Formation and diagenesis of macromolecular organic sulfur in Peru margin sediments

The geochemistry of organic sulfur in a suite of samples covering the upper 100 m of sediment from the Peruvian margin has been characterized by elemental analysis, X-ray absorption (XANES) spectroscopy and analytical pyrolysis. These data are compared with those obtained from a thermally immature, but consolidated, sulfur-rich sediment (Miocene Monterey Fm, CA). Results indicate that sulfur incorporation into organic matter takes place primarily within the upper few meters of sediment, but sulfur “in-growth” continues at greater depths. The major initial products are organic sulfides, with subordinate amounts of thiophenes. Sulfonic acids also comprise a significant portion of the organically-bound sulfur in the shallowest sediments. Analysis of chemically defined organic matter sub-fractions isolated from a surficial (<1 m) sample revealed no consistent relationship between molecular size and the proportion of sulfide bridges. In contrast, a strong correlation between these two parameters was observed for the Monterey Fm sample, implying that the degree of sulfur cross-linking may, in part, control molecular size and/or solubility. These findings have implications concerning the proposed role of sulfur in organic matter preservation through quenching selected labile biochemicals. The lack of any link between the formation of intermolecular sulfide bridges and the principal zone for remineralization of organic matter in sediments (i.e. the upper 1 m) suggests this process has only minor influence on the efficiency of organic carbon burial. Thus, if sulfur plays a role in the preservation of labile molecules, a different process must be responsible in Peru sediments. The relative proportion of kerogen-derived thiophenic pyrolysis products increases significantly over the 100 m depth interval and in the deepest samples approaches those of the Monterey sample. The distribution of thiophenic pyrolysis products in the deepest Peru sample also closely resembles that of the Monterey kerogen suggesting the presence of similar sulfur-bound (lipid) building blocks. Subtle differences include a higher proportion of products held to derive from steroid carbon skeletons in the Peru kerogens and a more significant contribution of products originating from osoprenoid carbon skeletons in the Monterey sample. The close similarity in organic sulfur composition for these samples lends support for the premise that the Peru margin is a modern analogue of the Monterey Fm