SULFURIZATION PATHWAYS IN MODERN EUXINIC LAKES: EXPLORING THE KEY CONSTRAINTS FOR ORGANO-SULFUR AND PYRITE FORMATION

Reduced sulfur (S) undergoes inorganic and organic reactions within water column and sediments. Studies have focused on modern euxinic lakes where S isotopes can potentially differentiate the two competitive sulfurization pathways that lead to pyrite and organo-S compounds (OSCs) formation, characteristic constituents of anoxic/euxinic environments. Mahoney Lake, Canada and Green Lake, NY, contain productive communities of phototrophic purple S bacteria and sulfate-reducers that drive an active S cycle. The variability in the levels of sulfate, sulfide and organic matter (OM) between the two lakes can help elucidate the parameters that influence these sulfurization reactions and further S burial. Despite the different geochemical characteristics between both lakes, the Δ³⁴S_SO4-H2S is identical within the water column (~50‰), expressing the dissimilatory sulfate reduction. Within the Mahoney Lake sediments, the δ³⁴S of pyrite and total organic sulfur (TOS) decrease with depth, supporting no diagenetic effects. The isotope offset between pyrite and TOS also changes downcore from ~15‰ down to nearly zero. In contrast, the corresponding δ³⁴S values within the surface sediments of Green Lake increase downcore with relative constant isotope offsets. We suggest thus that the hyper-euxinic environment of Mahoney Lake has changed through time including changes in the Fe availability, the nature of OM reactive to S and the relative kinetic rates between pyrite and OSCs formation. These parameters influence the partitioning of reduced S in these two pools of S burial and their S isotope compositions. Geochemical kinetic modeling evaluated the dominating pathway by varying the environmental parameters (C_org, H₂S, S₈, Fe) of the system. Initial results show that only significant levels of labile organic carbon (e.g. citral) can trigger organic matter sulfurization rates in levels similar to those of pyritization. Similar kinetic rates potentially lead to the incorporation of an isotopically similar S inorganic source to pyrite and TOS during the same timescales. FT-ICR-MS analysis of DOM in Mahoney water column revealed an abundant and diverse array of OSCs suggesting that rapid sulfurization of OM can occur syngenetically. We continue to explore constraints on the isotopic compositions of pyrite and TOS pool.