Abstract A closed sulfuretum was created in situ in duplicate box systems at a water depth of 10 m in the Baltic Sea. The isotopic fractionation of sulfur was documented during the course of the experiment as sulfur was cycled through several oxidation stages. Two contemporary fractionation cycles are postulated based on simulation models. The postulated mechanism may explain the anomalous negative δ34S values of sulfide in the natural environment that are reported in the literature. The kinetic fractionation constant of sulfate reduction was estimated to be — 1.020. Based on the most representative simulation model (model B), fractionation could not be verified either for the oxidation of sulfide to intracellular sulfur or for the oxidation of intracellular sulfur to sulfate.
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