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 comp...