World Geography / Oceanography
Salinity refers to excessive concentration of mineral salts in sea water. Different factors like latitude, ease of mixing, addition of fresh water, etc.
are affecting the salinity of ocean water of any particular region. Salinity is measured in grams per kg (minerals/sea water). The average salinity of
all oceans is 34.75 g/kg.
Spatial distribution of salinity is done through Isohaline lines, which connect the equal average salinity regions in the ocean. The
important salts in the oceans and seas are Chlorides, Sulphates, Carbonates of Sodium, Calcium, Potassium and Magnesium (important being NaCl, MgCl2,
Sources of Mineral Salts
- Terrestrial Sources (acquired from lands or continents) - They are brought by the agents like rivers, winds, glaciers, etc. through weathering of
rocks. Along with rubble material, mineral salts are added to the sea.
- Submarine Sources - They are added from within the sea. These sources are of two types, (a) Lava eruptions of submarine volcanoes, in which
huge amount of salts are present and (b) Submarine organisms, whose shells, secretions and skeletons provide wide variety of minerals when they die.
- Cosmic Sources - When cosmic bodies enter the Mesosphere, they burn and form into cosmic dust and fall into the ocean. They contain a variety of
mineral salts like Iron, Magnesium, etc.
Factors Influencing Salinity
Salinity is not uniform across all the seas. Dead Sea has a salinity of 375 g/kg and Baltic Sea has a salinity of 3-4 g/kg. Marginal seas, which are
close to the Poles, have least salinity.
- Latitude - With higher Latitude (high salinity in Tropics and Subtropics up to 300), there is rapid decline in salinity
from 300 towards Poles.
This is due to temperature. The higher the temperature, the higher will be the rate of evaporation and hence
higher the salinity.
- Addition of Fresh Water - If there is more addition of fresh water, less will be the salinity. Higher the rainfall, lower will be the salinity.
Higher the melting of ice or snow, higher will be the availability of fresh water and hence less will be the salinity.
As large scale melting of snow happens
near Poles, the salinity is less there, e.g. the salinity in Gulf of Bothnia is the lowest. More the rivers flowing into a sea, less will be the
- Ease of Mixing - If the mixing (free flow of water in sea) is easy, less will be the salinity. In open sea (free mix of water), salinity is low
and in inland and marginal seas, salinity is high due to no free mix of water, e.g. Mediterranean Sea has more salinity when compared to Atlantic Ocean.
Similarly, Red Sea has more salinity than Arabian Sea.