The composition of the precambrian ocean waters

The composition of the Precambrian oceans was different from the present composition; this is a direct consequence of a CO₂-atmosphere depleted in oxygen. At the beginning the mineralization was lower. The oceans waters were chloro-dicarbonated with a lower tenor of chlorine, richer in phosphates and they were depleted in sulphates; sulphur was in the reduced state of sulphides. The abundance of phosphates supported the expansion of life. The cations Na⁺ and Mg²⁺ coexisted with Ca²⁺, Fe²⁺ and Ba²⁺ whose levels were higher than they are today. The main trends of the evolution are schematized in the figure.