Stable carbon isotopic composition of biogenic carbonates

Abstract

The geology and sulfur isotopic composition of epigenetic pyrite of the Gas Hills district, Wyoming, indicate a bacterial origin for these sandstone-type uranium deposits. The δC 13 value of epigenetic calcite associated with organic materials, however, is heavier than the isotopic composition of calcite of biogenic native sulfur deposits. Stratigraphic and carbon isotopic evidence suggest that contamination by sedimentary carbonate is minimal in the Gas Hills samples. Field and laboratory data show that anaerobes produce isotopically heavier CO 2 and lighter methane than the associated organic material which they consume as an energy source. The δC 13 values of the salt-dome calcites overlap the methane field, the Gas Hills values overlap the CO 2 field, and analyses of Sicilian Sulfur limestones are intermediate. Deposits Gas Hills (calcites with organic material) Gulf Coast Cap rock Sicilian Sulfur limestone (gyngenetic calcites) Number of samples 20 35 27 Mean δC 13 −22.5 −37.9 −28.7 Sample standard deviation 4.0 6.7 8.9 Presumably a spectrum of values could be obtained by the oxidation of methane and contamination by the original bacterial CO 2 and sedimentary carbonate. The quantized values of the three deposits are, therefore, evidence of strong geologic control. Methane was stable under the reducing conditions (shown by the presence of uraninite and sulfides) of the Gas Hill, while methane was concentrated and oxidized in the cap rock of salt-domes (mineralogy includes native sulfur), and neither methane nor CO 2 was preferentially lost during the syngenetic precipitation of the Sicilian Sulfur limestone.