Reconstruction of secular variation in seawater sulfate concentrations

[摘要] Long-term secularvariation in seawater sulfate concentrations ([SO₄²⁻]_SW) isof interest owing to its relationship to the oxygenation history of Earth'ssurface environment. In this study, we develop two complementary approachesfor quantification of sulfate concentrations in ancient seawater and testtheir application to late Neoproterozoic (635 Ma) to Recent marine units.The "rate method" is based on two measurable parameters of paleomarinesystems: (1) the S-isotope fractionation associated with microbial sulfatereduction (MSR), as proxied by Δ³⁴S_CAS-PY, and (2) themaximum rate of change in seawater sulfate, as proxied by &partial; δ³⁴S_CAS/∂ t(max). The "MSR-trend method" isbased on the empirical relationship of Δ³⁴S_CAS-PY toaqueous sulfate concentrations in 81 modern depositional systems. For a givenpaleomarine system, the rate method yields an estimate of maximum possible[SO₄²⁻]_SW (although results are dependent on assumptionsregarding the pyrite burial flux, F_PY), and the MSR-trend methodyields an estimate of mean [SO₄²⁻]_SW. An analysis ofseawater 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 havevaried only slightly since 250 Ma. However, Phanerozoic seawater sulfateconcentrations may have been drawn down to much lower levels(~1–4 mM) during short (<~2 Myr) intervals of theCambrian, Early Triassic, Early Jurassic, and Cretaceous as a consequence ofwidespread ocean anoxia, intense MSR, and pyrite burial. The proceduresdeveloped in this study offer potential for future high-resolutionquantitative analyses of paleo-seawater sulfate concentrations.

[学科分类] 地球化学与岩石