1911 Encyclopædia Britannica/Ocean and Oceanography
OCEAN AND OCEANOGRAPHY. “Ocean” is the name applied to the great connected sheet of water which covers the greater part of the surface of the Earth. It is convenient to divide the subject-matter of physical geography into the atmosphere, hydrosphere and lithosphere, and in this sense the ocean is less than the hydrosphere in so far as the latter term includes also the water lying on or flowing over the surface of the land. The conception of an encompassing ocean bounding the habitable world is found in the creation myths of the most ancient civilizations. The Babylonians looked on the world as a vast round mountain rising from the midst of a universal sheet of water. In the Hebrew scriptures the waters were gathered together in one place at the word of God, and the dry land appeared. The Ionian geographers looked on the circular disk of the habitable world as surrounded by a mighty stream named Oceanus, the name of the primeval god, father of gods and men, and thus the bond of union between heaven and earth. The Greek word ὠκεανός is related to the Sanskrit açāyanas, “the encompassing.” Philologists do not know of any related word in Semitic languages. Pictet, however, recognizes allied forms in Celtic languages, e.g. the Irish aigean and Cymric eigiawn.
Since the Pythagorean school of philosophy upheld the spherical as against the disk-shaped world, some of the ancient geographers, including Eratosthenes and Strabo, looked upon the hydrosphere as forming two belts at right angles to each other, one belt of ocean following the equator, the other surrounding the earth from pole to pole as in the terra quadrifida of Macrobius; while others, including Aristotle and Ptolemy, looked upon the inhabited land, or oikumene, as occupying the greater part of the earth's surface, so that the Indian Ocean was an enclosed sea and India (i.e. eastern Asia) was only separated from Europe by the Atlantic Ocean. The latter view prevailed and was as a rule held by the Arab geographers of the middle ages, so that until the discovery of America and of the Pacific Ocean the belief was general that the land surface was greater than the water surface, or that at least the two were equal, as Mercator and Varenius held. Thus it was that a great South Land appeared on the maps, the belief in the prodigious extension of which certainly received a severe shock by Abel Tasman's voyage of circumnavigation, but was only overthrown after Cook's great voyages had proved that any southern land which existed could not extend appreciably beyond the polar circle. Only in our own day has the existence of the southern continent been demonstrated within the modest limits of Antarctica.
Oceanography is the science which deals with the ocean, and since the ocean forms a large part of the earth's surface oceanography is a large department of geography. The science is termed talassografia by the Italians, and attempts have been made without success to introduce the name “thalassography.” Of recent years the use of “hydrography” as the equivalent of physical oceanography has acquired a certain currency, but as the word is also used with more than one other meaning (see Surveying) it ought not to be used for oceanography.
Like geography, oceanography may be viewed in two different ways, and is conveniently divided into general oceanography, which deals with phenomena common to the whole ocean, and special oceanography, which has to do with the individual characteristics of the various divisions of the ocean. This article is restricted to general oceanography in its physical aspects, the closely-related meteorological, biological and economic aspects being dealt with elsewhere.
Methods of Research.—When research in oceanography began, the conditions of the sea were of necessity observed only from the coast and from islands, the information derived from mariners as to the condition of parts of the sea far from land being for the most part mere anecdotes bearing on the marvellous or the frightful. In recent times, especially since the rapid increase in the study of the exact sciences during the 19th century, observations at sea with accurate instruments have become common, and the ships' logs of to-day are provided with headings for entering daily observations of the phenomena of the sea surface. The contents of the sailors' scientific logs were brought together by the American enthusiast in the study of the sea, Matthew Fontaine Maury (1806-1873), whose methods and plans were discussed and adopted at international congresses held in Brussels in 1853 and in London in 1873. By 1904 more than 6800 of these meteorological logs with 7,000,000 observations had been accumulated by the Meteorological Office in London; 20,000 with 10,600,000 observations by the German Marine Observatory at Hamburg; 4700 with 3,300,000 observations by the Central Institute of the Netherlands at de Bilt near Utrecht. The Hydrographic Office of the United States had collected 3800 meteorological logs with 3,200,000 entries before 1888; but since that time the logs have contained only one observation daily (at Greenwich noon) and of these 2,380,000 entries had been received by 1904. In the archives of the French Marine in Paris there were 3300 complete logs with 830,000 entries and 11,000 abstract logs from men-of-war. The contents of these logs, it is true, refer more to maritime meteorology than to oceanography properly so-called, as their main purpose is to promote a rational system of navigation especially for sailing ships, and they are supplied by the voluntary co-operation of the sailors themselves.
While the sailors' logs supply the greater part of the scientific evidence available for the study of the surface phenomena of the ocean, they have been supplemented by the records of numerous scientific expeditions and latterly by publications embodying systematic observations on a permanent basis. Valuable observations were made in oceanography during the expeditions of Captain James Cook and the polar explorers, especially those of Sir John Ross in the north and Sir James Ross in the south, but the voyage of H.M.S. “Challenger” in 1872-1876 formed an epoch marking the end of the older order of things and the beginning of modern oceanography as a science of precision. The telegraph cable companies were quick to apply and to extend the oceanographical methods useful in cable-laying, and to their practical acuteness many of the most important improvements in apparatus are due. A second epoch comparable to that of the “Challenger” and resulting like it in a leap forward in the precision of the methods previously employed was marked by the institution in 1901 of the International Council for the Study of the Sea. This council was nominated by the governments of Norway, Sweden, Denmark, Finland, Russia, Germany, Great Britain, Holland and Belgium, with headquarters in Copenhagen and a central laboratory at Christiania, and its aim was to furnish data for the improvement of the fisheries of the North Sea and surrounding waters. In the course of investigating this special problem great improvements were made in the methods of observing in the deep sea, and also in the representation and discussion of the data obtained, and a powerful stimulus was given to the study of oceanography in all the countries of Europe. The efforts of individual scientific workers cannot as a rule produce such results in oceanography as in other sciences, but exceptions are found in the very special services rendered by the prince of Monaco, who founded the Oceanographical Institute in Paris and the Oceanographical Museum in Monaco; and by Professor Alexander Agassiz in the investigation of the Pacific.
Extent of the Ocean.—The hydrosphere covers nearly three-quarters of the earth's surface as a single and continuous expanse of water surrounding four great insular land-masses known as the continents of the Old World (Europe, Asia, Africa), America, Australia and Antarctica. As we are still ignorant of the proportions of land and water in the polar regions, it is only possible to give approximate figures for the extent of the ocean, for the position of the coast-lines is not known exactly enough to exclude possible errors of perhaps several hundred thousand square miles in estimates of the total area. Speaking generally, we may say with confidence that water predominates in the unexplored north polar area, and that it is very unlikely that new land of any great extent exists there. On the other hand, recent Antarctic exploration makes it practically certain that a great continent surrounds the south pole with a total area considerably more than Sir John Murray's estimate in 1894, when he assigned to it an area of 9,000,000 sq. km. (3,500,000 sq. statute miles). It is probable that the Antarctic continent measures about 13,000,000 sq. km. (5,000,000 sq. statute miles); and thus if we accept Bessel's figure of 509,950,000 sq. km. (196,900,000 sq. m.) for the whole surface of the sphere, there is a total land area of 148,820,000 sq. km. (57,460,000 sq. m.), and a total water area of 361,130,000 sq. km. (139,435,000 sq. m.), 29% of land and 71% of water, or a ratio of 1: 2.43.
Divisions of the Ocean.—The arrangement of the water surface on the globe is far from uniform, the ocean forming 61% of the total area of the northern and 81% of that of the southern hemisphere. Of the whole ocean only 43% (154.9 million sq. km.) lies in the northern hemisphere and 57% (206.2 million sq. km.) in the southern. If the globe is divided into hemispheres by the meridians of 20° W. and 160° E., as is usual in atlases, the eastern hemisphere, to which the Old World belongs, has 62% of its surface made up of water, while the western hemisphere, including America, has 81%. A great circle can be drawn upon a terrestrial globe in such a way as to divide it into two hemispheres, one of which contains the greatest amount of land and the other the greatest amount of sea of any possible hemispheres. The centre of the so-called land-hemisphere lies near the mouth of the Loire (47½° N. and 2½ W.), while the centre of the so-called water-hemisphere lies to the S.E. of New Zealand and eastward of Antipodes Island. Even in the land hemisphere the water area (134.5 million sq. km.) is in excess of the land area (121 million sq. km.), while in the water-hemisphere the amount of land is quite insignificant, being only 24.5 million sq. km. compared with 230.5 million sq. km. of water.
The outline of the water surface depends on the outline of the basins in which it is contained. The four great continental masses therefore give the ocean a distinctly tripartite form, the three great divisions being known as the Atlantic, the Indian and the Pacific Oceans, all three running together into one around Antarctica. Thus the connecting belt of water is narrow as compared with the extent of the oceans from north to south—Drake Strait south of South America is barely 400 m. wide, from Cape Agulhas to Enderby Land, 2200 m., and from Tasmania to Wilkes Land, 1550 m., while the meridional extension of the Indian Ocean is 6200 m., of the Pacific, 9300 m., and of the Atlantic, 12,500 m., measuring across the North Pole to Bering Strait. These proportions are not readily grasped from a map of the world on Mercator's projection, and must be studied on a globe. A simple, practical boundary between the three oceans can be obtained by prolonging the meridian of the southern extremity of each of the three southern continents to the Antarctic circle. A committee of the Royal Geographical Society—the deliberations of which were interrupted by the departure on his last voyage of Sir John Franklin, one of the members—suggested these meridians as boundaries; the north and south boundaries of the Atlantic and Pacific Oceans being the polar circles, leaving an Arctic and an Antarctic Ocean to complete the hydrosphere. We now know, however, that the Antarctic circle runs so close to the coast of Antarctica that the Antarctic Ocean may be left out of account. It has been found more convenient to take as northern boundaries the narrowest part of the straits near the Arctic circle, Bering Strait on the Pacific side, and on the Atlantic side the narrowest part of Davis Strait, and of Denmark Strait, then the shortest line from Iceland to the Faeroes, thence to the most northerly island of the Shetlands and thence to Cape Statland in Norway. It has also been found convenient to take the boundary between the Atlantic and Pacific, as the shortest line across Drake Strait, from Cape Horn through Snow Island to Cape Gunnar, instead of the meridian of Cape Horn. Possibly ridges of the sea-bed running southward from the southern continents may yet be discovered which would form more natural boundaries than the meridians. The committee of the Royal Geographical Society settled the existing nomenclature of the three great oceans. Some authors include the Arctic Sea in the Atlantic Ocean, and some prefer to consider the southern part of the Atlantic, Indian and Pacific Oceans as a Great Southern Ocean. Sir John Herschel took as the northern boundary of the southern ocean the greatest circle which could touch the southernmost extremities of the three southern continents. Such a circle, however, runs so near the coast of Antarctica as to make the southern ocean very small. Others, like Malte Brun (1803) and Supan (1903), take the loxodromes between the three capes and call the ocean to the south the Antarctic Ocean. G. V. Boguslawski suggested the parallel of 55° S. and Ratzel that of 40° S. as limits; but in none of these schemes has the coast of Antarctica been adequately considered, and they have all been too much influenced by the Mercator map. Each of the three oceans, Atlantic, Indian and Pacific, possesses an Antarctic facies in the southern part and a tropical facies between the tropics, and the Atlantic and Pacific an Arctic facies in their northern parts.
Where the ocean touches the continents the margin is in places deeply indented by peninsulas and islands marking off portions of the water surface which from all antiquity have been known as “seas.” These seas are entirely dependent on the ocean for their regime, being filled with ocean water, though subject to influence by the land, and the tides and currents of the ocean affect them to a greater or less extent. They owe their origin to depressions of the earth's crust of no very wide extent and not running very far into the continental mass, and geologically they are of recent age and still subject to change. In these respects they contrast with the great oceans which owe their origin to the most extensive and the profoundest depressions of the crust, date back at least to Mesozoic times, and have perhaps remained permanently in their present position from still remoter ages.
Seas may be classified according to their form either as “enclosed” or as “partially enclosed” (or “fringing”). Enclosed seas extend deeply into the land and originate either by the breaking through of the ocean or by the overflowing of a subsiding area. They are connected with the ocean by narrow straits, the salinity of the water contained in them differs in a marked degree from that of the ocean, and the tidal waves are of small amplitude. Four great intercontinental enclosed seas are included between adjacent continents—the Arctic Sea, the Central American or West Indian Sea, the Australo-Asiatic or Malay Sea and the Mediterranean Sea. There are also four smaller continental enclosed seas each with a single channel of communication with the ocean, viz. the Baltic Sea and Hudson Bay with very low salinity, the Red Sea and Persian Gulf with very high salinity.
The fringing or partially enclosed seas adjoin the great land masses and are only separated from the oceans by islands or peninsulas. Hence their tidal conditions are quite oceanic, though their salinity is usually rather lower than that of ocean water. The four fringing seas of eastern Asia, those of Bering, Okhotsk, Japan and East China, are arranged parallel to the main lines of dislocation in the neighbouring land-masses, and so are the Andaman Sea and the Gulf of California. On the contrary, the North Sea, the British fringing seas (English Channel, Irish Sea and Minch), and the Gulf of St Lawrence cross the main lines of dislocation.
In addition to these seas notice must be taken of the subordinate marginal features, such as gulfs and straits. Gulfs may be classified according tc their origin as due to fractures of the crust or overflowing of depressed lands. The former are either the extensions of oceanic depressions, e.g. the Arabian Sea, Bay of Bengal and Gulf of Arica, or such caldron-depressions as the Gulfs of Genoa and Taranto, or rift-depressions like the Gulfs of Aden and Akaba. Compound gulfs are formed seawards by fracture and landwards by the overflowing of depressed land, e.g. the Bay of Biscay, Gulf of Alaska and Gulf of the Lion. Gulfs formed by the overflowing of depressed lands lie upon the continental shelf, e.g. the Gulf of Maine, Bay of Fundy, Bay of Odessa, Gulf of Martaban.
Straits have been formed (1) by fracture across isthmuses, and such may be by longitudinal fracture as in the Strait of Bab-el-Mandeb, or transverse fracture as in the Strait of Gibraltar or Cook Strait; (2) by erosion, e.g. the Strait of Dover, the Dardanelles and Bosporus; (3) by overflowing through the subsidence of the land, as in the straits of Bering, Torres and Formosa.
Surface of the Ocean.—If the whole globe were covered with a uniformly deep ocean, and if there were no difference of density between one part and another, the surface would form a perfect ellipsoid of revolution, that is to say, all the meridians would be exactly equal ellipses and all parallels perfect circles. At any point a sounding line would hang in the line of the radius of curvature of the water surface. But as things are the water-surface is broken by land, and the mean density of the substance of the land is 2.6 times as great as that of sea-water, so that the gravitational attraction of the land must necessarily cause a heaping up of the sea around the coasts, forming what has been called the continental wave, and leaving the sea-level lower in mid-ocean. Hence the geoid or figure of the sea-surface is not part of an ellipsoid of rotation but is irregular. The differences of level between different parts of the geoid have been greatly overestimated in the past; F. G. Helmert has shown that they cannot exceed 650 ft. and are probably much less. Recent pendulum observations have shown that it is incorrect to assume a uniform density of 2.6 in the elevated part of the earth's crust, that on the contrary there are great local differences in density, the most important being a confirmation of Airy's discovery that there is a marked deficiency of mass under high mountains and a marked excess under the bed of the ocean. The intensity of gravity at the surface of the sea far from land has been measured on several occasions. During Nansen's expedition on the “Fram” in 1894-1895, Scott Hansen made observations with a Sterneck's half-seconds pendulum on the ice where the sea was more than 1600 fathoms deep and found only an insignificant deviation from the number of swings corresponding to a normal ellipsoid. In 1901 O. Hecker took the opportunity of a voyage from Hamburg to La Plata, and in 1904 and 1905 of voyages in the Indian and Pacific Oceans to determine the local attraction over the ocean by comparing the atmospheric pressure measured by means of a mercurial barometer and a boiling-point thermometer, and obtained results similar to Scott Hansen's. The inequalities of the geoid in no case exceed 300 ft. Distortion of the ocean surface may also arise from meteorological causes, and be periodic or unperiodic in its occurrence, but it does not amount to more than a few feet at the utmost. Solar radiation warms the tropical more than the polar waters, but, assuming equal salinity, this cause would not account for a difference of level of more than 20 ft. between tropical and polar seas. The annual range of temperature between summer and winter of a surface layer of water about 25 fathoms thick in the Baltic is as much as 20° F., but this only corresponds to a difference of level of 1¼ in. due to expansion or contraction.
Atmospheric precipitation poured into the sea by the great rivers must necessarily create a permanent rise of the sea-level at their mouths, and from this cause the level round the coasts of rainy lands must be greater than in mid-ocean. H. Mohn has shown how the inequalities of what he terms the density-surface can be found from the salinity and temperature; and he calculates that the level of the Skagerrak should be about 2 ft. higher than that of the open Norwegian Sea between Jan Mayen and the Lofoten Islands. The level of the Gulf of Finland at Kronstadt and of the Gulf of Bothnia at Haparanda should similarly be 15 in. higher than that of the Skagerrak. Recent levellings along the Swedish and Danish coasts have confirmed the higher level of the Baltic; and the level of the Mediterranean has also been determined by exact measurements to be from 15 to 24 in. lower than that of the Atlantic on account of evaporation. Apart from the effects of varying precipitation and evaporation the atmosphere affects sea-level also by its varying pressure, the difference in level of the sea-surface from this cause between two given points being thirteen times as great as the difference between the corresponding readings of the mercurial barometer. In the north tropical belt of high pressure south of the Azores the atmospheric pressure in January is 0.87 in. higher than in the Irminger Sea; hence the sea-level near the Azores is almost 1 ft. lower than in the northern sea. In the monsoon region, where the barometer rises 0.38 in. between July and January, the level of the sea falls in consequence by 5 in. Wind also gives rise to differences of level by driving the water before it, and the prevailing westerly wind of the southern Baltic is the chief cause of the sea-level at Kiel being 5½ in. lower than at Arkona on Rügen. Periodic variations of level due to meteorological causes account for the Baltic being fuller in the time of the summer rains than in winter, when the rivers and lakes are frozen and most of the precipitation on the land is in the form of snow. The range on the Arkona gauge is from 3.5 in. below mean level in April to 2.75 in. above the mean level in August. A similar range occurs on the Dutch coast in the North Sea, where the maximum level is reached in October, the month of highest rainfall, and there is a range of 8 in. to the minimum level at the time of least rainfall in early spring. In the monsoon regions the half-yearly change from on-shore to off-shore winds produces noticeable differences in level; thus fifteen years' observations at Aden show a maximum in May at the end of the north-east monsoon, and a rapid falling off after the beginning of the south-west monsoon to a minimum in August, the total range being 9½ in. The influence of wind on water-level is most remarkable in heavy storms on the flat coasts of the North Sea and Baltic, when the rise may amount to very many feet. In the region of tropical hurricanes the converging wind system of a circular storm causes a heaping up of water capable of devastating the low coral islands of the Pacific. On the 1st of November 1876 a cyclone acting in this way submerged a great area of the level plain of the Ganges delta to a depth of 46 ft.; here the influence of the difference of pressure within and without the cyclone acted in the same direction as the wind. The old speculations as to a great difference of level between the Mediterranean and the Red Sea, and on the two sides of the Isthmus of Panama, which hindered the projects for canals connecting those waters, have been proved by modern levelling of high precision to be totally erroneous.
Deep-sea Soundings.—The hand-lead attached to a line divided into fathoms was a well-known aid to navigation even in high antiquity, and its use is mentioned in Herodotus (ii. 5) and in the Acts of the Apostles (xxvii. 29). Greater depths than those usually sounded by a hand-line may possibly not have been beyond the reach of the earlier navigators, for Strabo says “of measured seas the Sardonian is the deepest with full one thousand fathoms” (i. 3, p. 53 Cas.). Yet we find that the great discoverers of the modern period were only familiar with the hand-lead, and the lines in use did not exceed 200 fathoms in length. Ingenious devices had indeed been tried in the 17th century and earlier, by which a lead thrown into the sea without a line detached a float on striking the bottom, and it was proposed to calculate the depth by the time required for the float to reappear. The earliest deep-sea sounding on record is that of Captain Phipps on the 4th of September 1773 in the Norwegian Sea, in 65° N. 3° E., on his return from his expedition to Spitsbergen. He spliced together all the sounding-lines on board, and with a weight of 150 ℔ attached he found bottom in 683 fathoms and secured a sample of fine soft blue mud. He detected the moment of the lead touching the bottom by the sudden slackening in the rate at which the line ran out. Polar explorers frequently repeated those experiments in deep-sea soundings, both William Scoresby and Sir John Ross obtaining notable results, though not reaching depths of more than 1200 fathoms. The honour of first sounding really oceanic depths belongs to Sir James Clark Ross, who made some excellent measurements in very deep water, though in a few instances he overestimated the depth by failing to detect the moment at which the lead touched bottom. The pursuit of these isolated investigations received a great impetus from the enthusiasm of the great American oceanographer Captain Matthew Fontaine Maury, U.S.N., who directed the whole impetuous strength of his character to the task of compelling the silent depths of the ocean to tell their tale. Instead of the expensive mile-long stout hemp lines used by Ross, Maury introduced a ball of strong twine attached to a cannon shot, which ran it out rapidly; when the bottom was reached the twine was cut and the depth deduced from the length of string left in the ball on board. The time of touching bottom was judged by timing each 100-fathom mark and noting the sudden increase in the time interval when the shot reached the bottom. Maury, however, recognized that in great depths the surest guarantee of bottom having been reached was to bring up a sample of the deposit. To do this with a heavy lead attached required a very strong hemp line, and the twine used in the American method was useless for this purpose. In 1854 J. M. Brooke, a midshipman of the U.S.N., invented the principle already foreshadowed by Nicolaus Cusanus in the 15th century and by Robert Hooke in the 17th, of using a heavy weight so hung on the sounding-tube that it was automatically released on striking the bottom and left behind, while the light brass tube containing a sample of the deposit was easily hauled up. This principle has been adopted universally for deep soundings, and is now applied in many forms. In 1855 Maury published the first chart of the depths of the Atlantic between 52° N. and 10° S. At this period an exact knowledge of the depths of the ocean assumed an unlooked-for practical importance from the daring project for laying a telegraph cable between Ireland and Newfoundland. Deep soundings were made in the Atlantic for this purpose by vessels both of the British and of the American navies, while in the Mediterranean and in the Indian Ocean many soundings were made in connexion with submarine cables to the East. Another stimulus came from the biologists, who began to realize the importance of a more detailed investigation of the life conditions of organisms at great depths in the sea. The lead in this direction was taken by British biologists, beginning with Edward Forbes in 1839, and in 1868 a party on board H.M.S. “Lightning” pursued researches in the waters to the north of Scotland. In 1869 and 1870 this work was extended to the Irish Sea and Bay of Biscay in H.M.S. “Porcupine,” and to the Mediterranean in H.M.S. “Shearwater.” The last-named vessel secured 157 trustworthy deep soundings, with samples of the deposits, and also observations of temperature and salinity in different depths, as well as dredging for the collection of the organisms of the deep sea.
These preliminary trips of scientific marine investigation were followed by the greatest purely scientific expedition ever undertaken, the voyage of H.M.S. “Challenger” round the world under the scientific direction of Sir Wyville Thomson and the naval command of Sir George Nares. This epoch-making expedition lasted from Christmas 1872 to the end of May 1876, and gave the first wide and general view of the physical and biological conditions of the ocean as a whole. Almost simultaneously with the “Challenger,” a German expedition in S.M.S. “Gazelle” conducted observations in the South Atlantic, Indian and South Pacific Oceans; and the U.S.S “Tuscarora” made a cruise in the North Pacific, sounding out lines for a projected Pacific cable. The successor of Sir Wyville Thomson in the editorship of the “Challenger” Reports, Sir John Murray, has rightly said that since the days of Columbus and Magellan no such revelation regarding the surface of our planet had been made as in that eighth decade of the 19th century. Since that time the British cable-ships have been busy in all the oceans making sections across the great expanses of water with ever increasing accuracy, and in that work the government surveying ships have also been engaged, vast stretches of the Indian and Pacific Oceans having been opened up to knowledge by H.M.SS. “Egeria,” “Waterwitch,” “Dart,” “Penguin,” “Stork,” and “Investigator.” American scientific enterprise, mainly under the guidance of Professor Alexander Agassiz, has been active in the North Atlantic and especially in the Pacific Ocean, where very important investigations have been made. The eastern part of the North Atlantic has been the scene of many expeditions, often purely biological in their purpose, amongst which there may be mentioned the cruises of the “Travailleur” and “Talisman” under Professor Milne-Edwards in 1880-1883, and since 1887 those of the prince of Monaco in his yachts, as well as numerous Danish vessels in the sea between Iceland and Greenland, conspicuous amongst which were the expeditions in 1896-1898 on board the “Ingolf.” The Norwegian Sea was studied by the Norwegian expedition on board the “Vöringen” in 1876-1878, and the north polar basin by Nansen and Sverdrup in the “Fram” in 1893-1896, the Mediterranean by the Italians on the “Washington” and by the Austrians on the “Pola” in 1890-1893, the latter carrying the investigations to the Red Sea in 1895-1898, while the Russians investigated the Black Sea in 1890-1893. For high southern latitudes special value attaches to the soundings of the German deep-sea expedition on the “Valdivia” in 1898-1899, and to those of the “Belgica” in 1897-1898, the “Gauss” in 1902-1903, and the “Scotia” in 1903-1904. The soundings of the Dutch expedition on the “Siboga” during 1899-1900 in the eastern part of the Malay seas and those of the German surveying ship “Planet” in 1906 in the South Atlantic, Indian and North Pacific Oceans were notable, and Sir John Murray's expedition on the “Michael Sars” in the Atlantic in 1910 obtained important results.
Modern surveying ships no longer make use of hempen lines with enormously heavy sinkers, such as were employed on the “Challenger,” but they sound instead with steel piano wire not more than 1 to 1 of an inch in diameter and a detachable lead seldom weighing more than 70 ℔. The soundings are made by means of a special machine fitted with a brake so adjusted that the revolution of the drum is stopped automatically the instant the lead touches the bottom, and the depth can then be read directly from an indicator. The line is hauled in by a steam or electric winch, and the sounding-tube containing a sample of the bottom deposit is rapidly brought on board. The sounding machines most frequently employed are those of Admiral C. D. Sigsbee, U.S.N., of Lucas, which was perfected in the Telegraph Construction and Maintenance Company's ships, and of the Prince of Monaco, constructed by Leblanc of Paris. All attempts to dispense with a lead and line and to measure the depth by determining the pressure at the bottom have hitherto failed when applied to depths greater than 200 fathoms; a new hydraulic manometer has been tried on board the German surveying ship “Planet.” A. Siemens has pointed out that a profile of the sea-bed can be delineated by taking account of the varying strain on a submarine cable while it is being laid, and the average depth of a section can thus be ascertained with some accuracy. All deep-sea measurements are subject to uncertainty because the sounding machine merely measures the length of wire which runs out before the lead touches bottom, and this agrees with the depth only when the wire is perpendicular throughout its run. It is improbable, however, that the smooth and slender wire is much influenced by currents, and the best deep-sea soundings may be taken as accurate to within 5 fathoms.
Relief of the Ocean Floor.—Recent soundings have shown that the floor of the ocean on the whole lies some 2 or 3 m. beneath the surface, and O. Krümmel has calculated the mean depth to be 2010 fathoms (12,060 ft.), while the mean elevation of the surface of the continents above sea-level is only 2300 ft. Viewed from the floor of the ocean the continental block would thus appear as a great plateau rising to a height of 14,360 ft. Nevertheless, the greatest depths of the ocean below sea-level and the greatest heights of the land above it are of the same order of magnitude, the summit of Mount Everest rising to 29,000 ft. above the sea-level, while the Nero Deep near Guam sinks to 31,600 ft. (5268 fathoms) below sea-level. Of course the area at great heights is very much less than the area at corresponding depths. Above the height of 15,000 ft. there are 800,000 sq. km. (310,000 sq. m.), and below the depth of 15,000 ft. there are 120,000,000 sq. km. (46,300,000 sq. m.); above the height of 20,000 ft. there are on the whole surface of the earth only 33,000 sq. km. (12,800 sq. m.), while below the depth of 20,000 ft. there are no less than 5,400,000 sq. km. (2,100,000 sq. m.). According to Krümmel's calculation the areas of the ocean beyond various depths are as follows:—
|Fathoms.||sq. km.||sq. st. m.|
On the whole the floor of the ocean is very smooth in its contours, and great stretches can almost be called level. Modern orometry has introduced the calculation of the mean angle of the slope of a given uneven surface provided that maps can be prepared showing equidistant contour lines. If the distance between the contour lines is h and the length of the individual contour lines l, the sum of their lengths ∑(l), and A the area of the surface under investigation, then the mean angle of slope is obtained from the equation
tan α = h∑(l).
Calculating from sheet A I of the Prince of Monaco's Atlas of Ocean Depths, Krümmel obtained a mean angle of slope of 0° 27′ 44″ or an average fall of 1 in 124 for the North Atlantic between 0° and 47° N., the enclosed seas being left out of account. In the same way a mean angle of slope of approximately half a degree was found for the Adriatic and the Black Sea. Large angles of slope may, however, occur on the flanks of oceanic islands and the continental borders. On the submarine slopes leading up to isolated volcanic islands angles of 15° to 20° are not uncommon, at St Helena the slopes run up to 38½° and even 40°, at Tristan d'Acunha to 33½°. E. Hull found a mean angle of slope of 13° to 14° for the edge of the continental shelf off the west coast of Europe, and off Cape Toriñana (43° 4′ N.) as much as 34°. Where the French telegraph cable between Brest and New York passes from the continental shelf of the Bay of Biscay to the depths of the Atlantic the angle of slope is from 30° to 41°. Such gradients are of a truly mountainous character, the angle of slope from the Eibsee to the Zugspitze is 30°, and that from Alpiglen station to the summit of the Eiger is 42°. Particularly steep slopes are found in the case of submarine domes, usually incomplete volcanic cones, and there have been cases in which after such a dome has been discovered by the soundings of a surveying ship it could not be found again as its whole area was so small and the deep floor of the ocean from which it rose so flat that an error of 2 or 3 m. in the position of the ship would prevent any irregularity of the bottom from appearing. While such steep mountain walls are found in the bed of the ocean it must be remembered that they are very exceptional, and except where there are great dislocations of the submarine crust or volcanic outbursts the forms of the ocean floor are incomparably gentler in their outlines than those of the continents. Being protected by the water from the rapid subaerial erosion which sharpens the features of the land, and subjected to the regular accumulation of deposits, the whole ocean floor has assumed some approach to uniformity. Still there are everywhere gentle inequalities on the smoothest ocean floor which give to its greater features a distinct relief.
In spite of the increase of deep-sea soundings in the last few decades, they are still very irregularly distributed in the open ocean, and the attempt to draw isobaths (lines of equal depth) on a chart of the world is burdened with many difficulties which can only be evaded by the widest generalizations. Bearing this caution in mind the existing bathymetrical charts, amongst which that of the prince of Monaco stands first, give a very fair idea of the great features of the bed of the oceans. A definite terminology for the larger forms of sub-oceanic relief was put forward by the International Geographical Congress at Berlin in 1899 and adopted by that at Washington in 1904. Equivalent terms, which are not necessarily identical or literal translations, were adopted for the English, French and German languages, the equivalence being closest and most systematic between the English and German terms.
The larger forms designated by special generic terms include the following. The continental shelf is the gentle slope which extends from the edge of the land to a depth usually about 100, though in some cases as much as 300 fathoms, and is there demarcated by an abrupt increase in the steepness of the slope to ocean depths. In the deep sea two types of feature are recognized under the general names of depression and elevation. The depression is distinguished according to form and slope as (1) a basin when of a roughly round outline, (2) a trough when wide and elongated, or (3) a trench when narrow and elongated lying along the edge of a continent. The extension of a basin or trough stretching towards the continent is termed an embayment when relatively wide and a gully when narrow. The elevation includes (1) the gently swelling rise which separates troughs and basins in the middle of the ocean, (2) the steeply sloping ridge which interposes a narrower barrier between two depressions, and (3) the plateau or wide elevation rising steeply on all sides from a depression. The deepest part of a depression is termed a deep, and the highest part of an elevation when not reaching the surface a height. In addition to these larger forms a few minor forms must be recognized. Amongst these are the dome, an isolated elevation rising steeply but not coming within 100 fathoms of the surface; the bank, an elevation coming nearer the surface than 100 fathoms, but not so near as 6 fathoms; and finally the shoal or reef, which comes within 6 fathoms of the surface, and so may constitute a danger to shipping. Similarly we may note the caldron or small steep depression of a round outline, and the furrow or long narrow groove in the continental shelf.
According to the resolutions of the International Geographical Congress the larger individual forms which have been described by generic terms shall have specific names of a purely geographical character; but in the case of the minor forms the names of ships and persons are considered applicable. In 1899 A. Supan published a chart of the oceans with a suggested nomenclature based on these principles; and the larger forms in the Prince of Monaco's great chart also are named in accordance with the rule. Although put forward by the highest international authority recognized by geographers the system of nomenclature has not been adopted universally. In particular Sir John Murray considers that only deeps exceeding 3000 fathoms in depth should be named, and in his charts he has named these deeps after persons whether the individuals thus honoured had themselves discovered or explored the deeps in question or not. Some of the “deeps” to which names have been given disappear or are divided into two or three smaller deeps when the contour lines representing hundreds of fathoms are translated into contour lines representing hundreds of metres. A similar change in the contour lines may result from the substitution of lines in fathoms for those originally drawn in metres, and hence it is extremely desirable that specific names should only be given to such features as are pronounced enough to appear on maps drawn with either unit. For the sake of uniformity it is to be hoped that the system of nomenclature recommended by the International Geographical Congress will ultimately be adopted.
The continental shelves are parts of the great continental blocks which have been covered by the sea in comparatively recent times, and their surface consequently presents many similarities to that of the land, modified of course by the destructive and constructive work of the waters. Waves and tidal currents produce their full effects in that region, and in high latitudes the effect of transport of materials by ice is very important; while in the warm water of the tropics the reef building animals and plants (corals and calcareous algae) carry on their work most effectively there. The continental shelves include not only the oceanic border of the continents but also great areas of the enclosed seas and particularly of the fringing seas, the origin of which through secular subsidence is often very clearly apparent, as for instance in the North Sea and the tract lying off the mouth of the English Channel. A closer investigation of the numerous long, narrow banks which lie off the Flemish coast and the Thames estuary shows that they are composed of fragments of rock abraded and transported by tidal currents and storms in the same way that the chalk and limestone worn off from the eastern continuation of the island of Heligoland during the last two centuries has been reduced to the coarse gravel of the off-lying Düne. Numerous old river valleys and furrows entrenched in the continental shelf bear witness to its land origin. Such valleys are very clearly indicated in the belts of the western Baltic by furrows a thousand yards wide and twenty to thirty fathoms deeper than the neighbouring sea-bed. Amongst the best known of the furrows of the continental shelf are the Cape Breton Deep, in the Bay of Biscay, the Hudson Furrow, southward of New York, the so-called Congo Canon, the Swatch of No Ground off the Ganges delta, the Bottomless Pit off the Niger delta, and numerous similar furrows on the west coast of North America and outside the fjords of Norway, Iceland and the west of Scotland, as well as in the Firth of Forth and Moray Firth.
The seaward edge of the continental shelf often falls steeply to the greatest depths of the ocean, and not infrequently forms the slope of a trench, a form of depression which has usually a steep slope towards a continent or an island-bearing rise on one side and a gentler slope towards the general level of the ocean on the other. All the greatest depths of ocean, i.e. all soundings exceeding 4000 fathoms, occur in trenches, and there are only a few small trenches known (on the west coast of Central America) in which the maximum depth is less than 3000 fathoms. Most trenches are narrow, but of considerable length, and their steeper side is believed to be due in every case to a great fracture of the earth's crust. Strong evidence of this is afforded by the association of some of the depressions, notably the Japan Trench and the Atacama Trench, with the origin of frequent submarine earthquakes. Troughs and rises are features of more frequent occurrence and are best described as they occur in the particular oceans.
In the Atlantic the prevailing meridional direction of the shore lines extends to the submarine features also. Captain Sherard Osborn in 1870 was the first to recognize that the North Atlantic Basin was divided by a central rise running generally from north to south into two parallel depressions. In 1876 the “Challenger” expedition found that a similar configuration exists in the South Atlantic also. As the result of all the deep-sea surveys now available we know that the central rise of the Atlantic starts from Iceland as the Reykjanes Ridge with less than 1000 fathoms of water over it in most parts and runs south-westward until in 51° N. it widens into what was called by Maury the Telegraph Plateau. Continuing southwards the rise joins the Azores Plateau, which has in parts a very marked relief, and runs thence southward almost exactly in the middle of the ocean, becoming gradually lower as it goes. As far as 29° N. the depth over it is less than 1500 fathoms, thence to 12° N. the depths are between 1500 and 2000 fathoms, and then it rises again to about 1500 fathoms and runs eastward under the name of the Equatorial Ridge. Crossing the equator in 13° W. the rise resumes a southerly direction and from Ascension to Tristan d'Acunha, the depth is in many places less than 1500 fathoms. The soundings of Bruce's Antarctic expedition in the “Scotia” showed that the rise cannot be traced beyond 55° S. where the depths increase rapidly to over 2000 fathoms. The whole length of the rise which divides the Atlantic into an eastern and a western basin may be taken as 7500 nautical miles. Between 30°and 40° S. two lateral ridges diverge from the great Atlantic rise, the Rio Grande ridge towards the north-west and the Walfisch ridge towards the north-east. The existence of the latter, which extends to the African continent, was announced by Sir Wyville Thomson in 1876 as a result of his discussion of the deep-sea temperature observations of the “Challenger” expedition, though the fact was not confirmed by soundings until many years later.
The West Atlantic Trough lying on the western side of the Central Rise widens in the north into the North American Basin, and its greatest depths appears to be in the Porto Rico Trench, where in 1882 Capt. W. H. Brownson, U.S.N., obtained a sounding of 4561 fathoms in 19° 36′ N., 66° 26′ W. The Brazilian Basin has also a large area lying at a depth greater than 2500 fathoms and culminates in the Romanche Deep close to the Equatorial Ridge in 0° 11′ S., 18° 15′ W. with a depth of 4030 fathoms. The Eastern Atlantic Trough cannot boast of such great depths though the Peake Deep with 3284 fathoms sinks abruptly from the Azores Plateau in 43° 9′ N., 19° 45′ W., and several soundings exceeding 2700 fathoms have been obtained in the Bay of Biscay east of the meridian of 5° E. The North African Basin has several deeps with more than 3300 fathoms to the north-west and the south-west of the Cape Verde Islands, but the South African Basin is less deep. In the South Atlantic there is no connexion between the Central Rise and the Antarctic Shelf, for the Indo-Atlantic Antarctic Basin stretches from near the South Sandwich Islands towards Kerguelen with depths exceeding 2500 fathoms and reaching in places 3100. The Cape Trough runs northward from this basin. It was long believed on the strength of a sounding of “4000 fathoms, no bottom” reported by Sir James Ross in 68° 22′ S., 12° 49′ W., that the Indo-Atlantic Basin was of enormous depth, but W. S. Bruce, in the “Scotia,” showed in 1904 that the real depth at that point is only 2660 fathoms.
In the Indian Ocean the Kerguelen Rise stretches broadly southward, east of the island which gives it a name, to the Antarctic Shelf with the greatest depths upon it usually less than 2000 fathoms, and it stretches northward beyond New Amsterdam to 30° S. This rise is separated from the Crozet Rise by a depression extending to 2675 fathoms, through which the Kerguelen Trough (which lies north of Kerguelen) is brought into free communication with the Indo-Atlantic Antarctic Basin. The greater part of the Indian Ocean is occupied by the great Indian Basin, which covers 35,000,000 sq. km. (13,500,000 sq. m.) and extends from the Chagos Islands eastward to Australia and south-eastward to Tasmania. The Australian Shelf rises steeply as a rule from depths of 2500 to 3000 fathoms. A broad depression with depths of from 3300 to 3500 fathoms lies to the east of the Cocos Islands and extends into the angle between the Malay Archipelago and Australia. On the north this depression sinks into the long and narrow Sunda Trench south of Java, and here in 10° 15′ S., 108° 5′ E., the German surveying-ship “Planet” obtained a sounding of 3828 fathoms in 1906. The Sunda Trench is distinguished by the wave-like configuration of its floor, and this wave-like character is continued to the westward of Sumatra with islands rising from the higher portions. The western part of the Indian Ocean has been shown by the surveys of H.M.S. “Sealark” and the German surveying-ship “Planet” to have a somewhat complicated configuration, the island groups and banks of atolls which occur there rising abruptly as a rule from depths of about 2000 fathoms or more. Between the Seychelles and Sokotra (0°-9° N.) there are great stretches of the ocean floor forming an almost level expanse at a depth of 2800 fathoms. The Arabian Gulf and Gulf of Aden are also very uniform with depths of about 1900 fathoms, while the floor of the Bay of Bengal rises very gradually northwards and is 1000 fathoms deep close up to the Ganges Shelf.
The Pacific Ocean consists mainly of one enormous basin bounded on the west by New Zealand and the Tonga, Marshall and Marianne ridges, on the north by the festoons of islands marking off the North Pacific fringing seas, on the east by the coast of North America and the great Easter Island Rise and on the south by the Antarctic Shelf. The total area of this basin is about 80,000,000 sq. km. (30,000,000 sq. m.), its surface being almost twice that of Asia. Half of this basin lies deeper than 2750 fathoms, and the greater part of it belongs to the northern hemisphere. From the floor of this vast and profound depression numerous isolated volcanic cones rise with abrupt slopes, and even between the islands of the Hawaiian group there are depths of more than 2000 fathoms. The Society Islands and Tahiti crown a rise coming within 1500 fathoms of the surface, two similar rises form the foundation of the Paumotu group Where Agassiz found soundings of 2187 fathoms between Marokau and Hao. This greatest of ocean basins contains also the largest and deepest trenches. The Tuscarora Deep of the Japan Trench (4655 fathoms in 44° 55′ N., 152° 26′ E.) was famed for many years as the deepest depression of the earth's crust. This great trench is continued along the Luchu Islands where the cable-steamer “Stephan” sounded in 4080 fathoms, and through the Bonin Trench (with a maximum of 3595 fathoms) to the famous Marianne Trench in which the U.S.S. “Nero” in 1899 found 5269 fathoms in 12° 43′ N., 145° 49′ E., the greatest depth yet measured. The northern part of the Marianne Trench leads to a wave-like configuration of the ocean floor, the depth to the east of Saipan being over 4300 fathoms, followed by a rise to 1089 fathoms and then a descent to 3167 fathoms. The trenches of Yap (4122 fathoms) and Palau (Pelew) (4450 fathoms) are not immediately connected with that of Marianne. To the east of the Philippines a sounding of 3490 fathoms is found close to the Strait of St Bernardino and north-east of Talaut there is a trench with 4648 fathoms. To the north-east the Japan Trench adjoins the Aleutian Trench, where a depth of 4038 fathoms has been found south-west of Attu. Trenches of great size also occur south of the equator. The Tonga and Kermadec trenches, both deeper than 4000 fathoms, stretch from the Samoa Islands southwards toward New Zealand for a distance of 1600 nautical miles. The deepest sounding obtained in the Tonga Trench is 5022 fathoms in 23° 39.4′ S., 175° 4′ W., and in the Kermadec Trench, 5155 fathoms, 30° 27.7′ S., 176° 39′ W. The steep western sides of these trenches often show an angle of slope of 7°.
The south-western part of the Pacific Ocean has a very rich and diversified submarine relief, abounding in small basins separated by ridges and rises. There are no depths, however, much exceeding 2500 fathoms amongst these depressions. The south-eastern part of the Pacific is mainly occupied by the Easter Island Rise with depths rarely so great as 2000 fathoms; but close to the continent of South America the Atacama Trench is a typical example of the deepest form of depression culminating with 4175 fathoms in 25° 42′ S., 71° 31.5′ W. The Pacific Antarctic Basin occupies the vast region south of 50° S. right up to the Antarctic Shelf, with depths ranging down to 2500-3000 fathoms, and communicating with the main Pacific Basin to the east of New Zealand.
The greatest of the intercontinental seas, the Arctic, comes nearest to oceanic conditions in the extent and depth of its depressions. The soundings of Nansen and Sverdrup on the “Fram” expedition indicate that northward from the Siberian Shelf the great North Polar Basin has an area of about 4,000,000 sq. km. (1,500,000 sq. m.) with depths down to 2200 fathoms. A rise between Spitsbergen and Greenland separates the Norwegian Trough (greatest depth 2005 fathoms in 68° 21′ N., 2° 5′ W.) which in turn is divided from the Atlantic by the Wyville Thomson Ridge which runs between the Faeroe and Shetland islands and is covered by only 314 fathoms of water at the deepest point. The ridge across Denmark Strait west of Iceland nowhere exceeds 300 fathoms in depth, so that the deeper water of the North Polar Basin is effectively separated from that of the Atlantic. A third small basin occupies Baffin Bay and contains a maximum depth of 1050 fathoms. Depths of from 100 to 300 fathoms are not uncommon amongst the channels of the Arctic Archipelago north of North America, and Bering Strait, through which the surface water of the Arctic Sea meets that of the Pacific, is only 28 fathoms deep.
The Central American Sea communicates with the Atlantic through the channels between the Antilles, none of which is quite 1000 fathoms deep, and it sinks to a depth of 2843 fathoms in the Caribbean Basin, 3428 fathoms in the Cayman Trench and 2080 fathoms in the Gulf of Mexico.
The Austral-Asiatic or Malay Sea is occupied by a great shelf in the region west of Borneo and north of Java, while in the east there are eight abruptly sunk basins of widely different size. The China Sea on the north has a maximum depth of 2715 fathoms off the Philippines, the Sulu Basin reaches 2550 fathoms, and the Celebes Basin 2795 fathoms. Some of the channels between the islands are of very great depth, Macassar Strait exceeding 1000 fathoms, the Molucca Passage exceeding 2000 fathoms, and the Halmahera Trough sinking as deep as 2575 fathoms. The deepest of all is the Banda Basin, a large area of which lies below 2500 fathoms and reaches 3557 fathoms in the Kei Trench. A depth of 2789 fathoms also occurs north of Flores. The borders of the Malay Sea are everywhere shallower on the side of the Indian Ocean than on that of the Pacific, and consequently water from the Pacific preponderates in the depths.
The Mediterranean Sea, the best-known member of the intercontinental class, is separated from the Atlantic Ocean by a ridge running from Cape Spartel to Cape Trafalgar on which the greatest depth is only 175 fathoms. The depth increases so rapidly towards the east that soundings exceeding 500 fathoms occur off Gibraltar. The Balearic Basin, between Spain and the rise bearing Corsica and Sardinia, has a maximum depth of 1742 fathoms, and the Tyrrhenian Basin between that rise, Italy and Sicily deepens to 2040 fathoms. The larger Eastern Mediterranean Basin stretches eastward from Sicily with large tracts more than 2000 fathoms below the surface, and the greatest depth ascertained during the detailed researches of the Austrian expedition on board the “Pola” was 2046 fathoms in 35° 44.8′ N., 21° 46.8′ E. The Adriatic Sea though very shallow in the north deepens southward to about 900 fathoms, and the Aegean Sea has a maximum depth of 1230 fathoms north of Crete. The Black Sea, connected with the Mediterranean by long and narrow channels, is occupied in the north by an extensive shelf on which lies the extremely shallow Gulf of Azov; but the greater part of the sea consists of a deep basin, the central part of which is an almost flat expanse at a uniform depth of 1220 fathoms.
The smaller enclosed seas are for the most part very shallow. The Persian Gulf nowhere exceeds 50 fathoms, the southern part of Hudson Bay does not exceed 100 fathoms except at one spot, though in the less-known fjords of the northern part depths up to 200 fathoms have been reported. The Baltic Sea exceeds 50 fathoms in few places except the broad central portion, though small caldron-like depressions here and there may sink below 200 fathoms. The Red Sea on the other hand, though shut off from the Indian Ocean by shallows of the Strait of Bab-el-Mandeb with little more than 100 fathoms, sinks to a very considerable depth in its central trough, which reaches 1209 fathoms in 20° N.
The fringing seas as a rule show little variety of submarine relief. The Bass Sea (Bass Strait), Irish Sea and North Sea lie on the continental shelf. In the North Sea the depth of 100 fathoms is only exceeded to any extent in the Norwegian gully, which has a maximum depth of 383 fathoms in the Skagerrack. Most of the other seas of this class are formed on a common plan. Towards the continent there is a broad shelf, and just before the chain of islands separating them from the ocean runs a narrow and deep trough. In the Bering Sea the trough north of Buldir in the Aleutian Islands sinks to 2237 fathoms, and in the Sea of Okhotsk, north-west of the Kuriles, to 1859 fathoms. Similar conditions prevail in the East China Sea and the Andaman Sea. The Sea of japan has a wide shelf only in the north, the central part forms a broad basin with depths of 1650 fathoms. The Laurentian Sea (Gulf of St Lawrence) has a narrow branching gully running between wide shelves, in which a depth of 312 fathoms is found south of Anticosti.
The area, general depth and total volume of the oceans and principal seas have been recalculated by Krümmel, and the accompanying table presents these figures.
Mean Depths of Oceans and Seas.
|sq. km.||sq. st. m.||cb. km.||cb. st. m.|
|Central American Sea||1143||4,584,570||1,770,170||9,579,490||2,298,400|
|Smaller Enclosed Seas||96||2,320,660||896,050||408,770||98,070|
|II. Enclosed Seas||674||32,350,200||12,490,890||39,879,530||9,568,170|
|East China Sea||97||1,242,480||479,740||219,820||52,700|
|III. Fringing Seas||531||8,113,810||3,132,860||7,879,860||1,890,630|
|Seas (Enclosed and Fringing)||645||40,464,010||15,623,750||47,759,390||11,458,800|
Oceanic Deposits.—It has long been known that the deposits which carpet the floor of the ocean differ in different places, and coasting sailors have been accustomed from time immemorial to use the lead not only to ascertain the depth of the water but also to obtain samples of the bottom, the appearance of which is often characteristic of the locality. In depths down to 100 fathoms the old-fashioned hand-lead, hollow below and “armed” with tallow, suffices to bring up a sample large enough to be recognizable. Captain Phipps in 1773 secured samples of soft blue clay in this manner from a depth of 683 fathoms, but as a rule when sounding in great depths the sample is washed off the tallow before it can be brought on board. Various devices have consequently been attached to leads intended to catch and hold the material when soft enough to be penetrated. One of the most effective early forms was the snapper or “deep-sea clamm” of Sir John Ross, a pair of powerful spring jaws held apart by an arrangement which when released on striking the bottom allowed the jaws to close, biting out and holding securely a substantial portion of the ground. A simpler form of collector, now almost universally used, is a plain brass tube which is driven into the bottom of the sea by the weight of the sounding lead, and in which the deposit may be retained by a valve or other contrivance, though in many cases friction alone suffices to hold the punched-out core. Larger quantities of deposit may be conveniently collected by means of the dredge, which can be worked in any depth and brings up large stones, concretionary nodules or fossils, of the existence of which a sounding-tube could give no indication.
The voyage of the “Challenger” supplied for the first time the nucleus of a collection of deep-sea deposits sufficient to serve as the basis for comprehensive classification and mapping. The “Challenger” collections supplemented by those of other expeditions and of many telegraph and surveying-ships were studied in detail by Sir John Murray and Professor A. Renard, whose monograph, published in 1891, laid the foundations and reared the greater part of the structure of our present knowledge on the subject. The classification adopted was a double one, taking account both of the origin and of the distribution in depth of the various deposits, thus:—
||Deep Sea Deposits (beyond 100 fathoms)||1. Red Clay.||A. Pelagic Deposits (formed in deep water remote from land)|
|2. Radiolarian Ooze|
|3. Diatom Ooze|
|4. Globigerina Ooze|
|5. Pteropod Ooze|
|6. Blue Mud||B. Terrigenous Deposits (formed in deep or shallow water close to land)|
|7. Red Mud|
|8. Green Mud|
|9. Volcanic Mud|
|10. Coral Mud|
|II.||Shallow Water Deposits (in less than 100 fathoms)||Sands, gravels, muds, &c.|
|III.||Littoral Deposits (between high and low-water marks)||Sands, gravels, muds, &c.|
Krümmel prefers to simplify this by grouping the deposits in a single category arranged according to their position into:
|(α) Littoral (including Murray and Renard's littoral and shallow water deposits [II. and III.]).|
|(β) Hemipelagic (including Nos. 6-10 of Deep Sea Deposits).|
|(γ) Eupelagic (including Nos. 1-5 of Deep Sea Deposits).|
As so defined the hemipelagic deposits are those which occur in general on the slope from the continental shelves to the ocean depths and also in the deep basins of enclosed and fringing seas. The eupelagic deposits are subdivided by Kriimrnel into two main groups; (a) epilophic, including the pteropod, globigerina and diatom oozes occurring on the rises and ridges and in the less deep troughs. (b) Abyssal, including the radiolarian ooze and red clay of the deepest abysses.
The littoral deposits include those of the actual shore on the wash of the waves and of the surface of the continental shelf.
Shore Deposits are the product of the waste of the land arranged and bedded by the action of currents or tidal streams. On the rocky coast of high latitudes blocks of stone detached by frost fall on the beach and becoming embedded in ice during winter are often drifted out to sea and so carry the shore deposits to some distance from the land. Similar effects are produced along the boulder-clay cliffs of the Baltic. Where the force of the waves on the beach produces its full effect the coarser material gets worn down to gravel, sand and silt, the finest particles remaining long suspended in the water to be finally deposited as mud in quiet bays. A particularly fine-grained mud is formed on the low coasts of the eastern border of the North Sea by a mixture of the finest sediment carried down by the slow-running rivers with the calcareous or siliceous remains of plankton. Pure calcareous sand and calcareous mud are formed by wave action on the shores of coral islands where the only material available is coral and the accompanying calcareous algae, crustacea, molluscs and other organisms secreting carbonate of lime. Recent limestones are being produced in this way and also in some places by the precipitation of calcium carbonate by sodium or ammonium carbonate which has been carried into the sea or formed by organisms. The precipitated carbonate may agglomerate on mineral or organic grains which serve as nuclei, or it may form a sheet of hard deposit on the bottom as occurs in the Red Sea, off Florida, and round many coral islands in the Pacific. Only the sand and the finest-grained sediments of the shore zone are carried outwards over the continental shelf by the tides or by the reaction-currents along the bottom set up by on-shore winds. The very finest sediment is kept in a state of movement until it drops into the gulleys or furrows of the shelf, where it can come to rest together with the finer fragments of the remains of littoral or bank vegetation. Thus are formed the “mud-holes” of the Hudson Furrow so welcome as guides telling their position to ship captains making New York harbour in a fog. Sand may be taken as the predominating deposit on the continental shelves, often with a large admixture of remains of calcareous organisms, for instance the deposits of maërl made up of nullipores off the coasts of Brittany and near Belle Isle. Amongst the most widely distributed of the deposits actually formed on the continental shelf are phosphatic nodules; these are especially abundant on the east coast of the United States and on the Agulhas Bank, where the amount of calcium phosphate in the nodules is as much as 50%. Sir John Murray finds the source of the phosphoric acid to be the decomposition of large quantities of animal matter, and he illustrates this by the well-known circumstance of the death of vast shoals of fish when warm Gulf-Stream water displaces the cold current which usually extends to the American coast. Glacial detritus naturally plays a great part in the deposits on the polar continental shelves.
Hemipelagic deposits are a mixture of deposits of terrigenous and pelagic origin. The most abundant of the terrigenous materials are the finest particles of clay and calcium carbonate as well as fragments derived from land vegetation, of which twigs, leaves, &c., may form a perceptible proportion as far as 200 m. from land. Blue mud, according to Murray and Renard, is usually of a blue or slaty or grey-green colour when fresh, the upper surface having, however, a reddish tint. The blue colouring substance is ferrous sulphide, the upper reddish layer contains more ferric oxide, which the predominance of decomposing organic matter in the substance of the mud reduces to ferrous oxide and subsequently by further action to sulphide. The proportion of calcium carbonate varies greatly according to the amount of foraminifera and other calcareous organisms which it contains. Blue mud prevails in large areas of the Pacific Ocean from the Galapagos Islands to Acapulco. In the Indian Ocean it covers the Bay of Bengal, the Arabian Gulf, the Mozambique Channel and the region to the south-west of Madagascar. In the Atlantic it is the characteristic deposit of the slopes of continental shelves of western Europe and of New England, being largely mixed with ice-borne material to the south of Newfoundland. It is particularly in evidence round the whole of the Antarctic Shelf, where it occurs down to depths of 2500 fathoms. It is the chief deposit, according to Nansen, of the North Polar Basin and, according to Schmelck and Böggild, of the Norwegian Sea also, where it is largely mixed with the shells of the bottom-living foramini er Biloculina. Max Weber states that blue mud occurs in the deep basins of the eastern part of the Malay Sea. In the form of volcanic mud it is common round the high volcanic islands of the South-Western Pacific.
Red mud may be classed as a variety of blue mud, from which it differs on account of the larger proportion of ochreous substance and the absence of sufficient organic matter to reduce the whole of the ferric oxide. This variety surrounds the tropical parts of the continental shelves of South America, South Africa and eastern China.
Green mud differs to a greater extent from the blue mud, and owes its characteristic nature and colour to the presence of glauconite, which is formed inside the cases of foraminifera, the spines of echini and the spicules of sponges in a manner not yet understood. It occurs in such abundance in certain geological formations as to give rise to the name of green-sand. Green mud abounds off the east coast of North America seawards of Cape Hatteras, also to the north of Cuba, and on the west off the coast of California. The “Challenger” expedition found it on the Agulhas Bank, on the eastern coasts of Australia, Japan, South America and on the west coast of Portugal. When the proportion of calcium carbonate in the blue mud is considerable there results a calcareous ooze, which when found on the continental slope and in enclosed seas is largely composed of remains of deep-sea corals and bottom-living foraminifera, pelagic organisms including pteropods being less frequently represented. The floors of the Caribbean, Cayman and Mexican Basins in the Central American Sea are covered with a white calcareous ooze, which is clearly distinguished from the eupelagic pteropod and globigerina oozes by the presence of abundant large mineral particles and the remains of land plants. In this deposit the occurrence of calcareous concretions is very characteristic, as L. F. de Pourtalès pointed out in 1870; they consist of remains of deep-sea corals, serpulae, echinoderms and mollusca united by a calcareous cement. Similar formations are found in the Mediterranean, where a dark mud predominates in the western part, passing into a grey, marly slime in the Tyrrhenian Basin and replaced by a typical calcareous ooze in the Eastern Basin. The bottom of the Black Sea is covered by a stiff blue mud in which Sir John Murray found much sulphide of iron, grains or needles of pyrites making up nearly 50% of the deposit, and there are also grains of amorphous calcium carbonate evidently precipitated from the water. The formation of the blue mud is largely aided by the putrefaction of organic matter, and as a result the water deeper than 120 fathoms is extraordinarily deficient in dissolved oxygen and abounds in sulphuretted hydrogen, the formation of which is brought about by a special bacterium, the only form of life found at depths greater than 120 fathoms in the Black Sea.
In the Red Sea the “Pola” expedition discovered a calcareous ooze similar to that of the Mediterranean, and the formation of a stony crust by precipitation of calcium and magnesium carbonates may be recognized as giving origin to a recent dolomite.
The terrigenous ingredients in the deposits become less and less abundant as one goes farther into the deep ocean and away from the continental margins. Still, according to Murray and Irvine, finely divided colloidal clay is to be found in all parts of the ocean however remote from land, though in very small amount, and there is less in tropical than in cooler waters. A minute fraction is always separating out of the water, and as a prodigious length of time may be accepted for the accomplishment of all the chemical and physical processes in the deep sea, we must take account of the gradual accumulation of even this infinitesimal precipitation. As well as the finest of terrigenous clay there is present in sea-water far from land a different clay derived from the decomposition of volcanic material. Volcanic dust thrown into the air settles out slowly, and some of the products of submarine and littoral volcanoes, like pumice-stone, possess a remarkable power of floating and may drift into any part of the ocean before they become waterlogged and sink. To this inconceivably slowly-growing deposit of inorganic material over the ocean floor there is added an overwhelmingly more rapid contribution of the remains of calcareous and siliceous planktonic and benthonic organisms, which tend to bury the slower accumulating material under a blanket of globigerina, pteropod, diatom or radiolarian ooze. When those deposits of organic origin are wanting or have been removed, the red clay composed of the mineral constituents is found alone. It is a remarkable geographical fact that on the rises and in the basins of moderate depth of the open ocean the organic oozes preponderate, but in the abysmal depressions below 2500 or 3000 fathoms, whether these lie in the middle or near the edges of the great ocean spaces, there is found only the red clay, with a minimum of calcium carbonate, though sometimes with a considerable admixture of the siliceous remains of radiolarians. Thus red clay and radiolarian ooze are distinguished as abyssal deposits in contradistinction to the epilophic calcareous oozes.
Globigerina ooze was recognized as an important deposit as soon as the first successful deep-sea soundings had been made in the Atlantic. It was described simultaneously in 1853 by Bailey of West Point and Ehrenberg in Berlin. Murray and Renard define globigerina ooze as containing at least 30% of calcium carbonate, in which the remains of pelagic (not benthonic) foraminifera predominate and in which remains of pelagic mollusca such as pteropods and heteropods, ostracodes and also coccoliths (minute calcareous algae) may also occur. Not more than 25% of the deposit may consist of bottom dwelling foraminifera, echini or Worm-tubes, and as a rule these make up only from 9 to 10%. These peculiarities, combined with the striking absence of mineral constituents, distinguish the eupelagic globigerina ooze from the hemipelagic calcareous mud. Out of 118 samples of globigerina ooze obtained by the “Challenger” expedition 84 came from depths of 1500 to 2500 fathoms, 13 from depths of 1000 to 1500 and only 16 from depths greater than 2500 fathoms. Viewed as a whole this deposit may be taken as a partial precipitation of the plankton living in the upper waters of the open sea. A small proportion of organic matter including the fat globules of the plankton is mixed with the calcium carbonate, the amount according to Gümbel's analysis being about 1 part in 1000. Secondary products, such as glauconite, phosphatic concretions and manganese nodules, occur though less frequently than in the hemipelagic sediments. Globigerina ooze is the characteristic deposit of the Atlantic Ocean, where it covers not less than 44,000,000 sq. km. (17,000,000 sq. statute m.). In the Indian Ocean the area covered is 31,000,000 sq. km. (12,000,000 sq. m.) and in the huge Pacific Ocean only 30,000,000 sq. km. (11,500,000 sq. m.).
Pteropod ooze is merely a local variety of globigerina ooze in which the comparatively large but very delicate spindle-shaped shells of pteropods happen to abound. These shells do not retain their individuality at depths greater than 1400 or 1500 fathoms, and in fact pteropod ooze is only found in small patches on the ridges near the Azores, Antilles, Canaries, Sokotra, Nicobar, Fiji and the Paumotu islands, and on the central rise of the South Atlantic between Ascension and Tristan d'Acunha.
Diatom ooze was recognized by Sir John Murray as the characteristic deposit in high latitudes in the Indian Ocean, and later it was found to be characteristic also of the corresponding parts of the Indian and Pacific covering a total area of about 22,000,000 sq. km. (8, 500,000 sq. m.). It has been found sporadically near the Aleutian Islands, between the, Philippines and Marianne Islands and to the south of the Galapagos group. It is made up to a large extent of the siliceous frustules of diatoms. It is usually yellowish-grey and often straw-coloured when wet, though when dried it becomes white and mealy.
Red clay was discovered and named by Sir Wyville Thomson on the “Challenger” in 1873 when sounding in depths of 2700 fathoms on the way from the Canary Islands to St Thomas. The reddish colour comes from the presence of oxides of iron, and particles of manganese also occur in it, especially in the Pacific region, where the colour is more that of chocolate; but when it is mixed with globigerina ooze it is grey. Red clay is the deposit peculiar to the abysmal area; 70 carefully investigated samples collected by the “Challenger” came from an average depth of 2730 fathoms, 97 specimens collected by the “Tuscarora” came from an average depth of 2860 fathoms, and 26 samples obtained by the “Albatross” in the Central Pacific came from an average depth of 2620 fathoms. Red clay has not yet been found in depths less than 2200 fathoms. The main ingredient of the deposit is a stiff clay which is plastic when fresh, but dries to a stony hardness. Isolated gritty fragments of minerals may be felt in the generally fine-grained homogeneous mass. The dredge often brings up large numbers of nodules formed upon sharks' teeth, the ear-bones of whales or turtles or small fragments of pumice or other volcanic ejecta, and all more or less incrusted with manganese oxide until the nodules vary in size from that of a potato to that of a man's head. A very interesting feature is the small proportion of calcium carbonate, the amount present being usually less as the depth is greater; red clay from depths exceeding 3000 fathoms does not contain so much as 1% of calcareous matter.
Murray and Renard recognize the progressive diminution of carbonate of lime with increase of depth as a characteristic of all eupelagic deposits. The whole collection of 231 specimens of deep-sea deposits brought back by the “Challenger” shows the following general relationship:—
Proportion of Calcium Carbonate in Deep-Sea Deposits.
|68||samples from||less than 2000||fathoms =||60-80 %|
|8||””||more than 3000||”||0.9 %|
In deep water there is a regular process of solution of the calcareous shells falling from the surface. Murray and Renard ascribe this to the greater abundance of carbonic acid in the deeper water, which aided by the increased pressure adds to the solvent power of the water for carbonate of lime. It is, however, a curious question how, considering the increase of carbonic acid by the decomposition of organic bodies and possible submarine exhalations of volcanic origin, the water has not in some places become saturated and a precipitate of amorphous calcium carbonate formed in the deepest water. The whole subject still requires investigation.
Amongst the foreign material found embedded in the red clay are globules of meteoric iron, which are sometimes very abundant. Derived products in the form of crystals of phillipsite are not uncommon, but the most abundant of all are the incrustations of manganese oxide, as to the origin of which Murray and Renard are not fully clear. The manganese nodules afford the most ample proof of the prodigious period of time which has elapsed since the formation of the red clay began; the sharks' teeth and whales' ear-bones which serve as nuclei belong in some cases to extinct species or even to forms derived from those familiar in the fossils from the seas of the Tertiary period. This fact, together with the extraordinarily rare occurrence of such remains and meteoric particles in globigerina ooze, although there is no reason to suppose that at any one time they are unequally distributed over the ocean floor, can only be explained on the assumption that the rate of formation of the epilophic deposits through the accumulation of pelagic shells falling from the surface is rapid enough to bury the slow-gathering material which remains uncovered on the spaces where the red clay is forming at an almost infinitely slower rate. Sir John Murray believes that no more than a few feet of red clay have accumulated in the deepest depressions since the close of the Tertiary period. The red clay is the characteristic deposit of the Pacific Ocean, where about 101,000,000 sq. km. (39,000,000 sq. m.) are covered with it, while only 15,000,000 sq. km. (5,800,000 sq. m.) of the Indian Ocean and 14,000,000 sq. km. (5,400,000 sq. m.) of the Atlantic are occupied by this deposit; it is indeed the dominant submarine deposit of the water-hemisphere just as globigerina ooze is the dominant submarine deposit of the land-hemisphere.
Radiolarian ooze was recognized as a distinct deposit and named by Sir John Murray on the “Challenger” expedition, but it may be viewed as red clay with an exceptionally large proportion of siliceous organic remains, especially those of the radiolarians which form part of the pelagic plankton. It does not occur in the Atlantic Ocean at all, and in the Indian Ocean it is only known round Cocos and Christmas Islands; but it is abundant in the Pacific, where it covers a large area between 5° and 15° N., westward from the coast of Central America to 165° W., and it is also found in patches north of the Samoa Islands, in the Marianne Trench and west of the Galapagos Islands.
The total areas occupied by the various deposits according to the latest measurements of Krümmel are as follows:—
Area of Submarine Deposits.
|Deposit.||Sq. km.||Sq. st. m.||%.|
Geologists are agreed that littoral and hemipelagic deposits similar to those now forming are to be found in all geological systems, but the existence in the rocks of eupelagic deposits and especially of the abysmal red clay, though viewed by some as probable, is totally denied by others. There is even some hesitation in accepting the continuity of the chalk with the globigerina ooze of the modern ocean. From the obvious rarity of true abysmal rocks in the continental area Sir John Murray deduces the permanence of the oceans, which he holds have always remained upon those portions of the earth's crust which they occupy now, and both J. Dana and Louis Agassiz had already arrived at the same conclusion. This theory accords well with the enormous lapse of time required in the accumulation of the red clay.
Salts of Sea-water.—Sea-water differs from fresh water by its salt and bitter taste and by its unsuitability for the purposes of washing and cooking. The process of natural evaporation in the salines or salt gardens of the margin of warm seas made the composition of sea-salt familiar at a very early time, and common salt, Epsom salts, gypsum and magnesium chloride were recognized amongst its constituents. The analyses cf modern chemists have now revealed the existence of 32 out of the 80 known elements as existing dissolved in sea-water, and it is scarcely too much to say that the remaining elements also exist in minute traces which the available methods of analysis as yet fail to disclose. Many of the elements such as copper, lead, zinc, nickel, cobalt and manganese have only been found in the substance of sea-weeds and corals. Silver and gold also exist in solution in sea-water. Malaguti and Durocher estimate the silver in sea-water as 1 part in 100,000,000 or 1 grain in 1430 gallons. If this estimate is correct there exists dissolved in the ocean a quantity of silver equal to 13,300 million metric tons, that is to say 46,700 times as much silver as has been produced from all the mines in the world from the discovery of America down to 1902. No quantitative determination of the amount of gold in solution is available. E. Sonnstadt detected gold by means of a colour test and roughly estimated the amount as 1 grain per ton of sea-water, and on this estimate all the projects for extracting gold from sea-water have been based.
The elements in addition to oxygen which exist in largest amount in sea salt are chlorine, bromine, sulphur, potassium, sodium, calcium and magnesium. Since the earliest quantitative analyses of sea-water were made by Lavoisier in 1772, Bergman in 1774, Vogel in 1813 and Marcet in 1819 the view has been held that the salts are present in sea-water in the form in which they are deposited when the water is evaporated. The most numerous analyses have been carried out by Forchhammer, who dealt with 150 samples, and Dittmar, who made complete analyses of 77 samples obtained on the “Challenger” expedition. Dittmar showed that the average proportion of the salts in ocean water of 35 parts salts per thousand was as follows (calculated asrparts per thousand of the sea-water, as percentage of the total salts. and per hundred molecules of magnesium bromide):—
The Salts in Ocean Water.
|Common salt, sodium chloride (NaCl)||27.213||77.758||112,793|
|Magnesium sulphate (MgSO4)||1.658||4.737||3,338|
|Gypsum, calcium sulphate (CaSO4)||1.260||3.600||2,239|
|Potassium sulphate (K2SO4)||0.863||2.465||1,200|
|Calcium carbonate (CaCO3) and residue||0.123||0.345||298|
|Magnesium bromide (MgBr2)||0.076||0.217||100|
As Marcet had foreshadowed from the analysis of 14 samples in 1819, the larger series of exact analyses proved that the variations in the proportion of individual salts to the total salts are very small, and all analyses since Dittmar's have confirmed this result. Although the salts have been grouped in the above table it is not to be supposed that a dilute solution like sea-water contains all the ingredients thus arbitrarily combined. There must be considerable dissociation of molecules, and as a first approximation it may be taken that of 10 molecules of most of the components about 9 (or in the case of magnesium sulphate 5) have been separated into their ions, and that it is only during slow concentration as in a natural saline that the ions combine to produce the various salts in the proportions set out in the above table. One can look on sea-water as a mixture of very dilute solutions of particular salts, each one of which after the lapse of sufficient time fills the whole space as if the other constituents did not exist, and this inter diffusion accounts easily for the uniformity of composition in the sea-water throughout the whole ocean, the only appreciable difference from point to point being the salinity or degree of concentration of the mixed solutions.
The origin of the salt of the sea is attributed by some modern authorities entirely to the washing out of salts from the land by rain and rivers and the gradual concentration by evaporation in the oceans, and some (e.g. J. Joly) go so far as to base a calculation of the age of the earth on the assumption that the ocean was originally filled with fresh water. This hypothesis, however, does not accord with the theory of the development of the earth from the state of a sphere of molten rock surrounded by an atmosphere of gaseous metals by which the first-formed clouds of aqueous vapour must have been absorbed. The great similarity between the salts of the ocean and the gaseous products of volcanic eruptions at the present time, rich in chlorides and sulphates of all kinds, is a strong argument for the ocean having been salt from the beginning. Two other facts are totally opposed to the origin of all the salinity of the oceans from the concentration of the washings of the land. The proportions of the salts of river and sea-water are quite different, as Julius Roth shows thus:—
The salts of salt lakes which have been formed in the areas of internal drainage in the hearts of the continents by the evaporation of river water are entirely different in composition from those of the sea, as the existence of the numerous natron and bitter lakes shows. Magnesium sulphate amounts to 4.7% of the total salts of sea-water according to Dittmar, but to 23.6% of the salts of the Caspian according to Lebedinzeff; in the ocean magnesium chloride amounts to 10.9% of the total salts, in the Caspian only to 4.5%; on the other hand calcium sulphate in the ocean amounts to 3.6%, in the Caspian to 6.9%. This disparity makes it extremely difficult to view ocean water as merely a watery extract of the salts existing in the rocks of the land.
The determination of salinity was formerly carried out by evaporating a weighed quantity of sea-water to dryness and weighing the residue. Forchhammer, however, pointed out that this method gave inexact and variable results, as in the act of evaporating to dryness hydrochloric acid is given off as the temperature is raised to expel the last of the water, and Tornöe found that carbonic acid was also liberated and that the loss of both acids was very variable. Tornöe vainly attempted to apply a correction for this loss by calculation; and subsequently S. P. L. Sörensen and Martin Knudsen after a careful investigation decided to abandon the old definition of salinity as the sum of all the dissolved solids in sea-water and to substitute for it the weight of the dissolved solids in 1000 parts by weight of sea-water on the assumption that all the bromine is replaced by its equivalent of chlorine, all the carbonate converted into oxide and the organic matter burnt. The advantage of the new definition lies in the fact that the estimation of the chlorine (or rather of the total halogen expressed as chlorine) is sufficient to determine the salinity by a very simple operation. According to Knudsen the salinity is given in weight per thousand parts by the expression S = 0.030 + 1.8050 Cl where S is the salinity and Cl the amount of total halogen in a sample. Such a simple formula is only possible because the salts of sea-water are of such uniform composition throughout the Whole ocean that the chlorine bears a constant ratio to the total salinity as newly defined whatever the degree of concentration. This definition was adopted by the International Council for the Study of the Sea in 1902, and it has since been very widely accepted.
Besides the determination of salinity by titration of the chlorides, the method of determination by the specific gravity of the sea-water is still often used. In the laboratory the specific gravity is determined in a pyknometer by actual weighing, and on board ship by the use of an areometer or hydrometer. Three types of areometer are in use: (1) the ordinary hydrometer of invariable weight with a direct reading scale, a set of from five to ten being necessary to cover the range of specific gravity from 1.000 to 1.031 so as to take account of sea-water of all possible salinities; (2) the “Challenger” type of areometer designed by J. Y. Buchanan, which has an arbitrary scale and can be varied in weight by placing small metal rings on the stem so as to depress the scale to any desired depth in sea-water of any salinity, the specific gravity being calculated for each reading by dividing the total weight by the immersed volume; (3) the total immersion areometer, which has no scale and the weight of which can be adjusted so that the instrument can be brought so exactly to the specific gravity of the water sample that it remains immersed, neither floating nor sinking; this has the advantage of eliminating the effects of surface tension and in Fridtjof Nansen's pattern is capable of great precision.
In all areometer Work it is necessary to ascertain the temperature of the water sample under examination with great exactness, as the volume of the areometer as well as the specific gravity of the water varies with temperature. All determinations must accordingly be reduced to standard temperature for comparison. Following the practice of J. Y. Buchanan on the “Challenger” it has been usual for British investigators to calculate specific gravities for sea-water at 60° F. compared with pure water at the maximum density point (39.2°) as unity. On the continent of Europe it has been more usual to take both at 17.5° C. (63.5° F.), which is expressed as “S17.5”, but for pyknometer work in all countries where the sample is cooled to 32° F. before weighing and pure water at 39.2° taken as unity the expression is (0°/4°). On the authority of the first meeting of the International Conference for the Study of the Northern European Seas at Stockholm in 1899 Martin Knudsen, assisted by Karl Forch and S. P. L. Sörensen, carried out a careful investigation of the relation between the amount of chlorine, the total salinity and the specific gravity of sea-water of different strengths including an entirely new determination of the thermal expansion of sea-water. The results are published in his Hydrographical Tables in a convenient form for use.
The relations between the various conditions are set forth in the following equations where σ0 signifies the specific gravity of the sea-water in question at 0° C., the standard at 4° being taken as 1000, S the salinity and Cl the chlorine, both expressed in parts by weight per mille.
|(1) σ0 = −0.093 + 0.8149 S − 0.000482 S2 + 0.0000068 S3|
|(2) σ0 = −0.069 + 1.4708 Cl − 0.00157 Cl2 + 0.0000398 Cl3|
|(3) S = 0.030 + 1.8050 Cl.|
The temperature of maximum density of sea-water of any specific gravity was found by Knudsen to be given with sufficient accuracy for all practical purposes by the formula θ = 3.95 - 0.266σ0, where θ is the temperature of maximum density in degrees centigrade. The temperature of maximum density is lower as the concentration of the sea-water is greater, as is shown in the following table:—
Maximum Density Point of Sea-Water of Different Salinities.
|Salinity per mile||0||10||20||30||35||40|
|Temperature θ° C||3.95°||1.86°||− 0.31°||− 2.47°||− 3.52°||− 4.54°|
Further Physical Properties of Sea-water.—The laws of physical chemistry relating to complex dilute solutions apply to sea-water, and hence there is a definite relation between the osmotic pressure, freezing-point, vapour tension and boiling-point by which when one of these constants is given the others can be calculated.
The most easily observed is the freezing-point, and according to the very careful determinations of H. T. Hansen the freezing-point τ° C. varies with the degree of concentration according to the formula
τ = −0.0086 − 0.0064633σ0 − 0.0001055σ02.
According to the investigations of Svante Arrhenius the osmotic pressure in atmospheres may be obtained by simply multiplying the temperature of freezing (τ) by the factor −12.08, and it varies with temperature (t) according to the law which holds good for gaseous pressure.
Pt = P0(1 + 0.00367t)
and can thus be reduced to its value at 0° C. Sigurd Stenius has calculated tables of osmotic pressure for sea-water of different degrees of concentration. The relation of the elevation of the boiling-point (t°) to the osmotic pressure (P) is very simply derived from the formula t = 0.02407P0, while the reduction of vapour pressure proportional to the concentration can be very easily obtained from the elevation of the boiling-point, or it may be obtained directly from tables of vapour tension.
Physical Properties of Sea-Water.
|Salinity per mille||10||20||30||35||40|
|Osmotic pressure P0 atmospheres||6.4||13.0||19.7||23.1||26.6|
|Elevation of boiling point (C.)||0.16||0.31||0.47||0.56||0.64|
|Reduction of vapour pressure (mm.)||4.2||8.5||13.0||15.2||17.6|
The importance of the osmotic pressure of sea-water in biology will be easily understood from the fact that a frog placed in sea-water loses water by exosmosis and soon becomes 20% lighter than its original weight, while a true salt-water fish suddenly transferred to fresh water gains water by endosmosis, swells up and quickly succumbs. The elevation of the boiling point is of little practical importance, but the reduction of vapour pressure means that sea-water evaporates more slowly than fresh water, and the more slowly the higher the salinity. Unfortunately no observations of evaporation from the surface of the open sea have been made and very few comparisons of the evaporation of salt and fresh water are on record. The fact that sea-water does evaporate more slowly than fresh water has been proved by the observations of Mazelle at Triest and of Okado in Azino (Japan). Their experiments show that in similar conditions the evaporation of sea-water amounts to from 70 to 91% of the evaporation of fresh water, a fact of some importance in geophysics on account of the vast expanses of ocean the evaporation from which determines the rainfall and to a large extent the heat-transference in the atmosphere.
The optical properties of sea-water are of immediate importance in biology, as they affect the penetration of sunlight into the depths. The refraction of light passing through sea-water is dependent on the salinity to the extent that the index of refraction is greater as the salinity increases. From isolated observations of J. Soret and E. Sarasin and longer series of experiments by Tornöe and Krümmel this relation is shown to be so close that the salinity of a sample can be ascertained by determining the index of refraction. According to Krümmel the following relations hold good at 18° C. for the monochromatic light of the D line of the sodium spectrum in units of the fifth decimal place.
Relation of Refractive Index and Salinity.
|For water of salinity (per mille)||0||10||20||30||35||40|
|Refractive index 1.33000 + units of 5th decimal place||308||502||694||885||981||1077|
The refractometer constructed by C. Pulfrich (of the firm of Zeiss, in Jena) has been successfully used by G. Schott and E. von Drygalski for the measurement of salinity at sea, and was found to have the same degree of accuracy as an areometer with the great advantage of being quite unaffected by the motion of the ship in a sea-way.
The transparency of sea-water has frequently been measured at sea by the simple expedient of sinking white-painted disks and noting the depth at which they become invisible as the measure of the transparency of the water. For the north European seas disks of about 18 or 20 in. in diameter are sufficient for this purpose, but in the tropics, where the transparency is much greater, disks 3 ft. in diameter at least must be used or the angle of vision for the reflected light is too small. In shallow seas the transparency is always reduced in rough weather. In the North Sea north of the Dogger Bank, for instance, the disk is visible in calm weather to a depth of from 10 to 16 fathoms, but in rough weather only to 6½ fathoms. Knipovitch occasionally observed great transparency in the cold waters of the Murman Sea, where he could see the disk in as much as 25 fathoms, and a similar phenomenon has often been reported from Icelandic waters. The greatest transparency hitherto reported is in the eastern basin of the Mediterranean, where J. Luksch found the disk visible as a rule to from 22 to 27 fathoms, and off the Syrian coast even to 33 fathoms. In the open Atlantic there are great differences in transparency; Krümmel observed a 6 ft. disk to depths of 31 and 36 fathoms in the Sargasso Sea, but in the cold currents of the north and also in the equatorial current the depth of visibility was only from 11 to 16½ fathoms. In the tropical parts of the Indian and the Pacific Oceans the depth of visibility increases again to from 20 to 27 fathoms. Some allowance should be made for the elevation of the sun at the time of observation. Mill has shown that in the North Sea off the Firth of Forth the average depth of visibility of a disk in the winter half-year was 4½ fathoms and in the summer half-year 6½ fathoms, and, although the greater frequency of rough weather in winter might tend to obscure the effect, individual observations made it plain that the angle of the sun was the main factor in increasing the depth to which the disk remained visible.
There are some observations on the transparency of sea-water of an entirely different character. Such, for instance, were those of Spindler and Wrangell in the Black Sea by sinking an electric lamp, those of Paul Regnard by measuring the change of electric resistance in a selenium cell or the chemical action of the light on a mixture of chlorine and hydrogen, by which he found a very rapid diminution in the intensity of light even in the surface layers of water. Many experiments have also been made by the use of photographic plates in order to find the greatest depth to which light penetrates. Fol and Sarasin detected the last traces of sunlight in the western Mediterranean at a depth of 254 to 260 fathoms, and Luksch in the eastern Mediterranean at 328 fathoms and in the Red Sea at 273 fathoms. The chief cause of the different depths to which light penetrates in sea-water is the varying turbidity due to the presence of mineral particles in suspension or to plankton. Schott gives the following as the result of measurements of transparency by means of a white disk at 23 stations in the open ocean, where quantitative observations of the plankton under 1 square metre of surface were made at the same time.
| Volume of
|Mean of 11 stations poor in plankton||85 cc.||14¼ fathoms|
|Mean of 12 stations rich in plankton||530 cc.||8¾ fathoms|
Any influence on transparency which may be exercised by the temperature or salinity of the water is quite insignificant.
The colour of ocean water far from land is an almost pure blue, and all the variations of tint towards green are the result of local disturbances, the usual cause being turbidity of some kind, and this in the high seas is almost always due to swarms of plankton. The colour of sea-water as it is seen on board ship is most readily determined by comparison with the tints of Forel's xanthometer or colour scale, which consists of a series of glass tubes fixed like the rungs of a ladder in a frame and filled with a mixture of blue and yellow liquids in varying proportions. For this purpose the zero or pure blue is represented by a solution of 1 part of copper sulphate and 9 parts of ammonia in 190 parts of water. The yellow solution is made up of 1 part of neutral potassium chromate in 199 parts of water, and to give the various degrees of the scale, 1, 2, 3, 4, &c., % of the yellow solution is mixed with 99, 98, 97, 96, &c., % of the blue in successive tubes. Observations with the xanthometer have not hitherto been numerous, but it appears that the purest blue (0-1 on Forel's scale) is found in the Sargasso Sea, in the North Atlantic and in similarly situated tropical or subtropical regions in the Indian and Pacific Oceans. The northern seas have an increasing tendency towards green, the Irminger Sea showing 5-9 Forel, while in the North Sea the water is usually a pure green (10-14 Forel), the western Mediterranean shows 5-9 Forel, but the eastern is as blue as the open ocean (0-2 Forel). A pure blue colour has been observed in the cold southern region, where the “Valdivia” found 0-2 Forel in 55° S. between 10° and 31° E., and even the water of the North Sea has been observed at times to be intensely blue. The blue of the sea-water as observed by the Forel scale has of course nothing to do with the blue appearance of any distant water surface due to the reflection of a cloudless sky. Over shallows even the water of the tropical oceans is always green. There is a distinct relationship between colour and transparency in the ocean; the most transparent water which is the most free from plankton is always the purest blue, while an increasing turbidity is usually associated with an increasing tint of green. The natural colour of pure sea-water is blue, and this is emphasized in deep and very clear water, which appears almost black to the eye. When a quantity of a fine white powder is thrown in, the light reflected by the white particles as they sink assumes an intense blue colour, and the experiments of J. Aitken with clear sea-water in long tubes leave no doubt on the subject.
Discoloration of the water is often observed at sea, but that is always due to foreign substances. Brown or even blood-red stripes have been observed in the North Atlantic when swarms of the copepod Calanus finmarchicus were present; the brown alga Trichodesmium erythraeum, as its name suggests, can change the blue of the tropical seas to red; swarms of diatoms may produce olive-green patches in the ocean, while some other forms of minute life have at times been observed to give the colour of milk to large stretches of the ocean surface.
On account of its salinity, sea-water has a smaller capacity for heat than pure water. According to Thoulet and Chevallier the specific heat diminishes as salinity increases, so that for 10 per mille salinity it is 0.968, for 35 per mille it is only 0.932, that of pure Water being taken as unity. The thermal conductivity also diminishes as salinity increases, the conductivity for heat of sea-water of 35 per mille salinity being 4.2% less than that of pure water. This means that sea-water heats and cools somewhat more readily than pure water. The surface tension, on the other hand, is greater than that of pure water and increases with the salinity, according to Krümmel, in the manner shown by the equation α = 77.09 + 0.0221 S at 0° C., where α is the coefficient of surface tension and S the salinity in parts per thousand. The internal friction or viscosity of sea-water has also been shown by E. Ruppin to increase with the salinity. Thus at 0° C. the viscosity of sea-water of 35 per mille salinity is 5.2% greater and at 25° C. 4% greater than that of pure water at the same temperatures; in absolute units the viscosity of sea-water at 25° C. is only half as great as it is at 0° C.
The compressibility of sea-water is not yet fully investigated. It varies not only to a marked degree with temperature, but also with the degree of pressure. Thus J. Y. Buchanan found a mean of 20 experiments made by piezometers sunk in great depths on board the “Challenger” give a coefficient of compressibility κ = 491 × 10−7; but six of these experiments made at depths of from 2740 to 3125 fathoms gave κ = 480 × 10−7. The value usually adopted is κ = 450 × 10−7. The compressibility is in itself very small, but so great in its effect on the density of deep water in situ that the specific gravity (0°/4°) at 2000 fathoms increases by 0.017 and at 3000 fathoms by 0.026. In other words, water which has a specific gravity of 1.0280 at the surface would at the same temperature have a specific gravity of 1.0450 at 2000 and 1.0540 at 3000 fathoms. If the whole mass of water in the ocean were relieved from pressure its volume would expand from 319 million cub. m. to 321.7 million cub. m., which for a surface of 139.5 million sq. m. means an increased depth of 100 ft. The rate of propagation of sound depends on the compressibility, and in ocean water at the tropical temperature of 77° F. the speed is 1482.6 metres (4860 ft.) per second, in Baltic water of 8 per mille salinity and a temperature of 50° F. it is 1448.5 metres (4750 ft.) per second, that is to say, 4½ times greater than the velocity of sound in air. This accounts for the great range of submarine sound signals, which can thus be very serviceable to navigation in foggy weather.
The electrical conductivity of sea-water increases with the salinity; at 59° F. it is given according to E. Ruppin's formula as L = 0.001465 S − 0.00000978 S2 + 0.0000000876 S3 in reciprocal ohms.
The radio-activity of sea-water is extraordinarily small; indeed in samples taken from 50 fathoms in the Bay of Danzig it was imperceptible, and R. T. Strutt found that salt from evaporated sea-water did not contain one-third of the quantity of radium present in the water of the town supply in Cambridge.
Dissolved Gases of Sea-water.—The water of the ocean, like any other liquid, absorbs a certain amount of the gases with which it is in contact, and thus sea-water contains dissolved oxygen, nitrogen and carbonic acid absorbed from the atmosphere. As Gay-Lussac and Humboldt showed in 1805, gases are absorbed in less amount by a saline solution than by pure water. The first useful determinations of the dissolved gases of sea-water were made by Oskar Jacobsen in 1872. Since that time much work has been done, and the methods have been greatly improved. In the method now most generally practised, which was put forward by O. Pettersson in 1894, two portions of sea-water are collected in glass tubes which have been exhausted of air, coated internally with mercuric chloride to prevent the putrefaction of any organisms, and sealed up beforehand. The exhausted tube, when inserted in the water sample and the tip broken off, immediately fills, and is then sealed up so that the contents cannot change after collection. One portion is used for determining the oxygen and nitrogen, the other for the carbonic acid. The former determination is made by driving out the dissolved gases from solution and collecting them in a Torricellian vacuum, where the volume is measured after the carbonic acid has been removed. The oxygen is then absorbed by some appropriate means, and the volume of the nitrogen measured directly, that of the oxygen being given by difference. In the second portion the carbonic acid is driven out by means of a current of hydrogen, collected over mercury and absorbed by caustic potash.
C. T. T. Fox, of the Central Laboratory of the International Council at Christiania, has investigated the relation of the atmospheric gases to sea-water by very exact experimental methods and arrived at the following expressions for the absorption of oxygen and nitrogen by sea-water of different degrees of concentration. The formulae show the number of cubic centimetres of gas absorbed by 1 litre of sea-water; t indicates the temperature in degrees centigrade and Cl the salinity as shown by the amount of chlorine per mille:—
O2 = 10.291 − 0.2809 t + 0.006009 t2 − 0.000632 t3 − Cl(0.1161 − 0.003922 t2 + 0.000063 t3)
N2 = 18.561 − 0.4282 t + 0.0074527 t2 − 0.00005494 t3 Cl(0.2149 − 0.007117 t2 + 0.0000931 t3)
In the case of ocean water with a salinity of 35 per mille, this gives for saturation with atmospheric gases in cc. per litre:—
|at 0° C.||15° C.||25° C.|
The reduction of the absorption of gas by rise of temperature is thus seen to be considerable. As a rule the amount of both gases dissolved in sea-water is found to be that which is indicated by the temperature of the water in situ. Jacobsen on some occasions found water in the surface layers of the Baltic supersaturated with oxygen, which he ascribed to the action of the chlorophyll in vegetable plankton; in other cases when examining the nearly stagnant water from deep basins he found a deficiency of oxygen due no doubt to the withdrawal of oxygen from solution, by the respiration of the animals and by the oxidation of the deposits on the bottom. When these processes continue for a long time in deep water shut off from free circulation so that it does not become aerated by contact with the atmosphere the water becomes unfit to support the life of fishes, and when the accumulation of putrefying organic matter gives rise to sulphuretted hydrogen as in the Black Sea below 125 fathoms, life, other than bacterial, is impossible. The water from the greatest depths of the Black Sea, 1160 fathoms, contains 6 cc. of sulphuretted hydrogen per litre.
The distribution of dissolved oxygen in the depths of the open ocean is still very imperfectly known. Dittmar's analysis of the “Challenger” samples indicated an excess of oxygen in the surface water of high southern latitudes and a deficiency at depths below 50 fathoms.
The facts regarding carbonic acid in sea-water are even less understood, for here we have to do not only with the solution of the gas but also with a chemical combination. On this account it is very difficult to know when all the gas is driven out of a sample of sea-water, and a much larger proportion is present than the partial pressure of the gas in the atmosphere and its coefficient of absorption would indicate. These constants would lead one to expect to find 0.5 cc. per litre at 0° C. while as a matter of fact the amount absorbed approaches 50 cc. The form of combination is unstable and apparently variable, so that the quantities of free carbonic acid, bicarbonate and normal carbonate are liable to alter. Since 1851 it has been known that all sea-water has an alkaline reaction, and Tornöe defined the alkalinity of sea-water as the amount of carbonic acid which is necessary to convert the excess of bases into normal carbonate. The alkalinity of North Atlantic water of 35 per mille salinity is 26.86 cc. per litre, corresponding to a total amount of carbonic acid of 49.07 cc. According to the researches of August Krogh, the alkalinity is greatly increased by the admixture of land water. This is proved by E. Ruppin's analysis of Baltic water, which has an alkalinity of 16 to 18 instead of the 5 or 6 which would be the amount proportional to the salinity, while the water of the Vistula and the Elbe with a salinity of 0.1 per mille has an alkalinity of 28 or more. Thus the alkalinity serves as an index of the admixture of river water with sea-water. Carbonic acid passes from the atmosphere into the ocean as soon as its tension in the latter is the smaller; hence in this respect the ocean acts as a regulator. The amount of carbonic acid in solution may also be increased by submarine exhalations in regions of volcanic disturbance, but it must be remembered that the critical pressure for this gas is 73 atmospheres, which is reached at a depth of 400 fathoms, so that carbonic acid produced at the bottom of the ocean must be in liquid form. The respiration of marine animals in the depths of deep basins in which there is no circulation adds to the carbonic acid at the expense of the dissolved oxygen. This is frequently the case in fjord basins; for instance, in the Gullmar Fjord at a depth of 50 fathoms with water of 34.14 per mille salinity and a temperature of 40.1° F., the carbonic acid amounts to 51.55 cc. per litre, and the oxygen only to 2.19 cc. Vegetable plankton in sunlight can reverse this process, assimilating the carbon of the carbonic acid and restoring the oxygen to solution, as was proved by Martin Knudsen and Ostenfeld in the case of diatoms. Little is known as yet of the distribution of carbonic acid in the oceans, but the amount present seems to increase with the salinity as shown by the four observations quoted:—
|Gulf of Finland of||3.2 per mille salinity = 17.2 cc. CO2||at 4.1° C.|
|Western Baltic of||14.2 per mille salinity = 37.0 cc. CO2||at 1.6° C.|
|North Atlantic of||35.0 per mille salinity = 49.0 cc. CO2||at about 10° C.|
|Eastern Mediterranean of||39.0 per mille salinity = 53.0 cc. CO2||at 26.7° C.|
Unfortunately the very numerous determinations of carbonic acid made by J. Y. Buchanan on the “Challenger” were vitiated by the incompleteness of the method employed, but they are none the less of value in showing clearly that the waters of the far south of the Indian Ocean are relatively rich in carbonic acid and the tropical areas deficient.
Distribution of Salinity.—A great deal of material exists on which to base a study of the surface salinity of the oceans, and Schott's chart published in Petermanns Mitteilungen for 1902 incorporates the earlier work and substantially confirms the first trustworthy chart of the kind compiled by J. Y. Buchanan from the “Challenger” observations. In each of the three oceans there are two maxima of salinity—one in the north, the other in the south tropical belt, separated by a zone of minimum salinity in the equatorial region, and giving place poleward to regions of still lower salinity. The three oceans differ somewhat between themselves. The North Atlantic maximum is the highest with water of 37.9 per mille salinity; the maximum in the South Atlantic is 37.6; in the North Indian Ocean, 36.7; the South Indian Ocean, 36.4; the South Pacific, 36.9; and the North Pacific has the lowest maximum of all, only 35.9. The comparatively fresh equatorial belt of water, has a salinity of 35.0 to 34.5 in the Atlantic, 35.0 to 34.0 in the Indian Ocean, 34.5 in the Western and 33.5 in the Eastern Pacific. Taking each of the oceans as a whole the Atlantic has the highest general surface salinity with 35.37.
The salinity of enclosed seas naturally varies much more than that of the open ocean. The saltest include the eastern Mediterranean with 39.5 per mille, the Red Sea with 41 to 43 per mille in the Gulf of Suez, and the Persian Gulf with 38. The fresher enclosed seas include the Malay and the East Asiatic fringing seas with 30 to 34.5 per mille, the Gulf of St Lawrence with 30 to 31, the North Sea with 35 north of the Dogger Bank diminishing to 32 further south, and the Baltic, which freshens rapidly from between 25 to 31 in the Skagerrak to 7 or 8 eastward of Bornholm and to practically fresh water at the heads of the Gulfs of Bothnia and Finland. The Arctic Sea presents a great contrast between the salinity of the surface of the ice-free Norwegian Sea with 35 to 35.4 and that of the Central Polar Basin, which is dominated by river Water and melted ice, and has a salinity less than 25 per mille in most parts. The average salinity of the whole surface of the oceans may be taken as 34.5 per mille.
The vertical distribution of salinity has only recently been investigated systematically, as the earlier expeditions were not equipped with altogether trustworthy apparatus for collecting water samples at great depths. Two main types of water-bottle for collecting samples have been long in use. The older, devised by Hooke in 1667, is provided with valves above and below, both opening upward, through which the water passes freely during descent, but which are closed by some device on hauling up. The newer or slip water-bottle type consists of a cylinder allowed to drop on to a base-plate when a sample is to be collected. The first form of slip water-bottle due to Meyer retained the water merely by the weight of the cylinder pressing on the base-plate. J. Y. Buchanan introduced an improved form on the “Challenger,” also remaining closed by weight, the cylinder being very heavy and ground to fit the bevelled base-plate very accurately. H. R. Mill in 1885 devised a self-locking arrangement by which the bottle once closed was automatically locked and rendered watertight; H. L. Ekman made further improvements; and, finally, O. Pettersson and F. Nansen perfected the instrument, adapting it not only for enclosing a portion of water at any desired depth, but by a series of concentric divisions insulating in the central compartment water at the temperature it had at the moment of collection. By means of a weight dropped along the line the water-bottle can be shut and a sample enclosed at any desired depth. The use of a sliding weight is not recommended in depths much exceeding 200 fathoms on account of the time required and the risk of the line sagging at a low angle and so stopping the weight. In deep water the closing mechanism is usually actuated by a screw propeller which begins to work when the line is being hauled in and can be set so as to close the water-bottle in a very few fathoms. A small but heavy water-bottle has been devised by Martin Knudsen, provided with a pressure gauge or bathometer, by which samples may be collected from any moderate depth down to about 100 fathoms, on board a vessel going at full speed. This has made it possible to obtain many samples from moderate depths along a long line in a very short space of time. Sigsbee's small water-bottle on the double valve principle actuated by a propeller requires extremely skilful handling to enable it to give good results.
As yet it is only possible to speak with confidence of the vertical distribution of salinity in the seas surrounding Europe, where there is a general increase of salinity with depth. For the open ocean the only quite trustworthy results are those obtained by the prince of Monaco in the North Atlantic, and by the recent Antarctic expeditions in the South Atlantic and South Indian Oceans. The observations made on the “Challenger” and “Gazelle,” though enabling some perfectly sound general conclusions to be drawn, require to be supplemented. It appears, as J. Y. Buchanan pointed out in 1876, that the great contrasts in surface salinity between the tropical maxima and the equatorial minima give place at the moderate depth of 200 fathoms to a practically uniform salinity in all parts of the ocean.
In the North Atlantic a strong submarine current flowing outward from the Mediterranean leaves the Strait of Gibraltar with a salinity of 38 per mille, and can be traced as far as Madeira and the Bay of Biscay in depths of from 600 to 2800 fathoms, still with a salinity of 35.6 per mille, whereas off the Azores at equal depths the salinity is from 0.5 to 0.7 per mille less. In the tropical and subtropical belts of the Atlantic and Indian Oceans south of the equator the salinity diminishes rapidly from the surface downwards, and at 500 fathoms reaches a minimum of 34.3 or 34.4 per mille; after that it increases again to 800 fathoms, where it is almost 34.7 or 34.8, and this salinity holds good to the bottom, even to the greatest depths, as was first shown by the “Gauss” and afterwards by the “Planet” between Durban and Ceylon.
Our knowledge of the Pacific in this respect is still very imperfect, but it appears to be less salt than the other oceans at depths below 800 fathoms, as on the surface, the salinity at considerable depths being 34.6 to 34.7 in the western part of the ocean, and about 34.4 to 34.5 in the eastern, so that, although the data are by no means satisfactory, it is impossible to assign a mass-salinity of more than 34.7 per mille for the whole body of Pacific water.
The causes of difference of salinity are mainly meteorological. The belt of equatorial minimum salinity corresponds with the excessively rainy belt of calms and of the equatorial counter current, the salinity diminishing towards the east. The tropical maxima of salinity on the poleward side of the trade-winds coincide with the regions of minimum rainfall, high temperature, strong winds and consequently of maximum evaporation. Evaporation is naturally greatest in the enclosed seas of the nearly rainless subtropical zone such as the Mediterranean and Red Sea. Where the evaporation is at a minimum, the inflow of rivers from a large continental area and the precipitation from the atmosphere at a maximum, there is necessarily the greatest dilution of the sea-water, the Baltic and the Arctic Sea being conspicuous examples.
Temperature of the Oceans.—There is no difficulty in observing the temperature of the surface of the sea on board ship, the only precautions required being to draw the water in a bucket which has not been heated in the sun in summer or exposed to frost in winter, to draw it well forward of any discharge pipes of the steamer, to place it in the shade on deck, insert the thermometer immediately and make the reading without delay. The measurement of temperature in the depths, unless a high-speed water bottle be used, involves stopping the ship and employing thermometers of special construction. Many forms have been tried, but only three types are in general use. The first is the slow-action thermometer which was originally used with good effect by de Saussure in the Mediterranean in 1780. He covered the bulb of the thermometer with layers of non-conducting material and left it immersed at the desired depth for a very long time to enable it to take the temperature of its surroundings. When brought up again the thermometer retained its temperature so long that there was ample time to take a correct reading. Since 1870 thermometers on this principle have been in use for regular observations at German coast and light-ship stations. Following the suggestion of Cavendish, Irving made observations of deep temperature on Phipps's Spitsbergen voyage of 1773 with a valved water-bottle, insulated by non-conducting material. A similar instrument gave excellent results in the hands of E. von Lenz on Kotzebue's second voyage of circumnavigation in 1823-1826. The last elaboration of the insulated slip water-bottle by Ekman, Nansen and Pettersson has produced an instrument of great perfection, in which the insulation is effected by layers of water between a series of concentric ebonite cylinders, all of which are closed both above and below when the apparatus encloses a sample, and each of which in turn must be warmed considerably before there is any rise of temperature in the chamber within. This can be used with certainty to .02° C. for water down to 250 fathoms, after taking account of the slight disturbance produced by the expansion of the greatly compressed deep water.
The second form of deep-sea thermometer is the self-registering maximum and~ minimum on James Six's principle. These instruments must be constructed with the greatest care, but when well made in accordance with J. Y. Buchanan's large model they can be trusted to give a good account of the vertical distribution of temperature, provided the water grows cooler as the depth increases. They would act equally well if the water grew continually warmer as the depth increases, but they cannot give an exact account of a temperature inversion such as is produced when layers of warmer and colder water alternate.
The third form is the outflow or reversing thermometer, first introduced by Aimé, who used a very inconvenient form in the Mediterranean in 1841-1845, but greatly improved and simplified by Negretti and Zambra in 1875. The principle is to have a constriction in the tube above the bulb so proportioned that when the instrument is upright it acts in every way as an ordinary mercurial thermometer, but when it is inverted the thread of mercury breaks at the constriction, and the portion above the point runs down the now reversed tube and remains there as a measure of the temperature at the moment of turning over. For convenience in reading, the tube is graduated inverted, and when it is restored to its original position the mercury thread joins again and it acts as before. Various modifications of this form of thermometer have been made by Chabaud of Paris and others. It has the advantage over the thermometer on Six's principle that, being filled with mercury, it does not require such long immersion to take the temperature of the water. A correction has, of course, to be made for the expansion or contraction of the mercury thread if the temperature of reading differs much from that of reversing. Magnaghi introduced a convenient method of inverting the thermometer by means of a propeller actuated on beginning to heave in the line, and this form is used for all work at great depths. For shallow water greater precision and certainty are obtained by using a lever actuated by a weight slipped down the line to cause the reversal, as in the patterns of Rung, Mill and others.
All thermometers sunk into deep water must be protected against the enormous pressure to which they are exposed. This may be done by the method suggested by Arago in 1828, introduced by Aimé in 1841 and again suggested by Glaisher in 1858, of sealing up the whole instrument in a glass tube exhausted of air; or, less effectively, by surrounding the bulb alone with a strong outer sheath of glass. In both forms it is usual to have the space between the bulb and the protecting sheath partly filled with mercury or alcohol to act as a conductor and reduce the time necessary for the thermometer to acquire the temperature of its surroundings.
The warming of the ocean is due practically to solar radiation alone; such heat as may be received from the interior of the earth can only produce a small effect and is fairly uniformly distributed. On account of the high specific heat of sea-water the diurnal range of temperature at the surface is very small. According to A. Buchan's discussion of the two-hourly observations on the “Challenger” the total range between the daily maximum and minimum in the warmer seas is between 0.7° and 0.8° F., and for the colder seas still less (0.2° F.), compared with 3.2° F in the overlying air. The maximum usually occurs between 1 and 2.30 P.M., the minimum shortly before sunrise. The temperature of the surface water is generally a little higher than that of the overlying air, the daily average difference being about 0.6° F., varying from 0.9° lower at 1 P.M. to 1.6° higher at 1 A.M. There are few observations available for ascertaining the depth to which warmth from the sun penetrates in the ocean. The investigations of Aimé in 1845 and Hensen in 1889 indicate that the amount of cloud has a great effect. Aimé showed that on a calm bright day in the Mediterranean the temperature rose 0.1° C. between the early morning and noon at a depth of about 12 fathoms. Luksch deduced a much greater penetration of solar warmth from the comparison of observations at different hours at neighbouring stations in the eastern Mediterranean, but his methods were not exact enough to give confidence in the result. The penetration of warmth from the surface is effected by direct radiation, and by convection by particles rendered dense by evaporation increasing salinity. Conduction has practically no effect, for the coefficient of thermal conductivity in sea-water is so small that if a mass of sea-water were cooled to 0° C. and the surface kept at a temperature of 30° C., 6 months would elapse before a temperature of 15° C. was reached at the depth of 1.3 metres, 1 year at 1.85 metres, and 10 years at 5.8 metres. Great irregular variations in radiation and convection sometimes produce a remarkably abrupt change of temperature at a certain depth in calm water; the layer in which this sudden change occurs has been termed the Sprungschicht. How closely two bodies of water at different temperatures may come together is shown by the fact that in the Baltic in August between 10 and 11 fathoms there is sometimes a fall of temperature from 57° to 46.5° F. Such a condition of things is only possible in very calm weather, the action of waves having the effect of mixing the water to a considerable depth. After a storm the whole of the water in the North Sea assumes a homothermic condition, i.e. the temperature is the same from surface to bottom, and this occurs not only south of the Dogger Bank, where the condition is normal, but also, though less frequently, in the deeper water farther north. Similar effects are produced in narrow waters by the action of tidal currents, and the influence of a steady wind blowing on- or off-shore has a powerful effect in mixing the water.
The warmest parts of the Indian Ocean and Western Pacific have a mean annual temperature of 82° to 84° F., but such high temperatures are not found in the tropical Atlantic. In the Indian Ocean between 15° N. and 5° S. the surface temperature in May averages 84° to 86° F., and in the Bay of Bengal the temperature is 86°, and no part of the Atlantic has so high a monthly mean temperature at any season. G. Schott's investigations show that the annual range of surface temperature in the open ocean is greatest in 40° N., with 18.4° F., and in 30° S., with 9.2° F.; on the contrary, near the equator it is less, only 4° F. in 10° N., and in high latitudes it is also small, 5.2° F. in 50° S. The figures quoted above are differences between the average surface temperatures of the warmest and of the coldest month. As to the absolute extremes of surface temperature, Sir John Murray points out that 90° F. frequently occurs in the western part of the tropical Pacific, while among seas the Persian Gulf reaches 96° F., only 2° under blood-heat, and the Red Sea follows closely with a maximum of 94°. The greatest change of temperature at any place has been recorded to the east of Nova Scotia, a minimum of 28° F. and a maximum of 80°, and to the north-east of Japan with a minimum of 27° F. and a maximum of 83°. In those localities, however, it is not the same water which varies in temperature with the season, but the water of different warm and cold currents which periodically occupy the same locality as they advance and retreat. The zones of surface temperature are arranged roughly parallel to the equator, especially in the southern hemisphere. Between 40° N. and 40° S. the currents produce a considerable rearrangement of this simple order, the belts of warm water being wider on the western sides of the oceans and narrower on the eastern.
The arrangement of the isotherms thus affords a basis for valuable deductions as to the direction of ocean currents. The surface temperature of the Atlantic is relatively lower than that of the other oceans when the whole area is considered. According to Krümmel's calculation the proportional areas at a high temperature are as follows:—
Percentage of Ocean Surface with Temperature.
|Over 77° F. (25° C)||22.4||38.0||40.1|
|Over 68° F. (20° C)||50.1||51.7||58.4|
This disparity results in some degree at least from the comparative narrowness of the inter-tropical Atlantic, and the absence of a cool northern area in the Indian Ocean. Krümmel calculates that the mean temperature of the whole ocean surface is 63.3° F., while the mean sea-level temperature of the whole layer of air at the surface of the earth is given by Hann as 57.8° F.
We are still ignorant of the depth to which the annual temperature wave penetrates in the open ocean, but observations in the Mediterranean enable us to form some opinion on the matter. The observations of Aimé in 1845 and of Semmola in the Gulf of Naples in 1881 show that the surface water in winter cools until the whole mass of water from the surface to the bottom, in 1600 fathoms or more, assumes the same temperature. Towards the end of summer the upper layers have been warmed to a depth which indicates how far the influence of solar radiation and convection have reached. Aimé estimated this depth at 150-200 fathoms, while the observations of the Austrian expedition in the eastern Mediterranean found it to be from 200 to nearly 400 fathoms. In the Red Sea, where a similar seasonal change occurs, the depth to which the surface layer warms up is about 275 fathoms. The great difference in salinity between the surface and the deep water excludes the possibility of effective convection in the seas of northern Europe, and in the open ocean the currents which are felt everywhere, and especially those with a vertical component, must exercise a very disturbing influence on convection.
The vertical distribution of temperature in the open ocean is much better known than that of salinity. The regional differences of temperature at like depths become less as the depth increases. Thus at 300 fathoms greater differences than 9° F. hardly ever occur between 50° N. and 50° S., in 800 fathoms the differences are less than 5.5° and in 1500 fathoms less than 2°. Even in the tropics the high temperature of the surface is confined to a very shallow layer; thus in the Central Pacific where the surface temperature is 82° F. the temperature at 100 fathoms is only 52° F. The whole ocean must thus form but a cold dwelling-place for the organisms of the deep sea. Sir John Murray calculates that at least 80% of the water in the ocean has a temperature always less than 40° F., and a recent calculation by Krümmel gave in fact a mean temperature of 39° F. for the whole ocean.
Fig. 1.—Diagram illustrating Distribution of Sea Temperature.
The normal vertical distribution of temperature is illustrated in curve A of fig. 1, which represents a sounding in the South Atlantic; and this arrangement of a rapid fall of temperature giving place gradually to an extremely slow but steady diminution as depth increases is termed anathermic (ἀνά, back, and θερμός, warm). Curve B shows the typical distribution of temperature in an enclosed sea, in this case the Sulu Basin of the Malay Sea, where from the level of the barrier to the bottom the temperature remains uniform or homothermic. Curve C shows a typical summer condition in the polar seas, where layers of sea-water at different temperatures are superimposed, the arrangement from the surface to 200 fathoms is termed dichathermic (δίχα, apart), from 1000 to 2000 fathoms it is termed katathermic (κατά, down). In autumn the enclosed seas of high latitudes frequently present a thermal stratification in which a warm middle layer is sandwiched between a cold upper layer and a cold mass below, the arrangement being termed mesothermic (μέσος, middle). The nature of the change of temperature with depth below 2500 fathoms is entirely dependent on the position of the sub-oceanic elevations, for the rises and ridges act as true submarine watersheds. As the Arctic Basin is shut off from the North Atlantic by ridges rising to within 300 fathoms of the surface and from the Pacific by the shallow shelf of the Bering Sea, and as the ice-laden East Greenland and Labrador currents consist of fresh surface water which cannot appreciably influence the underlying mass, the Arctic region has no practical effect upon the bottom temperature of the three great oceans, which is entirely dominated by the influence of the Antarctic. The existence of deep-lying and extensive rises or ridges in high southern latitudes has been indicated by the deep-sea temperature observations of Antarctic expeditions. Temperatures so low as 31.5° to 31.3° F. do not occur much beyond 50° S. The “Belgica” even found a temperature of 33.1° F. in 61° S., 63° W., at a depth of 2018 fathoms. The conditions of temperature in the South Atlantic are characteristic. South of 55° S. in approximately 3000 fathoms the bottom temperature is 31.1° F.; in the Cape Trough it is 32.7° in 45° S., and 33.8° to 34.3° in 35° S., while north of the Walfisch Ridge and east of the South Atlantic Rise bottom temperatures of 36° to 36.7° F. prevail right northwards across the equator into the Bay of Biscay, showing a steady rise, of bottom temperature as successive submarine elevations restrict communication with the Antarctic. On the other hand, in the more open Argentine Basin, which carries deep water far to the south, the bottom temperature in 40° S. is only from 32.2° to 32.7° F., and the same low temperature continues throughout the Brazil Basin to the equator; but in the North American Basin from the West Indies to the Telegraph Plateau no satisfactory bottom temperature lower than 35.6° F. has been reported. On the floor of the Indian Ocean temperatures of 33.3° to 33.6° occur south of 35° S. in depths of 2700 fathoms or more, but north of 35° S. the prevailing bottom temperatures are from 34.0° to 34.3°. In similar depths in the Pacific south of the equator temperatures of 33.8° to 34.5° are found, and north of the equator bottom temperatures at the same depth increase to 35.1° in the neighbourhood of the Aleutian Islands, again completely justifying the conclusion as to the Antarctic control of deep water temperature throughout the ocean.
The marginal rises and continental shelves prevent this cold bottom water from penetrating into the depths of the enclosed and fringing seas. Thus in the Central American Sea below 930 fathoms, the depth on the bar, no water is found at a temperature lower than that prevailing in the open ocean at that depth, viz. 39.6° F., not even at the bottom of the great Bartlett Deep in 3439 fathoms. Such homothermic masses of water are characteristic of all deep enclosed seas. Thus in the Malay Sea the various minor seas or basins are homothermic below the depth of the rim, at the temperature prevailing at that depth in the open ocean: in the China Sea below 875 fathoms with 36.5° F.; in the Sulu Sea (depth 2550 fathoms) below 400 fathoms with 50.5° F.; in the Celebes Sea below 820 fathoms with 38.6° F.; in the Banda Sea below 902 fathoms with 37.9° F. In other enclosed seas which are shut off from the ocean by a very shallow sill the rule holds good that the homothermic water below the level of the sill is at the lowest temperature reached by the surface water in the coldest season of the year, provided always that the stratification of salinity is such as to permit of convection being set up. To this group belongs the Arctic Sea; the Norwegian Sea is homothermic below 550 fathoms at 29.8° F., but this cold water does not penetrate into the Arctic Basin on account of the ridge between Spitsbergen and Greenland, and there the water below 1400 fathoms has a temperature of 30.6° to 30.7° F. because the surface layers of water are too light, on account of the low salinity due to ice-melting, to enable even the cold of a polar winter to set up a downward convection current. The Mediterranean Sea also belongs to this group; its various deep basins are homothermic (at the winter surface temperature) below the level of their respective sills—the Balearic Basin below 190 fathoms at 55° F.; the Eastern Basin below 270 fathoms at 55.9° F.; the Ionian Sea at 56.3° F.; and at 56.7° south of Cyprus. Similarly in the Red Sea the water below 380 fathoms is homothermic at 70.7° F.
An under-current flows out from the Red Sea through the Strait of Bab-el-Mandeb, and from the Mediterranean through the Strait of Gibraltar, raising the salinity as well as the temperature of the part of the ocean outside the gates of the respective seas. The action of the Red Sea water affects the whole of the Gulf of Aden and Arabian Sea, raising the temperature at the depth of 550 fathoms to 52° or 53° F. or 9 Fahrenheit degrees higher than the water of the Bay of Bengal at the same depth. The effect of the Mediterranean water in the North Atlantic does not require such large figures to express it, but is none the less extraordinarily far-reaching, as first indicated by the work of the “Challenger” and subsequently defined by H. N. Dickson's discussion of the observations of Wolfenden in the little sailing yacht “Silver Belle.” The temperature at 550 fathoms is raised to 49° or 50° F. between Madeira and the Biscay Shelf, i.e. 5.4° F . above the temperature at the same depth off the Azores.
In shallow seas such as the North Sea and the British fringing seas, where tidal currents run strong, there is a general mixing together of the surface and deeper water, thus making the arrangement of vertical temperature anathermic in summer and katathermic in winter, while at the transitional periods in spring and autumn it is practically homothermic. Thus at Station E2 of the international series at the mouth of the English Channel in 49° 27′ N., 4° 42′ W., the following distribution of temperature F. has been observed by Matthews:—
It is noticeable that there is a marked vertical temperature gradient only at the end of summer when a warm surface layer is formed, though in August 1904 that was only 8 fathoms thick. In small nearly land-locked basins shut off from one another by bars rising to within a short distance of the surface and affected both by strong tidal currents and by a considerable admixture of land water, the contrasts of vertical distribution of temperature with the seasons are strongly marked, and there are also great unperiodic changes effected mainly by wind, as is shown by the investigations of H. R. Mill in the Clyde Sea Area, and of O. Pettersson, J. Hjort and Helland-Hansen in the Scandinavian fjords.
Sea Ice.—The freezing-point of sea-water is lower as the salinity increases and normal sea-water of 35 per mille salinity freezes at 28.6° F. Experience shows that sea-water can be cooled considerably below the freezing-point without freezing if there is no ice or snow in contact with it. Freezing takes place by the formation of pure ice in flat crystalline plates of the hexagonal system, which form in perpendicular planes and unite in bundles to form grains so that a thick covering of ice exhibits a fibrous structure. It is only the water that freezes; the dissolved salts are excluded in the process in a regular order according to temperature. At temperatures about 17° F. sodium sulphate is the first ingredient of the salts to separate out, potassium chloride follows at 12° F., sodium chloride at −7.4° F., magnesium chloride at −28.5° F., and, as O. Pettersson was the first to point out, calcium chloride not until −67° F. During the rapid formation of ice the still unfrozen brine is often imprisoned between the little plates of frozen water; hence without some special treatment sea-ice is not suitable as a source of drinking water. After long continued frost the last of the included brine may be frozen and the salts driven out in crystals on the surface; these crystals are known to polar explorers by the Siberian name of rassol. Ice is a very poor conductor of heat and accordingly protects the surface of the water beneath from rapid cooling; hence new-formed pancake ice does not increase excessively in thickness in one winter, and even in the centre of the Arctic Basin the ice-covering only amounts to 6 or at most 9 ft. in the course of a year, while in the Antarctic regions the season's growth is only half as great; in the latter also the accumulated snow is an important factor in the thickness of the ice, and snow is an even worse conductor of heat. The influence of wind and tide breaks up the frozen surface of the sea, and sheets yielding to the pressures slide over or under one another and are worked together into a hummocky ice-pack, the irregularities on the surface of which, caused by repeated fractures and collisions, may be from 10 to 20 ft. high. Such formations, termed toross by the Russians, may extend under water, according to Makaroff's investigations, to at least an equal depth. Such old sea-ice when prevented from escaping forms the palaeocrystic sea of Nares; but, as a rule, it is carried southward in the East Greenland and Labrador currents, and melted in the warmer seas of lower latitudes. In the southern hemisphere the ice-pack forms a nearly continuous fence around the Antarctic continent. Pack-ice forms regularly in the inner part of the Baltic every winter, but not in the Norwegian fjords. Even in the Mediterranean sea-ice is formed annually in the northern part of the Black Sea, and more rarely in the Gulf of Salonica and at the head of the Adriatic off Triest. Hudson Bay is blocked by ice for a great part of the year, and the Gulf of St Lawrence is blocked every winter. Ice also clothes the continental shores of the northern fringing seas of eastern Asia. In addition to sea-ice, icebergs which are of land origin occur at sea. In the north, icebergs break off, as a rule, from the ends of the great glaciers of Greenland, and in the far south from the edge of the great Antarctic ice-barrier. The latter often gives birth to prodigious icebergs and ice islands, which are carried northward by ocean currents, nearly as far as the tropical zone before they melt. Thus in December 1906, an iceberg was seen off the mouth of the La Plata in 38° S., and in 1840 one was seen near Cape Agulhas in 35° S. The Antarctic icebergs are of tabular form and much larger than those of Greenland, but in either case an iceberg rising to 200 ft. above sea-level is uncommon, and one exceeding 300 ft. is very rare. The Greenland icebergs are carried by the Labrador current across the great banks of Newfoundland, where they are often very numerous in the months from February to August, when they constitute a danger to shipping as far south as 40° N. No icebergs occur in the North Pacific, and none has ever been reported nearer the coasts of Europe than off the Orkney Islands, and there only once, in 1836.
Oceanic Circulation.—Although observations on marine currents were made near land or between islands even in antiquity, accurate observations on the high seas have only been possible since chronometers furnished a practicable method of determining longitude, i.e. from the time of Cook, the circumnavigator. The difference between the position as determined astronomically and by dead-reckoning gives an excellent idea of the general direction and velocity of the surface currents. The first comprehensive study of the currents of the Atlantic was that carried out by James Rennell (1790-1830), and since that time Findlay in his Directories, Heinrich Berghaus, Maury and the officials of the various Hydrographic Departments have produced increasingly accurate descriptions of the currents of the whole ocean, largely from material supplied by merchant captains. Direct observations of currents in the open sea are difficult, and even when the ship is anchored the veering and rolling of the vessel produce disturbances that greatly affect the result. Such current—meters as those used by Aimé in 1841 and by Irminger since 1858 only gave the direction of the deeper current by comparison with the surface current at the time of observation. Later apparatus, such as Pettersson's bifilar current-meter or his more recent electric-photographic apparatus, and Nansen and Ekman's propeller current-meter, measure both the direction and the velocity at any moderate depth from an anchored vessel. One of the indirect methods of investigating currents is by taking account of the initial temperature of the current and following it by the thermometer throughout its course; hence the familiar contrast between warm and cold currents, of which the Gulf Stream and the Labrador current are types. Benjamin Franklin in 1775 and Charles Blagden in 1781, by means of numerous observations of temperature made on board the packets plying on the Atlantic passage, determined the boundaries of these two currents and their seasonal variations with considerable precision. The differences of salinity support this method, and, especially in the northern European seas, often prove a sharper criterion of the boundaries than temperature itself; this is especially the case at the entrance to the Baltic. Evidence drawn from drift-wood, wrecks or special drift bottles is less distinct but still interesting and often useful; this method of investigation includes the use of icebergs as indicators of the trend of currents and also of plankton, the minute swimming or drifting organisms so abundant at the surface of the sea.
The general lines of the currents of the oceans are fairly well understood, and along the most frequented ocean routes the larger seasonal variations have also been ascertained. The general scheme of ocean currents depends on the prevailing winds taken in conjunction with the configuration of the coast and its submarine approaches. The trade-wind regions correspond pretty closely with westward-flowing currents, while in the equatorial calm belts there are eastward-running counter-currents, these lying north of the equator in the Atlantic and Pacific, but south of the equator in the Indian Ocean. In the region of the westerly winds on the poleward side of 40° N. and S. the currents again flow generally eastward. A cyclonic circulation of the atmosphere is associated with a cyclonic circulation of the water of the ocean, as is well shown in the Norwegian Sea and North Atlantic between the Azores and Greenland. Where the trade-winds heap up the surface water against the east coasts of the continents the currents turn poleward. The north equatorial current divides into the current entering the Caribbean Sea and issuing thence by the Strait of Florida as the Gulf Stream, and the Antilles current passing to the north of the Antilles. Both currents unite off the coast of the United States and run northward, turning towards the east when they come within the influence of the prevailing westerly winds. In a similar manner the Brazil current, the Agulhas current and the East Australian current originate from the drift of the south-east trades, and in the North Pacific the Japan current arises from the north-east trade drift. The west-wind drifts on the poleward side carry back part of the water southward to reunite with the equatorial current, and thus there is set up an anticyclonic circulation of water between 10° and 40° in each hemisphere, the movement of the water corresponding very closely with that of the wind. The coincidence of wind and current direction is most marked in the region of alternating monsoons in the north of the Indian Ocean and in the Malay Sea.
The accordance of wind and currents is so obvious that it was fully recognized by seafaring men in the time of the first circumnavigators. Modern investigations have shown, however, that the relationship is by no means so simple as appears at first. Wemust remember that the ocean is a continuous sheet of water of a certain depth, and the conditions of continuity which hold good for all fluids require that there should be no vacant space within it; hence if a single water particle is set in motion, the whole ocean must respond, as Varenius pointed out in 1650. Thus all the water carried forward by any current must have the place it left immediately occupied by water from another place, so that only a complete system of circulation can exist in the ocean. Further, all water particles when moving undergo a deviation from a straight path due to the forces set up by the rotation of the earth deflecting them towards the right as they move in the northern hemisphere and towards the left in the southern. This deflecting force is directly proportional to the velocity and the mass of the particle and also to the sine of the latitude; hence it is zero at the equator and comes to a maximum at the poles. When the wind acts on the surface of the sea it drives before it the particles of the surface layer of water, and, as these cannot be parted from those immediately beneath, the internal friction of the fluid causes the propelling impulse to act through a considerable depth, and if the wind continued long enough it would ultimately set the whole mass of the ocean in motion right down to the bottom. The current set up by the grip of the wind sweeping over the surface is deflected by the earth's rotation about 45° to the right of the direction of the wind in the northern hemisphere and to the left in the southern. The deeper layers lag behind the upper in deflection and the velocity of the current rapidly diminishes in consequence. The older theory of the origin of drift currents enunciated by Zöppritz in 1878 was modified as indicated above by Nansen in 1901, and Walfrid Ekman subsequently went further. He showed that at a certain depth the direction of the current becomes exactly the opposite of that which has been imposed by deflection on the surface current, and the strength is reduced thereby to only one-twentieth of that at the surface. He called the depth at which the opposed direction is attained the drift-current depth, and he found it to be dependent on the velocity of the surface current and on the latitude. According to Ekman's calculation with a trade-wind blowing at 16 m. per hour, the drift-current depth in latitude 5° would be approximately 104 fathoms, in latitude 15°, 55 fathoms, and in latitude 45° only from 33 to 38 fathoms. A strong wind of 38 m. an hour would produce a drift-current depth of 82 fathoms in latitude 45°, and a light breeze of 3 m. an hour only 22 fathoms. It follows that a pure trade-wind drift cannot reach to any great depth, and this seems to be confirmed by observation, as when tow-nets are sunk to depths of 50 fathoms and more in the region of the equatorial current they always show a strong drift away from the side of the ship, the ship itself following the surface current. Ekman shows further that in a pure drift current the mean direction of the whole mass of the current is perpendicular to the direction of the wind which sets it in motion. This produces a heaping-up of warm water towards the middle of the anticyclonic current circulation between 10° and 40°, and on the other hand an updraught of deep water along the outer side of the cyclonic currents. The latter phenomenon is most clearly shown by the stripes of cold water along the west coasts of Africa and America, the current running along the coast tending to draw its water away seawards on the surface and the principle of continuity requiring the updraught of the cool deep layers to take its place. For this reason the up-welling coastal water is coldest close to the shore, and hence it only appears on the Somali coast during the south-west monsoon. On the flat coasts of Europe the influence of on-shore wind in driving in warm water, and of off-shore wind in producing an updraught of cold water, has long been familiar to bathers. In a similar way updraughts of cold water to the surface occur in the neighbourhood of the equator, especially in the Central Atlantic and Pacific.
When a drift-current impinges directly upon a coast there is a heaping up of surface water, giving rise to a counter-current in the depths, which maintains the level, and this counter-current, although subject to deflection on account of the rotation of the earth, is deflected much less than a pure drift-current would be. Such currents, due to the banking up of water, have a large share in setting the depths of the sea in motion, and so securing the vertical circulation and ventilation of the ocean.
The difference in density which occurs between one part of the ocean and another, shares with the wind in the production of currents. Vertical movements are also produced by difference of temperature in the water, but these can only be feeble, as below 1000 fathoms the temperature differences between tropical and polar waters are very small. If we assign to a column of water at the equator the density S4 = 1.022 at the surface and 1.028 at 1000 fathoms, or an average of 1.025, and to a column of water at the polar circle a mean density of 1.028, there would result a difference of level equal to (1.028 − 1.025) × 1000 = 3 fathoms in a distance from the equator to the polar circle of some 4600 m. A gradient like this, only 1 in 1,350,000, could give rise only to an extremely feeble surface current polewards and an extremely feeble deep current towards the equator. If there were strong currents at the bottom of the ocean the uniform accumulation of the deposit of minute shells of globigerina and radiolarian ooze would be impossible, the rises and ridges would necessarily be swept clear of them, and the fact that this is not the case shows that from whatever cause the waters of the depths are set in motion, that motion must be of the most deliberate and gentlest kind. In exceptional cases, when a strong deep current does flow over a rise, as in the case of the Wyville Thomson Ridge, the bottom is swept clear of fine sediment.
Strongly marked differences in density are produced by the melting of sea-ice, and this is of particular importance in the case of the great ice barrier round the Antarctic continent. O. Pettersson has made a careful study of ice melting as a motive power in oceanic circulation, and points out that it acts in two ways: on the surface it produces dilution of the water, forming a fresh layer and causing an outflow seaward of surface water with very low salinity; towards the deep water it produces a strong cooling effect, leading to increase of density and sinking of the chilled layers. Both actions result in the drawing in of an intermediate layer of water from a distance which takes part in the double system of vertical circulation as is indicated in fig. 2. The actual direction of this circulation is strongly modified by the influence of the earth's rotation. The existence of a layer of water of low salinity at a depth of 500 fathoms in the tropical oceans of the southern hemisphere is to be referred to this action of the melting ice of the Antarctic regions. Pettersson's view that ice-melting dominates the whole circulation of the oceans and regulates in particular the currents of the seas round northern Europe must, however, be looked on as carrying the explanation too far.
|Fig. 2.—Diagram of the stratification of temperature and the
vertical components of currents in high southern latitudes.
Differences of density between the waters of enclosed seas and of the ocean are brought about in some instances by concentration of the water of the sea on account of active evaporation, and in other instances by dilution on account of the great influx of land water. A very powerful vertical circulation is thus set up between enclosed seas and the outer ocean. The very dense water of the Red Sea and the Mediterranean makes the column of water salter and heavier and the level lower than in the ocean beyond the straits. Hence a strong surface current sets inwards through the Straits of Bab-el-Mandeb and Gibraltar, while an undercurrent flows outwards, raising the temperature and salinity of the ocean for a long distance beyond the straits. Through the Bosporus and Dardanelles at the entrance of the Black Sea, and through the sound and belts at the entrance of the Baltic, streams of fresh surface-water flow outwards to the salter Mediterranean and North Sea, while salter water enters in each case as an undercurrent. Wind and tide greatly alter the strength of these currents due to difference of density, and the surface outflow may either be stopped or, in the case of the belts, actually reversed by a strong and steady wind. Both outflowing and inflowing currents are subject to the deflection towards the right imposed by the earth's rotation.
Modern oceanography has found means to calculate quantitatively the circulatory movements produced by wind and the distribution of temperature and salinity not only at the surface but in deep water. The methods first suggested by H. Mohn and subsequently elaborated by V. Bjerknes have been usefully applied in many cases, but they cannot take the place of direct observations of currents and of the fundamental processes and conditions underlying them. The determination of the exact relationship of cause and effect in the origin of ocean currents is a matter of great practical importance. The researches of Pettersson, Meinardus, H. N. Dickson and others leave no doubt, for example, that the variations in the intensity of the Gulf Stream, whether these be measured by the change in the strength of the current or in the heat stored in the water, produce great variations in the character of the weather of northern Europe. The connexion between variations of current strength and the conditions of existence and distribution of plankton are no less important, especially as they act directly or indirectly on the life-conditions of food fishes.
Authorities.—General: M. F. Maury, The Physical Geography of the Sea and its Meteorology (New York and London, 1860); J. J. Wild, Thalassa: an Essay on the Depths, Temperature and Currents of the Ocean (London, 1877); C. D. Sigsbee, Deep-sea Sounding and Dredging (Washington, 1880); O. Krümmel, Handbuch der Ozeanographie (2 vols., Stuttgart, 1907); O. Krümmel, Der Ozean (Leipzig, 1902); J. Thoulet, Océanographie (2 vols., Paris), vol. i. Statique (1890), vol. ii. Dynamigue (1896); J. Thoulet, L'Océan, ses lois et ses problèmes (Paris, 1904); J. Thoulet, Guide de l'océanographie pratique (Paris, 1895); J. Walther, Allgemeine Meereskunde (Leipzig, 1893); Luigi Hugues, Oceanografia (Turin, 1904); Sir J. Prestwich, “Tables of Temperatures of the Sea at Different Depths . . . made between the years 1749 and 1868,” Phil. Trans. clxv. (1876), 639-670; A. Buchan, “Specific Gravities and Oceanic Circulation,” Trans. Roy. Soc. Edinburgh, xxxiv. (1896), 317-342; Sir John Murray, “Presidential Address to Section E (Geography),” British Association Report (Dover), 1899; M. Knudsen, Hydrographical Tables (Copenhagen, 1901); Sir John Murray, “Deep-Sea Deposits and their Distribution in the Pacific Ocean,” Geogr. Journal, 1902, 19, pp. 691-711, chart; “On the Depth, Temperature of the Ocean Waters and Marine Deposits of the South Pacific Ocean,” R. Geogr. Soc. of Australia, Queensland, 1907, pp. 71-134, maps and plates; J. Thoulet, Instruments et opérations d'océanographie pratique (Paris, 1908); Précis d'analyse des fonds sous-marins actuels et anciens (Paris, 1907); T. Richard, L'Océanographie (Paris, 1907); List of Oceanic Depths and Serial Temperature Observations, received at the Admiralty in the year 1888 (et seq.) from H.M. Surveying Ships, Indian Marine Survey and British Submarine Telegraph Companies (Official).
Important current and temperature charts of the ocean and occasional memoirs are published for the Admiralty by the Meteorological Office in London, by the U.S. Hydrographic Office in Washington, the Deutsche Seewarte in Hamburg, and also at intervals by the French, Russian, Dutch and Scandinavian Admiralties. Pilot Charts of the North Atlantic and North Pacific are issued monthly by the U.S. Hydrographic Office, and of the North Atlantic and of the Indian Ocean and Red Sea by the British Meteorological Office, giving a conspectus of the normal conditions of weather and sea.
Reports of Important Expeditions.—Sir C. Wyville Thomson, The Depths of the Sea (cruises of “Porcupine” and “Lightning”) (London, 1873); The Atlantic (cruise of “Challenger”) (London, 1877); Die Forschungsreise S.M.S. “Gazelle” in den Jahren 1874 bis 1876 (5 vols., Berlin, 1889-1890); Report of the Scientific Results of the Voyage of H.M.S. “Challenger” in the years 1872-1876 (50 vols., London, 1880-1895); A. Agassiz, Three Cruises of the U.S. Coast and Geodetic Survey Steamer “Blake” . . .from 1877 to 1880 (2 vols., Boston, Mass., 1888); S. Makaroff, Le Vitiaz el l'Océan Pacifique, 1886-1889 (St Petersburg, 1894); S. Makarofi, The Yermak in the Ice (in Russian) (St Petersburg, 1901); The Norwegian North Atlantic Expedition (on the “Vöringen”), 1876-1878 (Christiania, 1880-1900); Expéditions scientifiques du “Travailleur” et du “Talisman,” 1880-1883 (Paris, 1891 et seq.); Die Ergebnisse der Plankton-Expedition, 1889 (Kiel, 1892 et seq.); Résultats des champagnes scientifiques accomplies sur son yacht par Albert Ier Prince Souverain de Monaco (Monaco, from 1889); The Danish “Ingolf” Expedition, 1806 (Copenhagen, 1900); Prof. Luksch, Expeditionen S.M. Schiff “Pola” in das Mittelmeer und in das Rote Meer, Kais. Akad. Wissenschaften (Vienna, 1891-1904); Die Deutsche “Valdivia” Tief-See Expedition, 1898-1899 (Berlin, 1900); M. Weber, “Siboga Expedition,” Petermanns Mitteilungen (1900); Siboga Expeditié (Leiden, 1902 et seq.); F. Nansen, The Norwegian North Polar Expedition, 1893-1896 (Christiania and London, 1900); R. S. Peake, “On the Results of a Deep-sea Sounding Expedition in the North Atlantic Ocean during the Summer, 1899” (Extra Publ. Geogr. Soc., London); Bulletin des résultats acquis pendant les courses périodiques (Conseil permanent international pour l'exploration de la mer) (Copenhagen, 1902 seq.).
Reports of many minor expeditions and researches have appeared in the Reports of the Fishery Board for Scotland; the Marine Biological Association at Plymouth; the Kiel Commission for the Investigation of the Baltic; the Berlin Institut für Meereskunde; the blue books of the Hydrographic Department; the various official reports to the British, German, Russian, Finnish, Norwegian, Swedish, Danish, Belgian and Dutch governments on the respective work of these countries in connexion with the international co-operation in the North Sea; the Bulletin du musée océanographique de Monaco (1903 seq.); the Scottish Geographical Magazine; the Geographical Journal; Petermanns Mitteilungen; Wagner's Geographisches Jahrbuch; the Proceedings and Transactions of the Royal Societies of London and Edinburgh; the Annalen der Hydrographie; and the publications of the Swedish Academy of Sciences.
- Carte générale bathymétrique des océans dressée par ordre de S.A.S. le Prince Albert de Monaco, 24 sheets (Paris, 1904).
- “Challenger” Reports, “Deep Sea Deposits.”
- ἐπὶ λόφοις—on the threshold.
- Scot. Geog. Mag., vol. 16 (1900), p. 695.
- Comptes rendus, Acad. Sciences (Paris, 1859), 49, 463, 536.
- Chemical News (1870), vol. 22, pp. 25, 44; (1872) vol. 26, p. 159.
- Meddelelser om Grönland (Copenhagen, 1904), p. 331.