Age of the Oceans

Spindler and Wrangell in the Black sea studied transparency by sinking an electric lamp; experiments have also been made by using photographic plates to find the greatest depths to which light penetrates. Fol and Sarasin detected the last traces of sunlight in the western Mediterranean at a depth of 37o metres, and Murray and Hjort observed that photographic plates, after an exposure of two hours at a depth of 1,500 metres in mid-Atlantic, had become dark. The chief cause of the different transparency is the varying quantity of mineral particles in suspension or of plankton. Schott gives the following as the result of measurements of transparency by means of a white disc at 23 stations in the deep ocean, where quantitative observations of the plankton under 1 square metre of surface were made at the same time.

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 de termined by comparison with the tints of Forel's xanthometer which consists of a series of glass tubes filled with a mixture of blue and yellow liquids in varying proportions. Observations with the xanthometer have shown that the purest blue (o–I 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 tend ency 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 (o-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 o° and 31° E., and even the water of the North sea has been observed at times to be intensely blue. Over shallows even the water of the tropical oceans is always green. There is no relationship between colour or transparency on the one side and the temperature or salinity on the other, but a distinct relationship between colour, transparency and plankton or min eral particles in suspension 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 physical explanation for these facts,

according to the investigations of K. Grien near Capri, is the selective absorption of light in water. In the following table the originally existing amount of light is supposed to equal 1,000.

At a depth of 5o metres red and yellow are wholly absorbed, while of the blue and violet rays of the original amount of light still exist. Blue is the natural colour of water over the greater depths and the colour seen by divers. Clear sea water looks blue from the surface because some of the non-absorbed rays are re flected, but if the water contains mineral substances or plankton the red and yellow rays may also be reflected and the colour of the water is green. Discoloration is always due to foreign sub stances. Brown or even blood-red stripes have been observed in the north Atlantic when swarms of the copepod Ccslanus finmar chicus were present ; the brown alga Trichodesmium erythraeum, as its name suggests, can change the blue of the tropical seas to red ; swarms cf 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.

Other Properties.

On account of its salinity, sea water has a smaller capacity for heat than pure water. The specific heat diminishes as salinity increases, so that for io 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 internal friction or viscosity of sea water has been shown by E. Ruppin to increase with the salinity. Thus at o° 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 o° C.

The compressibility of sea water increases with decreasing tem perature, salt content and pressure. The difficult calculation of the amount of compressibility has recently been carried out by Ekman, Bjerknes and Sandstrom. The following series from the Philippine trench shows the variations.

One litre of water at this last depth therefore weighs not 1027.68 grams but actually 1071.23 grams. If the whole mass of water in the ocean were relieved from pressure its volume would expand from 1,369 million cub. km. to 1,381 million cub. km., which, for a surface of 361 million sq. km., means an increased depth of 32 metres.