atlantic, at about the latitude of Walfisch Bay, a somewhat similar tongue of 2,000-4,000 metre bottom extends out to the S.W. and actually joins the Central Rise between latitudes 30° to 40° S. Thus there is a kind of barrier or sill extending almost uninterruptedly across the South Atlantic between the parallels of 30° and 40° S. South from this the Central Rise terminates abruptly in a broad tongue of relatively high sea bottom, and south from which again is the South Polar basin.
|>o|
56 AN INTRODUCTION TO OCEANOGRAPHY
In describing these features of the Atlantic Ocean in respect of its depth, it is impossible to avoid conveying a somewhat exaggerated impression of the proportions of depth of water to distance. The sketch chart, Fig. 13, must necessarily do so, for the lateral scale is exceedingly small and the markings used to indicate the variations in depth do not suggest, in the least, the almost infinitesimal thickness of the water layer on the scale of the diagram. To correct this impression the reader must now consider Fig. 14.
In Fig. 14 the depth of water is graphed to the same scale as that of the horizontal distance. The earth is supposed to be diminished to a sphere of about 4 metres in radius and the actual curvature of this globe is represented on the diagram. On this scale (about 1 millimetre to the mile) the width of the Atlantic in the latitude chosen is about 3 metres, and so the section is broken up into segments, the left hand extremity of each joining on to the right hand of the segment just beneath. The section starts in Portugal and is so directed that it will pass through the Island of San Miguel in the Azores Archipelago. The relatively deep water off the European coast — the Cape Verde Basin — is well shown. Then comes the gradual shoaling towards the Azores Plateau on the Central Rise and the deepening again into the North American basin. The thickness of the black strip is proportional to the depth of water, and the thickness of the dotted parts at the two extremities and in the fifth segment is proportional to the height of the land above sea level. The numbers placed above the black strip represent distances in miles from the European coast.
The Pacific Ocean. — In most respects the Pacific differs remarkably from the Atlantic Ocean, and its depths do not show that approximate symmetry which is indicated in the latter region. Far less, however, is known about it, both with regard to its present condition and its geological
THE DEPTHS OF THE OCEAN
57
history, but nevertheless the contrast with the Atlantic
is very striking.
There are three evident regions (see also Fig. 58) :
(1) The South-W ester n Pacific. Here we have what has
been called the debris of a former South Asiatic Continent.
Fig. 15. Sketch Chart of the Pacrlic Ocean. The projection is a Mercator one and the depths are given in fathoms. The stippled areas represent -lea bottom where the depth is less than 1000 fathoms.
Australia and all the East Indian Islands are situated on the West Pacific continental shelf. On this region of sea- bottom there are, indeed, a number of small depressions where the depth may exceed 2,000 fathoms, but these do not obscure the obvious relations of the Australian and
58 AN INTRODUCTION TO OCEANOGRAPHY
Asiatic continental elevations. And though New Zealand is separated from Australia by a large depression of over 2,000 fathoms in depth, these Islands are also situated on a region of sea bottom with less than 1,000 fathoms of water over it, and this region is much greater in area than that of the New Zealand Islands. To the north of it are numerous outlying areas of sea with less than 1,000 fathoms of depth. Enclosing all this is the 2,000-fathom contour line which, on its south margin, passes into the continental shelf fringing the Antarctic Continent. Further, a prominent tongue of sea bottom, bounded by the 2,000- fathom contour line is thrust across and almost joins a similar tongue protruded out from the continental shelf of Western South America. A former connection with the Antarctic Continent is therefore suggested and, possibly, also one with South America.
(2) The Central Pacific. To the north and east of the vestigial (or abortive) Austral- Asiatic Continent are a number of patches of sea bottom where the water is less than 1,000 fathoms in depth, and on these are the multitudes of small islands that are so characteristic of the Pacific. These are isolated from each other and do not show that obvious continental relationship suggested by the Austral- Asiatic Shelf, but there is the strong suggestion in their distribution that they are outliers of the latter and belong to the same general region in the geological sense.
(3) The North and East Pacific. To the north, east and south-east of these Central Pacific shoal regions lies deeper water. Here also are most of the " deeps " : that is, depressions in the sea bottom of over 3,000, 4,000 and even 5,000 fathoms. This region and its significance we consider further in Chapters V and X.
The Indian Ocean. — Plainly the eastern and western regions of the Indian Ocean differ from each other in somewhat the same way as in the Pacific.
(1) Near Africa, and separated from it by water which is less than 2,000 fathoms in depth, is the large island of
THE DEPTHS OF THE OCEAN
59
Madagascar. Between this and India are a number of isolated patches of water that is less than 1,000 fathoms
in depth. These carry numerous islands : the Seychelles, Maldives, Laccadives, etc. Here also we seem to see outliers of a truly continental region.
60 AN INTRODUCTION TO OCEANOGRAPHY
(2) To the east are the deeper parts of the Indian Ocean. The eastern margin is formed by the continental shelf that bounds the Malay Peninsula, Sumatra, Java and Australia, and here the declivity into depths of over 2,000 fathoms is steeper than on the western side. Here, also, are the Indian Oceanic depressions of over 3,000 fathoms in depth.
(3) Centrally and rather to the south is an elevation of the bottom which approaches the South African continental shelf. On this are several groups of islands : the Crozets, Marions, St. Paul, Amsterdam, Kerguelen, etc. South of this again is the great Southern Ocean.
In some ways, then, there are strong suggestions of similarity between the Indian and Pacific Oceans and equally strong points of differences between both and the Atlantic. These resemblances and differences will become the more marked when we consider the kinds of continental margins that frame the oceans, as well as the general schemes of the latter. This we shall deal with in Chapters V and X.
CHAPTER IV THE SEA BOTTOM
It is held, in this book, that the present continental elevations and oceanic depressions are " permanent " in the sense that they have occupied their present positions throughout great parts of the geological period in Avhich the sedimentary rocks have been formed. This does not mean that the original oceanic floors, or the original continental elevations are now accessible to observation. Large tracts of the continental surfaces are covered by rocks that have certainly been deposited at the bottoms of shallow seas, and the ocean bottom in the region of the continental shelf is covered with materials of the same nature as those that are present on the continental land. There are sedimentary and " igneous " rocks that have been or are being eroded, and sediments that are in process of being consolidated into rocks like those that are exposed on the dry land.
So also the floors of the oceanic depressions are covered with deposits that conceal the original earth materials that existed there. Only about one to two feet of the depth of this layer of sea-bottom deposit is known, and that only at a very few spots. We have not even a hint, in the results of observations, as to the depth of these deposits, or as to the nature of the rock that lies underneath them. Very little is known as to the rate at which oceanic bottom deposits are being accumulated. Some few observations made by the engineers of the telegraph cable ships suggest that about one inch of deposit may be laid down in 10 years, but this is probably a maximum estimate and it is
61
62 AN INTRODUCTION TO OCEANOGRAPHY
certain that the rate of deposition is very much less in some other places. One of the most interesting and valuable results of oceanography would be a boring of the ocean floor to a considerable depth but, at present, there does not seem to be any practicable means of doing this.
The study of sea-bottom deposits. — The methods of collection have been described at the beginning of chapter III. A deep sea sounding usually involves the collection of a sample of the sea bottom at the same time, but this is only possible when the deposit is mud, or ooze, or sand capable of being forced up into the tubes of the sinkers (Fig. 9 (2)). The grab (Fig. 9 (5)) is able to lift up small pebbles, but any objects lying on the sea bottom and bigger than what we call pebbles cannot be collected by the apparatus described. Stones, boulders, large concre- tions, large bones, etc., can be obtained by dredging, and small dredges have been worked at practically all depths. The process of using such apparatus is, however, very laborious, and only an exceedingly small fraction of the oceanic abysses have been explored in this way. Dredge samples include everything that lies on the bottom and, as an estimate of the area traversed by the apparatus can be made, the sample obtained is roughly quantitative. A true sample of the muddy or oozy deposit cannot, however, be obtained by a dredge, as soft and fine material is usually washed out from the apparatus to some extent while it is being hauled up to the surface. For this purpose the cup dredge. Fig. 9 (6), can be used.
Shallow seas are sampled in just the same way but with modified apparatus — usually dredges with sharp, cutting, front blades that dig into the sea bottom to a small depth, and bags made of very closely woven, thick netting that retains fine material to a certain extent. In such samples the portion in the centre of the mass that fills the net is usually taken since this is less subject to w^ashing than the external parts. Grabs, such as that illustrated in Fig. 9 (5),
THE SEA BOTTOM 63
are, however, preferably employed in water of moderate depth. The foreshore is, of course, susceptible of direct observation and the methods of collection are obvious ones.
The description of a sea-bottom deposit is that of its constituent particles. Mineral substances come from the disintegration of rocks on the land, the broken-down particles being transported by currents or wave action, or blown into the sea from the atmosphere ; or they may result from the breaking down of volcanic materials ejected on to the sea bottom itself or reaching there after being water- or air-borne. Organic materials are the hard parts, or skelet-ons, of numerous species of plants and animals inhabiting the sea bottom itself or living in the upper layers of the ocean. The soft parts of these organisms putrefy and pass into solution, but the living or siliceous skeletons remain. Both mineral and organically formed deposits undergo chemical change when they lie for a long time on the sea bottom and so altered products, concretions, etc., are to be recognised.
The analysis of a sea bottom deposit includes, then, the identification of the constituent particles and of their mode of origin — that is, the kinds of minerals present, whether simply resulting from the disintegration of massive rocks, or chemically altered ; the species of plants and animals as deduced from a knowledge of the skeletons of the living organisms and the chemical composition of the deposit, as a whole, that is, for instance, the percentage of lime, silica, alumina, iron, manganese, etc., in the dried material. In addition to this treatment mechanical analyses may be made, that is, the material may be described in relation te the prevalent sizes of its particles, as, for example, in the case of the sands that form the bulk of the shore deposits. Such a description becomes necessary when we consider the transport of fine sands, muds and oozes by the agency of moving water.
The original nature of sea-bottom deposits.
(1) hiorganic Materials. This category includes
64 AN INTRODUCTION TO OCEANOGRAPHY
terrestrial, extra-terrestrial and chemically transformed substances.
Terrestrial materials are such as come from the continental land mass and are almost entirely the products of aqueous denundation carried down into the sea by rivers as muds and sands. They are shore-erosion materials resulting from wave action on the rocks of the shores and distributed mainly by tidal and wind currents ; volcanic maierials coming from the disintegration of lavas ejected on to the ocean floor by submarine volcanoes, pumice (either itself or in disintegrated forms) settling on the sea bottom after floating in the water, and volcanic dusts carried by winds and finally settling on the surface of the sea.
Extra-terrestrial materials are very rare though of immense interest. They are meteoritic dust coming into the earth's atmosphere from outer space. They are recognised in ocean-bottom materials as the " cosmic spherules," Fig. 19. They are microscopic, roughly spherical particles, black or bronze in colour, laminated or crystallised in texture. They usually consist of iron or of magnetic iron oxide. They are only to be found, as a rule, in those deeply-laid deposits that accunmlate with exceeding slowness, and they are recognised there because of the relative scarcity of materials of other origin.
The chemically altered materials we consider later.
(2) Organic 7naferials. Those of vegetable origin are the remains of calcareous algae such as the corallines, and broken-down coccospheres and rhabdospheres. These are relatively rare. Diatom oozes, made up of the siliceous skeletons or frustules of these minute plants, occur much more frequently.
Calcareous materials of animal origin are very much more abundant. Here we have molluscan shells, and a great variety of other calcareous skeletons. Siliceovs tnaterials of animal origin are not so abundant, but nevertheless constitute an important category of ocean-bottom deposits.
THE SEA BOTTOM
65
Sea-bottom deposits : the Challenger classification. — The
classification made by Murray and Renard ought to be known. It is :
Deep-sea deposits laid down in water over 100 fathoms in depth
Shallow- water deposits laid down in M^ater less than 100 fathoms in depth Littoral deposits laid down on the foreshore
Red clay Radiolarian ooze Diatom ooze Globigerina ooze Pteropod ooze Blue mud Red mud Green mud Volcanic mud I Coral mud
Sands
Gravels
Muds
J Sands I Gravels I Muds
Pelagic deposits
Terrigenous deposits
This classification considers both the immediate place of origin of the deposits and the place of deposition in respect of the depth of water. Terrigenous deposits are the results of disintegration of the rocks that occur on the land. These erosion-products are transported from the land by rivers, ice and winds, and are then distributed on the sea bottom near the land by wave-action, ocean currents, tidal streams and winds. Therefore they are not found in bulk at a very great distance from the land because of the relative feebleness of these transporting agencies. Nevertheless, very fine muds of land origin may be found at considerable distances from the shore and thus the terrigenous deposits are placed both inside and outside the 100-fathom contour line.
66 AN INTRODUCTION TO OCEANOGRAPHY
The pelagic deposits, on the other hand, have their place of origin, not on the land but in the sea itself ; they consist of the remains of animals and plants that live there, and they are generally associated with masses of water of great depths. But the organisms whose skeletons form the pelagic deposits are really more abundant in relatively shallow water than they are in the ocean over great depths, and such materials as compose (say) globigerina and diatom oozes are probably laid down on quite shallow sea bottoms in much greater quantity than they are on abysmal ocean floors. In shallow water, however, they are covered up by the much greater quantities of terrigenous materials resulting from land erosion and so they are almost unrecognisable.
Here we are considering the oceanic depressions on the great scale as contrasted with the great continental elevations. Further, we are considering the continental shelf, not as any particular gradient of sea bottom, or sea floor of any particular limits of depth : it is the transition region between the oceanic depressions and the continental elevations and, during the vicissitudes of geological history, it may have been now sea bottom and again land surface. A classification of sea-bottom deposits which presents a more illuminating point of view may, therefore, be substituted for that of Murray and Renard.
Categories of sea-bottom deposits. —
fLittoral deposits
(1) Continental deposits -i Shallow- water deposits
[Terrigenous muds. ["Shallow- water neritic
(2) Neritic deposits < deposits
[Oceanic neritic deposits.
(3) Pelagic deposits The deep-sea oozes.
These various categories we now consider further. Obviously the antecedent to the accumulation of very extensive shore and shallow sea-bottom deposits is the
THE SEA BOTTOM 67
existence of a vast land area over which disintegration of earth materials is proceeding on the great scale. That means continental land, or at the least, extensive island land areas ; great rivers with large volumes of water, fairly rapid flow and extensive catchment areas ; large glaciers capable of transporting heavy materials and with notable power of erosion ; desert areas from which light sand and dust may be blown by strong winds ; shallow, marginal continental seas in which there are rapid tidal streams capable of shifting bottom deposits, and where wave-action may be exerted to the maximum degree — all or some of these conditions. Obviously they are not present on oceanic islands, and round these we find nothing quite like the continental shelf and the characteristic continental shallow-water deposits. Continental deposits are not defined very precisely by the depth of sea in which they are laid down or by the distance from the shore at which they are found. There will be a grading of the materials in respect of the average sizes of the constituent particles, and this will depend on the strength of the transporting agencies.
Neritic deposits are quite different in their nature. They are organic in mode of origin, consisting of the dead skeletons of marine animals and plants. There are very clear distinctions between such deposits as shelly gravels, or " coral " bottoms, on the one hand, and mud or sand- banks, on the other. The former are called " neritic " and the latter terrigenous. Neritic deposits laid down off continental shores consist predominantly of the dead and partially or wholly disintegrated skeletons of marine animals and plants which have their most favourable environment in shallow seas of relatively low salinity. They are there in abundance because of the proximity of the land. Neritic deposits off oceanic islands may be very different in their nature, again because of distance from extensive land areas. The differences between continental and oceanic neritic deposits traces back to the natural
68 AN INTRODUCTION TO OCEANOGRAPHY
habits of the organisms whose dead skeletons form the materials in question.
Pelagic deposits are due to organisms which have their most favourable environment in truly oceanic regions. They are independent of the land, living and reproducing generally at or near the surface of the ocean.
The various kinds of sea-bottom deposits may now be described, premising that for details of distribution and identifications of the species of minerals and organic remains the original memoirs must be consulted.
Littoral deposits. — These are the materials present on the foreshore, that is, the region between high and low water tide-marks. Characteristically they consist of boulders, gravels, sands and muds. It will be convenient to reserve a description of them for the following chapter, where we deal with the continental margins.
Shallow- water deposits. — The most characteristic, most abundant and most widely distributed sea-bottom material is sand. In general this is the bottom material present in most shallow seas. Its main source is the shore-land itself, because the most abundant constituent of the latter is generally a quartz- containing rock of some kind. Wave- action, assisted by atmospheric agencies, breaks down the shore materials, whether rock cliffs, boulder clay, or whatever they may be. Stones and boulders suffer attrition by being moved against each other by wave action. Disintegration of the mineral constituents occurs, and many of the latter undergo chemical alteration. The more resistant species remain and undergo grading. Of these resistant minerals the most abundant is quartz sand which is, therefore, the characteristic bottom material in shallow seas.
The grading of the shallow-water deposits is the most important question involved in our discussion, but so little adequate investigation has been made so far that little can be said. In a very general way, however, we find that the coarser materials, gravels of various grades,
THE SEA BOTTOM 69
occur nearest to the foreshore ; the coarser sands further out to sea and the finer sands still further away from the land. This is what is to be expected when we consider the transporting agencies. The effect of the waves is felt most strongly on the beach itself, then on the foreshore and, finally, on the shallow sea bottom just off the land. To what degree wave-action diminishes with increasing depth of water is not precisely known, but in a sea of over 50 fathoms in depth the effect becomes slight. Tidal streams are most rapid in shallow water near the land, and particularly in bays and narrow estuaries, and they diminish in velocity very greatly as the distance from the land and the depth of water increase. Wind currents are also most pronounced in water that is very shallow. In general, then, the power of the movements of the sea to shift and transport stones, gravels and sands decreases with distance from the land. Further there is a resultant effect of all these agencies. The general pounding action of waves will take place at a certain angle to the shore line and this angle will depend on local conditions. Tidal streams reverse themselves in direction with every high and low water change, but not completely, so that they tend to transport materials along certain resultant paths. Wind currents, too, have prevalent seasonal directions, and it must generally happen that they also have a resultant effect, taking the whole year into consideration, on each part of a coast line.
Therefore, waves, tidal streams, wind currents, and the marine channels of rivers transport the materials of the shallow sea bottom ; so that they roll stones, gravels, sands and muds in this way and that but, on the whole, in a certain general direction dependent on the sum of all the local conditions. The general directions will vary from place to place, although they may, to some extent, be seasonal. The same particles will be moved more rapidly near the coast than off shore. Coarser particles will in general, tend to remain on or near the beach and foreshore.
70 AN INTRODUCTION TO OCEANOGRAPHY
and finer ones will tend to be transported into deeper water.
Large inequalities of the coast line, that is bold promontories or headlands shaping out bays and estuaries, influence the directions and velocities of the tidal streams. Thus " tidal races " or local rapidly-running streams, or slack ones, or eddies are established. On the sea bottom beneath rapidly running tidal streams there will generally only be rock, or boulders, or stones, or gravel, while beneath large tidal eddies fine sand and mud will tend to accumulate. Again unperiodic changes occur. Exceptional spates, or freshets, or melting snow-falls may greatly increase the volume and velocities of river streams in their estuarine portions, and so sand-banks, embankments, etc., may be temporarily breached and lead to changing river and tidal channels. These react on the directions of the tidal streams and so on the places in which coarser and finer bottom materials tend to accumulate.
A grading into accumulations of coarse and fine gravels, sands and muds occurs, then, as the result of the disintegrating and transporting agencies which we have very briefly mentioned. The distribution of the shallow- water deposits in a particular sea area is, therefore, the result not only of the nature of the rocks composing the coast line, but also of the average effect of the transporting agencies. The processes involved will be, as a rule, exceeding complex, and their investigation must always be difficult and laborious in the extreme.
The composition of a shallow-water deposit is studied by identification of the rock fragments or mineral particles contained in it. Quantitative separation of the species of mineral particles may often be effected by the use of heavy liquids of suitable densities ; by the use of an electro- magnet where magnetic particles are concerned ; by chemical analyses, etc. Grading of particles which are mineralogically similar is effected by the use of metallic sieves with apertures of known size (where the particles
THE SEA BOTTOM 71
are large) or by elutriation in water currents of known velocities (when the particles are small ones).
The Terrigenous Muds — Muds are, in general, deposits in which the particles are very small and rather heterogeneous in respect of their chemical composition. Very fine sands, with minute particles, are distinguishable from muds and silts in that they consist predominantly of quartz particles. In muds and silts alumina is present to a much greater extent than in sands : this and the small- ness of the particles is, perhaps, the only notable character in which they differ. Clays are finer still than muds and " bind " to a greater degree in drying. It must be noted, however, that there are no satisfactory class-characters that can generally be utilised in distinguishing between muds, silts and clays.
Deposits that are variously coloured, very fine in respect of the sizes of their particles, that remain easily in suspension in sea water and discolour the latter, occur very generally on the foreshore and at the bottoms of deep and shallow seas. Neglecting the foreshore, in the meantime, we consider the muds of the off-shore regions. Their origin is, in general, the disintegration of continental and volcanic rock, both sedimentary and plutonic. The condition for their occurrence on the sea bottom is relatively still water : still because of great depth, the absence of a current or tidal stream, or the presence of an eddy, in the central portion of which fine suspended particles settle to the sea bottom. But there are no precise boundaries, as a rule, to a mud patch and the transition between a typical mud and a clean, angular, or rounded sand may be a very gradual one. Both on and off the foreshore muddy sands and silts, of very indefinite compositions as regards the fineness of the particles and the proportions of the various grades present, may occur. In shallow bays and estuaries muddy water, during times of rough sea, or when strong spring tidal streams are running, is familiar to every one. This is the result of a natural process of levigation,
72 AN INTRODUCTION TO OCEANOGRAPHY
the lighter particles of the rather heterogeneous sea bottom being borne by the motion of the water. The process would, of course, lead to an ultimate grading of the sea bottom in respect of the sizes of the particles were it not that the heterogeneity along the coastal sea bottom is continually maintained by the disintegration of more earth materials.
Off the mouths of rivers, mud is also formed by a process of physical agglutination. Fresh water entering the sea in rivers contains colloidal " clay," that is the aluminous particles are in a state which is not that of mere physical suspension. They are too small to fall to the bottom as sediments but they are not in a state of true solution. When this river water mixes with that of the sea, containing ionised sodium chloride in solution (see Chap. VI) electrical changes occur and the colloidal clay passes into the form of a true suspension. The particles of the latter then sink as a sediment and the water becomes " muddy " until the sedimentation is complete.
Outside the region of " shallow " water, where wave motion becomes very feeble at sufficient depths and where the tidal streams have very small velocities, fine particles in suspension finally sink to the sea-bottom. There is, then, a " mud line " round the continents and this we may take to be somewhere outside the 100-fathom con- tour— where exactly will vary to a great extent. As a rough approximation we may put it near the outer margin of the zones of " terrigenous " deposits on Figures 17 and 18, but it must be noted that observations on the distribu- tion of sea-bottom deposits near the land are far too few to enable us to chart the mud line in detail in any sea area. However, somewhere towards the 1,000-fathom line the gravels, sands and silts of the zone of terrigenous deposits disappear and are replaced by the various muds of the table on p. 65. But, in exceptional cases the terrigenous muds may be found at far greater depths than 1,000 fathoms.
THE SEA BOTTOM 73
Blue Muds. These are fine muds that are black or bluish-black in colour. Their limits of depth are so variable that nothing more precise can be said than that they occur towards the edges of the continental shelves. Their origin is as we have stated above — that is, they are the finer detritus resulting from the disintegration of the shore materials or they are the result of the distribution seawards of the colloidal clay brought to the sea by river water. The colour is due to a process of chemical reduction leading to the formation of ferrous sulphide. The typical blue muds are very widely distributed.
Green Muds. These are only varieties of the general deep-sea mud fringing the continental shelves. The colour (which is rarely strikingly " green ") is due to the presence of the interesting mineral called glauconite. This is a hydrated silicate of aluminium, iron and potassium but its chemical constitution is not satisfactorily established. As a rule it occurs as casts filling the cavities of the shells of foraminifera (see fig. 19) and thus in rather small particles. Its mode of origin is not well understood but it is very probable that it results from a process of organic putrefaction followed by a chemical synthesis. It has much significance inasmuch as it points to the way in which potassium compounds are withdrawn from solution in the sea. It is rather rare, far more than the typical blue muds, but its distribution is wide. It is known to occur off the Cape of Good Hope, off Eastern Australia, off Japan, off the Atlantic coasts of the United States of America and in the Ceylon Seas. In general it is said (by Murray) to occur on the Continental slopes off high and bold coasts where there is a seasonal alteration in the strength of the ocean currents.
Red Muds. These are simply a variety of the general continental slope mud coloured reddish by iron oxide. Typical localities are the Yellow Sea and the continental shelf off the coast of Brazil. Relatively to the blue-black muds, the red variety is rare.
74 AN INTRODUCTION TO OCEANOGRAPHY
Volcanic Muds. The characteristic minerals in these deposits are those arising from the disintegration of volcanic rocks — thus there are liparite, basaltic minerals, andesite, volcanic glass, broken down pumice, etc. Where volcanic rocks occur on the coasts the weathering of these produces volcanic sands and muds in the immediate vicinity. So also the loci of submarine volcanoes are also those of volcanic muds. But plutonic materials are very widely distributed on the sea bottom. Dust is ejected into the atmosphere during eruptions and may settle everywhere on the surface of the sea. Pumice may float on the sea for prolonged periods before becoming water-logged and sinking to the bottom to disintegrate.
Neritic Deposits — The sea-bottom deposits considered so far are mainly inorganic in nature : they consist of land detritus, the disintegration products of " clastic " agencies. They are transported from the land by rivers and are distributed on the sea bottom by movements of the ocean. The distributing agencies grade the deposits to a certain extent.
Nevertheless almost any sample of sand or mud taken from the foreshore or the adjacent shallow sea is sure to contain materials that are organic in their origin and do not come from the detritus of the land. Almost any such sample will contain, perhaps, a few per cent, of calcium carbonate, and microscopic examination will show, as a rule, fragments or particles which can often be identified as broken down shells or skeletons of various animals and plants. Such mineral matter of organic origin may be briefly mentioned : —
Molluscan shells and their fragments ;
The limy plates and spines of starfishes, sea-urchins,
ophiurids and crinoids ; The calcareous tests, or skeletons, of alcyonaria,
polyzoa, gorgonids, etc. ; The limy tubes of marine worms ; The calcareous tests of foraminifera ;
THE SEA BOTTOM 75
The limy spicules of sponges, alcyonaria and tunicates ; The calcareous exoskeletons, or carapaces, of Crustacea ; Teeth, earbones, etc., of whales and sharks. On a great scale — the calcareous skeletons of solitary and reef-building corals. The above are animal remains, and they consist of calcium carbonate in the form of aragonite or calcite. Siliceous animal remains are : — ■ The skeletons of radiolaria ; The spicules of sponges. There are also calcareous plant remains. These are : — Calcareous skeletons of algae — the " corallines " ; Coccospheres and rhabdospheres with their dis- integrated products — the coccoliths and rhabdoliths. (These are also calcareous algae). The siliceous plant remains are: —
The skeletons or frustules of diatoms. These materials are not restricted to any particular sea bottom, with respect to depth : they occur both in continental or pelagic deposits. In shallow water in particular and, in general, on the continental shelf they are smothered by the huge quantities of sand and mud laid down at the same time.
In certain places, and on practically all coasts, however, there are deposits which consist predominantly of some of the kinds of organic remains mentioned above. This is even the case with the deposits of the foreshore, but we consider these in the next chapter. The accumulations of calcareous materials of organic origin that we find here and there on the sea bottom near the land are —
The Shallow-Water Neritic Deposits. The principal deposits of this kind are the shelly gravels and sands that are to be found at the sea bottom in depths out to (say) 50 fathoms. The characteristic remains in such are the whole or broken valves of bivalve and univalve shellfish.
It is, of course, very rarely that a neritic deposit will consist entirely of such materials and when this is the case
76 AN INTRODUCTION TO OCEANOGRAPHY
there is sure to have been some process of segregation at work. To the same category of deposits belong shelly sands which are the result of the attrition of shelly gravels.
Nullipore gravels and sands. Nullipores are calcareous algae, that is, plants like the ord'nary seaweeds but having the soft plant tissues impregnated with calcium carbonate to the degree that the plant body has become stony in texture. The two principal forms in British waters are Lithothamnium and the ordinary nullipore — Corallina.
Polyzoan gravels and sands. Polyzoa are colonial animals of small size which have much the same external appearance as the zoophytes. As a rule they form incrustations on the surface of stones, gravels and even coarse sands. They are never very abundant in a shallow- water deposit in point of mass, but they may be very noticeable because of the way in which they form thin crusts on the surface of truly terrigenous materials.
The distribution of the shallow-water neritic deposits. This is not arbitrary. Just as the distribution of the various grades of gravel, sand and mud on the continental shelf depends on the nature of the adjacent land, and on the prevalent currents, tidal streams, winds, etc., so there are factors which rule the distribution of the neritic deposits. These are called the "ecological factors." There is something in the geological nature of the inorganic materials of the sea bottom ; or in that of the adjacent land ; some particular set of currents or tidal streams that carry food materials ; some favourable admixture of fresh and salt water ; some particular materials in solution in the rivers entering the sea in the neighbourhood ; a particular annual range of temperature, or winter minimal, or summer maximal values of the latter ; some set of currents or tidal streams that carries eggs or larvae to a certain part of the sea. All or some of these conditions may lead to the establishment, throughout a great number of years, of colonies of molluscs, nullipores, echinoids, etc., on
THE SEA BOTTOM 77
restricted parts of the sea bottom. In course of time their skeletons accumulate to form a neritic deposit — one which does not come from the materials of the land but which is, to a predominant degree, conditioned by the local peculiarities of the adjacent land.
The study of such ecological conditions has become one of the most attractive departments of biology. The methods are the ordinary ones of the field naturalist, but they include rather specialised apparatus for collecting the living animals and the deposits from the sea bottom. The use of the fisherman's trawl-net (but with a very thick foot-rope which will bump over stones and rough ground), or of the old fashioned naturalists' dredge gives us qualitative information as to the kinds of organisms that inhabit the sea bottom. Quantitative estimates are made by means of the Petersen " bottom-grab," an apparatus that scoops up a given patch of soft sea bottom, say one-quarter square metre. The deposits so obtained are then examined at leisure and the mineral and organic materials present are identified, measured, counted, etc. Most of the animals and plants living on the sea bottom {demersal species) at some early stage in their life-history live swimming or passively drifting in the water and usually near the surface — this is their pelagic or planktonic stage — and they are collected by using a small conical net made of fine (miller's) bolting- silk cloth. Such nets are towed slowly from boats, and either at the surface of the sea, or at any convenient depth down to the bottom. They can be so constructed as to give approximately the volume of water that passes through their meshes and the individual animals or plants caught by them can be identified and counted. Finally the physical conditions of the sea can be studied : the directions of tidal streams by means of floats, etc., the temperature of the water at the surface and various depths by means of specially constructed thermometers ; the salinity of the water and its composition in respect
78 AN INTRODUCTION TO OCEANOGRAPHY
of substances necessary for life (see Chapter VI), are determined by chemical analysis. All such investigations are seasonal ones since the conditions are always changing.
Thus the investigation of marine' animal and plant communities comes into relation with the study of the neritic deposits now forming on the sea bottom. From the results of such studies (which have not, so far, been very extensively made) it becomes possible to reconstruct the ecological conditions that prevailed in the past when neritic deposits, such as may be found in the fossil form, were being laid down.
Oceanic Neritic Deposits. — These occur in the neigh- bourhood of oceanic islands. The main character of the latter, from one point of view, is that they have no continental shelf — that is no flat, or relatively fiat margin of sea bottom round them. As a rule the sea in the immediate vicinity of an oceanic island sinks down more steeply towards the ocean bottom than does that along a continental margin. The slope is never "precipitous " to the oceanic abyss but it may often be 1 in 10 and rarely as steep as 1 in 1 . The islands themselves are usually of volcanic origin ; they are small ; there is no great surface exposed to subaerial or submarine erosion and great rivers are absent. Their material, therefore, makes but a small contribution to the sea bottom in the neighbourhood, and the latter is what we term neritic. There are volcanic sands and muds, and along with these materials are the remains of organisms that live in the water of the ocean and have much the same general biological characters as those that contribute to the formation of the deep sea oozes. In fact what we have in the neighbourhood of an oceanic island is an oceanic- insular, rather than a continental-terrigenous, inorganic sea bottom, along with a pelagic deposit. The pelagic constituents are not, as a rule, disintegrated to the same extent as are those that we fmd at the bottom of the deep
THE SEA BOTTOM 79
oceans. The shells and other remains are usually cleaner and more nearly entire and there is less muddy or clayey constituents. Such sea-bottom materials in the immediate vicinity of oceanic islands are, therefore, mixtures in various proportions, of volcanic and coral sands.
Coral Formations. — True corals, as apart from the "corals" that are often marked on the official charts, or the deposits so named by fishermen (which are often nullipores or polyzoan remains) are animals belonging to the group called Hydrozoa. They are often solitary, living on the sea bottom as units. Mostly the hydrozoa are the common " zoophytes, " that is, colonial groups. The individuals or zooids, or polyps, are connected together by a common fleshy material and are arranged in variously formed communities simulating plant forms. The common, fleshy, connecting material, and the greater parts of the bodies of the polyps are invested by a stiff, flexible cuticle formed of chitin. The colonies are rooted to stones on the sea bottom. Zoophytes are an important group of animals in shallow water. Because of the perishable nature of the chitinous investing skeleton they leave little or no remains, to contribute to the formation of the deep or shallow-water deposits.
Instead of a chitinous cuticle many of these animals secrete a calcareous one (just as many marine algae, or polyzoa do). These are the calcareous corals. A number of them form very massive colonial skeletons which grow to form the typical reefs.
Reef- building corals have rather a limited distribution, as will be seen by consulting Figures 17 and 18 at the end of this chapter. They are quite absent over the greater part of the Atlantic, being found only in the West Indian region and of? Bermuda. They are highly characteristic of the Australian Seas and of what we have called the Central parts of the Pacific and Indian Ocean. Figs. 17 and 18 give only their general distribution : on a larger scale chart very numerous patches of coral sea bottom would be
80 AN INTRODUCTION TO OCEANOGRAPHY
marked. But, in broad terms, coral reef formations are characteristic of the Central and Western regions of^jth^^ Pacific and Indian Oceans and curiously deficrent in the Eastern regions. They are quite absent in the Northern and Southern latitudes. This distribution is bound up with that of oceanic temperature as we shall see in Chapter VII.
Coral Reefs. These are to be regarded rather as great geological features than neritic bottom formations, nevertheless their nature is described by the latter term. Massive coral reefs are present in the gigantic scale as the Great Australian Barrier Reef. This runs for 1,200 miles along the north-eastern coast of Australia at an average distance of about 20-30 miles from the continental land. Inside the Barrier there is a channel with a general depth of about 15 to 20 fathoms, and within this again, and forming its continental margin are a line of inner reefs. Between the inner reefs and the land are shoal channels.
Fringing Reefs have much the same characters represented on a much smaller scale than the great Barrier Reefs. They may be placed on a foundation of sedimentary or plutonic rock, or on a basis of old coral rock. Abrasion of the foundation by wave action results in the formation of a very shallow submarine flat bounded shoreward by a low flat beach often terminated by cliffs which may be undercut. On the outer edge of the submarine flat the fringing reef is built up by the polyps. As it is formed it becomes broken down by wave action and fragments of the dead coral rock fall down into the sea to form what is called a " talus." Often this seaward slope of the fringing reef is called a "coral precipice," but the terms " talus " and " precipice " ought not to suggest the land features to which they were originally applied. The actual slopes of the " talus " and " precipice " (when these can really be stated numerically) are, as a rule, far less than the corresponding land ones. Such a slope as that formed by loose stones and earth lying at their
THE SEA BOTTOM 81
*' angles of repose " on a hillside can hardly exist in the sea, for wave action will break up and distribute the materials of the "talus" and spread them out over the sea bottom at an angle which may be a rather small one. Precipices on the land may be actually vertical, but such features are exceedingly rare on the margins of continents and oceanic islands.
Between the fringing reef and the land the sub- marine flat is usually deepened to the extent that it forms a shallow channel navigable by smaTMboats. Just how this deepening occurs is not quite clear. It jsascribed to slow disintegration of the old coral rock foundation, with the formation of coral mud. When there are tidal streams these run with increased velocity, along the submarine flat inside the fringingj"eef — the latter, it may be stated, is usually broken do^^ii here and there so that there is access of the sea to the shallow region inside the reef. The disintegrated coral mud may, therefore, be scoured away by rapid tidal stream, or even removed in solution.
On the outside of the reef the coral polyps continue to live and build fresh material because there is a plentiful supply of food and oxygen — more than there is on the inside margin. The deposition of new coral rock will keep pace with the destruction of the material of the reef by wave action. Abraded rock falls down the seaward " talus^" continually adding to the latter, and evidently the reef will tend both to increase in mass and to grow seaward from the land. Thus the barrier form of reef comes into existence. But when the formation becomes sufficiently great its character must alter. The boat channel between the fringing reef and the land will deepen as coral mud is removed, and tend to become a passage for oceanic water, and new reef building corals will tend to, establish themselves so that the inner reefs become formed.
Atolls are reefs of roughly circular, crescentic, or arcuate form. The profile of a typical coral atoll, that of Funafuti,
82 AN INTRODUCTION TO OCEANOGRAPHY
is represented, in section, in Fig. 10, 4. The gradient on the outer margin of the reef is relatively steep, more so at the extreme margin than ofE shore and, as a rule, a depth of over 1,000 fathoms occurs on the convex side of the atoll at a distance of a mile or m6re. On the concave side, that is, within the encircling reef, the slope is slight and the depth is only a few fathoms. Inside the lagoon so formed the bottom consists predominantly of living corals, but outside there is the usual " talus " formed by the disintegration of the reef by wave action. This is covered by coral sands containing the remains of pelagic organisms.
Modes of Origin of Coral Reefs — The well-known hypotheses of the mode of origin of coral reefs in general are((lpthat of Charles Darwin and (2) the Murray-Semper one. According to Darwin's hypothesis reefs are built up on ocean bottoms that are undergoing depression. The usually cited case is that of a volcanic cone which is slowly being submerged. Round the margin of the cone reef-building polyps are living and are raising the shallow sea-bottom to near the surface and as the cone keeps on sinking the growing reefs keep pace with it and maintain their level at a fathom or two beneath the surface. Wave action leads to disintegration and fragments of coral rock are piled up to form a beach. Inside the latter a shallow flat forms and this becomes a channel in the way that we have indicated above. Thus a fringing reef is established but with continued subsidence of the enclosed cone the fringing reef becomes an atoll encircling a shallow lagoon. It is not clear why the growing corals do not gradually fill up the latter : to explain this we have to assume death of the enclosed corals, disintegration of their calcareous matrix and removal of coral mud either by scour, or by solution, or by both processes. The hypothesis involves a general depression of an enormous regiorL_gf the Central and Western Pacific, but the assumption fits in with the general idea formed from a broad survey of
THE SEA BOTTOM 83
the ocean depths — that in this region we see the " debris of a drowned continental elevation " and so also, perhaps, with the other great coral sea, that between l^Iadagascar^ and India, Clear evidence is, however, still wanting
^EEafsuch is actually the case. In the Fiji Islands of the Pacific it is, however, probable that extensive submarine
Tiats on which coral reefs are situated have actually been formed by a downward tilting of the ocean floor.
The Semper-Murray hypothesis dispenses with the assumption of subsidence of the ocean floor in the regions where reefs are being formed. Let there be an elevation of the latter to begin with : the dead skeletons of pelagic organisms rain down on this and, in time, raise its level near to the surface. This assumes, be it lioted, a general elevation of an ocean floor on which the skeletons of pelagic organisms subside but, given a local elevated region there will be a tendency for such remains to accumulate there more rapidly than in the adjacent deeper water, where they will dissolve to a greater extent than in the relatively shallow water over the elevation. But an apparently fatal objection to the hypothesis is contained in the now established theory of isostatic equilibrium of oceanic depressions and continental elevations. The weighting of the ocean floor (whether
"over a local elevation or elsewhere) by the accumulation of pelagic deposits ought to lead to subsidence. (See the following chapter).
The theory of isostasy, however, was not formulated when the Semper-Murray hypothesis of coral reefs was in general acceptance. Let, then, the accumulation of pelagic deposits gradually raise the level of a submarine elevation : by and by the latter will come so near to the surface that coral polyps become able to establish themselves and build encircling or fringing reefs. To account for the existence of the lagoon, or shallow channel, it is assumed that solution of the lime of the reefs occurs ; that there is less food and oxygen in these still enclosed
84 AN INTRODUCTION TO OCEANOGRAPHY
waters and that abrasion and removal of disintegrated material proceeds. Here again the results of recent research cause difficulties: in such enclosed waters as those inside a fringing reef or atoj'l deposition of lime, rather than solution and removal, ought to occur.
Are Coral reefs formed on subsiding regions ? This is the main problem and it is one for geological investigation. Where such investigation has been most successful it seems to be established that reef areas are also, in general, areas of subsidence of the ocean floor.- In the West Indies, for instance, there are satisfactory evidences of depression or downward tilting of a large area of sea bottom in the seaward direction : there are deeply indented bays and valleys, steeply sloping sea bottoms, drowned escarpments and submerged peat deposits. Most important of all, there is geological evidence (from the West Indies and Southern United States) that fossil reefs occur and that these rest unconformahly on eroded submarine platforms. There is also evidence that this is the case with living reefs. Coral reefs appear, therefore, to become established on submerged rock platforms. This is the case with fringing and barrier reefs and also with atolls. There is no evidence that these grow up round subsiding cones, but rather that they are based on submarine flat summits.
The Glacial Control hypothesis of Coral Reefs. There ought, therefore, to be a flat platform underneath reefs, whether barriers, fringing reefs or atolls and this should show signs of erosion. How to account for the formation of these submerged platforms which existed prior to the deposition of reefs upon them, is a problem that must be solved before constructing a general theory. According to Daly, Andrews, Humphreys and others the formation of the platforms, and the subsequent growth of reefs on them, are due to changes in water level rather than actual movements of the earth crust relative to the earth as a whole. During periods of extensive glaciation enormous volumes of water have been withdrawn from
THE SEA BOTTOM 85
the ocean and deposited on the continental elevations as the great ice-caps. This, and the consequent gravitative effect of the increased continental masses, has been estim- ated to lower the ocean level by as much as 36 fathoms. At the same time the ocean temperature became lowered and the conditions for coral growth may have become unfavourable except in a narrow equatorial zone. Land surfaces unprotected by growing reefs were thus exposed to wave action and wide and fiat terraces were cut out round continental lands and large islands, while the tops of smaller islands were planed down to a general level. On the melting of the ice-caps the sea temperature was probably raised as the result of the reversal of operation of whatever causes led to the glaciation and, at the same time, the water level became raised by the melting of the accumulated ice. Corals then began to build on the edges of the terraces and flat submerged island summits. In the former cases fringing and barrier reefs became formed and in the latter, atolls. In the channels between the fringes and barriers and the land, and in the spaces encircled by the atolls, conditions for coral growth must have been rather less favourable, nevertheless it goes on there, and both channels and lagoons are assumed to be filling up rather than being deepened by scouring and solution.
Ecology of Coral organisms. The problem is, therefore, one for geology on the one hand, and marine biology on the other. Coral polyps are plankton-feeders, capturing and ingesting'minute pelagic organisms in the usual way, but they have also the holophytic mode of nutrition. That is, they contain in their fleshy tissues green cells, which are really symbiotic algae with which the polyps become infected at certain stages in their development. These symbiotic algae are able, by reason of the chlorophyll that they contain, to take up carbon dioxide from its solution in the sea water and then to synthesise carbohydrate from this in essentially the same way as
86 AN INTRODUCTION TO OCEANOGRAPHY
the ordinary green plant does. Obviously this manner of feeding must be reckoned with in any discussion as to the conditions in which corals thrive best. ;- As to the general depths at which the polyps live, reproduce and build : we must distinguish between the deep and shallow species. The former may be found in water of 50 fathoms or over, but the massive reef builders live best in water of 27 fathoms or less. They ^prefer water which is free from sediment although they have limited powers of removing sediment from their exposed parts. They can only settle on a hard sea bottom. The most favourable temperature is about 18° C. and the salinity that appears to be most suitable is about ,27 to 30 per mille (see Chap. VI). This is a fairly wide range and must include a variety of hydrographic conditions.
Ajrather brisk circulation of the sea water is favourable, as in the case of all sessile, marine animals. A fairly strong light is favourable and in the complete absence of sunlight many, but not all, corals die. The importance of light is obvious when we consider the role of the symbiotic algae in the tissues of the polyps, for the chlorophyll of these can only function in the presence of light above a minimum intensity. At a depth of 50 fathoms the lighting is favourable. The red is still present, though weak, and the blue and ultraviolet constituents are still powerful.
In the lagoons and shallow channels within fringing reefs, the conditions must be rather less favourable than on the seaward sides of the reefs. The water circulation is less brisk so that nutritive matter may be more scanty than on the outside margins. There must be a tendency for the withdrawal of carbon dioxide from solution, not only by the green plants present but also by the algal cells of the coral polyps. There may be increased evaporation where the water is still. There will be a Mgher temperature.
THE SEA BOTTOM 87
Now the shallow water in such regions is usually saturated with hme. This does not exist in the form of calcium carbonate (which is sparingly soluble) but in the form of the hydrogen calcium carbonate (which is much more soluble). There is an equilibrium between theJIJOg in the atmosphere and that in solution in the sea water. The latter is not present so much in ordinary solution as in combination with calcium carbonate to form the acid-salt, CaH2(C0g)2, or some such compound. If the sea takes up CO 2 from the atmosphere, there will be a tendency for solution of ordinary limestone so as to form more of the hydrogen calcium salt. If, on the other hand, CO 2 is withdrawn from the water in any way some of^tjie^bicarbonate will dissociate and normal calcium carbonate will be precipitated. Also if the alkalinity of the sea water (see p. 14^) is increased, the same precipitation must occur. In the lagoons and shaUow channels there may be enormous numbera^q^f denitrifying bacteria. Certain ordinary bacteria can reduce proteid matter (arising from decomposed animal and plant substance) to nitrate and nitrite, and it is in such forms that inorganic nitrogen usually exists in solution in sea water. The denitrifying bacteria are able to reduce nitrates to nitrites, nitrites to ammonia and ammonia to elementary nitrogen which then returns to the atmosphere. But there will be a tendency for the ammonia, as soon as it is formed, to combine with CO 2 in solution in the sea water. Then the calcium bicarbonate will dissociate into COg and calcium carbonate (CaCOg) which will be precipitated as minute balls of aragonite.
In lagoons and on the leeward sides of reefs there will therefore, be a tendency to the deposition of calcium carbonate precipitates. This will cause sediments and lead to the result that reproduction and growth of the reef-building corals will be less active there than on the outside margins of channels and lagoons. The net effect will be — not solution and deepening of the
88 AN INTRODUCTION TO OCEANOGRAPHY
lagoons and channels — but deposition of sediments and filling up. "
The processes in operation are, however, very numerous and complex, and far too little is known about them to utilise in the elaboration of a satisfactory theory of coral formation.
The Pelagic Deposits. — Categories of marine organisms. From our present point of view it is convenient to arrange all marine organisms — plants and animals — in three groups : the Benthos, Nekton and Plankton. To the Benthos belong all rooted, sessile, sedentary or semi- sedentary organisms. These include the rooted shore and sea bottom algae, the zoophytes, corals (massive reef -building and solitary species), the alcyonarians, sea-anemones, sponges, polyzoa, all starfishes, sea-urchins and crinoids, nearly all the molluscan shellfish, most of the larger Crustacea, all the barnacles and very many of the marine worms. These animals live, either attached to hard objects on the sea bottom, or burrowing on the sand and mud, or moving freely on the sea bottom though incapable of making long journeys. Many of them, as we have seen, have calcareous or siliceous skeletons which accumulate to form the neritic deposits.
To the Nekton belong the whales, other marine mammals, all the adult fishes, some of the cephalopods (the larger squids) and some of the Crustacea. They are animals that have well-developed locomotory organs and they can make long migrations. They mostly have calcareous skeletons and they contribute both to the neritic and pelagic deposits.
To the Plankton belong nearly all the unicellular plants and animals (the Infusoria, Flagellate Protozoa, Peridinians, Diatoms, etc.), all the smaller Crustacea (Copepods, Ostracods, Schizopods, Amphipods, etc.), many marine worms, some of the Coelenterates (the Medusae, Siphonophores, etc.), many molluscs (the Heteropods, Pteropods and some of the Cephalopods) and nearly all
THE SEA BOTTOM 89
the eggs and larvae of the Benthos and Nekton. These organisms contribute to the pelagic sea bottom deposits.
Density of life in the Sea. The most prolific region is that of the shallow water just beyond the littoral zone and extending out to about 50 fathoms. The littoral zone itself teems with life (sand and mud-living molluscs, sea-weeds, barnacles, etc.). The littoral and shallow water regions contain most of the algae and it is exceptional for these to inhabit deeper water. In the case of the greater Laminarians, however, marine algae may grow up towards the surface from water that is as deep as 50 fathoms. The characteristic region of the Benthos is the sea between the foreshore and the 50 fathom contour line, but the general abundance of life there depends largely on the nature of the sea bottom and that of the adjacent land ; upon the sea temperature ; upon the nature and origin of the neighbouring ocean currents and tidal streams and upon the presence of fresh water coming down from the land. This region is that of the greatest abundance of fishes and also of the planktonic organisms.
Beyond the 50 fathom contour-line demersal (or bottom- living) algae rapidly diminish in abundance because of the great diminution in the intensity of light which reaches the sea bottom. Many benthonic and nektonic animals are herbivorous and so depend directly upon plant organisms for their nutrition. Carnivorous animals feed on other animals, whether herbivores or carnivores, but obviously all animal life must depend directly or indirectly on plant substance. There is, then, a poverty of plant life on the sea bottom which is deeper than 50 fathoms, and this density of plant life becomes less as the water becomes deeper and has a lower degree of illumination. Benthonic animal life therefore decreases in much the same ratio.
The Abysmal life. In the greatest depths (or, in general, everywhere outside the continental shelves) Lfe of all kinds becomes very scanty on the sea bottom. There is
90 AN INTRODUCTION TO OCEANOGRAPHY
no plant life at all, because of the darkness. All the principal groups of animal life are represented in the abysmal fauna, but the prevalent species of fishes, Crustacea, echinoderms, molluscs, worms, and sponges are different from those that inhabit shallow water. So far, however, as their evolution can be traced they are closely related to the shallow- water species and have, in all probability, come from the shore areas originally and have become adapted to the abysmal conditions.
In all cases the mode of nutrition differs from that of the shallow-water species. Abysmal animals subsist largely by eating the bottom deposits ; at the very low temperature (see p. 191) of the water on the oceanic floors putrefaction becoming greatly retarded. Pelagic animals living in the surface layers of the ocean die and fall down to the sea bottom. There the fleshy parts decompose slowly — so slowly that they form the only renewable source of food for the abysmal animals. This poverty of food material is the principal reason for postulating a very low density of demersal life in the great ocean.
Categories of planktonic life. A description of the organisms that make up the oceanic plankton cannot be given here and only the main groups that contribute to the sea-bottom deposits can be mentioned. The planktonic plants are, of course, holo phytic in their mode of nutrition : they synthesise starch and sugar from the CO 2 in solution in the water. Their nitrogen is obtained from nitrates, nitrites and ammonia (also in solution). Lime and silica and other inorganic constituents have the same source. In all respects their condition of life is pelagic, appertaining to oceanic conditions and not depending, except remotely, on the land.
The Heteropods and Pteropods are small, pelagic molluscs which drift about with oceanic and wind currents at the surface of the sea. They have delicate calcareous shells. They exi3t in enormous shoals and furnish one of the
THE SEA BOTTOM 91
principal sources of food of the whalebone whales. The Foraminifera are protozoa, the protoplasmic cell bodies of which are enclosed in thin calcareous " tests " or chambered cells. They are holozoic in their mode of nutrition, that is they ingest the organic substances of other planktonic animals and plants, or they may perhaps be saprozoic or saprophytic, that is, they may absorb organic food materials from solution in the sea water.
The Peridinians are unicellular organisms which are animal-like in their structure. But they contain chlorophyll and have the same mode of nutrition (holophytic) as have the typical green plants. They have cellulose tests impregnated with silica.
The Diatoms are unicellular plants which are, of course, holophytic as regards their nutrition. They have siliceous shells, or frustules.
The Copepods are minute Crustacea which are (as are all the above groups of organisms) extraordinarily abundant in the sea.
Density and Distribution of the Marine plankton. In general all the planktonic groups are more abundant in shallow water near the land than in the truly oceanic regions far out at sea, and this is because of the more abundant food materials brought down into the sea by rivers. It is because of this wealth of planktonic life in shallow water and the nutritive influence of the rivers, that the ordinary life of the shallow seas is more abundant than that of the deep oceans. The plankton is much more abundant in cold polar, subpolar and temperate sea zones than it is in the warmer tropical and subtropical oceanic zone. This is because the denitrifying bacteria are more abundant, and function more rapidly in warmer than in colder seas. The effect of these organisms is to break down nitrates, nitrites and ammonia into elementary nitrogen. Thus these materials, which are the indispensable sources of nutriment for all marine plants
92 AN INTRODUCTION TO OCEANOGRAPHY
and animals, are destroyed to a much greater extent in the warmer than in the colder seas.
Finally the plankton inhabits the upper, illuminated strata of the ocean.
The reader is now prepared to consider the various deep sea oozes.
Pteropod Ooze. — This material is characterised by the presence of the entire and broken shells of Pteropod molluscs. Of all the deep-sea oozes it is the least abundant. It exists in two or three small patches in the Western and Central Pacific ; in a few larger (but still relatively small) patches in the Atlantic, mainly on or near to the Central Rise, and in the Mediterranean. Generally it is found in relatively shallow water of about 400 to 1,500 fathoms in depth far from the land, and therefore on low elevations of the sea bottom. On these places the temperature of the water is fairly high and has a restricted annual range. The deposit is not an individualised one and easily passes into some form of globigerina ooze. In it about 35 species of Pteropods and 32 species of Heteropods have been found. When dried it is a coarse white powder containing about 90% of calcium carbonate. See Fig. 19 for the appearance of this and other deposits and Figs. 17 and 18 for their approximate regions of distribution.
Globigerina Ooze. — This deposit is, as a rule, a dirty white, rather coherent powder when dried. About a half to two-thirds of its weight is due to calcium carbonate, mainly present in the form of detritus. When the finer materials are washed away a coarsely granular residue is left, and this is the " ooze " that is usually represented in figures (as, for instance, in our Fig. 19). From one half to a third of the substance is insoluble in acid and one or two per cent, of this consists of the siliceous skeletons of radiolarians and diatoms. Recognisable minerals, such as quartz particles, volcanic glass, plaglioclase, augite, magnetite, mica, etc., are
THE SEA BOTTOM
93
\gg \60
Wo /so
Fig. 17. The deposits on the bottom of the Atlantic Opean. The area of terrigenous deposits is stippled. Elsewhere G=globigerina ooze; P=pteropod ooze ; R = red clay; Ra = radiolarian ooze ; D= diatom ooze and C= coral formations.
94 AN INTRODUCTION TO OCEANOGRAPHY
usually present in very small quantity. The rest of the material is fine amorphous clay.
Globigerina ooze has a very wide distribution, being found in all the oceans but particularly in the Atlantic. Pelagic Foraminifera are, in fact, of universal occurrence, though they are least abundant, and the number of species is also least, in polar waters. In equatorial zones about a couple of dozen species have been recognised. The ooze occurs all over the Atlantic, close up to the Eastern and Western continental shelf deposits of mud. It extends, in the Atlantic region to about 72° N. lat. and to about 60° S. lat. a difference which is due to the European Stream circulation and the consequent higher sea temperature in the North. It occurs to a limited extent in the deposits of the deeper part of the Norwegian Sea, It is, however, nearly absent in the Antarctic south of 50° to 60°. It occurs all over the Indian Ocean except on the eastern margin opposite to the Asiatic- AustraHan continental shelf. It is present in the Pacific but again mainly on the eastern side. The southern limit of its distribution in the Pacific is about 60° but in the Indian Ocean it does not occur in quantity much below about 43° to 45°.
Its vertical range extends from about 400 to about 3,000 fathoms. In the Atlantic it is even found at 3,500 fathoms, but 2,800 to 2,900 represents the extreme limits in the Indian and Pacific Oceans. Typically, however, Globigerina ooze is a deposit characteristic of sea bottoms that lie between about 1,200 to 2,200 fathoms in depth.
Radiolarian Ooze. — This deposit is characterised by the presence of the siliceous skeletons of the protozoan organisms called Radiolarians. It is a dirty grey powder drying to a very coherent clay-like substance. It always contains much clay and the siliceous remains, even when the diatom and sponge skeletons are considered, are not so predominant as are the foraminiferal tests in Globigerina ooze. It contains silica and calcium carbonate, but the
THE SEA BOTTOM
95
96 AN INTRODUCTION TO OCEANOGRAPHY
proportions vary remarkably in samples taken from various ocean bottoms. No typical Radiolarian oozes occur in the Atlantic and there is only a small patch on the eastern side of the Indian Ocean. Even in the Pacific, where it is most abundant, it only occurs centrally and as a belt on the eastern side between about 5° and 15° N. lat.
It is not known to occur in sea bottoms of less than about 2,000 fathoms in depth and it may go down to 5,000 fathoms.
Diatom Ooze. — The characteristic of Diatom ooze is the presence, in great abundance, of the siliceous frustules of pelagic diatoms. It is a fine, white, coherent powder when dried. With the diatom remains are numerous sponge spicules and some radiolarian skeletons and the inevitable, amorphous, clayey constituents.
It does not occur at all in the Atlantic and Indian Oceans, nor in the North Polar basin. Along the extreme north of the Pacific, between latitude about 40° to about 55°, there is a broad band of diatom ooze sloping northerly from West to East and just outside the zone of continental muds. It is highly characteristic of the Antarctic Ocean, where a broad band encircles the earth. This band has a maximum breadth of about 20° in longitude 20° E. and it is estimated to have an area of about 10 J millions of square miles.
The extreme vertical range of diatom ooze is about 600 to 4,000 fathoms, the lower limit being found in the Antarctic and the higher one in the North Pacific. The usual range of depths is 600 — 2,000 fathoms.
Red Clay. — Red Clay is, when dried, a firm coherent powder which is usually red-brown in the Atlantic and chocolate-brown in the Pacific. It is, chemically, a hvdrated aluminium silicate and its origin is mainly the decomposition of pumice and other volcanic minerals. Lime is always very scarce, and in red clays derived from the deepest sea bottoms lime may be totally absent.
Nerific -. Shelly and Null!fyore^ra\/el
W^-W%
and GlohiCerina cxQR
1^
Diatom oose
Man0onesp nodule
Neritic: Nu/lipo re gravel
Pelagic: Globi^erina ooje
Farbone or w/ja le
Shark's tooth
Aferitic - Coral Sand
'Pelagic: J^adiolarian ooje
Cosmic sl:>herulp
Fig. 19. Typical deep-sea and shallow-water deposits. The shelly and nulliijore gravels are reduced ; the coral sand, pteropod and globigerina oozes are moderately magnified ; the radiolarian and diatom oozes are highly magnified. The cosmic spherule is highly magnified while the manganese nodule, the whale earbone and the shark's tooth are slightly reduced.
THE SEA BOTTOM 97
Red clay is essentially a residue — the end product of a series of solutions and decompositions.
It is characteristic in that it contains the rare sea bottom materials — the earbones of whales, the teeth of sharks, synthetic minerals like phillipsite, manganese-iron nodules and the very peculiar cosmic spherules. The earbones of whales, like the teeth of sharks, consist of very hard, highly resistant materials and they are " common " in red clay (relatively to the other deep sea oozes) because the rate of formation of red clay is so much slower than that of any other deposit. This is also the case with regard to the cosmic spherules : these can hardly be found in any of the other oozes but may he expected in the red clay. Phillipsite and the other synthetic products (the peculiar manganese nodules, for instance) are probably to be associated with the long series of decompositions of which red clay itself is a terminus.
It is the most widely distributed of all the deep sea bottom deposits, being found in all the oceans. It occurs on both sides of the Atlantic between latitudes of 40° N. and S. It is widely distributed over the Pacific and Indian Oceans but mainly on the eastern sides. Something like it occurs in the North Polar Basin but it is shut off from the Antarctic Continental area by the broad band of Diatom ooze which extends close up to the outer margin of the continental terrigenous deposits. It occurs, in general, everywhere at depths greater than about 2,700 fathoms and it is present at the bottoms of the "deeps." To this statement there are, of course, exceptions and it is often difficult to distinguish between a Radiolarian ooze and a Red clay.
The Pelagic Oozes in general. — The two sketch charts Figs. 17 and 18 give only the very broadest idea of the distribution of the deep sea oozes. Of necessity the latter are regarded as being quite distinct from each other, but this is not really the case. Pteropod ooze passes gradually into the surrounding Globigerina ooze and samples may
98 AN INTRODUCTION TO OCEANOGRAPHY
he examined which it is impossible to place with certainty in either category. Radiolarian ooze shades off into Red Clay and so on. The hard contours drawn in Figs. 17 and 18 are therefore quite artificial and are intended to represent only the general distributional features.
The vertical range is equally vague. In general terms the serial arrangement is Pteropod ooze (on the shallower bottoms), Diatom ooze, Globigerina ooze, Radiolarian ooze and Red Clay on the deepest bottoms). To some extent the vertical' distribution depends on solution of the materials. If the ratios to each other of Pteropods, Foraminfera, Diatoms, Radiolarian and volcanic debris at the superficial strata of the ocean were everywhere the same, then a sorting out of these materials would occur as they sank slowly to the bottom. The fragile shells of the Pteropods would dissolve first of all, then those of the Foraminifera, then the Diatom frustules and lastly the Radiolarian skeletons. That means that lime becomes the less abundant the deeper is the sea bottom, for its solution is apparently accelerated by increasing water pressure. At a certain rough limit of depth, then, no lime will be found and this is actually the case. Then the siliceous skeletons of the Radiolaria, Diatoms and Sponge also dissolve with great depth and so they disappear completely, as organically formed structures, in the very deep ocean basins. In the Red Clay we have the insoluble residues of the calcareous and siliceous skeletons, as well as the end-products of the volcanic materials and the synthetic products for which a low temperature, a high pressure and, no doubt, some other conditions are essential.
The distribution at the surface of the ocean, of Pteropods, Foraminifera, Diatoms, Radiolaria, etc., is not, however, everywhere the same for the ecological con- ditions there vary from region to region. So also the differences that we know do obtain in the ocean with respect to the vertical and horizontal water movements affect the paths along which these organic remains fall towards
THE SEA BOTTOM 99
the bottom. The result is irregularity of distribution such as is not representable on small-scale charts. Also the observations at our disposal are far too few to justify us in drawing the contours limiting the regions of deposition in any other way than very roughly.
Finally we may again remind the reader that the figures of deep sea oozes given in the books are usually representations of the characteristic organic remains rather than of the oozes themselves as they come from the sounding tubes. What one sees in the latter is a mass of general detritus resulting from the attrition of the characteristic shells, etc. This contains fragmented shells, tests, frustules, spicules, etc., some of them entire and it is from the presence of these that the deposit is identified.
CHAPTER V THE OCEANIC MARGINS
On any ordinary small-scale map there appears to be a very distinct boundary line between the land and the sea. On a large-scale chart, however, this line of demarca- tion may disappear completely, revealing a region of transition which is sea at one time and land at another.
On p. 101 we have three representations of the same area, Morecambe Bay, on the West Coast of England, but on different scales. (I) is a reproduction in line of the ordinary small-scale map and we see that a quite distinct boundary line apparently separates the water from the adjacent land area. The Bay is here depicted as a sea-area. (II), however, represents a small part of the same region on a mu6h bigger scale and the markings are those that we find on an Admiralty Chart. The approximate boundary is the line which is marked " L.W.O.S.," that is, low water of ordinary spring tides, and this is the average limit of the sea during one day or so in each fort- night. A little distance landward of this line would be another (not usually marked on the Admiralty Charts) giving the limit, low water of ordinary neap tides, and this would be the average limit of the sea on one day (or thereabout) every fortnight and intermediate between the days of highest spring tides (see p. 101).
Near the arbitrary line representing the " shore " on the Chart ought to be another pair of boundary lines — the high water marks of ordinary spring and neap tides. The water of the sea, then, may be anywhere between the high and low water tide marks in normal conditions. At certain times, twice or so during the year, however, the high water marks are higher than usual while the
100
THE OCEANIC MARGINS
101
low water marks are lower than usual. This occurs regularly during the periods of " equinoctial tides," but it also occurs exceptionally at other times when unusual gales of wind may raise or lower the expected heights of the tides.
Fig. 20. I is a small-scale map of the West Coast of England ; II is a sketch copy of the Admiralty chart representing the part of I marked by circle 2 ; III is also a copy of the Admiralty large-scale chart marked in I by the circle 1.
In addition to this broader region of shifting of the water level, the coast line may still be more indefinite
102 AN INTRODUCTION TO OCEANOGRAPHY
Fig. 20 (III) represents a " salt-marsh " from the same general area. We see here that the shore is cut up in a most complex manner by a multitude of small, sinuous channels up which ordinary flood tides penetrate. At very high spring tides most of these channels are obliterated and the sea covers much of the area represented by them and the miniature islands, capes, straits, etc., of the vegetation-covered part of the marsh.
This indefiniteness of the land boundary is characteristic of a large part of the world : wherever there is a significant rise and fall of the tides. In some parts (Liverpool and in the Bristol Channel, for instance) this extreme rise and fall may be about 30 to 40 feet. In other places (as in the Firth of Clyde) the rise and fall may not exceed 10 feet. In the Mediterranean it is only two or three feet.
Further, a very little observation will show that on many parts of the coast the average high water tide markings are not constant — even in the space of time throughout which one person may observe them. That is, there is significant erosion of the coast line in some places or filling up by the deposition of material elsewhere. Such changes are quite noticeable when maps, made at various times during a century or two, are compared : they are due to erosion, or deposition of land materials. Channels, ports and river courses may become silted up, or new channels may be eroded out. Such changes may be slow, progressive ones, or they may be catastrophic (when, for instance, they are caused by very exceptional tides, or floods in rivers entering estuaries). They may be variable, reversing themselves because of conditions that are exceedingly difficult to trace.
Finally, when we consider the changes of land and sea that occur in those periods of time which we call geological ones we see that they occur on a great scale, altering the outlines of the continents themselves. We have now to do with earth movements of enormous magnitude and acting very slowly but with huge effects.
THE OCEANIC MARGINS 103
The Foreshore. — On most parts of the oceanic margins there is, then, a zone situated between the tide-marks. The latter are conventional, being taken to be the junctions of land and sea at the moments of mean high and mean low water at intervals of approximately 15 days. The levels to which the tides rise and fall at these fortnightly intervals are variable and so the marks " L.W.O.S.T." on the Admiralty charts represent the average water levels. (To this matter we return in Chapter VIII). The zone between high and low water marks is called the Foreshore, or sometimes the Strand. Below it, in this country, is the legally conventional " sea." Above the average level to which the tide rises once a fortnight, at springs, is the legally conventional " land." The foreshore is a kind of no-man's land regarded as the property of the Crown, unless there are immemorial, prescriptive, or manorial titles to its partial monopoly for some purpose or another.
The area of the foreshore depends not only on the vertical range of the tides but also on the shore gradients. The vertical range of the tides can only be given an average value for it changes from day to day. The area of fore- shore, therefore, undergoes a continuous change : (1) daily, being greatest at low water ; (2) fortnightly, being greater still at low water of spring tides ; and (3) six-monthly, being greatest at low water of the highest equinoctial spring tides. It is least at the corresponding high waters of the above tidal conditions. It depends also on the shore gradients, vanishing altogether on those coasts formed by rocky cliffs. There the vertical change of tide is marked in various ways on the face of the cliff but there may be no change at all in the sea area of such coasts. On those coasts, then, where the land is flat and low and easily eroded, and where there is a large tidal range, the extent of the foreshore may be very great. In Morecambe Bay it amounts to about 100 square miles.
High water tide-marks are nearly always recognisable on such flat coasts by lines of debris (sea-weeds, etc.) on
104 AN INTRODUCTION TO OCEANOGRAPHY
the foreshore. On falling tides, that is, during the few days following the highest springs, several such temporary tide-marks can be seen, each showing the level to which the sea rose on a particular day. The levels of the highest tides of all are usually indicated by some fairly permanent markings, such as low sand-hills, deposits of shingle, low earthy cliffs, etc.
Low water tide-marks are not so easily recognised, though they can often be indicated by the presence of typical animals and plants. Thus the large Laminarian sea-weeds mark the approximate levels (on suitable shores, of course) of the lowest tides.
The Foreshore Gradients. — On a rocky coast the land surface immediately adjacent to the sea is usually a low plateau bounded seaward by a cliff. The latter may be the typical, precipitous wall of rock, usually inclined backward at a small angle to the perpendicular ; rarely overhanging or undercut but often with caves due to wave action. At its base, and exposed by low tides, there is often a rocky shelf (though as often, perhaps, the cliff may be continued below the sea level), or there may be a terrace of shingle or boulders or gravel. The cliff may be of softer material than rock : boulder clay deposits, for instance, and then there is often a terrace at its foot, partly above and partly below high water level. This terrace corresponds to the talus at the foot of an escarpment on the land, but here its materials have been disintegrated and spread out horizontally by waves and tidal streams. We may find, then, on coasts of these, or similar materials a rather steep, or even an approximately vertical slope from the land plateau to the upper limit of the foreshore.
Even where the land is flat and low and composed of easily eroded materials, something similar to this rather steep gradient may exist. The coastal substance is disintegrated by wave action, but the stones, shingle, etc., which are separated out tend to become piled up on the
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Upper Figure— A vertical section of the Lancashire coast near Formby. The
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Lower Figure— A vertical section passing out nearly due west from Bradda
Head m Isle of Man. The dark part represents the cliff and sea-bottom. The
coast IS rocky, composed of slate.
The scale is the same in both Figures and is identical for horizontal and vertical
dimensions. It is 1 millimetre for every 10 feet.
106 AN INTRODUCTION TO OCEANOGRAPHY
upper margin of the foreshore as a low rise, rather than a cliff. The rest of the shore materials become abraded ; muds are removed by the tidal streams and deposited elsewhere, and quartz sand is spread out on the lower levels of the foreshore or on the sea bottom adjacent to the latter. But even when the land is exceedingly easily eroded and stones and boulders resulting therefrom are rare, the loose sand at the upper margin of the foreshore becomes blown by winds on to the land and then compacted together here and there by vegetation. Such wind-blown sand may even accumulate behind a foreshore consisting of shingle or boulders, the latter being left behind after the separation of the sand by wind action. Thus there arise the low sand-hills so characteristic of many coasts. These give to the shore-line a fairly large gradient down to the foreshore.
The latter has, itself, a gradient which is always slight but is variable with a host of conditions. It will depend, for instance, on the nature of the land, the tidal range, the velocity of the tidal streams, the amount of shelter from heavy seas, etc. We must, however, remember that it is an average gradient. There are " features " on the foreshore : shallow channels and gutters through which land water drains ; " lakes " which depend in some way upon scouring eddies ; patches of raised stones or boulders covered by sea- weed. There are soft parts or " quick- sands " : these are generally found near to the land and not often far down the foreshore, and they appear to depend on land water oozing up through the sand on to the surface of the latter. Thus there is considerable diversity of feature on the foreshore as we see from Fig. 20, or almost any large-scale Admiralty chart of a coastal area where there is a considerable rise and fall of the tide.
The Shallow-Water Bottom Gradient. — At the lower margin of the foreshore the gradient again increases slightly : that is, the sea usually deepens rather rapidly at first and then more slowly. Then there is the gradient
THE OCEANIC MARGINS 107
of the upper part of the continental shelf : this upper part may be taken as the sea bottom where the water is less than 100 fathoms in depth. Beyond that it is usually said that the gradient again increases, though there are remarkably few good series of soundings that illustrate this point. We must note particularly now that the gradient of the upper, shallow part of the continental shelf must also be thought about as an average one. In reality there is, here again, very considerable diversity of bottom relief. Particularly in shallow bays and estu- aries do we find this variety : sinuous channels, sand-banks, bars, ridges, etc. It is all on a very small scale when compared with those features of ocean bottom which we considered in Chapter III, but it is interesting, and of immense practical importance for navigation and fishery. It may be illustrated by almost any Admiralty chart of the sea in the immediate neighbourhood of an important harbour. There the soundings are usually very numerous so that the natural detail is all represented.
This variety of feature that the shallow-water seas exhibit contrasts strongly with the bottom relief of the ocean basins. But we must always remember that the soundings on the latter regions are very few indeed when compared with those that have been made on the sea bottom within the 100 fathom line. Even, however, when we bear this in mind, and entertain the possibility that more detailed examination of the oceanic abysses might reveal a greater variety than is known so far, it is, neverthe- less, probable that the latter regions are monotonous in their flatness.
The Marginal Seas. — All the region of sea bottom within the 1,000-fathom, or the 2,000-metre contour lines (for the limit of depth cannot be precisely stated) we are regarding as the continental shelf. This corresponds roughly with the zone of continental sea-bottom deposits and there are structural reasons why we regard it as belonging rather to the areas of earth-elevation than to those of earth-depres-
108 AN INTRODUCTION TO OCEANOGRAPHY
sion on the big scale. On this continental shelf margin, then, are situated a number of smaller seas. Some of these are partially land-encircled ; others are partially separated from the oceanic basins by chains of continental islands while others again are depressions of the shelf itself, areas of the latter where the depths exceed those roughly limiting ones that we have mentioned. These water areas have been called the Marginal Seas and there are several categories of them.
The Epeiric Seas. These are the smaller sea-areas that have, as a rule, very limited depths (not much exceeding 100 fathoms and usually a great deal less) and which present the characters of great gulfs or bays with relatively narrow communications with the ocean. Ex- amples from the Atlantic region are Hudson's Bay, the Gulf of St. Lawrence, Baffins Bay, the Baltic, the North Sea and the White Sea. It is difficult to differentiate them always from the great bays or straits such as, for instances, the Bay of Biscay or the Irish Sea, and some of their individuality must be traced to economic or historical reasons. They are mostly very shallow sea- areas with depths of 50 to 100 fathoms. They are also characterised by a low salinity and large temperature ranges, on the physical side, and with marked faunal features, from the biological side. Their presence is rather characteristic of the North Atlantic region.
The Epi-Continental Seas. Just as the Epeiric seas characterise the Atlantic margins so do the Epi-continental seas seem to belong typically to the Pacific. They are the Behring Sea, partially enclosed by the Aleutian Islands and the Kamschatka Peninsula ; the Sea of Okhotsk, between Siberia, Kamschatka and Sakhalin and bounded towards the Pacific by the Kurile Islands; the Sea of Japan ; the Yellow Sea, which is a great gulf with a shoaling of the bottom emerging to form the arc of the Lu Chu Islands, across its opening ; the China Sea, partially enclosed towards the Pacific by
THE OCEANIC MARGINS 109
the Philippines and Formosa ; the Celebes Sea, between Borneo, Celebes and the Philippines, and the Banda Sea, between Celebes, New Guinea and Australia (See Figs. 58-9, Chap. X). Fairly well-marked characters distinguish the Epi-continental Seas : depths which, in places, exceed 1,000 fathoms and boundaries towards the ocean formed by chains of islands arranged in arcs with their con- vexities outwards from the continental land. Doubtless there are physical and biological characters, but as to these much detail is not yet available.
The Epi-continental seas are rather characteristic of the Western sides of the Pacific and Atlantic Oceans. On the Eastern side of the former there is no example except the untypical one of the Gulf of California. This Eastern side of the Pacific, has, we shall see, an altogether different structure. Nor is there anything in the Indian Ocean quite like them, and the only examples in the Atlantic are the Caribbean Sea and the Gulf of Mexico. The Norwegian Basin may, perhaps, be regarded as such a sea bounded to the south by Iceland, the Faeroes and Northern British Islands, but the pecuUar arcuate boundary with its concavity facing continental land is wanting in this case.
The Mediterraneans and Relict Seas. — These may be mentioned here although they do not belong to the general category of marginal seas.
The Mediterraneans are the Roman Mediterranean and the North Polar Basin. The Classical Mediterranean- Black Sea region is almost completely enclosed, for the communication through the Straits of Gibraltar is very restricted and permits of only a very peculiar water cir- culation. The Caribbean and Gulf of Mexico basins have been called a Mediterranean Sea but, obviously, they are separated from the Atlantic in the same way as are the Epi-continental Pacific Seas — ^that is by an arc of numerous islands, the Greater and Lesser Antilles. The North Polar Mediterranean has been called the " Arctic Ocean "
110 AN INTRODUCTION TO OCEANOGRAPHY
for so long that it is difficult to think about it by another name : it is, however, separated from the Pacific in the same way as the Roman Mediterranean is separated from the Atlantic and it is separated from the Atlantic by an extensive region of continental shelf. The Mediterraneans have depths that belong to the order of the truly oceanic abysses and their bottom deposits may have the pelagic character. Further, there are indications that they have belonged in the past, to the oceanic earth-depression regions rather than to those of elevation — which latter character distinguishes the Epeiric and Epi-continental Seas. The latter we consider to be of the same nature as the seas that have transgressed on the land during the geological periods : their loci have alternately been land and sea. The Mediterraneans, on the other hand, share with the great oceans the character of relative permanence.
The Relict Seas are vestigial sea-areas that have lost their connection with the ocean and have become land- locked, or nearly so. The Epeirics, Epi-continentals and Mediterraneans which have been mentioned are, it should be noted, in working communication with the ocean and have some importance when the general circulation , of the latter is concerned : Relicts have no such influence on the water circulation of the ocean. They are the remains of seas that have transgressed upon the continental land during periods of depression of the latter and have then become isolated from the ocean as the result of subsequent continental elevation.
They are the Black Sea, Caspian, Sea of Aral, Lake Ontario, Lake Champlain, probably Lake Tanganyika, Lough Neagh (in Ireland) and doubtless many other smaller lake areas. In all these cases the water is fresh because the salt has been removed by drainage. In the Northern part of the Baltic, for instance, the water has a very low salinity : there is little evaporation, much inflow from rivers and a deficient inflow from the North Sea area. Isolation of the Baltic by elevation across the
THE OCEANIC MARGINS 111
Sound and Belts would therefore leave a relict sea that would speedily become fresh. Salt water land-locked areas — as for instance, the American Great Salt Lake, the Bitter Lake in the Suez Canal, the Dead Sea, etc., are not necessarily relicts because of their high salinity but simply terminal basins suffering removal of water by evaporation and continuous addition of saline materials in solution.
Indications of the original nature of the Relicts are the geological history of their general land areas, as in the case of the Black Sea ; elevated beaches with marine fossils (Lake Champlain) ; the rehct marine fauna (squids in Lake Onondaga, which was a tributary to Ontario) ; the fishes of the Caspian (sturgeon, salmon, herring) ; sponges in Lake Tanganyika, etc.
The Marginal Sea-bottom Deposits. — In general these are the sea-bottom deposits of the Continental shelf, that is, Murray's terrigenous deposits and the shallow-water neritic materials. This region of sea-bottom is that on which the greater mass of the sedimentary rocks of the earth's crust has been deposited, and on which future sedimentaries are, of course*, now forming. The general limits of depth of the marginal deposits we may take to be those of the Continental shelf, but this is necessarily a rough approximation. Muds of land origin may be carried much further out to sea before they subside to the bottom : thus blue muds may be found at 2,000- 3,000 fathoms. In general, however, the two limits — that of the Continental shelf in about 1,000 fathoms, and that of the oceanic distribution of continental sea- bottom deposits will be much the same.
The Foreshore Terrigenous Deposits. For the most part the material lying on the foreshore is quartz sand, but even a very limited degree of observation will show many other kinds of deposit. In the neighbourhood of rock shores the intertidal zone may be clean, or nearly clean rock covered with weed ; off boulder-clay cliffs
112 AN INTRODUCTION TO OCEANOGRAPHY
there will be rounded water-worn stones of all sizes passing into quartz sand ; elsewhere there may be sand and mud, or mud alone. The conditions are obviously the nature of the adjacent land, the strength and direction of the tidal streams, the volumes of rivers and streams entering the sea, the configuration of the coast line — as in bays of difTerent forms, estuaries, straits, etc. Obviously the variety of these conditions will be very great.
The Foreshore Neritic Deposits. Quite different materials may, however, be seen in the deposits due to molluscan shells, nuUipores, the sandy tubes of marine worms ; remains of polyzoa, etc. '{See Fig. 19). Coral materials driven on to the intertidal zone, or beyond this, by wave action we have already considered. Shelly gravels and sands are very common, but such accumulations on the foreshore, or on the adjacent shallow sea bottom are, as a rule, not the results of the growth of mollusca in situ. Wave action, assisted by peculiarities in the local tidal streams and in the form of the coast line, drive the dead shells of molluscs living beyond the low water marks on to the foreshore (as one sees by examining the debris of the tide marks). Even in a clean sandy foreshore thickly populated by cockles we do not usually find many dead shells in the sand. As a rule foreshore shell deposits, then, have originated elsewhere in the neighbourhood of the locality where they are seen. On the other hand, highly characteristic fore- shore neritic deposits formed in situ are the sandy tubes of the worm Sabellaria : here we have true " Annelid Reefs " of quite a massive kind.
Shallow Water Neritic Deposits. It would more com- monly be the case that the extensive deposits of molluscan shells found on sea bottoms less than 50 fathoms in depth had originated in situ from animals living and dying there. Possibly examination of the shell fragments from a neritic deposit would indicate whether the accumulation had been made on the region inhabited by the living animals or had been transported on the sea bottom, or
THE OCEANIC MARGINS 113
driven by wave action on to the foreshore. Evidence of wear by rubbing among stones and sand on the beach, or rolling gently on the sea bottom, the marks of corrosion by solution, or those of boring by marine molluscs or sponges might indicate deposition in situ. But there is need for prolonged and careful observations on these matters.
Neritic shallow- water deposits will, of course, vary in nature according to the local faunas. Thus the gravels and sands composed characteristically of the tests of Foraminifera {Orbulites) or of calcareous algae {Halimeda^ for instance) are typical of some tropical sea bottoms — as off Ceylon. Here too, we may mention such limy deposits as may be found on the continental shelf in the West Indian Seas. These may be very largely the result of precipitation of calcium carbonate from solution in the sea water, and they are neritic only in the sense that they are, to a great extent, due to the action of certain species of marine bacteria.
The deposits of the foreshore and the shallow water immediately adjacent to this are, then, exceedingly varied — far more so than those laid down in the oceanic abysses. But terrigenous materials of continental origin must always predominate except, of course, on those marginal areas (as on the North-East coast of Australia) where coral formations on the great scale occur. By studying the ways in which shallow water neritic deposits form and are transported, much useful data might become available for elucidating the conditions in which many marine sedimentary rocks have been formed.
The Continental Margins. — We may now neglect the great variety of feature which is exhibited by the bottom deposits in the transitional zone between the continental elevations and the oceanic depressions. On a chart showing whole oceans and continents this transitional zone is, after all, a very narrow one, and what one looks at are the great continental and oceanic regions
114 AN INTRODUCTION TO OCEANOGRAPHY
of the earth's surface. Something, then, must be said as to the general nature of this continental-oceanic margin on the great scale and from the point of view taken in Chapter II — the origin of the oceanic abysses and their assumed permanence as great earth features. It is assumed that the general positions on the earth of the oceanic depressions and continental elevations were marked out quite early in the history of the planet and that the continental margins represent zones of weakness in the superficial earth layers. These margins have always been, and still remain zones of weakness because movements of the earth body as a whole still continue. There are causes that lead to instability of these surface layers : strains set up by the tide-generating force in an earth body which is not perfectly rigid ; loss of heat which has been locally generated ; erosion of the continental areas ; deposition on the sea bottom and possibly other factors. Therefore,* there is instability and this has led, and still leads to alterations in the shapes of the continents as marked out by the land-sea margin, by the surface relief of the land and by its elevation above sea level. This means that extensive areas of the submarine contin- ental shelf have, at various times in the past, been dry land, while far more extensive areas of the present continental regions have been shallow sea bottoms. It does not follow, however, that correspondingly large areas of the present oceanic regions have ever been dry land. There is geological evidence that the greater part of the North American Continent, for instance, has been submerged beneath shallow seas, but evidence of the uplifting of a truly oceanic basin is not nearly so strong.
The physical condition of the interior of the Earth. — As a whole the .evidence indicates that the interior of the earth, generally, is not highly heated. There are local regions of high temperature but these are probably superficial in their situation and the heat is locally generated. The temperature gradients are very irregular
THE OCEANIC MARGINS 115
and the observations apply only to a very restricted depth. The conclusion that has been made from these observations : that a high temperature exists towards the earth's centre is unjustifiable because it involves extrapolation from the data of the surface temperature gradients and this extrapolation itself involves an hypothesis — that the rate of increase of temperature continues. The main reason for the assumption of a very high internal temperature is, of course, the postulation of a gaseous-molten origin for the earth, and this we have no good reasons for accepting. We take it, then, that the whole interior mass of the earth body, down to the centre, is not, as a whole, highly heated.
The Density of the Earth. The mean density of the earth as a whole is about 5' 6 times that of water — this result depends on gravity measurements. But the most superfi- cial layer of earth substance — the atmosphere — is very much less in density than water. The next layer — the ocean (or hydrosphere) — is very little greater in density than water (the ratio is about r03 to 1*00). Then comes the rocky part of the earth (the lithosphere) and the density of this is about 2*7 times that of water. Since the density of the earth as a whole is about 5'6, it follows that the nuclear part (the centrosphere) must have a density that is greater than the mean : it has been taken to be anything from about 7 to 11.
The Rigidity of the Earth. The rigidity is the measure of the resistance which the earth body opposes to external causes which would lead to its deformation. Obviously the atmosphere opposes very little such resistance since it is greatly deformed (in