Popular Science Monthly/Volume 44/February 1894/Physical Conditions of the Deep Sea
By SIDNEY J. HICKSON, M. A., D. Sc.
IT is not surprising that the naturalists of the early part of the present century could not believe in the existence of a fauna at the bottom of the deep seas. The extraordinary conditions of such a region—the enormous pressure, the absolute darkness, the probable absence of any vegetable life from want of direct sunlight—might very well have been considered sufficient to form an impassable barrier to the animals migrating from the shallow waters and to prevent the development of a fauna peculiarly its own.
The fragmentary accounts of animals brought up by sounding lines from great depths might, it is true, have thrown doubts on the current views; but they were not of sufficient importance in themselves, nor were the observations made with such regard to the possibility of error, as to withstand the critical remarks that were made to explain them away.
The absence of any evidence obtained by accurate systematic research, together with the consideration of the physical character of the ocean bed, were quite sufficient to lead scientific men of that period to doubt the existence of any animal life in water deeper than a few hundred fathoms. We now know, however, that there is a very considerable fauna at enormous depths in all the great oceans, and we have acquired, moreover, considerable information concerning some of those peculiar physical conditions of the abyss that fifty years ago were merely matters of speculation among scientific men.
The relation between animals and their environment is now a question of such great interest and importance that it is necessary in any description of the fauna of a particular region to consider its physical conditions and the influence that it may be supposed to have had in producing the characteristics of the fauna.
The peculiar physical conditions of the deep seas may be briefly stated to be these: It is absolutely dark so far as actual sunlight is concerned, the temperature is only a few degrees above freezing point, the pressure is enormous, there is little or no movement of the water, the bottom is composed of a uniform fine soft mud, and there is no plant life. All of these physical conditions we can appreciate except the enormous pressure. Absolute darkness we know, the temperature of the deep seas is not an extraordinary one, the absence of movement in the water and the fine soft mud are conditions that we can readily appreciate; but the pressure is far greater than anything we can realize. At a depth of twenty-five hundred fathoms the pressure is, roughly speaking, two and a half tons per square inch—that is to say, several times greater than the pressure exerted by the steam upon the piston of our most powerful engines. Or, to put the matter in other words, the pressure per square inch upon the body of every animal that lives at the bottom of the Atlantic Ocean is about twenty-five times greater than the pressure that will drive a railway train.
A most beautiful experiment to illustrate the enormous force of this pressure was made during the voyage of H. M. S. Challenger. I give the description of it in the words of the late Prof. Moseley: "Mr. Buchanan hermetically sealed up at both ends a thick glass tube full of air, several inches in length. He wrapped this sealed tube in flannel, and placed it, so wrapped up, in a wide copper tube, which was one of those used to protect the deep-sea thermometers when sent down with the sounding apparatus. This copper tube was closed by a lid fitting loosely, and with holes in it, and the copper bottom of the tube similarly had holes bored through it. The water thus had free access to the interior of the tube when it was lowered into the sea, and the tube was necessarily constructed with that object in view, in order that in its ordinary use the water should freely reach the contained thermometer.
"The copper case containing the sealed glass tube was sent down to a depth of two thousand fathoms and drawn up again. It was then found that the copper wall of the case was bulged and bent inward opposite the place where the glass tube lay, just as if it had been crumpled inward by being violently squeezed. The glass tube itself, within its flannel wrapper, was found, when withdrawn, reduced to a fine powder, like snow almost. What had happened was that the sealed glass tube, when sinking to gradually increasing depths, had held out long against the pressure, but this at last had become too great for the glass to sustain. and the tube had suddenly given way and been crushed by the violence of the action to a fine powder. So violent and rapid had been the collapse that the water had not had time to rush in by means of the holes at both ends of the copper cylinder and thus fill the empty space left behind by the collapse of the glass tube, but had instead crushed in the copper wall and brought equilibrium in that manner. The process is exactly the reverse of an explosion, and is termed by Sir Wyville Thomson an "implosion." It is only reasonable to suppose that the ability to sustain this enormous pressure can only be acquired by animals after generations of gradual migrations from shallow waters. Those forms that are brought up by the dredge from the depths of the ocean are usually killed and distorted by the enormous and rapid diminution of pressure in their journey to the surface, and it is extremely probable that shallow-water forms would be similarly killed and crushed out of shape were they suddenly plunged into very deep water. The fish that live at these enormous depths are, in consequence of the enormous pressure, liable to a curious form
Fig. 1.—Diagram illustrating the Passage of an Ocean Current across a Barrier (A).
of accident. If, in chasing their prey or for any other reason, they rise to a considerable distance above the floor of the ocean, the gases of their swimming bladder become considerably expanded and their specific gravity very greatly reduced. Up to a certain limit the muscles of their bodies can counteract the tendency to float upward and enable the fish to regain its proper sphere of life at the bottom; but beyond that limit the muscles are not strong enough to drive the body downward, and the fish, becoming more and more distended as it goes, is gradually killed on its long and involuntary journey to the surface of the sea. The deep-sea fish, then, are exposed to a danger that no other animals in this world are subject to namely, that of tumbling upward. That such accidents do occasionally occur is evidenced by the fact that some fish, which are now known to be true deep-sea forms, were discovered dead and floating on the surface of the ocean long before our modern investigations were commenced.
Until quite recently, every one agreed that no rays of sunlight could possibly penetrate the sea to a greater depth than a few hundred fathoms. Moseley says that "probably all is dark below two hundred fathoms excepting in so far as light is given out by phosphorescent animals," and Wyville Thomson speaks of the "utter darkness of the deep-sea bottom."
Within the last few years a few authors have maintained that it is quite possible that a few rays of sunlight do penetrate even to the greatest depths of the ocean—a view mainly based on the fact that so many deep-sea animals possess extremely perfect and complicated eyes and very brilliant colors. Verrill says: "It seems to me probable that more or less sunlight does actually penetrate to the greatest depths of the ocean, in the form of a soft sea-green light, perhaps at two or three thousand fathoms equal in intensity to our partially moonlight nights, and possibly at the greatest depths equal only to starlight. It must be remembered that in the deep sea far away from land the water is far more transparent than near the coast." Packard is of a similar opinion.
There seem to me to be very slight grounds for this view. The fact that, comparatively speaking, shallow-water fish avoid nets that are rendered phosphorescent by entangled jellyfish does not justify us in assuming that deep-sea fish avoid regions where there are phosphorescent Gorgonians or Pennatulids. It is not by any means certain that fish avoid sunken nets on account of their phosphorescence. Most fish possess, as is well known, a very acute sense of smell, and it is very probable that they avoid such nets on account of the putrid odors of the dead animals that remain attached to them.
Nor is there much strength in the further argument that it can hardly be possible that there can be an amount of phosphorescent light regularly evolved by the few deep-sea animals, having this power, sufficient to cause any general illumination, or powerful enough to have influenced, over the whole ocean, the evolution of complex eyes, brilliant and complex protective colors, and complex commensal adaptations.
We have no sound information to go upon to be able to judge of the amount of light given off by phosphorescent animals at the bottom of the deep sea. The faint light they show on deck after their long journey from the depths in which they live to the surface may be extremely small compared with the light they give in their natural home under a pressure of two tons and a half to the square inch. The complex eyes that many deep-sea animals exhibit were almost certainly not evolved as such, but are simple modifications of eyes possessed by a shallow-water ancestry.
The more recent experiments that have been made tend to show that no sunlight whatever penetrates to a greater depth, to take an extreme limit, than five hundred fathoms. Fol and Sarasin, experimenting with very sensitive bromo-gelatin plates, found that there was no reaction after ten minutes' exposure at a depth of four hundred metres on a sunny day in March. But although it is very highly probable that not a glimmer of sunlight ever penetrates to the depths of the ocean, there is in some places, undoubtedly, a very considerable illumination due to the phosphorescence of the inhabitants of the deep waters.
All the Alcyonarians are, according to Moseley, brilliantly phosphorescent when brought to the surface. Many deep-sea fish possess phosphorescent organs, and it is quite possible that many of the deep-sea protozoa, tunicates, jellyfish, and crustacea are in their native haunts capable of giving out a very considerable amount of phosphorescent light. If we may be allowed to compare the light of abysmal animals with that of surface forms, we can readily imagine that some regions of the sea may be as brightly illuminated as a European street is at night—an illumination with many very bright centers and many dark shadows, but quite sufficient for a vertebrate eye to distinguish readily and at a considerable distance both form and color.
To give an example of the extent to which the illumination due to phosphorescent organisms may reach, I may quote a passage from the writings of the late Sir Wyville Thomson: "After leaving the Cape Verd Islands the sea was a j)erfect blaze of phosphorescence. There was no moon, and although the night was perfectly clear and the stars shone brightly, the luster of the heavens was fairly eclipsed by that of the sea. It was easy to read the smallest print, sitting at the after-port in my cabin, and the bows shed on either side rapidly widening wedges of radiance so vivid as to throw the sails and rigging into distinct lights and shadows."
A very similar sight may frequently be seen in the Banda seas, where on calm nights the whole surface of the ocean seems to be a sheet of milky fire. The light is not only to be seen where the crests of waves are breaking, or the surface disturbed by the bows of the boat, but the phosphorescence extends as far as the eye can reach in all directions. It is impossible, of course, to say with any degree of certainty whether phosphorescence such as this exists at the bottom of the deep sea, but it is quite probable that it does in some places, and hence the well-developed eyes and brilliant colors of some of the deep-sea animals. On the other hand, the entire absence or rudimentary condition of the eyes of a very considerable proportion of deep-sea animals seems to prove that the phosphorescent illumination is not universally distributed, and that there must be some regions in which the darkness is so absolute that it can only be compared with the darkness of the great caves.
It may be stated then with some confidence that in the abysmal depths of the ocean there is no trace of sunlight. It is highly improbable, on the face of it, that any ray of light could penetrate through a stratum of water four miles in thickness, even if the water were perfectly pure and clear, but when we remember that the upper regions, at least, are crowded with pelagic organisms provided with skeletons of lime and silica, we may justly consider that it is impossible.
The temperature of the water in the abyss is by no means constant for a constant depth, nor does it vary with the latitude. It Fig. 2.—Sicyonis crassa: M, mouth; S, ciliated groove; T, tentacles. Each tentacle is perforated by a single large aperture. (After Hertwig.) is true that, as a rule, the water is colder at greater depths than in shallower ones, and that the deeper the thermometer is lowered into the sea, the lower the mercury sinks. This is consistent with physical laws. If there is any difference at all in the temperature of a column of water that has had time to settle, the thermometer will always reach its highest point at the top of the column and its lowest at the bottom, for the colder particles being of greater specific gravity than the warmer ones will sink, and the warmer ones will rise. The truth of this will be clear if we imagine a locality at the bottom of a deep ocean with a source of great heat such as an active volcano.
Such a source of heat would, it is true, raise the temperature of the water in its immediate vicinity, but the particles of water thus heated would immediately commence to rise through the superjacent layers of colder water, and colder particles would fall to take their places. Thus the effect of an active volcano at the bottom of the deep sea would not be apparent at any very great distance in the same plane. In fact, unless the bottom of the ocean was closely studded with volcanoes we should expect to find, as indeed we do find, that the temperature of the sea rises as the water shallows.
If then we were to consider a great ocean as simply a huge basin of water, we should expect to find the water at the surface warmer than the water at the bottom. The temperature of the surface would vary constantly with the temperature of the air above it. That is to say, it would be warmer at the equator than in the temperate regions. The temperature at the bottom would be the same as the lowest temperature of the basin, that is, of the earth that supports it. The great oceans, however, can not be regarded as simple basins of water such as this. The temperature of the surface water varies only approximately with the latitude. It is, generally speaking, hottest at the equator and coldest at the poles, but surface currents in the intermediate regions produce many irregularities in the surface temperature.
Again, although we have no means of knowing what the temperature of the earth is at one thousand fathoms below the surface of the ocean, it is very probable that in the great oceans the temperature of the deepest stratum of water is considerably lower than the true earth temperature. This is due to currents of cold water constantly flowing from the poles toward the equator. If these polar currents were at any time to cease, the temperature of the lowest strata of water would rise. Although the polar currents can not be actually demonstrated nor their exact rapidity be accurately determined, the deduction from the known facts of physical geography that they do actually exist is perfectly sound and beyond dispute. A few considerations will, I think, make this clear.
If the ocean were a simple basin somewhat deeper at the equator than at the poles, the cold water at the poles would gradually sink down the slopes of the basin toward the latitude of the equator, and the bottom temperature of the water would be constant all the world over, A few hills here and there would not affect the general statement that for a constant depth the temperature of the lowest stratum of water would be constant. But in some places ridges occur stretching across the ocean from continent to continent, and these ridges shut off the cold water at the bottom of the sea on the polar side from reaching the bottom of the sea on the equator side. If A (Fig. 1) represents a ridge stretching from continent to continent across an ocean, and the arrow represents the direction of the current, then the water that flows across the ridge from the polar side to the equator side will be drawn from the layers of water lying above the level of the ridge, and consequently none of the coldest water will ever get across it, and from the level of the ridge to the bottom of the sea on the equatorial side the water will have the same temperature as the water at the level of the ridge on the polar side. It follows from this that in j)laces where there are deep holes in the bed of the ocean surrounded on all sides by considerable elevations, the temperature of the water at the bottom will be the same as the temperature of the water on the summit of the lowest ridges that surrounds them.
This explains why it is that we find that the bottom temperature for a given depth is frequently less in one place than it is in another, even in places of the same parallel of latitude. One or two examples may be taken to illustrate these points. The temperature off Rio Janeiro in latitude 20º south was found by the Challenger to be 0·6º C. at a depth of 2,150 fathoms. In a similar latitude north of the equator at a depth of 2,900 fathoms the temperature was found to be 2·2º C, and at a point near Porto Rico there is a deep hole of 4,561 fathoms, with a bottom temperature of 2·2º C.
Again it has been shown by the American expedition that the temperature of the water at the deepest point in the Gulf of Mexico, 2,119 fathoms, is the same as that of the bottom of the Straits of Yucatan, 1,127 fathoms, namely, 4·1º C. And, passing to another part of the world altogether, we find in the small but deep sea that lies between the Philippines and Borneo that, at a depth of 2,550 fathoms, the temperature is 10·2º C. These facts then show that, although at the bottom of the deep seas the water is always very cold, the degree of coldness is by no means constant in the same latitude for the same depth.
We must now return to the polar currents. We have assumed above that these currents do exist, and it is probable that by this time the reader must have seen why they are assumed to exist. The water at the bottom of the ocean is exceedingly cold. Where does this coldness come from? It is obvious that in temperate and tropical climes it does not come from the surface. Nor is it at all probable that it comes from the earth upon which the water rests; for, if it were so, the temperature for water of a given depth would always be the same. We should not find the bottom temperature of 2·4º C. at 2,900 fathoms off Rio de la Plata and a temperature of 2·2º F. in 4,561 fathoms off Porto Rico.
In fact, the only hypothesis that can with any show of reason be put forward to account for the temperature of the bottom of the ocean is that which derives its coldness from the polar ice.
Perhaps it is of the nature of an assumption to say that there are no rapid currents and tides in the abysmal depths of the ocean, for we have no means of demonstrating or even of calculating the rate of flow of these waters. But it is a reasonable hypothesis and one that we may well use until the contrary is proved.
A fact of some importance that supports this hypothesis, as regards some parts of the ocean at least, is presented by the sea-anemones. Many of the shallow-water actinians are known to possess minute slits in the tentacles and disk, affording a free communication between the general body cavity or cœlenteron and the exterior. In many deep-sea forms the tentacles are considerably shorter and the apertures larger than they are in shallow-water forms. It is difficult to believe that such forms, perforated by, comparatively speaking, large holes, could manage to live in rapidly flowing water, for if they did so they would soon be smothered by the fine mud that composes the floor of all the deep seas. In fact, anemones of the type presented by such forms as Sicyonis crassa are only fitted for existence in sluggish or still water.
Another character that must be taken into consideration is that presented by the floor of the great oceans. The floor of the ocean, if it were laid bare, would probably present a vast undulating plain of fine mud. Not a rock, not even a stone, would be visible for miles. The mud varies in different parts of the globe according to the depth, the proximity to land, the presence of neighboring volcanoes, or the mouths of great rivers.
The globigerina ooze is perhaps the best known of all the different deep-sea deposits. It was discovered and first described by the officers of the American Coast Survey in 1853. It is found in great abundance in the Atlantic Ocean in regions shallower than
Fig. 3.—Globigerina Ooze. (After Agassiz.)
2,200 fathoms. Deeper than this it gradually merges into the "red mud." It is mainly composed of the shells of foraminifera, and of these the different species of globigerina are the most abundant. It is probably formed partly by the shells of the dead foraminifera that actually live on the bottom of the ocean and partly by the shells of those that live near the surface or in intermediate depths and fall to the bottom when their lives are done. So abundant are the shells of these protozoa that nearly ninety-five per cent of the globigerina ooze is composed of carbonate of lime. The remaining five per cent, is composed of sulphate and phosphate of lime, carbonate of ammonia, the oxides of iron and manganese, and argillaceous matters. The oxides of iron and manganese are probably of meteoric origin; the argillaceous matter may be due to the trituration of lumps of pumice stone and to the deposits caused by dust storms.
Globigerina ooze may be found on the floor of the ocean at depths ranging from 500 to 2,800 fathoms of water in equatorial and temperate latitudes. The reason that it is not found in arctic seas may be that the cold surface waters of these regions do not bear such an abundant fauna of foraminifera. This is supported by the fact that it extends ten degrees farther north than south in the Atlantic, the warm water of the Gulf Stream bearing a richer fauna than the waters of a corresponding degree of latitude in the southern sea.
The pteropod ooze has only twenty-five per cent of carbonate of lime. It contains numerous shells of various pteropods, heteropods, and foraminifera, but nearly fifty per cent of its substance is composed of the siliceous skeletons of radiolaria and the frustules of diatoms. According to Murray, it is found in tropical and subtropical seas at depths of less than 1,500 fathoms.
The radiolarian ooze is found only in the deepest waters of the central and western Pacific Ocean. In some of the typical examples not a trace of carbonate of lime was to be found, but in somewhat shallower waters a few small fragments occurred. A diatom ooze, mainly composed of the skeletons of diatoms, has also been found in deep water near the Antarctic Circle, but it has not apparently a very wide range.
Of all the deep-sea deposits, however, the so-called "red mud" has by far the widest distribution. It is supposed to extend over one third of the earth's surface. It is essentially a deep-sea deposit, and one that is found in its typical condition at some considerable distance from continental land. Like the globigerina ooze it is never found in inclosed seas. To the touch it is plastic and greasy when fresh, but it soon hardens into solid masses. When examined with the microscope it is seen to be composed of extremely minute fragments, rarely exceeding 0·05 millimetre in diameter. It contains a large amount of free silica that is probably formed by the destruction of numerous siliceous skeletons, and a small proportion of silicate of alumina. It usually contains the remains of diatoms, radiolaria, and sponge spicules, and occasionally lumps of pumice stone, meteoric nodules, and, in colder regions, stones and other materials dropped by passing icebergs.
In the great oceans, then, we find in the deepest places red mud, or, where there is an abundant radiolarian surface fauna, radiolarian ooze; in water that is not deeper than about 2,000 fathoms, we find the globigerina ooze; in shallower waters and in some localities only pteropod ooze. It must not be supposed that sharp limits can anywhere be drawn between these different kinds of deposits, for they pass gradually into one another and present many intermediate forms.
It is probable that the sea water, by virtue of the free carbonic acid it contains in solution, is able to exert a solvent action upon the calcium carbonate shells of animals as they sink to the bottom, and during the long and very slow journey from the surface to the bottom of the deepest seas these shells are completely dissolved. The first to be dissolved would be the thin, delicate shells of the pteropods and heteropods, for besides the fact that they present a wider surface to the solvent action of the water they are probably influenced more by tide and currents, sink more slowly and erratically, and thus have a longer journey to perform. Then the smaller but more solid and compact shells of the foraminifera are dissolved, and lastly, in the deepest water only the siliceous skeletons of the radiolaria and diatoms are able to reach their last resting place at the bottom of the ocean.
These four oozes then are characteristic of the floor of the deep oceans. In the proximity of land and in inland seas where deep water occurs, other muds are found differing from one another in accordance with the character of the coasts in their vicinity.
One more character of the deep-sea region must be referred to, and that is the absence of vegetable life. It has not been determined yet with any degree of accuracy where we are to place the limit of vegetable life, but it seems probable that below a hundred fathoms no organisms, excepting a few parasitic fungi, are to be found that can be included in the vegetable kingdom. While then the researches of recent times have proved beyond a doubt that there is no depth of the ocean that can be called azoic, they have but confirmed the perfectly just beliefs of the older naturalists that there is a limit where vegetable life becomes extinct. It is not difficult to see the reason for this. All plants, except a few parasites and saprophytes, are dependent upon the influence of direct sunlight, and as it has been shown above that the sunlight can not penetrate more than a few hundred fathoms of sea water, it is impossible for plants to live below that depth.
Noticing the proceedings of the recent meeting of the British Association at Nottingham, the London Spectator remarks upon "a singular deficiency in those careful descriptions of the precise position of any science which have so frequently wakened up ordinary men to careful thought." There is a popular side to the association's work which is not less important than the one hy which it seeks to advance science. The aim of that side is "to arouse such general interest in science that the minds which are fitted for such study will be inclined to devote themselves to it. To obtain the ablest in any pursuit we need a vast reservoir of men who are more or less interested in it. You can not have your Napoleon of science without an army to draw him from, and the work of increasing the area of recruiting is not unworthy a great association. Of course, 'interesting papers' often add little to positive science; but then, neither do music and banners and tine uniforms add to military force. But they bring recruits, without which such force remains latent and useless."