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

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 thesecarbonates and associated sulphur in five former sulphur mines to subject them to a detailed petrographic andgeochemical study in order to explore their different modes of formation. Native sulphur formed in conjunctionwith 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 matterby sulphate reduction increased alkalinity, inducing precipitation of secondary carbonate minerals. A set ofauthigenic limestones lacking sulphate minerals, but characterized by pseudomorphs after gypsumand high δ18Ovalues (as high as +9‰ vs. V-PDB) reflects syngenetic mineral formation within evaporitic settings. Low δ13Cvalues (as low as −52‰ vs. V-PDB) reveal that these carbonate phases were formed by microbial sulphatereduction coupled to the oxidation of biogenic methane. Another set of authigenic carbonates that replacedsulphate minerals is typified by lowδ18O values (as lowas−4‰). These carbonates formed epigenetically duringlater diagenesis following compaction. Dissolution of gypsum or anhydrite by meteoric waters delivered thesulphate for microbial sulphate reduction. Lowcarbon isotope values of these carbonates (−29 to−5‰) indicatethat carbonatewas derived fromthe oxidation of crude oil and possibly minormethane, partly involving differentdegrees of admixture of dissolved carbonate from other sources. Although the studied rocks with their vastamounts of secondary carbonate minerals and sulphur seem to indicate a similar genesis at first glance – havingformed by biogeochemical transformations of sulphate and hydrocarbons – this study reveals that theseprocesses can occur at different times in variable geological environments.