Computer Science
Scientific paper
Jul 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.1991c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 7, pp.1991-2010
Computer Science
14
Scientific paper
Late Devonian (Frasnian) abiotic marine calcite has been microsampled and analyzed for 87 Sr / 86 Sr ratios, 18 O and 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 18 O values (-4.8%. ± 0.5, PDB), 13 C values (+2.0 to +3.0%., PDB), 87 Sr / 86 Sr ratios (0.70805 ± 3), and Sr/Mg weight ratios (0.04 to 0.05). Marine 87 Sr / 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 18 O values which are 3 to 4%. lower than those of modern marine cements. These low 18 O values are best explained by precipitation from 18 O-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 D Sr on Mg content was based on a compilation of Sr and Mg data from Holocene abiotic marine calcite ( D Sr = 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% MgCO 3 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.
Carpenter Scott J.
Halliday Alex N.
Holden Peter
Huston Ted J.
Lohmann Kyger C.
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