Anomalous concentrations of iron sulfides found at roll fronts are believed to result from limited oxidation and mobilization of reduced sulfur species from earlier formed pyrite within the more extensively oxidized core of the roll. Laboratory experiments and chemical theory suggest that the reactions need not be biogenic, and that the sulfur of the reconstituted pyrite could be isotopically indistinguishable from biogenic sulfur. Sulfite formed by limited oxidation slowly decomposesto sulfate and sulfides, and because the sulfate-producing reaction is irreversible at low temperature, only the reduced sulfur species are available for further oxidation-reduction reactions.

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