Diurnal sulfur isotope patterns in a stratified euxinic lake

Abstract:

The distribution of sulfur isotopes in the environment is controlled by fractionations imparted during microbial sulfate reduction that commonly range between 19‰ and 66‰. In addition, microbial sulfide oxidation and subsequent disproportionation of intermediate phases of sulfur may contribute to the net isotopic offset observed between sulfate and sulfide. Thus, it has been proposed that the cycling of sulfur, comprising repeated cycles of reduction, oxidation, and disproportionation, is responsible for producing offsets up to 65‰ between the δ³⁴S values of sulfate and sulfide. Such large fractionations are often observed in nature though disproportionation is not necessarily required to produce such large offsets. To address these questions, we report on initial results from inorganic sulfur cycling in the water column and pore waters of euxinic Fayetteville Green Lake (FGL), New York. Water column samples were collected during the day when phototrophic sulfide oxidation is likely active and during the night when chemolithotrophy is likely operative. The sulfur isotopic offsets between sulfate and sulfide at the sediment water interface (57‰) are strikingly similar to those reported for the bottom waters of FGL (55‰; Zerkle et al., GCA, 4953-4970, 2010). The offset increases to nearly 70‰ at 25 cm within the sediments, suggesting ongoing microbial sulfur cycling or possibly differential diffusion during sulfate reduction within the sediments. We will compare the day/night isotope patterns with those of the pore waters to focus on the net isotope effects of sulfate reduction and sulfide oxidation.