Sodium is the second most abundant element dissolved in seawater. It has a conservative distribution that varies only due to differences in evaporation or precipitation in the water that sank to form intermediate or deep waters in the sea .
Sodium is present in seawater as the Na+ ion. The sodium residence time was calculated from the global average river input after correcting for mans influence on the composition of modern rivers (Berner and Berner, 1987).