A comprehensive sulfur and oxygen isotope study of sulfur cycling in a shallow, hyper-euxinic meromictic lake

Mahoney Lake is a permanently anoxic and sulfidic (euxinic) lake that has a dense plate of purple sulfur bacteria positioned at mid-water depth (similar to 7 m) where free sulfide intercepts the photic zone. We analyzed the isotopic composition of sulfate (delta S-34(SO4) and delta O-18(SO4)), sulfide (delta S-34(H2S)), and the water (delta O-18(H2O)) to track the potentially coupled processes of dissimilatory sulfate reduction and phototrophic sulfide oxidation within an aquatic environment with extremely high sulfide concentrations (> 30 mM). Large isotopic offsets observed between sulfate and sulfide within the monimolimnion (delta S-34(SO4-H2S) = 51 parts per thousand) and within pore waters along the oxic margin (delta S-34(SO4-H2S) > 50 parts per thousand) are consistent with sulfate reduction in both the sediments and the anoxic water column. Given the high sulfide concentrations of the lake, sulfur disproportionation is likely inoperable or limited to a very narrow zone in the chemocline, and therefore the large instantaneous fractionations are best explained by the microbial process of sulfate reduction. Pyrite extracted from the sediments reflects the isotopic composition of water column sulfide, suggesting that pyrite buried in the euxinic depocenter of the lake formed in the water column. The offset between sulfate and dissolved sulfide decreases at the chemocline (delta S-34(SO4-H2S) = 37 parts per thousand), a trend possibly explained by elevated sulfate reduction rates and inconsistent with appreciable disproportionation within this interval. Water column sulfate exhibits a linear response in delta O-18(SO4)-delta S-34(SO4) and the slope of this relationship suggests relatively high sulfate reduction rates that appear to respond to seasonal changes in the productivity of purple sulfur bacteria. Although photosynthetic activity within the microbial plate influences the delta O-18(SO4)-delta S-34(SO4) relationship, the biosignature for photosynthetic sulfur bacteria is restricted to the oxic/anoxic transition zone and is apparently minor relative to the more prevalent process of sulfate reduction operative throughout the light-deprived deeper anoxic water column and sediment pore waters. (C) 2016 Elsevier Ltd. All rights reserved.