The sulfur (S) isotope difference between sulfates and sulfides preserved in sedimentary rocks (δ34S) has been utilized to reconstruct ancient marine sulfate levels with implications for oxygenation of the Earth surface and biogeochemical cycling. S isotope data from modern, low-sulfate euxinic systems illustrate that preserved δ34S values are positively correlated with sulfate concentration. However, absolute constraints on the range of low-sulfate levels over which preserved δ34S values vary with sulfate concentration remain poorly constrained. Here, we present a compilation of S isotope data for modern euxinic systems demonstrating that preserved δ34S values increase with sulfate concentration at low sulfate levels and approach values that are similar to in situ S isotope fractionation values from microbial sulfate reduction at high sulfate levels. We compare these results to a closed system model of S isotope cycling in a euxinic ocean in order to evaluate when the size of the sulfate reservoir is sufficiently small that Rayleigh fractionation affects the preservation of S isotope signatures. We conclude that the reservoir effect places constraints on δ34S values deposited in euxinic settings at sulfate concentrations <5mM. Thus, over this range, δ34S values can be used to evaluate ancient sulfate levels. At higher sulfate levels (>10mM), δ34S values are similar to the kinetic isotope fractionation due to microbial sulfate reduction and therefore provide information about biological and environmental controls on sulfate reduction rates and location of pyrite formation. The results of this compilation provide an improved model for the use of δ34S records to evaluate paleoenvironmental conditions in euxinic depositional environments.
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We are grateful to the authors of the studies included in this compilation; this work would not be possible without the wealth of previously published sulfur isotope data for modem euxinic systems. This work was improved by discussions with Neal Blair, Brian Kristall, Bradley Sageman, Andrew Masterson, and Matthew Jones. We would also like to thank David Fike, two anonymous reviewers, and Associate Editor James Farquhar for comments that greatly improved the manuscript. This work was supported by NASA Earth and Space Science Fellowship Program Grant Planet09F-0042 (Gomes), Northwestern University Graduate School Presidential Fellowship (Gomes), and National Science Foundation Grant EAR-0955969 (Hurtgen).
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