CHANGES IN THE SR/CA AND MG/CA RATIOS OF CRETACEOUS SEAWATER DERIVED FROM THE COMPOSITION OF BIOLOGICAL LOW-MG CALCITE

A large database (>1100 samples) of chemical compositions of Cretaceous rudist bivalve shells shows significant changes in Sr and Mg concentrations of skeletal calcite from the Berriasian to the Maastrichtian. Rudist shells provide excellent material as the thick outer shell layer of low-Mg calcite of several groups has a high potential to preserve original chemical and isotopic compositions. Numerical ages of the studied specimens were obtained by Sr isotope stratigraphy.

In sclerochronological sections which allow for the evaluation of intra-shell variations, Sr concentrations show only minor variations, so that an increase from mean values of 900 µg/g in Aptian-Albian samples to 1500 µg/g in Campanian samples is believed to reflect changes in the Sr/Ca ratio of seawater. Applying partition coefficients for modern mollusk calcite results in a Sr/Ca ratio of Campanian seawater that was 1.5 times that of modern seawater. This is consistent with data derived from benthic foraminifera by Lear et al.

Estimating Mg/Ca ratios of seawater from Mg concentrations in biological calcite is difficult due to a pronounced temperature control. At the same d¹⁸O, Early Cretaceous specimens have much less Mg than Late Cretaceous samples. This is consistent with an increasing Mg/Ca ratio of seawater from the Barremian to the Maastrichtian, also reported by others from studies of fluid inclusions in halite. Importantly, the seawater Mg/Ca ratio was low before sea-floor spreading rates are believed to have increased during the mid-Cretaceous. This indicates that hydrothermal alteration of oceanic crust was not the major controlling factor of the seawater Mg/Ca ratio.

Changing Sr/Ca ratios of Cretaceous seawater are consistent with patterns of aragonite versus calcite predominance in marine carbonates, suggesting that the changing mode of carbonate sedimentation, triggered by the seawater Mg/Ca ratio, controlled the Sr/Ca ratio of seawater.