SALINITY OF THE OCEANS

Almost every known chemical element can be found in varying proportions in the oceans; whose most characteristic feature is the Salinity. It is the degree of saltiness of the ocean water. It is usually expressed as parts per thousand (ppt) or in % variations.
All sea water contains large amounts of dissolved mineral matter of which sodium chloride (NaCl) alone constitutes more than 77%. The other more important compounds include magnesium, calcium and potassium. 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 or salinity does vary appreciably in different areas.

DISTRIBUTION OF SALINITY

The distribution of salinity is expressed by “ISO-HALINES”, 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 thousand parts of water. According to Dittmar, the composition is as:

Sodium Chloride--------NaCl------27.2 g
Magnesium Chloride----MgCl2-----3.8 g
Magnesium Sulphate----MgSO4---1.6 g
Calcium Sulphate-------CaSO4---1.26 g
Potassium Sulphate-----K2SO4---0.8 g
Calcium Carbonate------CaCO3---0.123 g
Magnesium Bromide-----MgBr2----0.07 g
Total-----------------------------35 g

The degree of concentration of salts varies appreciably in different areas. The variation is due to three causes:
1. Supply of fresh water
2. Rapidity of fresh water
3. Degree of water mixing by current

Open Seas

Since warm water will dissolve more of a given substance than cold water it would be expected that the areas of greatest salinity would be near the equator. This is not so sue to following reasons:

1. In the equatorial areas, the rainfall is heavy and occurs almost daily, and the relative humidity of the atmosphere is high so that there is little evaporation. In addition, there are rivers of large volume (e.g., Congo, Zaire and Amazon), which constantly supply fresh water. As a result, the salinity is not high but below normal, i.e., below 35 per thousand.

2. North and south of the equatorial area are the trade wind belts (trade winds are drying winds). Therefore, evaporation is rapid in these latitudes (about 20o to 30o north and south of the equator). On the land, in these latitudes, are the great deserts, sol that there are relatively few rivers to add a supply of fresh water. In these belts, the salinity rises to 37 per thousand and over. Where really large rivers enter the sea in these belts, the salinity is lower, viz. at the mouth of the Zambesi, the Ganges, the Mississippi and the rivers of Indo-China.

3. Proceeding polewards from the trade wind belt, the salinity gradually decreases until, in the poleward sections of the Arctic Ocean, it is only from 20 to 30 per thousand. Here there is less rapid evaporation, more rain, more rivers and a large supply of fresh water from melting ice.

Partially Enclosed Seas

The Mediterranean Sea has a very high salinity, i.e., 40 per thousand. It is partially enclosed and its waters do not circulate freely with those of the open ocean. It is in a region of rapid evaporation, particularly during the summer, when too many of the rivers become almost dry. The one really large river, the Nile, adds less and less fresh water year by year because of the increased demands for irrigation purposes. For similar reasons, very high salinities are found in Red Seas and Persian Gulf. In Red Sea, the average salinity increases to 39 per thousand.
The Baltic Sea contrasts with the Mediterranean. It, too, is nearly enclosed, but it is a cooler region, with a lower rate of evaporation. Some large rivers (Oder, Vistula) empty into the Baltic, and numerous streams flow from the snow-clad mountains of Scandinavia, bringing a constant supply of fresh water. As a result the salinity is only 2 per thousand, and the waters nearly fresh. One of the important results of this low salinity is that the Baltic Sea freezes readily.
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, Yenisey and Mackenzie.

Inland Seas and Lakes

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 Israel / Jordan, a salinity of 250 per thousand has been recorded. This is due to several features:
1. Temperature in the Dead Sea region are high
2. The rainfall and atmospheric humidity is low
3. The lake receives water from only a small drainage area
4. The rate of evaporation is high

The highest salinity is perhaps, that of Lake Van, in Turkey, 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 any where else!
In enclosed seas and lakes, the composition of the dissolved salts is not the same as ordinary sea water.