1922 Encyclopædia Britannica/Oceanography

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OCEANOGRAPHY (see 19.967[1]). — The period following the year 1910 was not productive of notable additions to knowledge of general oceanography. Several expeditions were made just before that year and in the period between then and the World War. The most important were: the Australian Antarctic expedition of 1911-4 under Sir Douglas Mawson; the Danish Oceanographical expeditions in the Mediterranean and adjacent seas of 1908-10; a short cruise made by Sir John Murray and Dr. Johan Hjort in the Norwegian Fishery exploring vessel “Michael Sars” in 1910, the general results of which were published as The Depths of the Ocean (1912) by the leaders of the expedition; and a short special cruise made by the “Scotia” in 1913 (after the loss of the “Titanic”) under the leadership of Dr. Matthews, which made observations upon the distribution of ice in the North Atlantic.

Generally, oceanographic work at sea was brought to a stop by the outbreak of war in 1914. A good deal of special investigation relating to naval and especially submarine warfare was carried on during 1914-8, but the results of this confidential work were not published. The very important activities of the Conseil Permanent International pour l'Exploration de la Mer were suspended during the war except in a few local seas. Fortunately the continuity of the organization was maintained, largely through the mediation of the British Government, and the council held its first post-war meeting in London in 1920. Its work is primarily that of the investigation of the fisheries of northern Europe, but its general methods are oceanographical, and its published results have formed an immense contribution to the science. Germany and Russia had, temporarily at all events, withdrawn from the coöperation, but France came in for the first time in 1920, and it was understood that the United States was likely to join in the scheme of investigation. An entirely new project was an international survey of the Mediterranean and adjacent seas, from the fishery and oceanographical standpoints, by France, Italy, Spain and Portugal, but in 1921 no definite programme had been put in operation. The International Research Council formed just after the war constituted a section for Physical Oceanography, which held its first meeting in Paris in 1921. In 1920 a very influential movement began, in England, for the despatch of a new “Challenger” expedition on a great scale, but it was suspended in 1921 for lack of funds. On the whole, oceanographical research was being taken up most actively in Europe, but much important work was also begun in America, for instance the fine hydrographical research in the Pacific by the Scripps Institute of the university of California.

Methods of Investigation. — Little change occurred subsequently to 1910 with regard to the methods of oceanographical investigation except a continual refinement and an increasing improvement in the apparatus used: in this direction the activities of the Central Bureau of the International Council were very noteworthy. The instruments — current-meters, sounding apparatus, water-collecting bottles, thermometers, hydrometers, etc. — were all elaborated and improved. Hydrodynamical methods received increased attention and the investigation of the movements of the ocean by means of physico-mathematical devices developed as a result of the older work of Bjerknes, continued chiefly by Helland-Hansen and Sandström. It became fairly certain, however, that theory had outrun observational work, and that the latter must again receive renewed attention.

The empirical data on which the hydrodynamical investigations are based are: (1) observed velocities and directions of oceanic currents and drifts; (2) salinity; (3) density; (4) temperature of the sea water in situ; (5) oceanic soundings. Given that such observations at the surface of the sea, at intermediate levels and at the bottom are sufficiently numerous and are of a high degree of precision, general conclusions as to the movements of the ocean may be deduced from established theorems in hydrodynamics. But detailed studies of the circulation of the water in any small area show deviations from the calculated results that are to be expected: thus Nansen's investigation of the Norwegian sea shows that the main directions of streaming of the water are broken up by numerous large and small vortices. So also any exhaustive survey of the temperature and salinity of the sea at a great number of points on and below the surface reveals a complexity of conditions that may defy mathematical analysis and could not easily be predicted. A very large amount of local detailed observation in the various sea-areas must be the next important work to be undertaken: this means current-observations by direct readings of metres, by the employment of drift-bottles and numerous determinations of temperature and salinity at all seasons.

Variations in Oceanic Circulation. — The general scheme of oceanic circulation was made out prior to 1910. The excess of heat received in equatorial regions expands the water, but at the same time excess of evaporation concentrates it, so that the density increases. The heating effect is, however, the more significant, and so the water of the ocean tends to flow N. and S. from the equator towards the poles. In intermediate latitudes there is a loss of heat and then the increased density due to equatorial concentration becomes a factor. The water sinks below the surface and continues to flow along the sea bottom back towards the equator. In the polar areas the melting of sea-ice and of ice formed by precipitation lowers the density of the sea-water and causes a difference of level which sets up streaming movements towards the equator. This surface drifting water is cold and as it enters into intermediate zones it remains colder than the water in situ there and is therefore denser; it sinks below the surface and continues to flow along the bottom either back to the polar regions or towards the equator.

This main scheme is complicated in various ways: (1) by the rotation of the earth, which continually deflects currents of water or air to the right in the northern or to the left in the southern hemisphere; (2) by the conformation of the land masses (as in the case of the equatorial stream which is banked up in the Gulf of Mexico and flows out through the Straits of Florida); (3) by the varying depth of the ocean, for currents tend to flow more readily through deep than in shallow waters (as in the case of the main Atlantic drift, which flows most strongly through the deep channel between Shetland and the Faroe Is.); and (4) by the driving force of the winds acting on the surface of the sea (thus the drift of water from the equator is not N.E., as one might expect, but from E. to W., because of the impelling force of the N.E. and S.E. trade-winds).

All ocean currents vary from year to year in their strength of flow and the main interest of physical oceanography in recent years has been the tracing-put of these variations and the search for the causes. The variations themselves are detected by the method of seasonally repeated hydrographic soundings. Samples of water are collected periodically from a number of places in a large sea-area (the North or Norwegian seas, or the English Channel, for instance) at the surface, bottom and a number of intermediate levels. At the same time temperature observations are made. Stations which are placed in a straight line across a sea are then connected and “sections” are made. These show the magnitudes of the layers of different salinity and temperature beneath the surface, and when a number of sections are compared the differences from season to season and from year to year can be seen. So far only the North Atlantic has been at all well studied and evidence of seasonal and periodic variations extending over a number of years has been obtained in this area. Water drifting into the North Atlantic from the equatorial stream has a relatively high salinity (from 36 000 to 36.5 000) and a high temperature (from 15 °C. to 20 °C.), and when the distribution of salinity from season to season is studied it is seen that the area of dense water (salinity 36 000) extends farther to the N. in Nov. than in March. A large area of the North Atlantic is thus covered with relatively warm and dense water and this would slowly drift N. until it cooled sufficiently to sink beneath the surface. The prevailing W. and S.W. winds, however, drive it towards the N.E., where it impinges on the shallow seas and shore of northern Europe.

Taking such an easily surveyable area as the North Sea, the quantity of relatively warm and dense Atlantic water entering it from year to year can be estimated by the method of hydrographic sections. It can thus be seen that Atlantic water enters the North Sea round the N. of Shetland and (to a far less extent) through the English Channel. The flow culminates about March in each year, when a considerable part of the North Sea is covered with water of 35 000 salinity, but in Nov. the area so covered is very much less. Therefore the inflow waxes and wanes from season to season throughout the year, but it also varies in the same season in different years. There is no doubt about the latter variation, but with regard to its periodicity — that is, the number of years elapsing between one maximum and the next — much still remains to be done.

Farther to the N. of the British Isles the superficial drift of Atlantic water ceases, the temperature having fallen so much that the inflowing water becomes denser than that in situ, so that it sinks beneath the surface. It still flows on, however, as a deep current and it then becomes a factor of immense importance with regard to the fisheries in the regions into which it penetrates. The sinking-down occurs in the Kattegat when the inflowing Atlantic water enters the Baltic as an undercurrent which is both warmer and denser than that on the surface. The same thing occurs as the Atlantic stream rounds North Cape: there it breaks up into branches which are irregularly distributed and, sooner or later, sink below the surface and flow on as submarine currents. Entering the Barents Sea (that is, the area between the ice and the northern coast of Europe), these currents flow along the bottom. The inflowing Baltic undercurrent carries with it herrings and other fish from the North Sea outside, and the submarine current entering the Barents Sea also carries with it such fish as plaice. It is mainly because these fisheries are seasonal that the periodicity has been noticed, and because of the economic interests involved the study of the seasonal and longer periodicities has become very important.

As to the causes of the changes in the strength of the current from year to year much investigation has still to be made. The connexion that seemed to be first established was between variations in the quantity of water transported from the tropical to the sub-polar Atlantic and variations in the intensity of solar radiation. Helland-Hansen and Nansen traced a periodicity in the flow of Atlantic water along the W. coast of Norway: every ten to twelve years this flow appeared to reach a maximum and a graph of the variations showed a certain resemblance to the well-known graphs showing the numbers of spots on the sun from year to year. Not only so, but a similar variation was traced in the productivity of the great Lofoten (Lofoden) cod-fisheries. It was difficult to be sure as to the variations in the actual number of fish caught, but it was easy to show that there was a real variability in the yield of cod-liver oil (an important product of the fishery). Tracing, then, the quantities of oil given per 1,000 fish from year to year, they seemed to establish a connexion between the variation in “condition” of the fish, the variation in the inflow of Atlantic water, and the variation in the number of sunspots from year to year.

The relation appeared, however, to be far more complicated than was at first supposed. Helland-Hansen and Nansen showed later that it was improbable that variations in the northerly drift of Atlantic water could be traced directly to variations in the quantity of heat received by the sea from solar radiation. Of the total quantity of energy incident on the earth about 40% is reflected back from the earth's atmosphere. Of the 60% that penetrates only about one-third actually heats up the surface of the land or sea and the rest is absorbed by the atmosphere. The heating of the latter causes great differences of pressure, which in turn set up changes of atmospheric circulation. Now it is probable that the main cause of oceanic circulation is the driving force of the winds upon the superficial layers of water; hence periodic and irregular changes in the direction and velocities of ocean currents are probably due to changes in atmospheric circulation traceable to changes in the quantities of heat absorbed from the sun by the earth's atmosphere.

Later still Hjort showed that the study of the variability in the productivity of a fishery is always a complex matter — far more so than was formerly supposed. It appeared that the quantity of oil contained in the liver of a cod (per unit of weight) increases with the age of the fish. Detailed study of the cod shoals also showed that their composition was continually changing: in some years the shoal is composed of younger or older fish than the average and with this latter variation there are changes in the quantities of oil yielded per 1,000 fish. The changes in the composition of the shoals, as regards the proportions of the various “year-classes,” are to be correlated with oceanographical changes (see below). It is proper, however, to point out at once how very complicated may be the relationships between oceanographical and strictly biological phenomena, though, of course, the latter are ultimately dependent on the former.

Long-range Periodicities in Oceanographical Changes. — More and more the science seeks to discover periodicities and to correlate these with others. In these attempts new methods are elaborated and in their criticism contributory phenomena are discovered. An interesting example is the discussion, by Otto Pettersson, of the effects of long-range fluctuations in the tide-generating force: this memoir was published about 1914, but has only recently become available to English readers.

The tide-generating force is due to the attraction of the waters of the ocean by sun and moon. There are two gravitational fields which sometimes reinforce and at other times diminish each other and the effect is always a resultant one. There are therefore maxima and minima in the value of the tide-generating force, depending on the relative positions of the sun, earth and moon. The orbits of earth and moon are elliptical, so that the earth is sometimes nearer, sometimes farther away from the sun, and the same is the case with the moon in relation to the earth. The orbital planes of earth and moon are inclined to each other at an angle of 50.8° and at two points only in its orbit can the moon be situated in the plane of the ecliptic: the line joining these two points is called the “line of nodes.” A line joining the moon in perigee and in apogee is called the “line of apsides.” Now such a constellation as the following must sometimes exist: the earth is in perihelion; the line of nodes coincides with the line of apsides and both lie in the line joining earth and sun. The line of nodes rotates in a period of 18.612 years and the line of apsides in a period of 8.84 years. Such a constellation can be shown to occur at intervals of about 1,800 years and about those times the tide-generating force will be at an absolute maximum. Working out the calculations, Pettersson finds that the favourable constellation occurred and will occur in 3500 B.C., 1900 B.C., 250 B.C., 1433 A.D., 3300 A.D., and so on. In addition to these there are subsidiary maxima at intervals of 4½, 9 and 84-93 years.

Given, then, that the variations in tide-generating force are big enough, the periods when the maxima occur will be critical with regard to oceanographical and meteorological phenomena. About the time of the maxima there must be a longer tidal range (that is, a greater rise and fall than the average); the difference between neap tides and spring tides will also be increased, and as results of these conditions there must be great tidal floods breaking over low-lying coasts and producing extensive denudation. Pettersson further deduces sharp extremes of climate and great temperature contrasts. Far inland he supposes there will be devastating droughts. An effect of the greater tide-generating force will also be instability of the liquid magmas underlying volcanic areas, leading to violent eruptions and earthquakes. There will be great outbursts of polar ice, but this will melt at higher latitudes than in the periods when the tide-generating force is minimal.

It is shown to be probable that such effects actually occurred about the time of the last maximum (A.D. 1433). There is evidence that, towards the close of the mediaeval period, great storms and tidal inundations occurred on the shores of the North Sea and Baltic, and in the course of these floods, culminating in 1297, the Zuider Zee was formed from a lake that existed in its neighbourhood, by the breaking down of dykes. (Similar effects can be seen on a small scale, even in our own times, as the result of exceptionally big tides.) Severe winters were experienced and the Baltic was frequently frozen over so that there was solid ice communication between Sweden and Denmark across the Belts and Sound: this happened in the 13th, 14th and 15th centuries but not in the l6th. There have been great differences in the seas round Iceland and Greenland with regard to the presence of ice: from the 9th to the 12th centuries there is no evidence (in contemporary accounts) of the presence of much ice in the sea off Greenland, nor was much ice carried by the Labrador current, but from the 13th century onwards we do have evidence that there was very troublesome ice off Greenland. Hence from the 10th to the 12th centuries there was great intercourse with Iceland and Greenland on the part of the English, Swedish and Danish, but at the end of the 13th century some change occurred, resulting in the southerly emigration of the Eskimos and the extinction of European civilization in Greenland. At the present time the S. and E. coasts are icebound, and the W. coast, though icebergs are present in the adjoining sea, is clear. Many economic changes probably occurred in consequence of the variations in tide-generating force, as, for instance, the decline in the mediaeval Baltic herring fisheries controlled by the Hanseatic League.

Hydrobiology. — The study of marine life has in recent years become more general, and has become associated with very precise investigations into the chemical composition of sea-water, changes in chemical equilibrium, the effect of variations in salinity and temperature, the processes set up by marine bacteria, and so on. The investigation of the microscopic pelagic life of the sea has also developed to a great extent. Several decades ago all marine organisms became grouped together in three great categories: (1) the Benthos, or bottom-living, rooted or sedentary forms; (2) the Nekton, or actively swimming animals; and (3) the Plankton, or drifting (usually) microscopic organisms, which have little power of locomotion (see 21.720). The plankton is divided into (a) the Zoö-plankton (such as the minute crustacea and the eggs and larvæ of fishes and many other marine animals); and (b) the Phyto-plankton, that is, the minute algae, diatoms, peridinians, some flagellate protozoa, spores of algæ, etc. The investigation of the plankton from a new point of view, begun by Hansen in 1889, was continued by Lohmann at Kiel, by Cleve in Sweden, by Gran and Ostenfeldt in Norway and Denmark, and by Herdman, Allen and others in England. Hansen's early results were much criticized and the original methods very greatly modified and improved. It became clear that only very rough estimates of the numbers of planktonic organisms in a volume of sea-water as large as (say) 10 cubic metres could be made, but that these estimates could nevertheless be trusted to show very marked regional and seasonal differences.

Distribution of the Plankton. — In general the plankton — and especially the phyto-plankton of the polar and temperate seas — is much more abundant than is that of the sub-tropical and tropical zones. All forms of plankton are more abundant in the shallow coastal waters of relatively low salinity. Finally, the plankton (and again the vegetable forms in particular) are practically restricted to the upper hundred fathoms or so of the sea. Deeper than this, microscopic life is scanty; there is practically no reproduction and growth. These facts of distribution are due to certain conditions that govern the production of organic substance in the oceans.

Holozoic and Holyphytic Organisms. — These terms relate to the modes of nutrition. Typical animals are holozoic, that is, they obtain their food by eating the tissues of other animals and plants: they take their food substances in the organized forms of proteids, fats and carbohydrates. Typical plants are hplophytic, that is, they obtain their food substances from purely mineral sources. Water and carbonic acid are synthesized, under the action of sunlight, to form sugar, starch or some other carbohydrate and this is then combined with simple nitrogenous salts to form proteid. Fats doubtless originate by the “cleavage” of the synthetically formed proteids, or from carbohydrates. Now dead animal substance and the excreta of animals decompose in the long run into carbonic acid, water and mineral salts, and so there is a continual destruction of animal substance both on the land and in the sea. Animals cannot make use of these decomposition products, but the plants can. Therefore all life in the sea (as on land) depends on the power which the holophytic organisms possess of synthesizing mineral substances into organized tissues. This is mainly effected, in the sea, by the phyto-plankton.

Ultimate Food Substances in the Sea. — These are the materials which are utilized by the vegetable plankton in the synthesis of living material: they are water, carbonic acid, nitrates and nitrites of calcium, magnesium and other earthy and alkaline metals, phosphates, silica, traces of salts containing iron, sulphur, potassium and a few other elements. Except the water, all are present in the sea in exceedingly small proportion. The source of the carbon of organic tissues is carbonic acid; that of the nitrogen in the proteids is the nitrates, nitrites and salts of ammonia dissolved in sea-water; the material of the shells or other skeletons is the silica, phosphate and calcium of the salts of sea-water (and, in rare cases, the salts of strontium). All these substances exist as only a fraction of one part or, at most, a few parts, per million of water. Carbonic acid is the most abundant and it may be contained in sea-water in the proportion of about 50 milligrammes per litre (that is, 50 per million). All of this is not available, for carbonic acid is present as such in solution, as bicarbonate (of magnesium mainly) and as normal carbonate. Only the “free” carbonic acid and that of the bicarbonate can be utilized in the process of photosynthesis by the diatoms and algæ.

Mineral nitrogenous compounds (nitrates, nitrites and ammonia) are much more rare. The distribution is very interesting and it has been shown that the water of the Antarctic Ocean contains about 0.5 part per million of nitrogen in the above forms. The North Atlantic contains, on the average, about 0.15 part per million and the equatorial seas little more than about 0.1 part per million. The proportion varies with the temperature. There is more inorganic nitrogen in the sea near the land than in mid-ocean and there is more at the sea bottom than near the surface; finally, there is more in the later winter than at any other season. Silica (which is required for the skeletons of diatoms, radiolaria, peridinians, etc.) is present in about the same concentration, but it is now suspected that a source of this substance may be clay washed down from the land and present in the sea in the colloidal form. Phosphates, necessary for the formation of skeletons and also for the nucleo-proteid of cells, are about as scarce as nitrogen. In the case of all these substances the quantities involved are so very small, and the difficulties of estimation are therefore so great, that the information we possess is by no means satisfactory. Clearly, however, the vast quantity of living substance in the ocean is built up from materials that are present in the sea-water as an exceedingly dilute solution, and the solution is dilute just because organisms are incessantly utilizing it. It follows, too, that when there is a number of substances, all essential for the elaboration of living material, and when one of these is present in minimal proportion, that one substance rules the production, just as the effective strength of a chain depends on the weakest link. This is Liebig's “law of the minimum.”

Seasonal Periodicities of Life in the Sea. — In the temperate seas the two great features are: (1) the outburst of vegetable life in the spring; and (2) the vernal or summer phase of reproduction among animals. The low temperature of the winter allows (indirectly) an accumulation of the essential nitrogenous mineral salts, but as the minimal temperature is passed (in Feb. or March) and the days begin to lengthen the phyto-planktonic organisms begin to reproduce. Carbonic acid is taken from the water and synthesized (by the mediation of light energy) into carbohydrate. The carbonic acid is taken from solution and then bicarbonate (usually that of magnesium) dissociates into carbonic acid and normal carbonate, and the process of photosynthesis ceases when there is no more bicarbonate in solution. The result of this is that the alkalinity of the sea-water increases and the hydrogen-ion concentration decreases. Perfectly pure distilled sea-water dissociates, to an infinitesimal degree, into hydrogen (H) and hydroxyl (HO) ions, so that one litre of such water contains 1 × 10-7, or 110,000,000 part of a gram-molecule of either hydrogen or hydroxyl (a gramme-molecule of hydrogen is 2 grammes, or of hydroxyl 17 grammes). Pure water, then, has a hydrogen-ion concentration of 10-7 but sea-water gives (because of the mixture of the salts in solution) the concentration 10-8.2 and when photosynthesis by the larger algae, or diatoms, is very active this figure falls to about 10-9.1. That is, the concentration of H-ions decreases and that of the HO-ions increases; the water becomes more alkaline because the carbonic acid of the bicarbonate has been abstracted by the phyto-plankton to the extent that normal carbonate is left. When that condition is attained photosynthesis slows down and ceases.

The spring outburst of plant life in the sea culminates about April, just about the time when the temperature of the water begins to rise rapidly. The increasing temperature raises the rate of animal metabolism, while the higher alkalinity is a stimulus to cell-division. Therefore the animal organisms, as a rule, reproduce in the spring or early summer just after the vernal phyto-plankton maximum. From then onwards the plant organisms diminish because they are eaten by the animal larvæ.

The numerical values are, it is to be noted, exceedingly small. Experiments made by Moore and Whitley at Port Erin in the Isle of Man show that the hydrogen-ion concentration falls from about 10-8.1 in Dec. to about 10-8.4 in April. This corresponds to an increased alkalinity represented by about 2 c.c. of N100 standard alkali, and that difference means that the carbon of about 8-8 milligrammes of carbonic acid has been built up (by photosynthesis) into carbohydrate during the period during which the change in alkalinity proceeded. If it occurs uniformly overthe sea to a depth of only one metre it leads to a production of about 6 tons of carbohydrate per sq. km. of sea.

Following the great spring production of plant substance there is, therefore, a summer outburst of animal life. Following that again is a less well-marked maximum of phyto-plankton in the autumn, occurring just after the period of highest sea temperature. The temperature then falls rapidly and there is a gradual slackening in the production of organic substance and a general lethargy of life. The plankton, both animal and vegetable, attains its minimal values and many of the larger forms of animal life pass into a kind of condition of hibernation.

The Transport of Essential Food Substances. — First of all we consider inorganically combined nitrogen (as nitrates and nitrites chiefly), since upon this depends all the life of the ocean. The concentration of these substances is least in the warm equatorial seas and greatest near the poles. The temperature is, however, only an indirect cause of this variation and the direct cause is now known to be the activity of the nitrogen-bacteria. The nitrogen-bacteria that concern us here are of two main categories: (1) those that assimilate elementary nitrogen from its solution in sea-water, building it up into combination with carbohydrate as proteid; and (2) those that break down nitrate into nitrite, nitrite into ammonia and ammonia into elementary nitrogen. Two antagonistic processes proceed simultaneously, the fixation of atmospheric nitrogen and the reverse change, and either process is accelerated by an increase and retarded by a decrease in temperature. It is maintained by Brandt and others belonging to the Kiel school of marine biologists that the process of denitrification is, on the whole, more significant in the sea than that of nitrogen-fixation.

If this is admitted the poverty of tropical sea-water in mineral nitrogen compounds is explained by the higher temperature, which accelerates the activity of denitrifying bacteria. Since there is less of the indispensable food material in the warmer seas there is, therefore, less phyto-plankton. This is really the case, for all observations show that the Antarctic and Arctic ice-bound seas are enormously rich in diatom life when compared with temperate and tropical regions: the great Antarctic zone of sea-bottom deposit, in which the skeletons of diatoms predominate, covers some ten millions of square miles. The relative abundance of nitrates and nitrites at the bottom of deep oceans as compared with the surface can be explained in the same way, for at the bottom the temperature is about zero Centigrade and the activities of the denitrifying bacteria are practically suspended. The dead bodies of organisms fall down from the surface and are slowly resolved into products of putrefaction, which gradually pass into the mineral forms, nitrates, carbonic acid and ash. The bottom water is relatively rich in these substances as well as in decaying organic matter, and would become progressively richer but for the slow drift towards the equator and the welling-up of bottom water to the surface in these latitudes.

It would seem that, on the whole, nitrogen compounds in the ocean (whether existing in the organic or inorganic forms) remain constant in amount. Nitrogen is always being synthesized from the atmosphere (by plants, and by electrical discharges which combine nitrogen and oxygen), and this combined nitrogen is either utilized by land organisms or is washed down into the sea in the water of the rivers. In the end much inorganic nitrogen salts must be added to the sea both in the above way and as the result of the putrefaction of the dead substance of terrestrial animals and plants.

As a general rule the sands in the immediate vicinity of the shore contain organic matter resulting from land drainage (particularly near great centres of human population) and from the remains of dead plant and animal organisms. At the same time the denudation of rocks sets free iron compounds which dissolve in the sea to a slight extent and permeate the littoral sands which contain organic matter. The putrefaction of the latter sets free sulphuretted hydrogen, which then acts on the iron compounds, precipitating ferrous sulphide. The latter discolours the sand and so one finds, round the coast and towards the upper margin of the zone between high- and low-water marks, an under layer of black sand formed in this way. On the surface, where the sand is bathed by the tidal water, the ferrous sulphide becomes oxidized and the sand is bleached, but underneath it is dense black or grey, as the case may be.

A considerable degree of denitrification must, therefore, take place in the ocean, for the concentration of combined nitrogen is always excessively small. The regional differences, as we have seen, can oe explained by the regional difference of temperature.

The quantities of oxygen and carbonic acid in the sea are nearly constant so far as we can determine. The former gas is continually being evolved by the plants and absorbed by the animals, and precisely the reverse actions occur in the case of carbonic acid. Further, the ocean and the atmosphere stand in equilibrium with each other; if there is excess of carbonic acid anywhere in the sea it is absorbed by the atmosphere and vice versa, and so also with the oxygen. Differences of temperature and atmospheric pressure must disturb this equilibrium, but the movements of both ocean and atmosphere lead to a high degree of uniformity in both envelopes as regards their gaseous constitutions.

Silica is continually being added to the ocean. Land masses are denuded and minerals containing silicates are carried down to the sea as sediments. The coarser particles of the sediments are deposited near the shore as gravels, sand and muds, but the very fine particles remain in suspension in the colloidal form, and some of this may be acted upon by marine bacteria or (it is surmised) even utilized by diatoms as a source of silica. The silica, in the form of diatom or radiolarian skeletons, is eventually deposited on the ocean floor after the death of the organisms. Most of the fine colloidal clay is, however, deposited as river-sludges when the fresh water carrying it mixes with denser sea-water. The colloidal particles are electrically charged and become discharged by the ions of sodium, magnesium and calcium present in the sea-water. This “coagulation” leads to the formation of the river-sludges that form deltas.

Lime is transported in solution as sulphate and bicarbonate, both of which salts are soluble to some extent in water. The water of the ocean is usually nearly saturated with calcium salts, which must continually be removed since they are always being added in the water brought down from the land. Lime is, in fact, absorbed to an enormous extent by fishes, molluscs, crustacea, calcareous algæ and sponges, starfishes, sea-urchins and feather stars, many polyzoa and a multitude of protozoa (mainly the foraminifera). All these animals have calcareous skeletons or shells of some form and they secrete the calcium from its solution as sulphate, converting it into carbonate. Some unicellular organisms are said to segregate salts of strontium from sea-water.

Coral Formations. — Coral reefs remove calcium from solution in the sea on a vast scale. During recent years the controversies with regard to the modes of formation of these structures have entered on a new phase. The theories of Darwin, Agassiz, Dana, Semper, Murray and others had led to apparently interminable discussion, and the great boring experiments at Funafuti atoll, which were expected to be crucial, gave results that backed both the rival theories of Darwin and Murray. On the other hand, Wayland Vaughan (see Annual Report of the Smithsonian Institution, 1917) has shown clearly that the problem is essentially a biochemical one and may finally be solved by the methods of the latter science.

It is not at all certain that the masses on which coral reefs are built consist entirely of the remains of the skeletons of reef-forming organisms and it is probable that chemically precipitated carbonate of lime predominates. The water in shallow seas, off the shores of islands or in lagoons, is saturated with calcium bicarbonate and if the amount of carbonic acid in solution be reduced by any means, normal carbonate must be precipitated. Therefore a reduction in the partial pressure of the gas in the atmosphere, or a rise in the temperature of the water, or a violent agitation of the sea itself, will lead to precipitation of calcium carbonate. Evaporation of the water and anything that lowers the hydrogen-ion concentration have the same effect.

Therefore an increase in photosynthesis caused by the multiplication of plant microorganisms will lead to the precipitation of calcium carbonate, for carbonic acid will be withdrawn from solution to take part in carbohydrate synthesis by the plants. Denitrifying bacteria will raise the alkalinity (or reduce the H-ion concentration) by forming ammonia, which will combine with the carbonic acid in solution and so throw down normal carbonate of lime. Drew found as many as 160 millions of denitrifying bacteria per c.c. of sea- water on the W. side of Andros I. in the Bahamas. There are, therefore, a number of agencies, all of which operate in shoal waters on the lee side of islands, or in shallow lagoons in such regions as the Bahamas, and the result of all these is to throw down calcium carbonate from solution in sea-water as minute needle-shaped crystals or little balls of aragonite. Such material, it is suspected, may form the massive bases on which barrier or fringing or atoll reefs are built up.

The “Glacial Control” Theory. — Interesting speculations as to the periods of origin of great coral reefs have been made by Wayland Vaughan, Andrews and Daly and Humphreys. (The causes or conditions of glaciation, it may be noted here, are no better known than in 1910. It has been suggested, however, that a prolonged period of volcanic activity may reduce the air temperature to a marked degree by throwing large quantities of dust into the atmosphere: this will act by preventing the penetration of solar radiation.) During a period of prolonged glaciation water becomes withdrawn from the ocean, for rainfall goes to form solid ice-caps that accumulate upon polar and continental land areas. Daly estimates that the maximum lowering of ocean level due to this cause would only amount to 36 fathoms, but even that would be the cause of very marked geological effects. In Pleistocene times, then, when there were prolonged glacial ages, the sea-level was lowered and at the same time there was a reduction in sea temperature, so that the rate of reproduction of the coral polypes, and so the growth of reefs, was diminished. The protection of the shore may therefore have been decreased, with the result of increased land erosion and the formation of extensive shallow submarine plateaux. When the warmer interglacial periods recurred the polar and continental ice-caps melted and the sea-level became raised again — that is, there was submergence of the eroded plateaux formed as indicated above. Corals would now grow luxuriantly in these shallow coastal waters of increasing temperature, forming reefs and extensive coral flats. These new structures would rest uncomfortably upon eroded formations and this, Wayland Vaughan points out, is what we actually observe in the case of living and fossil coral reefs. In so far as it depends on solution of calcareous rock the Semper-Murray theory of coral reefs is unsatisfactory.

Bibliography.Books: Sir J. Murray, The Ocean (1913); The Science of the Sea (1912); H. R. Mill, The Realm of Nature (1913); Jenkins, Oceanography (1921); J. Johnstone, Conditions of Life in the Sea (1908); Murray and Hjort, Depths of the Ocean (1912); J. Y. Buchanan, Comptes Rendus and Accounts Rendered (1918 and 1920). Special Papers: Rolf Witting, “Die Meeresoberfläche,” in Fonnia, vol. xxxix., No. 5 (Helsingfors 1918), noticed in Nature Aug. 21 1919, deals with the problem of mean sea-level; Svenska Hydrog. Biolog. Kommission Skrifter, No. 5 (Copenhagen 1914); H. and O. Pettersson's papers on tide-generating force are published in Publications de Circonstance, Conseil Internat. pour l'Exploration de la Mer, No. 65 (1913); Meddelelser f. Kommissioner f. Havundersögelser, Serie Hydrografi (Copenhagen 1904-20), contain important papers; the publications of the university of California (Zoölogy) deal with the work of the Scripps Inst. for Marine Biology. Recent papers on coral reefs are published in the Annual Report of the Smithsonian Institution, 1917; and F. W. Clark, Data of Geochemistry, Bulletin No. 693, U.S. Geological Survey (ed. 4, 1920), gives numerous references. (J. J.)

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