Reduced sulfur in euxinic sediments of the Cariaco Basin: sulfur isotope constraints on organic sulfur formation

Abstract

Reduced sulfur accumulation in Holocene and latest Pleistocene euxinic marine sediments from the Cariaco Basin, Venezuela, was investigated to constrain the timing and possible pathways of organic matter (OM) sulfurization. Data were collected for a diverse suite of sulfur species, including concentrations and sulfur isotope compositions of pore-water sulfide, pore-water sulfate, pyrite sulfur, total organic sulfur (TOS), kerogen sulfur (KS), and polar bitumen sulfur (PBS). Results suggest that there was a period during which almost no diagenetic pyrite formed in the sediments of the Cariaco, coincident with a shift from high to lower sedimentation rates and a concomitant change in the delivery of organic matter to the sediments. The sulfur isotope composition of organic matter was predicted based on assumed pathways using weighted isotopic mass balance calculations and compared to measured isotope values for organic sulfur. These results indicate that organic sulfur is derived primarily from pore-water sulfide, with minor contributions from primary bio-sulfur (e.g., in proteins derived from algae and bacteria). The predicted sulfur isotope values of organic sulfur compounds (OSC) suggest that pore-water sulfide is the ultimate source of reduced sulfur for incorporation into organic matter. It is possible, however, that reactive sulfur intermediates such as elemental sulfur or polysulfides react directly with organic matter. These intermediate sulfur species are likely formed through partial oxidation of sulfide by anaerobic sulfide-oxidizing microbes living in the sediments.