Microbially induced sulphur and carbonate authigenesis in Messinian sulphate-rich strata

Authigenic carbonates accompanied by native sulphur are present within the sulphate-rich Messinian deposits in Sicily and southern Spain. Large sulphur isotope fractionation between sulphate and sulphur (as high as 60) demonstrates the major role of microbial sulphate reduction for sulphur precipitation. Bicarbonate for carbonate precipitation is derived from crude oil carbon but also from methane, as reflected by low carbon isotope values (as low as -52 ). The former presence of archaea and bacteria, which mediated the sulphate-dependant anaerobic oxidation of methane within the methane-influenced deposits, was validated by specific 13C-depleted molecular lipids. In lacustrine settings as the Hellín basin in southern Spain sulphate for microbial sulphate reduction is provided by weathering of gypsum deposits in the underlying basement. Here the oxidation of sulphide, produced by sulphate reducing microorganisms, to native sulphur was most likely mediated by sulphur bacteria.