Formation of secondary carbonates and native sulphur in sulphate-rich Messinian strata, Sicily

Abstract

Microbially formed authigenic carbonates accompanied by native sulphur are present in the ‘Calcare Solfifero’ below a thick succession of gypsum deposited during the Messinian salinity crisis in Sicily. We sampled these carbonates and associated sulphur in five former sulphur mines to subject them to a detailed petrographic and geochemical study in order to explore their different modes of formation. Native sulphur formed in conjunction with microbial sulphate reduction,which is reflected in its depletion in 34S (δ34Svaluesas lowas−2‰vs. V-CDT) and an enrichment of 34S in the residual sulphate (δ34S values as high as+61‰). The oxidation of organic matter by sulphate reduction increased alkalinity, inducing precipitation of secondary carbonate minerals. A set of authigenic limestones lacking sulphate minerals, but characterized by pseudomorphs after gypsumand high δ18O values (as high as +9‰ vs. V-PDB) reflects syngenetic mineral formation within evaporitic settings. Low δ13C values (as low as −52‰ vs. V-PDB) reveal that these carbonate phases were formed by microbial sulphate reduction coupled to the oxidation of biogenic methane. Another set of authigenic carbonates that replaced sulphate minerals is typified by lowδ18O values (as lowas−4‰). These carbonates formed epigenetically during later diagenesis following compaction. Dissolution of gypsum or anhydrite by meteoric waters delivered the sulphate for microbial sulphate reduction. Lowcarbon isotope values of these carbonates (−29 to−5‰) indicate that carbonatewas derived fromthe oxidation of crude oil and possibly minormethane, partly involving different degrees of admixture of dissolved carbonate from other sources. Although the studied rocks with their vast amounts of secondary carbonate minerals and sulphur seem to indicate a similar genesis at first glance – having formed by biogeochemical transformations of sulphate and hydrocarbons – this study reveals that these processes can occur at different times in variable geological environments.