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

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• 作者：William P. Gilhooly III^a ; ^b ; ^{wgilhool@iupui.edu" class="auth_mail" title="E-mail the corresponding author} ; Christopher T. Reinhard^a ; ^c ; Timothy W. Lyons^a
• 关键词：Anoxygenic photosynthesis ; Photic Zone Euxinia ; Sulfur and oxygen isotopes ; Sulfate reduction rates ; Mahoney Lake
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 September 2016
• 年：2016
• 卷：189
• 期：Complete
• 页码：1-23
• 全文大小：1113 K

文摘

Mahoney Lake is a permanently anoxic and sulfidic (euxinic) lake that has a dense plate of purple sulfur bacteria positioned at mid-water depth (∼7 m) where free sulfide intercepts the photic zone. We analyzed the isotopic composition of sulfate (δ³⁴S_SO4 and δ¹⁸O_SO4), sulfide (δ³⁴S_H2S), and the water (δ¹⁸O_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 (δ³⁴S_SO4-H2S = 51‰) and within pore waters along the oxic margin (δ³⁴S_SO4-H2S > 50‰) 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 (δ³⁴S_SO4-H2S = 37‰), 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 δ¹⁸O_SO4–δ³⁴S_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 δ¹⁸O_SO4–δ³⁴S_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.