Ocean's Salinity and Salt Budget

The salinity of ocean waters is measured by the amount of dissolved solids present in a unit weight of ocean water, usually expressed in parts per thousand by weight or grams per kilogram.

Salinity of Ocean

The salinity of ocean water is usually around 35 parts per thousand on an average at zero degrees Celsius. This implies that in the total weight of ocean water, dissolved salts amount to 3.5 percent. Sodium chloride or the common salt is the most common among all the dissolved salts in the sea. The chemical composition of ocean water is as follows.


Sources of salinity

The salts dissolved in the ocean waters have their origin on the continental landmasses. They can be carried into the oceans by rain, rivers, groundwater table, sea-waves, winds, and glaciers. However, some of the dissolved salts have their origin from the ocean bottom. The layers of the earth beneath the crust contain minerals in a molten state which can reach the crust either due to volcanic activity or due to their outgassing (continuous emission in the form of gasses) from the fissures present at the bottom of the ocean. In addition, the dead and decomposing organic matter also adds to the salinity of the oceans.

Determinants of the salinity of the oceans:

Ocean salinity is dependent upon several factors and it keeps varying with the place and time of measurement. The main determinants of ocean salinity include,

  • Evaporation: In general, salinity is higher at places with high rates of evaporation. The tropical seas such as the Red Sea, Persian Gulf etc., have the highest rates of evaporation. Consequently, the waters of these seas close to the Tropic of Cancer have some of the highest rates of ocean salinity.
  • Temperature: Temperature and ocean salinity share a direct relationship. In general, regions with high temperatures are also the regions with high salinity. Also, the Torrid zone (hot, tropical regions) has higher salinity than the Frigid zone (cool, temperate regions).
  • Precipitation: Precipitation and salinity share an inverse relationship. In general, regions with higher levels of precipitation have lower levels of salinity. This is the reason why though the equatorial region is as hot as the sub-tropics, it records lower salinity than the sub-tropics since the former receives heavy precipitation in a day.
  • Ocean Currents: They play an important role in the spatial distribution of dissolved salts in ocean waters. The warm currents near the equatorial region push away the salts from the eastern margins of the oceans and accumulate them near the western margins. Similarly, ocean currents in the temperate regions increase the salinity of ocean waters near the eastern margins. For instance, Gulf Stream in the North Atlantic Ocean increases the salinity of ocean waters along the western margins of the Atlantic Ocean. The North Atlantic Drift, on the other hand, increases the salinity of waters in the North Sea.
  • The influx of Fresh Water: Salinity is relatively lower in areas where major rivers meet the oceans. For instance, at the mouths of rivers such as Amazon, Congo, Ganga etc., the ocean surface salinity is found to be lower than the average surface salinity. Similarly, in the polar regions, when the glaciers melt during the summers, there is an influx of fresh water into the surrounding ocean which reduces the surface salinity.

Horizontal Distribution of Salinity:

The surface salinity of oceans decreases on either side of the tropics. For instance, the surface salinity along the Tropic of Cancer is around 36 parts per thousand (ppt) while at the equator it's around 35 parts per thousand. On the basis of their salinity levels, seas across the world can be categorized as follows:

  • Seas with salinity levels below the normal: They have a low salinity due to the influx of fresh water. They include the Arctic Ocean, Southern Ocean, Bering Sea, Sea of Japan, Baltic Sea etc. Their surface salinity can be as low as 21 ppt.
  • Seas with normal salinity levels: These have a salinity in the range of 35 to 36 ppt. They include the Caribbean Sea, Gulf of Mexico, Gulf of California, Yellow Sea etc.
  • Seas with salinity levels above the normal: They have higher levels of salinity because of their location in regions with higher temperatures leading to greater evaporation. They include the Red Sea (39 - 41 ppt), Persian Gulf (38 ppt), Mediterranean Sea (37 - 39 ppt) etc.

Vertica Distribution of Salinity:

There is no definite trend in the variation of salinity with depth. Instances of increase, as well as a decrease in salinity levels, have been found with increasing depth.

From the above graph, it is evident that,

  • Salinity decreases with increasing depth at the equator as well as near the tropics.
  • At higher latitudes, salinity is found to increase with increasing depth.

Salt Budget

It is also known as the salt cycle. It involves all the processes through which salt moves from the ocean into the lithosphere, to a certain extent into the atmosphere, and back into the oceans.

  • Moving water, including groundwater, leaches minerals from the rocks through the process of surface erosion. The mineral-laced water joins the rivers and streams which finally reach the oceans. These minerals add to the salinity levels of the ocean waters.
  • Some of the salts in the ocean waters accumulate at the ocean bottom through the process of sedimentation turning into mineralized rocks. Over a period of millions of years, some of these rocks get raised above the ocean surface due to plate tectonics, or due to volcanic activity. This brings the salt back to the lithosphere in the form of minerals (rocks).
  • Salt from the oceans also gets sprayed into the atmosphere due to the action of wind. This salt returns to the lithosphere mixed with precipitation. However, this constitutes a tiny fraction of salt moving from the land to the sea and vice versa.
  • Salt cycle operates over a very long period of time.

Every year, around 3 billion tons of salt gets added to the oceans from the land. A tiny fraction of this salt is extracted by humans for daily consumption.

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