[delta]18O values, 87Sr/86Sr and Sr/Mg ratios of Late Devonian abiotic marine calcite: Implications for the composition of ancient seawater

Abstract

Late Devonian (Frasnian) abiotic marine calcite has been microsampled and analyzed for 87Sr/86Sr ratios, [delta]18O and [delta]13C values, and minor element concentrations. Portions of marine cement crystals from the Alberta and Canning Basins have escaped diagenetic alteration and preserve original marine [delta]18O values (-4.8%. +/- 0.5, PDB), [delta]13C values (+2.0 to +3.0%., PDB), 87Sr/86Sr ratios (0.70805 +/- 3), and Sr/Mg weight ratios (0.04 to 0.05). Marine 87Sr/86Sr ratios are globally consistent and can be correlated within the Alberta Basin, and among the Alberta, Canning, and Williston Basins. Correlation of isotopic and chemical data strengthen the conclusion that marine cements from the Leduc Formation preserve original marine [delta]18O values which are 3 to 4%. lower than those of modern marine cements. These low [delta]18O values are best explained by precipitation from 18O-depleted seawater and not by elevated seawater temperature or diagenetic alteration.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 x 10-6 (ppm Mg) + 6.20 x 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.