Salinity of the ocean

Salinity of the ocean

Salinity is defined as the ratio between the weights of dissolved salts (in grams) per 1000 grams of water. It is expressed as part per thousand (‰) and has no units. Example: 30‰ means 30 grams in 1,000 grams of sea water. The average ocean salinity is 35‰.

Sources of salt in the ocean: Sea water is a weak but complex solution made up of many things including mineral salts and decayed biological marine organisms. Most of the ocean salts are derived from weathering and erosion of the earth’s crust by the rivers. Some of the ocean salts have been dissolved from rocks and sediments below the sea floor, while others have escaped from the earth’s crust through volcanic vents as solid and gaseous materials.

Fact File

Depth of water is measured in the unit ‘Fathom’. One fathom is equal to 1.8 metre (six feet)

Factors affecting the salinity of ocean water

The salinity of ocean water depends upon

1.        The rate of evaporation

2.        Amount of precipitation,

3.        Addition of fresh water flow from rivers

4.        Ice in Polar Regions

5.        Upwelling of deep water initiated by prevailing winds and

6.        Mixing of water by ocean currents.

Distribution of salinity

On an average the salinity decreases from equator towards the poles. The highest salinity is observed between 20° and 40° north latitudes because this zone is characterized by high temperature, high evaporation but less rain than the equatorial region.

The marginal areas of the oceans bordering the continents have lower salinity than their interior due to addition of fresh water to the marginal areas through the rivers (Figure 5.15).

Very high salinity is recorded in Lake Von, Turkey (330‰ ) Dead Sea (238‰) and Great Salt Lake, Utah, USA (220‰).

Activity

Identify regions of high salinity and low salinity.

Compare the salinity of Arabian Sea and Bay of Bengal and find the reason for the same.

Find out the reason for low salinity on east coast of Asia and West coast of North America.(Figure 5.15)

Why does the salinity vary along the west coast of South America?

Fact File

Isohaline is an imaginary line drawn to join places having equal salinity.

Salinity of Dead Sea is 8.6 times saltier than other oceans. The shore of Dead Sea is 423m below sea level. It has the lowest elevation on land. The sea is 377m deep. The high salt content will make people float on the sea. The high salt content has made the Dead Sea devoid of life in it.