Paleoecological problems of Mg and Sr in biogenic calcites in light of recent thermodynamic data

Abstract

Calcitic shells of three species of the ostreid pelecypods, Exogyra cancellata, E. costata (Late Cretaceous) and Crassostrea virginica (Recent) were analyzed for Mg and Sr. Shells of Exogyra contain 0.5–1.5 mole per cent MgCO 3, 0.04–0.13 mole per cent SrCO 3; shells of Crassostrea—1.0–1.6 mole per cent MgCO 3, 0.08–0.09 mole per cent SrCO 3. These calcites are not at equilibrium with the (a Mg ++ ) (a Ca ++ ) ratio as in the Recent sea water. The concentrations of Sr in the skeletons of these species and of Belemnitella americana (Late Cretaceous) can, at least in part, be accounted for by Nernst's distribution equilibria with respect to the Sr Ca ratio as in the Recent seas. A combined record of Mg and Sr, treated as an incomplete ternary free energy surface, may accentuate differences between geographically distinct populations. Interpretation of such differences is at present impeded by a lack of exact knowledge of the genetic and environmental effects on the skeletal chemistry. Approximate calculation of the activity coefficients of the components in magnesian calcites at 25°C is given in the Appendix.