WHY DID THE COMPOSITION OF SEAWATER VARY DURING THE PHANEROZOIC?

The evidence for significant variations in the composition of seawater during the Phanerozoic is very strong. The concentration of Mg²⁺, Ca²⁺, and SO₄^2- has varied in two cycles, which are nearly synchronous with those of sea level changes. It was thought for some time that this correlation is due to changes in the rate of seafloor spreading. However Rowley (2002) has shown that the rate of seafloor spreading has been nearly constant during the past 180 Ma, while sea level has fluctuated markedly. This implies that the variations in the stand of sea level and the composition of seawater have been produced by factors other than changes in the rate of seafloor spreading.

The low concentration of Mg²⁺ and SO₄^2-, and the high concentration of Ca²⁺ in seawater during periods of high sea level stand are probably related to widespread evaporative concentration of seawater in shallow continental seas, which led to extensive dolomitization and CaSO₄ precipitation. The high Mg²⁺ concentration in seawater during the later parts of the Permian may be due to the narrowness of the continental shelves at that time, to the limited space for biogenic CaCO₃ precipitation, to increased supersaturation of seawater with respect to CaCO₃, and to inorganic precipitation of CaCO₃ which was not readily dolomitized. The increase in the Mg²⁺ content of seawater during the Tertiary is almost certainly due to the increase in the quantity of foraminiferal and coccolith remains that were deposited on the ocean floor.

The variations in the composition of seawater during the Phanerozoic therefore seem to be related mainly to tectonic processes that affected sea level, to sedimentologic and climatic changes, and to evolutionary processes that changed the distribution of marine CaCO₃.