Sulphur Isotopes and Microbial Sulphur Cycling in Sediments

Abstract

The relative abundances of the main isotopes of sulphur (325, 34S) in sediments are influenced only by kinetic fractionations, which principally occur during the the formation of H25 by microbial sulphate reduction. The isotopic composition of the H25 being produced at any instant varies with rates of oxidation recycling of sulphate (which determine the maximum fractionation) and rates of sulphate supply relative to rates of sulphate reduction (which determine the isotopic composition of the sulphate being reduced).The isotopic composition of seawater sulphate fluctuates over geological time, consistent with variations in the amounts of sulphate removed to form pyrite by microbial sulphate reduction. Analogous variations are seen in the composition of marine pyrite through time. By contrast variations in the isotopic composition of sulphate oxygen are affected by sulphate reduction and also by sulphid oxidation (since oxygen can be supplied both from atmospheric 02 or water). Future research is likely to focus increasingly on the role of oxidation processes in modifying sulphur isotope and sulphate oxygen isotope signatures.