Seasonal changes in sulfur biogeochemistry of a dilute, dimictic Arctic lake: Implications for paired sulfur isotope records from ancient oceans

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© 2018 Elsevier B.V. Calibration of ancient marine sulfate levels has been done largely using experimental studies of the kinetic isotope effect associated with microbial sulfate reduction and work from modern ocean basins or high-sulfate lakes that are largely restricted and meromictic. The sulfur isotope record of sulfates and sulfides from sedimentary sequences have been used to reconstruct sulfate levels throughout the geologic record and indicate that sulfate concentrations were low relative to modern oceans (28 mM) for most of Earth's history. Despite the higher potential for modern low-sulfate systems to be a better analog for ancient oceans, there are few sulfur isotope studies that have been carried out in these environments. Here, we present δ34S systematics of a modern low-sulfate (~330–600 μM) euxinic lake on the ice-free margin of southwestern Greenland. We find large isotope fractionations (>20‰) between water column sulfate and sulfides, with this fractionation increasing from 23.9‰ during open-water conditions to 42.0‰ under annual ice-cover. While these large kinetic isotope effects associated with microbial sulfate reduction (εSR) are expressed in the water column of this lake, the underlying sedimentary sulfides preserve a notably smaller range of δ34S values (13.0–26.6‰). Geochemical modeling of our data suggests that the δ34S of water column sulfate and sulfides, along with sedimentary sulfides are primarily controlled by εSR and a reservoir effect established under strong thermal stratification during open-water conditions. Under ice-covered conditions εSR appears to be the dominant control on δ34S values, and when combining both seasonal data sets εSR increases as a function of sulfate levels in the lake. Sulfur isotopic data presented here highlight the complexity of seasonal biogeochemical cycling of sulfur in low-sulfate systems, and how system openness affects δ34S fractionations in modern euxinic depositional environments. Specifically, a reservoir effect is only documented in the water column δ34S data under open-water conditions, however a reservoir effect is apparent from sedimentary sulfides under both ice-cover and open-water conditions due to time averaging pyrite δ34S values. Our results add to the growing body of work on modern low-sulfate systems to help further constrain the use of ancient δ34S records for paleoenvironmental reconstructions.

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Chemical Geology

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