Organic Matter Sulfurization in the Water Column and Its Effect on Sedimentary Processes

An essential characteristic of marine organic matter (OM) is its lability, which is typically taken to mean its susceptibility to remineralization by microorganisms. OM that is more highly condensed or polymerized is less available for heterotrophs and is therefore associated with lower rates of microbial metabolisms and inorganic carbon release than fresh, lower molecular weight material. Accordingly, understanding the mechanisms driving the transformation of OM into recalcitrant ‘proto-kerogen’ will greatly improve our understanding of sedimentary metabolic rates and carbon burial. One important set of reactions driving proto-kerogen formation in O₂-limited environments are abiotic reactions between sulfide and/or polysulfides and functionalized OM. We investigate this process in sinking particles from the modern Cariaco basin. We find evidence for rapid sulfurization in the water column based on the concentrations and sulfur-isotopic compositions of organic fractions and individual organosulfur compounds. More extensive sulfurization in Cariaco Basin is associated with high OM export from the surface ocean, low terrestrial inputs, and high concentrations of both elemental S and the dominant non-polar organosulfur compound, C₂₀ thiophene. Depending on the frequency of high OM export events, we estimate that water column sulfurization delivers roughly half of the total organic S present at 5 cm depth in underlying sediments. We compare these data to results from Santa Barbara Basin and find that similar processes may be responsible for low rates of heterotrophy in this environment. OM sulfurization during sedimentation and very early diagenesis will have many implications for biogeochemical cycles in sediments during later diagenesis and burial.