The Encyclopedia Americana (1920)/Deep-Sea Exploration
DEEP-SEA EXPLORATION. While the beginnings of our knowledge of the surface of the sea date back to the earliest recorded voyage of discovery, the physical conditions obtaining in the depths of the great oceans have been ascertained chiefly during the past half century.
The modern science of oceanography (q.v.) is based largely upon the voyages of a score or more of vessels sent out by various governments for the special purpose of making physical and biological investigations of the oceanic basins.
As late as Captain Cook's time the ocean had been sounded no deeper than a few hundred fathoms. For scientific attempts to ascertain depths far from land, it is scarcely necessary to go back farther than a century and a half, and it was not until the day of submarine cables that systematic soundings were taken.
The dredge has been employed by naturalists in obtaining marine forms in shallow water for a century and a half, but its use in ocean depths has been limited to about half that period.
The governments that have participated largely in modern deep-sea exploration are the English, French, German, Austrian, Dutch, Russian, Norwegian, Swedish, and by no means the least, that of the United States. The foundations of oceanography as a science were laid by the British in the publication of the results of the famous Challenger Expedition. The investigations of this vessel were followed by those of American vessels, one of which, the Albatross, is perhaps better equipped for such work than any other vessel afloat, and has been in commission for more than thirty years.
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The Albatross Dredging at Sea.
Deep-sea explorations have now been carried into all seas and the general conditions of temperature, depths, and animal life in the great oceanic areas are no longer unknown, while the literature of oceanography occupies an important place in science.
The deep-sea exploring vessel of the present time might properly be described as a floating laboratory, equipped for hydrographical, meteorological, geological and biological investigations. The equipment would include not merely that of a general scientific laboratory, but also the heavy apparatus necessary for sounding and dredging. The ship machinery for deep-sea work has been gradually brought to a state of high efficiency. Soundings formerly made with rope are now made with a single strand of wire, and instead of the stout rope cable for dredging, light wire rope of great strength is employed. Various self-recording instruments have gradually been perfected for quick and accurate work in taking deep sea temperatures and in the collecting of small forms of life at intermediate depths.
Sounding.— During the course of deep-sea exploration, many soundings have been made in depths exceeding 3,000 fathoms. More than 50 great depressions, or “deeps” as they have been called, are now known, varying from 3,000 to 5,000 fathoms. Ten of these exceed 4,000 fathoms. It is only in the western Pacific that depths exceeding 5,000 fathoms have been found. The greatest depth known was discovered in July 1913 by the German survey ship Planet, about 40 miles east of northern Mindanao, where a sounding was made of 5,348 fathoms, or 406 feet more than six miles. This depth exceeds by 3,000 feet the height of the loftiest mountain peak. The deeps now known are distributed over the Atlantic, Pacific and Indian oceans. While many of them are far out in great ocean basins, some of them are close to the continental slopes. One exceeding 3,000 fathoms (Bartlett Deep) lies in the Caribbean Sea between Honduras and Cuba. Some of the deeps are now known to extend over thousands of square miles while others are apparently mere holes. Enough sounding has already been done to indicate that the greatest depths have probably been discovered.
Deep-sea sounding has shown that the floor of the ocean has not only its depressions, but its elevations. Many islands are merely the projecting summits of submarine peaks, their bases often resting on an ocean floor lying 3,000 fathoms deep. Submarine ridges of great extent have been discovered in the north and south Atlantic. The average depth of the ocean is probably not less than 2,200 fathoms.
Several types of sounding machines are in use, but whatever the pattern, each contains a reel wound with thousands of fathoms of piano wire. In the operation of sounding a detachable shot weighing from 30 to 60 pounds is employed to draw the wire rapidly and steadily from its reel. It is secured to the sounding rod at the end of the line in such a manner as to become detached when the bottom is reached. Several instruments are sent down at each operation. The sounding rod is provided with a device for bringing up a specimen of the bottom mud. A few feet above it are attached the deep-sea thermometer and water bottle. The latter secures its sample of water at the bottom only. All deep-sea instruments are necessarily self-acting at the bottom and thermometers are so constructed that they will not be effected by higher temperatures on being brought toward the surface. While the sounding instruments are carried down by the heavy shot, the wire is reeled in by steam power. It takes about one hour to make a sounding three miles deep and reel in the line with its attached instruments.
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| Fig. | 1.—Showing net in position for lowering and towing: a, Tow line; b, Tripping arm; c, Tripping ring; d, Opening slings; e, Closing slings; f, Folding net ring; g, Closing weight; h, Closing messenger. (See also Fig. 3.) |
| Fig. | 2.—Showing net closed for heaving in. |
| Fig. | 3.—Closing messenger. |
| Fig. | 4.—Showing heavy tripping arm used on steamer Albatross, with 60-pound sounding shot attached as sinker for proper strain on wire tow line in deep-sea work. |
| i. Common sounding shot - 60 pounds. |
Dredging.— For the collecting of animal life from the bottom, the dredge is employed. This may be described as a bag-shaped net about 20 feet long, fitted with an iron-framed mouth from 8 to 12 feet wide and 3 or 4 feet high. It is towed by a wire cable about one-third of an inch in diameter, its length depending upon the region to be investigated. The cable is operated by a powerful engine on the main deck, and the cable itself is coiled upon a reel sufficiently large to contain the whole of it. The dredge sinks by its own weight, which is sufficient to carry down the dredge line with it, the only weights ordinarily used being small ones to carry the end of the net down below its iron-framed mouth and prevent its fouling. When dragging along the bottom, it usually fills rapidly. Very often the hauls of animal forms are large, but the dredge is more often heavily loaded with oozes of various kinds in which animal forms are imbedded. The dredge rope is passed over the side of the ship from the end of a boom, the top of which is guyed to the mast with a spring to relieve strain. The spring or accumulator is graduated and often shows the dredge to be pulling thousands of pounds. The net is sometimes too heavily loaded to be lifted, and may be torn from its frame. Much of the deep-sea mud or ooze washes away during the progress of the dredge toward the surface and under ordinary conditions its load is no greater than can be lifted from the water and swung on board with safety.
Dredges are of several forms, the commonest being that of the beam trawl employed by fishing vessels in shoal water, but with a shorter beam. For convenience we shall refer to all appliances used for collecting at the bottom as dredges. For the exploration of intermediate depths, several types of nets are employed which can be opened and closed by devices operated by metal messengers sent down the dredge line. Various devices are used for collecting forms of life in deep water, such as the otter net and other appliances well known to fishermen. The surface and intermediate depths of the sea bear forms of life for the capture of which fine-meshed tow nets are used.
Before using the dredge or beam trawl, the depth is ascertained by sounding. In dragging the dredge upon the bottom it is necessary to let out considerably more line than the actual depth, and the dragging of the dredge along the bottom can often be felt by merely placing the hand upon the dredging cable. The deepest dredge haul ever made (that taken by the Albatross off the Tonga Islands in the South Pacific) was 4,173 fathoms, and the time required from the moment the dredge was put overboard until brought to the surface was about 10 hours. On very rough bottom, where dredging nets may be torn from their frames, the “tangle” is often used. This apparatus consists of bunches of shredded rope attached to iron bars, and is effective in obtaining starfishes, sea urchins, crinoids and other creatures which have their surfaces sufficiently rough to become entangled in the loose strands of rope. Fish traps of special design have been used successfully at great depths, and the Albatross has succeeded in taking fishes at a depth of a thousand fathoms with ordinary gill nets.
Deep-Sea Life.— As plant life does not exist in the sea at depths below the influence of sunlight, we find at the bottom and at intermediate depths, forms of animal life only. These consist of fishes, mollusks, crustaceans, medusæ, echinoderms and representatives of practically all of the classes of marine life to be found living in shallow waters under the influence of sunlight, but of very different genera and species. When deep-sea investigations began, it was hoped that archaic forms would be discovered; but this hope has not been realized except in slight degree. Animal forms are sometimes brought up by the dredge in great abundance. It is not an uncommon thing to obtain several barrels of crustaceans, starfishes, mollusks or echinoderms, at a single haul. The dredge of the Albatross once brought up 800 fishes from a depth of 1,770 fathoms. The fact that the Albatross obtained living sponges at 4,178 fathoms indicates that there is probably no depth too great for the existence of animal life.
Surface Life.— The surface life of the sea consists of both plant and animal forms, the latter in great variety. It is usually minute, and much of it phosphorescent. It is the small life at or near the surface and now collectively called plankton, which makes the phosphorescence with which travelers at sea are familiar. It is more abundant in some regions than in others, and constitutes the food not only of many of the larger animals living at the surface, but is the basis of the food supply of all creatures living at the bottom and intermediate depths. These small forms are constantly dying and falling to the bottom. Indeed, their dead shells and skeletons cover great tracts of the ocean floor in the regions where they abound. The crustacea are doubtless the most abundant of these pelagic forms, but almost all classes are represented, including mollusks, worms, cœlenterates, tunicates, protozoans, fishes and even reptiles. In the colder waters of the ocean the tow nets used in taking plankton fill very rapidly with certain forms which swarm in cold waters; but in tropical waters the variety of forms is greater, although usually less abundant. Several forms of nets are used in taking the plankton, but all are lined with fine gauze or silk bolting cloth. Under the most favorable conditions the dragging of such nets at or near the surface often yields enormous quantities of surface forms. When the ship is at rest in favorable weather, surface life is readily attracted to insulated electric lights lowered a few inches below the surface of the water where it can be gathered from the deck with long-handled gauze-lined dip nets.
Great currents like the Gulf Stream, drift the surface life of warm regions into colder waters where it is killed by the lower temperature and deposited in great quantities upon the bottom. In such places the dredges have taken bottom forms in greatest abundance. There are, however, wide areas such as those of mid-Pacific regions, which are almost devoid of surface forms, and where the bottom is correspondingly lacking in animal life.
Marine Deposits.— Geology, like zoology, is also dependent upon the deep-sea dredge for what is known of the floor of the ocean. The deposits may be referred to three groups: Those derived from the land through the action of rivers and currents and called terrigenous deposits. They consist of muds of various kinds which are characteristic of the adjacent lands from which they have been derived. Farther off, and beyond the influence of rivers, are the pelagic deposits which are made up of dead organisms originating at the surface. These are usually found in the form of diatom, radiolarian, or globigerina oozes, depending upon the character of the surface life prevailing above them. Still farther from land, in the deeper parts of the ocean, are the red clay deposits, which are believed to cover about half of the floor of the ocean. These deposits receive the minimum of dead matter from the surface and are far beyond the influence of matter washed from the continents. The red clay deposits are believed to have formed very slowly, and to have been subjected to little change during long periods of time.
Light.— The most recent investigations have shown that light does not penetrate deeper than five or six hundred fathoms, and even there is very dim. Experiments with the photometer have shown that light penetration is greater in tropical than in northern waters. A photographic plate exposed at a depth of 900 fathoms for two hours, was in no way affected. Below the levels reached by sunlight, the sea is in total darkness, and plant forms, with the exception of diatoms, are unknown. There is, however, some phosphorescent light, as the deep-sea dredge has brought up many fishes and invertebrates with phosphorescent organs. Such light must, however, be very faint and unevenly distributed.
Notwithstanding the absence of light, a considerable number of deep-sea forms are brilliantly colored, brilliant reds being often found. Most of the deep-sea fishes are, however, black or grayish. Starfishes and crustaceans are frequently red or pink, while holothurians are often violet.
There is a great variation in the size of the eye among deep-sea fishes, which is doubtless dependent to some extent upon the penetration of sunlight or the existence of phosphorescence, their eyes ranging from very minute to abnormally large. A number of the deep-sea species are totally blind.
Temperature.— The influence of the warmth to be found in the surface waters does not extend below a few hundred feet, even at the equator where such warmth is greatest. Below 900 fathoms the temperature is always within three or four degrees of the freezing point of fresh water. The low temperature at the bottom is caused by the settling of cold surface water in polar regions. The cold water thus slowly distributed over the floor of the ocean carries with it the oxygen necessary for the maintenance of life in the depths. The bottom of the Black Sea, receiving no such supply of polar water, is devoid of animal life. All deep-sea life would doubtless perish were it not for the air thus brought down.
Salinity.— Studies of the salinity of the ocean, based on samples of water secured by special forms of apparatus from all depths, have shown various degrees of salinity in different seas. Sea saltness in the Atlantic is higher than in other great open oceans, and highest of all in the Red Sea and in the Mediterranean. It is remarkably low in the Black Sea and in the Baltic, where there are many rivers but no great evaporation.
Pressure.— Sea pressure amounts to about a ton to the square inch with each 1,000 fathoms of depth. At 1,000 fathoms it amounts to 180 atmospheres, while at the greatest depth known (over 5,000 fathoms), the pressure would be about six tons to the square inch. It is evident that animal forms of the surface regions could not endure such conditions, but the tissues of deep-sea animals are so permeated by fluids, that a balance is maintained. Most of the deep-sea forms are so soft that when brought to the surface they require various laboratory hardening processes for their preservation as specimens. Both fishes and invertebrates, when brought up from the greater depths, are always dead, and probably die before being dragged far from the bottom. It is interesting to note, however, that the Albatross has taken alive fishes from 590 fathoms, and large Lithodes crabs from 735 fathoms.
Size of Deep-Sea Forms.— The dredges hitherto used in deep-sea exploration have secured no fishes or invertebrates of large size. The largest fishes taken seldom exceed four or five feet in length. It is not unlikely that by using larger dredges, with wider-meshed nets less liable to become overloaded with mud, larger animals could be captured.
Bibliography (British and American only).— Agassiz, A., ‘Three Cruises of the Blake’ (Boston 1888); ‘Bulletins and Memoirs of Museum of Comparative Zoology’ (Cambridge, Mass.); ‘Challenger Expedition’ (50 vols., ed. by Sir John Murray, Edinburgh); Fowler, ‘Science of the Sea’ (1912); Hickson, ‘Fauna of the Deep Sea’ (London 1894); Murray, Sir John, and Hjort, Dr. Johan, ‘The Depths of the Ocean’ (London 1912); ‘Reports and Bulletins’ (United States Bureau of Fisheries); Tanner, ‘Deep Sea Exploration’ (Washington 1897); Townsend, ‘Records and Bibliography of the Albatross’ (Washington 1901); Thomson, ‘Voyage of the Challenger’ (1878).