The isotopic composition of native sulfur from deposits in the late Miocene gypsum-sulfur series of Sicily (Italy), an extensively deformed lagoonal evaporitic sequence of diatomite, limestone, gypsum, and marl beds, indicates enrichment in S 32 characteristic of the isotope fractionation occurring during reduction of sulfate to H 2 S by bacterial action. Corroborative evidence for biogenic origin of the deposits is provided by delta C 13 determinations which show an enrichment of C 12 in the carbonate of the sulfur-bearing limestone relative to that of over- and underlying sulfur-barren horizons. Oxidation of the H 2 S to form native sulfur is considered to have been essentially contemporaneous with sedimentation in a restricted marine environment where reducing conditions obtained near the bottom, but whose waters were more oxygenated in the upper zone.

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