It is a term used for the total body of water of the earth. In other word”, all the natural waters occurring on or below the surface of the earth is known as hydrosphere. Thus the term includes the oceans, seas, lakes, rivers, snow and ice, underground and atmospheric water.
An enormous mass of water is concentrated in oceans and seas which occupy approximately 71 per cent (i.e 361,059,000 sq km out of 510,000.000 sq km.) of the surface of the entire globe.
Oceans and seas communicate with each other and form a single mass of water called the World Ocean. Its average depth is about 3800 metres. The total volume of the world-occan is about 1.4 billion cubic kilometres i.e. about 97 percent of the world’s fiee water is constituted by the world ocean.
The distribution of the oceans and seas is highly irregular in different latitudinal belts, as well as in the Northern and Southern Hemispheres. There are four major oceanic bodies:- (i) the Pacific, (ii) the Atlantic, (iii) the Indian and (iv) the Arctic.
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The first three oceans together constitute 90 per cent of the total area of water bodies The Pacific is the largest occan, both in surface area and volume.
Composition of Sea-Water
The sea waters are more variable in composition than the atmosphere, including large proportions of mineral matter as well as water and gases. Sea-water is a solution of salts.
Dissolved salts, or solutes are added to the sea water from the erosion of the rocks of the earth’s surface and from the eruption of volcanic materials espacially along oceanic ridges. These are lost by precipitation to ocean-floor sediments.
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These salts result in the property of salinity, but the degree of salinity is not the same everywhere. The concentration of solutes in the water is affected by temporal and regional variations in erosion, precipitation and also by surface evaporation and the addition of water from rain and rivers.
Thus, in the North Sea, for example, the percentage of salt is less than that of the Atlantic, in the Baltic it is very much less. In the Mediterranean, on the other hand, the proportion of salt is considerably greater than in any part of the open ocean.
The average salinity of the sea water is 35 parts per thousand i.e
on the average 1000 grams of sea-water contains 35 grams of dissolved solids. According to Dittuiar. the proportion of these solids are as follows:
Sodium chloride 27.213
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Magnesium chloride 3.807
Magnesium sulphate 1.658
Calcium sulphate 1.260
Potassium sulphate 0.863
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Calcium carbonate 0.123
Magnesium bromide 0.076
The ingredients of sea water have maintained approximately fixed proportion over a considerable span of geologic time. Of the various elements combined in these salts, chlorine alone makes up 55 per cent by weight of all the dissolved matter and sodium makes up 31 per cent.
Magnesium, calcium, sulphur and potassium are the other tour major elements in these salts. Sea-water also holds in solution small amounts of all the gases of the atmosphere.
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According to Brian Mason (Principles of Geochemistry, 1952) common elements present in the ocean-water arc as follows
Elements Weight Percentage
Oxygen 85.79
Hydrogen 10.67
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Chlorine 1.898
Sodium 1.056
Magnesium 0.127
Sulphur 0.088
Calcium 0.040
Potassium 0.038
Bromine 0.007
Carbon (inorganic) 0.003
Strontium 0.001
99.718
The rest is made up by other dissolved gases As we know, water absorbs oxygen more intensely than nitrogen. Different gases are differently absorbed by water. Thus while the oxygen to nitrogen ratio in the air is 1:4, in water these gases are usually found in a 1:2 ratio.
Sea water derives oxygen from the air and also through photosynthesis by marine plants. The carbon dioxide content of sea water is also high. Its sources are the atmosphere, river waters, the life activity of marine animals and volcanic-eruptions.