Abstract
Abstract The accumulation of oxygen in Earth’s atmosphere and oceans in the late Archean had profound implications for the planet’s biogeochemical evolution. Oxygen impacts sulfur cycling through the oxidation of sulfide minerals and the production of sulfate for microbial sulfate reduction (MSR). The isotopic signature of sulfur species preserved in the geologic record is affected by the prevailing biological and chemical processes and can therefore be used to constrain past oxygen and sulfate concentrations. Here, in a study of a late Archean analogue, we find that the sulfur isotopic signature in the water column of a seasonally stratified lake in southern China is influenced by MSR, whereas model results indicate that the isotopic signature of the underlying sediments can be best explained by concurrent sulfate reduction and sulfide oxidation. These data demonstrate that small apparent sulfur isotope fractionations (δ34Ssulfate-AVS = 4.2‰–1.5‰; AVS—acid volatile sulfides) can be caused by dynamic sulfur cycling at millimolar sulfate concentrations. This is in contrast to current interpretations of the isotopic record and indicates that small fractionations do not necessarily indicate very low sulfate or oxygen.
Highlights
Evidence for oxygen production in the oceans dates from 2.8 b.y. ago (Farquhar et al, 2011), oxygen did not significantly accumulate in the atmosphere until the Great Oxidation Event (GOE) 2.4 b.y. ago (Farquhar and Wing, 2003; Bekker et al, 2004)
One way of constraining oxygen levels is through the stable isotope composition of sulfur preserved in the geologic record, as increasing atmospheric oxygen concentrations led to an increase in oxidative weathering processes that enhanced the delivery of sulfate to the oceans and allowed a more dynamic, microbially driven sulfur cycle
This approach assumes that the isotopic signature formed in the water column is preserved in the sediment, and has led to estimates of Archean seawater sulfate concentrations of a few to hundreds of micromolar (Crowe et al, 2014; Habicht et al, 2002)
Summary
Alyssa J. Findlay1,2*†, Valeria Boyko1, André Pellerin2, Khoren Avetisyan1, Qingjun Guo3*†, Xi Yang3, and Alexey Kamyshny, Jr.1 1Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, 653 Beer Sheva, Israel 2Center for Geomicrobiology, Department of Bioscience, Aarhus University, 8000 Aarhus C, Denmark 3State Key Laboratory of Resources and Environmental Information Systems, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China
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