Reconstruction of secular variation in seawater sulfate concentrations

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 Δ³⁴S_CAS-PY, and (2) the maximum rate of change in seawater sulfate, as proxied by &partial; δ ³⁴S_CAS/∂ t(max). The "MSR-trend method" is based on the empirical relationship of Δ³⁴S_CAS-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, F_PY), 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 (<~2 Myr) intervals of the Cambrian, Early Triassic, Early Jurassic, and Cretaceous 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 paleo-seawater sulfate concentrations.