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 characterize d by elemental analysis, X-ray absorption (XANES) spectroscopy and ana lytical pyrolysis. These data are compared with those obtained from a thermally immature, but consolidated, sulfur-rich sediment (Miocene Mo nterey Fm, CA). Results indicate that sulfur incorporation into organi c matter takes place primarily within the upper few meters of sediment , but sulfur ''in-growth'' continues at greater depths. The major init ial products are organic sulfides, with subordinate amounts of thiophe nes. Sulfonic acids also comprise a significant portion of the organic ally-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 co ncerning the proposed role of sulfur in organic matter preservation th rough quenching selected labile biochemicals. The lack of any link bet ween the formation of intermolecular sulfide bridges and the principal zone for remineralization of organic matter in sediments (i.e. the up per 1 m) suggests this process has only minor influence on the efficie ncy of organic carbon burial. Thus, if sulfur plays a role in the pres ervation of labile molecules, a different process must be responsible in Peru sediments. The relative proportion of kerogen-derived thiophen ic pyrolysis products increases significantly over the 100 m depth int erval and in the deepest samples approaches those of the Monterey samp le. The distribution of thiophenic pyrolysis products in the deepest P eru sample also closely resembles that of the Monterey kerogen suggest ing the presence of similar sulfur-bound (lipid) building blocks. Subt le differences include a higher proportion of products held to derive from steroid carbon skeletons in the Peru kerogens and a more signific ant contribution of products originating from isoprenoid carbon skelet ons in the Monterey sample. The close similarity in organic sulfur com position for these samples lends support for the premise that the Peru margin is a modern analogue of the Monterey Fm.