Almost every known chemical element can be found in varying proportions in the oceans whose most characteristic feature is their salinity, in contrast to the fresh water of lakes and streams. All sea water of which sodium chloride or common salt alone constitutes more than 77 per cent.
The other more important elements include magnesium, calcium and potassium, while the rest are distinguishable only in traces of very minute quantities. Due to the free movement of ocean water, the proportions of different salts remain remarkably constant in all oceans and even to great depths. But the degree of concentration of the salt solution in oceans does vary appreciably in different areas. This is expressed as salinity, the degree of saltness of the water, either as a percentage or more often in parts per thousands. Variations are shown in salinity distribution maps by isohalines, lines joining places having an equal degree of salinity.
Generally speaking, the average salinity of the oceans is 35.2 per thousand, about 35 parts of salt in 1000 parts of water. In the Baltic Sea, where there is much dilution by fresh water and melting ice, the salinity is much lower, only about 7 per thousand. In the Red Sea where there is much surface evaporation and fewer rivers to bring in fresh water, the average salinity increases to 39 per thousand. In enclosed seas, which are areas of inland drainage, such as the Caspian Sea, the salinity is very high, 180 per thousand, and in the Dead Sea of Palestine, a salinity of 250 per thousand has been recorded. The highest salinity is, perhaps, that of Lake Van, in Asia Minor, with 330 per thousand. It is a salt lake, and salts are collected from its shores. The density of the water is so high that in Lake Van or the Dead Sea it is almost impossible to sink. Beginner-swimmers will find it much easier to float here than anywhere else! The variation of salinity in the various seas and oceans is affected by the following factors (Fig. 12.7).
1) The rate of evaporation: The waters fringing the High Pressure Belts of the Trade Wind Deserts, between 20° and 30°N and S, have high salinity because of the high rate of evaporation caused by high temperature and low humidity.
The temperate oceans have lower salinity due to the lower temperature and a lower rate of evaporation.
2) The amount of fresh water added by precipitation, streams and icebergs: Salinity is lower than the average 35 per thousand in equatorial waters because of the heavy daily rainfall and high relative humidity. Oceans into which huge rivers like the Amazon, Congo, Ganges, Irrawaddy and Mekong drain, have much of their saltiness diluted and have a lower salinity. The Baltic, Arctic and Antarctic waters have a salinity of less than 32 per thousand because of the colder climate with little evaporation and because much fresh water is added from the melting of icebergs, as well as by several large poleward-bound rivers, e.g. Ob, Lena Yenisei and Mackenzie.
3) The degree of water mixing by currents: In wholly or partially enclosed seas such as the Caspian Sea, Mediterranean Sea, Red Sea and Persian Gulf, the waters do not mix freely with the ocean water and they are not penetrated by ocean currents. Salinity is high, often over 37 per thousand. In areas of inland drainage without links with the oceans, continuous evaporation under an almost cloudless sky causes the accumulation of salt around the shores. In the open oceans where currents freely flow, salinity tends to be near the average 35 per thousand or even a little lower. The range of salinity is negligible where there is free mixing of water by surface and sub-surface currents.