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Abstract

Despite the recognition that both organic sulfur and pyrite form during the very early stages of diagenesis, and that the amount of H2S generated in bacterial sulfate reduction primarily limits their formation, the mechanisms and the active species involved still are not clear. In this study, we quantified the major forms of sulfur distributed in sediments to assess the geochemical mechanisms involved in these transformations. XANES spectroscopy, together with elemental analysis, were used to measure sulfur speciation in the organic-rich sediments from the Bay of Concepcion, Chile. Organic polysulfides constituted the major fraction of the organic sulfur, and occurred maximally just below the sediment surface (1-3 cm), where intermediates from H2S oxidation were likely to be generated most abundantly. Sulfonates, which could be formed through the reactions of sulfite and thiosulfate, also showed a sub-surface maximum in the vicinity of the ''oxic-anoxic interface''. These results strongly suggest a geochemical origin for organic polysulfides and sulfonates, and illustrate that intermediates from H2S oxidation play a dominant role in incorporating sulfur into organic matter. Pyrite was absent in the surficial layer, and first appeared just below the H2S maximum, where organic polysulfides began to decrease in abundance. From these results, we argue, that an iron monosulfide precursor formed first from reactions with H2S, and then reacts with organic polysulfides, completing the synthesis of pyrite in the sediment column.