SEAWATER STRONTIUM/CALCIUM RATIO INDICATES PLATE TECTONIC CONTROL ON THE EVOLUTION OF CARBONATE SEDIMENTATION

Major changes in global carbonate sedimentation occurred during the Cretaceous. Producers of platformal sediments changed from Early Cretaceous aragonitic coral-algal to Late Cretaceous calcite-dominated rudist bivalves. Calcitic pelagic carbonate sedimentation increased in importance, and hemipelagic chalks are characteristic sediments of the Late Cretaceous. These general patterns of carbonate deposition reflect a global shift from aragonite-dominated during most of the Early Cretaceous to calcite-dominated in the Late Cretaceous. This change is also seen in the calcite/aragonite ratio of the shells of rudist bivalves which were among the major carbonate producers on mid- and Late Cretaceous carbonate platforms. As aragonite sedimentation is the most important sink of seawater Sr, the Sr/Ca ratio of seawater responded to decreasing aragonite sedimentation and increased significantly from the Barremian to the Late Campanian. This is seen in Sr concentrations in biological low-Mg calcite of more than 900 samples from rudist bivalves and belemnites (Steuber, 2002).

The evaluation of a large database of Sr concentrations in more than 1100 brachiopods (Veizer et al., 1999) shows that the Ordovician-Cretaceous curve of seawater Sr/Ca ratios is consistent with previously advocated phases of calcite and aragonite seas. Episodes when aragonite was the dominant marine carbonate correspond to low Sr/Ca ratios of seawater while high Sr/Ca ratios occurred during episodes of calcite seas. This curve covaries with estimates of seafloor accretion rate so that the effect of plate tectonics on the composition of seawater, probably via the changing Mg/Ca ratio of seawater, is assumed to be the controlling factor. Model simulations show that changes in the Phanerozoic hydrothermal and continental weathering fluxes cannot account for the observed changes in seawater Sr/Ca ratios, while changing rates of Sr burial in marine carbonates provides the most reasonable mechanism to explain the analytical data.

References

Steuber, T., 2002, Plate tectonic control on the evolution of Cretaceous platform-carbonate production: Geology, v. 30, p. 259-262.

Veizer, J., et al., 1999, ⁸⁷Sr/⁸⁶Sr, d¹³C and d¹⁸O evolution of Phanerozoic seawater: Chemical Geology, v. 161, p. 59-88.