COMPOUND-SPECIFIC SULFUR ISOTOPE CONSTRAINTS ON THE PATHWAY(S) OF DIAGENETIC SULFURIZATION OF ORGANIC MATTER

The sequestration of various forms of reduced sulfur in sediments has been shown to be a significant sink in the coupled global biogeochemical cycles of C, S, and O. Organic sulfur is thought the second largest pool of reduced sulfur in sediments, after pyrite. In addition, the formation of organic sulfur compounds during diagenesis has been shown to preserve functionalized biomarker compounds in sediments on geological timescales, thereby affecting molecularly based paleoenvironmental reconstructions. Despite such importance, however, the pathways of organic matter sulfurization are not well constrained. It is generally thought that organic sulfur is derived ultimately from pore water sulfide (or water column sulfide), but whether incorporated directly or via some other reactive intermediate (such as polysulfides or elemental sulfur) is a subject of current debate. One reason for this debate is that the cycling of reduced sulfur in the sedimentary environment is an extremely complex biogeochemical process, comprised of purely inorganic reactions, biological reactions, and biologically mediated inorganic reactions. Sulfur isotope systematics is one tool that is useful for deconvolving the myriad of sulfur-related reactions occurring in the sedimentary environment. Until recently, however, studies of sedimentary sulfur isotope systematics were limited by the ability to measure bulk forms of the isotopic comosition of reduced sulfur (such as pyrite sulfur or total organic sulfur). Herein we present results from the first ever measurements of the sulfur isotopic composition of specific biomarker compounds. In addition, we compare measured compound specific sulfur isotope values with those predicted based on theoretical relationships and reactions rates as determined for the sulfurization of organic compounds in sediments of the euxinic Cariaco Basin. Discrepancies between measured and predicted compound-specific sulfur isotopic compositions suggest that the sulfur incorporated into organic matter is probably derived from reactive intermediates, such as polysulfides.