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. | |
| Oxygen | 8.03 | 5.84 | 4.93 |
| Nitrogen | 14.40 | 11.12 | 9.78 |
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,[1] 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:—
| Water from | ||
| 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.
- ↑ Meddelelser om Grönland (Copenhagen, 1904), p. 331.
