Reconstruction of secular variation in seawater sulfate concentrations

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, but quantitative approaches to analysis of this variation remain underdeveloped. In this study, we develop two complementary approaches for assessment of the [SO42–] of ancient seawater and test their application to reconstructions of [SO42–]SW variation since the late Neoproterozoic Eon (< 650 Ma). The first approach 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 ∂ δ34SCAS / ∂ t (max). This "rate method" yields an estimate of the maximum possible [SO42–]SW for the time interval of interest, although the calculated value differs depending on whether an oxic or an anoxic ocean model is inferred. The second approach is also based on Δ34SCAS-PY but evaluates this parameter against an empirical MSR trend rather than a formation-specific ∂ δ34SCAS / ∂ t (max) value. The MSR trend represents the relationship between fractionation of cogenetic sulfate and sulfide (i.e., Δ34Ssulfate-sulfide) and ambient dissolved sulfate concentrations in 81 modern aqueous systems. This "MSR-trend method" is thought to yield a robust estimate of mean seawater [SO42–] for the time interval of interest. An analysis of seawater sulfate concentrations since 650 Ma suggests that [SO42–]SW was low during the late Neoproterozoic (< 5 mM), rose sharply across the Ediacaran/Cambrian boundary (to ~ –10 mM), and rose again during the Permian to levels (~ 10–30 mM) 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 (&amp;lesssim; 2 Myr) intervals of the Cambrian, Early Triassic, Early Jurassic, and possibly other intervals as a consequence of widespread ocean anoxia, intense MSR, and pyrite burial. The procedures developed in this study offer potential for future high-resolution quantitative analyses of paleoseawater sulfate concentrations.