Sea level, sediments and the composition of seawater

The composition of seawater changed significantly during the course of the Phanerozoic in concert with long-term changes in sea level, which were largely the result of changes in the volume of mid-ocean ridges. These reflect changes in the rate of seafloor spreading and determine the rate of seawater cycling through midocean ridges. Spencer and Hardie (1990) and Hardie (1996) proposed that the changes in the cycling rate of seawater through mid-ocean ridges have been responsible for the changes in the composition of seawater during the Phanerozoic. This explanation cannot account for the major changes in the composition of seawater during the Cenozoic, which was a time of nearly constant seafloor spreading rate. However, changes in the rate of deposition of shallow water carbonates and their penecontemporaneous dolomitization can explain the Cenozoic changes in seawater composition. This indicates that sedimentologic parameters have played a major role in the chemical evolution of seawater. The more rapid cycling of seawater through MORs during the Mesozoic can explain semiquantitatively the fairly minor differences between the composition of seawater in the Eocene and in the Jurassic. Changes in the composition of seawater during the Paleozoic were probably determined both by changes in the rate of dolomite and gypsum/anhydrite deposition and by changes in the rate of seawater cycling through mid-ocean ridges. If the causes of these compositional changes are the same as those during the Cenozoic and Mesozoic, then sedimentologic factors have been significantly more important than the rate of cycling of seawater through MORs in shaping the chemical evolution of seawater during the Phanerozoic.