Abstract

Abstract. Long-term secular variation in seawater sulfate concentrations ([SO42−]SW) is of interest owing to its relationship to the oxygenation history of Earth's surface environment. In this study, we develop two complementary approaches for quantification of sulfate concentrations in ancient seawater and test their application to late Neoproterozoic (635 Ma) to Recent marine units. The "rate method" is based on two measurable parameters of paleomarine systems: (1) the S-isotope fractionation associated with microbial sulfate reduction (MSR), as proxied by Δ34SCAS-PY, and (2) the maximum rate of change in seawater sulfate, as proxied by &partial; δ 34SCAS/∂ t(max). The "MSR-trend method" is based on the empirical relationship of Δ34SCAS-PY to aqueous sulfate concentrations in 81 modern depositional systems. For a given paleomarine system, the rate method yields an estimate of maximum possible [SO42−]SW (although results are dependent on assumptions regarding the pyrite burial flux, FPY), and the MSR-trend method yields an estimate of mean [SO42−]SW. An analysis of seawater sulfate concentrations since 635 Ma suggests that [SO42−]SW was low during the late Neoproterozoic (<5 mM), rose sharply across the Ediacaran–Cambrian boundary (~5–10 mM), and rose again during the Permian (~10–30 mM) to levels that have varied only slightly since 250 Ma. However, Phanerozoic seawater sulfate concentrations may have been drawn down to much lower levels (~1–4 mM) during short (

Highlights

  • Oceanic sulfate plays a key role in the biogeochemical cycles of S, C, O, and Fe (Canfield, 1998; Lyons and Gill, 2010; Halevy et al, 2012; Planavsky et al, 2012)

  • An analysis of seawater sulfate concentrations since 635 Ma suggests that [SO24−]SW was low during the late Neoproterozoic (< 5 mM), rose sharply across the Ediacaran–Cambrian boundary

  • The upper limits on [SO24−]SW imposed by the rate method may have limited utility for assessment of Phanerozoic seawater sulfate, this method may be of greater value in analyzing Archean and Proterozoic seawater sulfate concentrations, which are thought to have been quite low (< 1 mM; Kah et al, 2004; Canfield et al, 2007; Planavsky et al, 2012)

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Summary

Introduction

Oceanic sulfate plays a key role in the biogeochemical cycles of S, C, O, and Fe (Canfield, 1998; Lyons and Gill, 2010; Halevy et al, 2012; Planavsky et al, 2012). The first method calculates a maximum possible [SO24−]SW based on a combination of two parameters that are readily measurable in most paleomarine systems: (1) the S-isotope fractionation between cogenetic sedimentary sulfate and sulfide ( 34SCAS-PY), and (2) the maximum observed rate of variation in seawater sulfate δ34S (∂δ34SCAS / ∂t) This rate-based method is an extension of earlier modeling work by Kump and Arthur (1999), Kurtz et al (2003), Kah et al (2004), Bottrell and Newton (2006), and Gill et al (2011a, b). Our results suggest that large-scale empirical relationships may exist that are not highly sensitive to influences such as organic substrate type, sulfate reduction rates, strain-specific fractionation, and other factors We envision such local influences, as they become more completely understood, being mapped onto, and integrated with, the broad first-order relationships documented in this study

The rate method
The MSR-trend method
Controls on fractionation by microbial sulfate reducers
General considerations and modeling protocol
Long-term variation in seawater sulfate concentrations
Conclusions
Relationship of rate of seawater sulfate change to sulfate residence time
Sources of sulfide δ34S data
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