Chemical properties of sea water & Residence time

Chemical properties of sea water:

The oceans  cover an area of 361 x 106 km2, or 70.8% of the earth's surface. From the mean depth of 3800 m. the volume of the ocean waters can be calculated to be 1370 x 106 km3. At the surface the density of sea water of normal salinity at 0o C is 1.028, and it increases with depth because of the slight compressibility of water under increased pressure. Seawater thus comprises about 91% of the weight of the hydrosphere and its composition can therefore be taken without serious error as giving an average composition of the hydrosphere.
The circulation of the waters in the hydrosphere provides a mechanism for geochemical changes through weathering and transport of dissolved and solid materials. 
About 68% of the land area is drained to the Atlantic Ocean but the constancy of ocean composition indicates that seawater mixing erases all (except local) variation in its chemical composition.

Reference: Principles of Geochemistry (Mason and Moore). 

•  In the open ocean the salinity is average about 35 parts per thousand (g/Kg) with a regional range between 32 – 37 ppt.

• It is max. 41ppt in Red sea & Persian Gulf where evaporation is high.

• It will be low where a river / melting ice enters the ocean 

     Although the total mass of terrestrial water is in significant, in comparison with the total mass of hydrosphere, these water are important geochemically. Because the solvent power of the river water, its is never pure H2O, it may contains dissolved substance from land. The amount of dissolved substance will change from time to time and from place to place. The average composition is given in the table. 

    It has been estimated that the total rainfall on the land surface is 123 X 1018 g, of which 27.4 X 1018 g is drains off to the ocean. Clarke estimate that the water 27.35 X 1014 g of dissolved substance with it.

 Major dissolved constituents of the river water

Reference: Principles of Geochemistry (Mason and Moore).

• Comparison shows that the water in river & in ocean are opposite in chemical character. In sea water, Na > Mg > Ca & Cl > SO4 > HCO3; in river water, Ca > Na > Mg & HCO3 > SO4 > Cl. 

• It is evident that the factors other than the supply of dissolved material in runoff from the land regulate the composition of seawater. Many reactions take place in the sea to alter the balance of dissolved substances.

Residence time

 

    It is generally believed that the ocean is, to a large degree, in a steady-state condition, whereby the amount of an element introduced per unit time is balanced by an equal amount deposited in the sediments.

 

    The degree to which the concentration of any substance in the sea water is controlled by the residence time of the substance. Residence time of any element in the ocean is the average time that element spends in the ocean. In its simplest form the residence time, or replacement time, is the length of time (τ) it would take for the input of a substance to replace the amount present.

 Conclusion: From the above explanation it is clear that the concentration of elements in ocean is depended on, two factors,

 

Its supply from the surface water & Its residence time.