Extreme sulfur isotopic fractionation between sulfate of carbonate fluorapatite and authigenic pyrite in the Neocomian sequence at Wąwał, Central Poland

The isotopic study (S, O, C) of carbonate fluorapatite (CFA) and pyrite in the Neocomian sequence at Wąwał, Polish Lowland, provides a rare occasion to understand factors that controlled fractionation between these phases in the process of bacterial sulfate reduction (BSR). Sulfur isotopic composition of CFA-sulfate is moderately enriched in 34S, but pyrite is highly depleted in 34S with respect to initial Neocomian seawater. We found that the extent of sulfur fractionation between CFA sulfate and sulfur in accompanying framboidal sulfides is larger (≈70‰) than can be derived by bacterial activity (≈46‰). Isotopic composition of CFA-sulfate (δ18O(SO42−)) indicated that SO4–H2O equilibration could proceed relatively easily in early diagenetic events of Neocomian sediments, due to probable bacterial activity. We speculate how the isotopic data δ34S(SO42−), δ18O(SO42−), δ13C(CO32−) translates into the successive oxidation–reduction processes coupled to CFA and pyrite precipitation.