Sea

Sea. All primitive peoples appear to have experienced a feeling of dismay when they braved the dangers of the sea. The great majority of the civilised nations of antiquity took but little interest in the physical phenomena of the ocean. The few facts that were known with reference to the sea were limited to maritime nations like the Phœnicians. Among the learned men of antiquity two views were held with reference to the distribution of land and water. The Homeric school—to which Eratosthenes and Strabo belonged—regarded the old world as a single island surrounded by the ocean. The Ptolemaic school, on the other hand, looked on the Atlantic and Indian Oceans as enclosed seas like the Mediterranean, and held that the east and west points of the known world approached so close to each other that a ship sailing from Spain might easily reach the eastern coasts of Asia. This mistaken notion led, fourteen centuries after Ptolemy, to the discovery of America by Columbus. The discoveries of Columbus, Vasco da Gama, and Magellan, during the thirty years from 1492 to 1522, added a hemisphere to the chart of the world. These voyages doubled at a single bound all that was previously known of the surface of the globe, and in a special manner enlarged our knowledge of the sea. Down to the time of the Challenger and similar recent scientific expeditions all our knowledge of the sea might, literally speaking, be called superficial; it was limited to the upper layers of oceanic waters and to the shallower depths surrounding continents and islands. Owing to the recent introduction of improved apparatus and methods, the most profound depths of the ocean have been examined with precision and success, so that we now possess a large amount of definite information concerning the physical and biological conditions of all regions of the ocean.

Area.—The waters of the sea cover about 143,259,300 sq. m., or about five-sevenths of the surface of the earth. The areas of the main divisions of the ocean are estimated as follows :

Depth.—The solid globe or lithosphere, viewed as to its superficial aspect, may be regarded as divided into two great planes : one of these corresponds to the dry land or upper surface of the continental masses, and occupies about two-sevenths of the earth's surface ; the other, corresponding to the abyssal regions of the ocean, is depressed over 2½ miles below the general level of the continental plane, and occupies about four-sevenths of the earth's surface. The transitional area, uniting these two planes, forms the sides or walls of the ocean basins, and occupies about one-seventh of the earth's surface. The depressed regions of the globe, represented by the ocean basins, are filled with sea-water up to within about 375 fathoms (2250 feet) of the general level of the continents, the average depth of the water in the ocean basins being on the other hand about 2080 fathoms (12,480 feet). Were the solid crust of the earth to be reduced to one uniform level by removing the elevated continental masses into the depressed abyssal areas, the surface of the earth would then be covered by a universal ocean or hydrosphere with a depth of about 2 miles. This depth of 2 miles below the present sea-level has been called by Dr Mill the mean sphere level. The average depths of the main divisions of the ocean are :

The greatest depth recorded is 5155 fathoms, in the South Pacific (1896), near the Kermadec Islands ; the Challenger sounded in 4475 fathoms, in the North Pacific, north-west of the Caroline Islands. In the South Pacific a depth of 4428 fathoms was found just south of the Friendly Islands, and 4170 fathoms off the west coast of Chili. In the Atlantic the greatest depth is 4561 fathoms, off Porto Rico, West Indies. Ross records a sounding in the Antarctic Ocean where he found no bottom at 4000 fathoms. By far the larger portion of the sea-floor lies between the depths of 1000 and 3000 fathoms, equal to nearly 78 per cent., while about 17½ per cent. is found in depths less than 1000 fathoms, and about 4½ per cent. in depths greater than 3000 fathoms. The bulk of water in the whole ocean is estimated at 323,800,000 cubic miles.

Temperature.—The temperature of the surface-waters of the ocean varies from 28° F. in the polar regions to 85° or 86° in equatorial regions. In many places the surface-layers are subject to great annual changes due to the seasons and the direction of the wind. The temperature of the water at the bottom of the ocean over the abyssal areas ranges from 32.7° F. to 36.8° F. In some large basins separated from each other by low ridges the temperature may differ to the extent of one or two degrees, but the temperature is apparently constant at any one spot throughout the year. The great mass of the ocean consists of cold water—i.e. of water below 40° or 45° F.; at a depth of little over half a mile the water in the tropics has generally a temperature below 40° F. In the open ocean the temperature usually decreases as the depth increases, the coldest water being found at the bottom. In enclosed or partially enclosed seas, cut off by barriers from the great ocean basins, the temperature remains uniform from the height of the barrier down to the bottom ; for instance, in the Mediterranean the temperature is about 56° from 200 fathoms down to 2000 fathoms ; in the Sulu Sea, 50.5° from 400 fathoms to 2500 fathoms ; in the Celebes Sea, 38.6° from 800 fathoms to the bottom in 2600 fathoms. In regions where there are heavy rains, or where rivers pour fresh water into the sea, alternating layers of colder and warmer water have been observed within a hundred fathoms from the surface.

Circulation.—The circulation of oceanic waters is maintained by the action of the prevailing winds on the surface-layers. In the oceanic areas the prevailing winds are governed by the large anticyclonic areas situated towards the centres of the North and South Atlantic and North and South Pacific. The winds blow out from and around these anticyclonic areas. For instance, in the southern hemisphere the warm salt water of the tropical regions is driven to the south along the eastern coasts of South America, Africa, and Australia, till on reaching a latitude of between 50° and 55° S. it sinks on being cooled, and spreads slowly over the floor of the ocean to the north and south. A similar circulation takes place in the northern hemisphere, although much modified by the peculiar configuration of the land-masses ; for instance, the cold salt water at 30° F. which occupies the deeper parts of the Arctic basin is largely made up of the dense Gulf Stream water, which sinks to the bottom on being cooled in the Norwegian Sea. The water evaporated from the sea-surface is borne to the land-masses and condensed on the mountain-slopes. It is estimated that over 6500 cubic miles of this water is returned to the sea by rivers annually, bearing along with it a burden of soluble salts and earthy matters in suspension ; in this way the ocean has in all probability become salt in the course of ages. The saltiest waters are found in the regions of greatest evaporation ; for instance, in the Red Sea, Mediterranean, and in the trade-wind regions of the great ocean basins. It has been shown that by the action of off-shore winds cold water is brought up from the deep sea to supply the place of the surface-water carried seawards off the west coast of Africa and America, and off Cape Guardafui on the east coast of Africa ; and the absence of coral-reefs off these coasts is believed to be due to the great range and variation in the temperature of the water thus produced. The present writer has shown that a similar vertical circulation takes place in the lochs of the west of Scotland.

Composition of Sea-water.—It is probable that every element is in solution in sea-water; the great majority are, however, present only in exceedingly minute traces. If the average density of sea-water be taken at 1027, pure water being 1000, then the following would represent the composition of 1000 cubic centimetres of sea-water:

Each base is probably in combination with each acid, so that there are really sixteen salts altogether from the mixture of the four bases and four acids. The total amount of sea-salts may vary greatly in different samples of sea-water, but it has been shown by hundreds of carefully conducted experiments that the ratio of the constituents of sea-salts is nearly everywhere constant, with one significant exception, that of lime, which is in slightly greater proportion in the water from the deeper parts of the ocean basins. Owing to the constant circulation in the ocean, the gases of the atmosphere, which are everywhere absorbed at the surface of the sea, are carried down to the greatest depths, and thus living organisms may flourish throughout the whole extent of the ocean. Nitrogen remains at all times and places nearly constant; not infrequently the proportion of oxygen is much reduced in deep water, owing to the processes of oxidation and respiration. Carbonic acid free or loosely combined is abundant, and plays a most important rôle in the economy of the ocean, combining with and rendering soluble normal carbonates of lime and magnesia to solution in the form of bicarbonates. Water, as is well known, is but slightly compressible, and almost any substance that will fall to the bottom of a tumbler of water will in time fall to the bottom of the deep ocean. Still the compressibility of water must not be neglected in oceanographical questions. In the deeper parts of the ocean the pressure amounts to four or five tons per square inch; hence, in an ocean with a depth of 5 miles, were the action of gravity suddenly to cease, the ocean waters would rise 500 feet above their present level from expansion. There is evidence of very extensive chemical action on some regions of the sea-floor, and it has been suggested that this action is much intensified by the great pressure in the deeper parts of the ocean. It is probable, however, that all the reactions here alluded to may be accounted for by the decomposition of organic substances on the seabed in the presence of the sulphates in sea-water, and the long periods of time to which the materials on the sea-bed have been exposed to the action of sea-water in regions where there is a slow rate of deposition.

Life.—The colour of pure sea-water is a light shade of blue; it has, however, frequently various shades of green and brown, owing to the presence of organisms and matters in suspension. It has been definitely established that life in some of its many forms is universally distributed throughout the ocean. It has long been known that marine plants and animals abound in the shallow waters surrounding continents and islands. Algae disappear from the sea-bed at depths between 100 and 200 fathoms, but a great abundance of animals have been procured in the greater depths. A

Challenger trawling in a depth of over a mile (1000 fathoms) yielded 200 specimens of living animals belonging to 79 species and 55 genera. A haul in about two miles (1600 fathoms) yielded 200 specimens belonging to 84 species and 75 genera. A trawling in about three miles depth (2600 fathoms) yielded 50 specimens belonging to 27 species and 25 genera, not counting Protozoa. Even in depths of over four miles fishes and animals belonging to all the chief invertebrate groups have been procured. The term 'Benthos' is now used for all the animals and plants which live attached to or creep over the bottom of the ocean, 'Plankton' being the term for all the plants and animals which live in, and are carried along by the currents of, the ocean. In the great body of oceanic waters life is most abundant in the surface and sub-surface waters down to about 100 fathoms. Pelagic algæ, such as diatoms and oscillatoria, are abundant in this region, and are the principal and original source of food for many pelagic and nearly all deep-sea animals. In the intermediate depths of the ocean life though present is less abundant. Within a few hundred fathoms of the bottom life again becomes more abundant, crustaceans and cuttle-fish being especially numerous. A very large number of the organisms which belong to the pelagic Plankton, such as diatoms, radiolaria, foraminifera, and molluscs, secrete silica or carbonate of lime to form their shells and skeletons: these in falling to the bottom after the death of the organisms make up a large part of the marine deposits in many regions of the ocean.

Deposits.—The explorations of the Challenger and other expeditions have resulted in a great extension of our knowledge of marine sediments, especially of those now forming in the deep sea. All marine deposits may be divided into two classes—viz. those made up principally of the debris from the solid land of the globe, laid down in greater or less proximity to the shores of continents and islands, called 'terrigenous' deposits, and those in which this continental debris is nearly or quite absent, laid down in the abyssal regions of the ocean, called 'pelagic' deposits. Commencing with the former, there are first the littoral and shallow-water deposits, forming around the land-masses from the shore down to a depth of about 100 fathoms, consisting of sands, gravels, and muds derived almost entirely from the disintegration of the neighbouring lands. The littoral deposits, laid down between tide-marks, cover about 63,000 sq. m., and the shallow-water deposits, between low-water mark and 100 fathoms, about 10,000,000 sq. m. Proceeding seawards from an average depth of about 100 fathoms, the deposits gradually change in character, the proportion of land detritus decreasing, while the remains of oceanic organisms increase in abundance, until at a considerable distance from land and in comparatively deep water the terrigenous deposits pass insensibly into truly pelagic deposits. The terrigenous deep-sea deposits—i.e. those formed at depths greater than 100 fathoms—may be briefly summarised as follows:

Blue Mud, the most extensive, is grayish or bluish in colour, with usually a thin reddish upper layer, and is characterised by the presence of fragments of rocks and mineral particles coming from the disintegration of the land, of which quartz is the principal species; the remains of marine organisms may be present in varying proportions, increasing with depth and distance from the land. Blue mud is found along the coasts of continents and continental islands, and in all enclosed and partially enclosed seas; in some places, as in the Yellow Sea, but notably off the coast of Brazil, the mud may be of a red colour from the large amount of ferruginous matters brought down by the rivers, and it is then called Red Mud. Blue mud is estimated to cover about 14,500,000 sq. m. of the earth's surface—4,000,000 in the Arctic, 3,000,000 in the Pacific, 2,500,000 in the Antarctic, 2,000,000 in the Atlantic, 1,500,000 in the Indian, and 1,500,000 in the Southern Ocean. Red mud covers about 100,000 sq. m. off the coast of Brazil.

Green Mud and Sand are similar to the blue muds, but are characterised by the presence of the mineral glauconite in isolated grains or in small concretions; the dead shells of calcareous organisms are usually filled with the glauconite, which gives the green colour to the deposits. The sands occur in the shallower water nearer the coast, and in them the grains are larger than in the muds. These deposits are found usually off high and bold coasts where no very large rivers enter the sea; for instance, off the east coast of Australia, off South Africa, and off the west coast of North America. Green mud and sand cover about 850,000 sq. m.—300,000 in the Atlantic, 250,000 in the Pacific, 150,000 in the Indian, 90,000 in the Southern, and 60,000 in the Antarctic.

Volcanic Mud and Sand are deposited around the oceanic islands of volcanic origin, and the name is derived from the presence of fragments and particles of volcanic rocks and minerals, which are larger and more numerous nearer the islands, when the deposit is called a sand, becoming smaller and mixed with a larger percentage of pelagic organic remains in the deeper water farther removed from the coast, when the deposit is called a mud. Volcanic mud and sand cover about 600,000 sq. m.—300,000 in the Pacific, 200,000 in the Atlantic, and 100,000 in the Indian Ocean.

Coral Mud and Sand occur similarly around the oceanic coral islands and off those coasts and islands fringed by coral-reefs, and are characterised by the greater or less abundance of coral fragments from the reefs. The sands are found in the shallower water nearer the reefs, as in the case of the volcanic sands. Coral mud and sand cover about 2,557,000 sq. m.—1,417,000 in the Pacific, 760,000 in the Atlantic, and 380,000 in the Indian Ocean.

Of pelagic deposits there are five types, four of organic origin, receiving their designations from the distinctive presence of the remains of calcareous or siliceous organisms, the fifth and most extensive being of inorganic origin.

Globigerina Ooze is so called from the presence of the dead shells of pelagic Foraminifera, those belonging to the genus Globigerina predominating, which live in the surface and sub-surface waters of the ocean, being especially abundant in tropical regions, and the shells of which after death fall to the bottom and there accumulate in moderate depths. The percentage of carbonate of lime in the deposit due to these shells and other calcareous fragments varies from 30 to over 90, and there is usually an admixture of minute mineral particles and remains of siliceous organisms. The depth at which Globigerina ooze is found varies from less than 500 to over 2500 fathoms, the average depth being about 2000 fathoms; but there is a marked difference between a sample collected in comparatively shallow water near land and one from deep water towards the central regions of the ocean basins, the point of union being the presence of a considerable proportion of carbonate of lime shells. Globigerina ooze covers about 49,520,000 sq. m.—17,940,000 in the Atlantic, 11,300,000 in the Pacific, 10,560,000 in the Southern, and 9,720,000 in the Indian Ocean.

One of the facts brought out by recent oceanographical researches is the gradual disappearance of these calcareous shells from the deposits of the deep sea with increasing depth in regions where they may appear to be equally abundant at the surface. In depths of over 3000 fathoms these shells are rare, and often there is not a trace of carbonate of lime even in lesser depths, the Globigerina ooze being then replaced by one of the other kinds of pelagic deposits.

Pteropod Ooze resembles Globigerina ooze in all respects, except that there is a greater abundance of the dead shells of pelagic Mollusca, such as Pteropods and Heteropods; it is usually found in lesser depths than the Globigerina ooze. Pteropod ooze covers about 400,000 sq. m. in the Atlantic.

The names applied to these oozes are not intended to convey the idea that they are wholly made up of the organisms indicated by the names, or that these organisms form a preponderating proportion, for no deep-sea deposit can be said to be strictly homogeneous. Neither is there a sharp dividing line between the various kinds of deposits; they merge insensibly the one into the other. Often it is difficult to locate a sample, one or other of the names being equally applicable.

Diatom Ooze is distinguished by the presence of numerous remains of siliceous organisms, principally Diatoms, though fragments of siliceous Sponge spicules and Radiolaria and Foraminifera are rarely absent. It is found in the Antarctic and Southern Oceans and also in the north-west Pacific. Diatom ooze covers about 10,880,000 sq. m.—10,000,000 in the Southern, 840,000 in the Antarctic, and 40,000 in the Pacific.

Radiolarian Ooze in like manner contains a varying proportion of siliceous remains, in this case principally Radiolaria and their fragments. Calcareous organisms and mineral particles are nearly always present in both these oozes, being usually more numerous and the mineral particles larger in the Diatom ooze than in the Radiolarian ooze, which latter generally occurs in greater depths than the former. The Challenger's deepest sounding, already mentioned, brought up a typical Radiolarian ooze, and it is found in the deeper water of the central Pacific and Indian Oceans. Radiolarian ooze covers about 2,290,000 sq. m.—1,161,000 in the Pacific, and 1,129,000 in the Indian Ocean.

Red Clay occupies nearly the whole of the deeper abysses of the ocean, occurring in its most characteristic form in the central regions of the Pacific, far removed from continental land. It is of a reddish or chocolate colour, due to the presence of the oxides of manganese and iron. Fragments of calcareous organisms are sometimes tolerably abundant in the shallower depths, but in a typical red clay there is usually not more than a trace of carbonate of lime. Siliceous remains are generally present, and there is a small proportion of minute mineral particles of volcanic origin, principally derived from disintegrated pumice. Mineral particles of secondary origin, arising from the decomposition of volcanic debris, are associated with the red clay, and in some regions of the central Pacific isolated crystals and spheroidal groups of phillipsite of secondary origin formed in situ make up a considerable quantity of the deposit. Concretions of manganese and iron are very characteristic of the red clays, and may be of all sizes, sometimes a large quantity of the size of marbles, and sometimes the size of potatoes, being procured. These concretions are formed around various nuclei, such as sharks' teeth, earbones of whales, and pieces of pumice. The Challenger sometimes procured over one thousand sharks' teeth and sixty earbones of whales in a single haul. The presence of the remains of vertebrates, some of them belonging to extinct species lying alongside others belonging to existing species, as well as the formation of manganese nodules and zeolitic crystals in situ, and the presence of metallic and chondritic spherules of cosmic origin, appear to indicate that the red clay accumulates at a very slow rate. Red clay covers about 51,500,000 sq. m.—37,230,000 in the Pacific, 5,800,000 in the Atlantic, 4,350,000 in the Southern, and 4,120,000 in the Indian Ocean.

See the articles in this work on the Arctic, Antarctic, Atlantic, Pacific, and Indian Oceans; those on the Red Sea, Dead Sea, &c.; also CHALLENGER, GULF STREAM, GEOGRAPHY, PHOSPHORESCENCE, POLAR EXPLORATION, SAND, SOUNDING, STORMS, TIDE, WAVE, WINDS, WRECKS.

THE SOVEREIGNTY OF THE SEA.—Blackstone lays it down that the main or high seas are part of the realm of England, as the Courts of Admiralty have jurisdiction there. But the law of nations, as now understood, recognises no dominion in any one nation over the high seas, which are the highway of all nations, and governed by the public law of the civilised world. Such a right has, however, long been claimed over the four seas surrounding the British Isles. It was strongly asserted by Selden, and denied by Grotius, and measures were taken to vindicate the right in the reign of Charles I. The Dutch claimed the supremacy of the seas in Cromwell's time, but were worsted by Blake (q.v.). Every nation has undoubtedly a right to the exclusive dominion of the sea within a certain distance from the shore, now fixed at three miles. This right of lordship includes the right to free navigation, to fishing, to taking wrecks, the forbidding passage to enemies, the right of flag, of jurisdiction, &c. By the law of England the main sea begins at low-water mark; and between low and high water mark the common law and admiralty have a divided jurisdiction, one on land when left dry, the other on the water when it is full sea. The right of seal-fishing in the Behring Sea has been the subject of lengthened diplomatic controversy and arbitration between the United States and Great Britain (see SEAL). For inshore fishery regulations, see FISHERIES. See further INTERNATIONAL LAW, BLOCKADE, ENEMY, NEUTRALITY, SEASHORE; also RULE OF THE ROAD.