δ18O values, 87Sr86Sr and Sr/Mg ratios of Late Devonian abiotic marine calcite: Implications for the composition of ancient seawater

For comparison with Devonian data, analogous data were collected from Holocene Mg-calcite and aragonite marine cements from Enewetak Atoll, Marshall Islands. Mg-calcite and aragonite marine cements are in isotopic equilibrium with ambient seawater, and Mg-calcite cements are homogeneous with respect to Sr and Mg contents. Empirically derived homogeneous distribution coefficients for Mg and Sr in modern, abiotic Mg-calcite from Enewetak Atoll are 0.034 and 0.15, respectively. An equation describing the dependence of DSr on Mg content was based on a compilation of Sr and Mg data from Holocene abiotic marine calcite (DSr = 3.52 × 10−6 (ppm Mg) + 6.20 × 10−3). Unlike that derived from experimental data, this Sr-Mg relation is consistent over a range of 4 to 20 mol% MgCO3 and may represent precipitation phenomena which are minimally controlled by kinetic effects. Comparison of Sr and Mg contents of analogous Devonian and Holocene marine cements suggests that the Mg/Ca ratio of Late Devonian seawater was significantly lower and that the Sr/Ca ratio was significantly higher than that of modern seawater.