The salinity of Ocean water | Why is the Ocean water saline?

Water comprises over 70% of Earth’s area. Close to 97% of the total water on Earth is ocean water, meaning most of the water on Earth is saline. What is the average salinity of ocean water? Why is the Ocean water saline? Let’s find out.

What is the average salinity of ocean water?

Saltiness or salinity is typically measured in g/Kg. The average salinity of ocean water is around 35 g/Kg. This means on average there are 35 parts of salts per 1000 parts of water in the ocean.

The salinity in different areas of the ocean varies. For example, the eastern coast of India (Bay of Bengal) is relatively less saline than the Western coast (Arabian Sea). This is because the Bay of Bengal receives more rain and discharge from the rivers increasing the amount of freshwater.

The enclosed water bodies tend to have higher salinity if the freshwater intake is low and the rate of evaporation is high. For example, the Dead Sea has an incredibly high salinity of over 330 g/Kg.

Why is the ocean water saline?

The Ocean water is saline because of 4 main reasons.

  • Rainwater reacts with the Carbon Dioxide in the air to form Carbonic Acid, making the rain slightly acidic. The rocks on land are weathered by this slightly acidic rain. The mineral ions thus formed are transported to the rivers and consequently to the Ocean. These ions combine together to form Salts. Over millions of years, the salts have accumulated making the Ocean saline.
  • When the ocean water seeps through cracks deeper under the ocean bed, it is heated by geothermal energy. The heated water is later released back into the ocean through hydrothermal vents. The released water is rich in minerals and salts (which are picked up from under the ocean bed) which contribute to the salinity of the Ocean.
  • Underwater volcanic eruptions also add to the salinity of the ocean.
  • Over millions of years due to constant evaporation and constant discharge from the rivers, the salt concentration has increased in the ocean.

There are a number of natural processes through which salt is removed from the Ocean. As a result, an equilibrium is achieved and the average salinity of the ocean stays around 35 g/Kg.


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