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This study uses selective extractions in combination with S K-edge X-ray absorption near-edge structure (XANES) spectroscopy to investigate the influence of eutrophic conditions on reduced inorganic S and organic S speciation in estuarine sediments. In order to address this objective, we carried out a detailed geochemical analysis of porewater and solid phase parameters on sediment transects collected from a eutrophic estuary located in Western Australia. Our results show that the study system, which is typical in terms of eutrophic estuarine conditions, contained anomalously high concentrations of sedimentary Fe monosulfide (evident by chemical extraction and XANES spectroscopy), with a median acid-volatile sulfide (AVS) concentration of 191μmol/g, and a range of 34–335μmol/g. We observed a dominance of Fe monosulfide over other S species, as evident by a generally high ratio of AVS to total S …