On the chemical properties of sedimentary sulfur in estuarine environments

The composition of iron, sulfur and organic matter and their distributions with depth have been used as sensitive indicators of diagenetic processes occurring in anoxic sediments derived from the Authie and Seine bays in northern France. The contents of acid volatile sulfurs (AVS), chromium reduced sulfurs (CRS), total organic carbon and reactive Fe present in the sedimentary solid phase have been reported. These investigations have revealed that degree of pyritization (DOP) values remain nearly constant in the two sites, suggesting that pyrite formation is limited by the reactivity/availability of iron in the bulk sediment; whereas neither AVS or organic matter are important determinants of the degree of pyritization. The elemental composition of humic acids extracted from these anoxic sediments has also been examined with X-ray photoelectron spectroscopy (XPS). Diagenetic enrichment of sedimentary organic matter with sulfur occurs with depth in Authie Bay sediments, as evidenced by the XPS ratios S/C, whereas a depletion takes place with depth in Seine Bay sediments. Such a phenomenon results mainly from the availability of inorganic reduced sulfurs generated through bacterial sulfate reduction along the sedimentary column and, to a lesser extent, from the absence or weak re-supply of “active organic matter”.