THE ROLE OF ORGANIC SULFUR IN GLOBAL SULFUR CYCLING: LINKS TO INORGANIC SULFUR AND MICROBIAL PROCESSES

The importance of organic sulfur (OS) to global sulfur cycling is only beginning to be recognized. For example, the sequestration of various forms of reduced sulfur in sediments is a significant sink in the coupled global biogeochemical cycles of C, S, and O, thereby regulating atmospheric CO2 and O2 concentrations on geological time scales. OS is thought to be the second largest pool of reduced sulfur in sediments after pyrite, but has generally been neglected in models of the C, S, and O cycles. In addition, the sulfurization of organic matter (OM) during diagenesis has been shown to remove that OM from the reactive pool, thereby preserving functionalized biomarker compounds in sediments and affecting molecularly based paleoenvironmental reconstructions. Finally, OS in petroleum products lead to increased costs of refining and environmental damage. Thus, it is clear that we need a better understanding of the processes involved in OS formation, as well as its interactions with inorganic sulfur species and microbial processes in the natural environment. One reason that our understanding of the processes associated with OS formation remains incomplete is that the cycling of reduced sulfur in the sedimentary environment is a complex set of biogeochemical processes, comprised of purely inorganic reactions, organic/inorganic reactions, purely biological reactions, and biologically mediated inorganic and organic reactions. Furthermore, the extremely reactive nature of many key intermediate inorganic sulfur species makes sampling and storage difficult. Recent analytical advances, particularly in the area of stable sulfur isotopic analysis, have provided avenues of investigation previously unavailable. The measurement of the sulfur isotope composition of various forms of OS, particularly of specific compounds, in conjunction with the measurement of the sulfur isotope composition of inorganic sulfur species has enhanced our understanding of the complex role of OS in sedimentary sulfur cycling. This presentation will review our knowledge of the processes surrounding OS formation, its relevance to inorganic and microbial processes in the sedimentary and aqueous environments, and potential impacts on the global sulfur cycle.