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

Late Devonian (Frasnian) abiotic marine calcite has been microsampled and analyzed for {sup 87}Sr/{sup 86}Sr ratios, {delta}{sup 18}O and {delta}{sup 13}C 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}{sup 18}O values ({minus}4.8{per thousand} {plus minus} 0.5, PDB), {delta}{sup 13}C values (+2.0 to +3.0{per thousand}, PDB), {sup 87}Sr/{sup 86}Sr ratios (0.70805 {plus minus} 3), and Sr/Mg weight ratios (0.04 to 0.05). Marine {sup 87}Sr/{sup 86}Sr 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}{sup 18}O values which are 3 to 4{per thousand} lower than those of modern marine cements. These low {delta}{sup 18}O values are best explained by precipitation from {sup 18}O-depleted seawater and not be 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. 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.