Decline in oceanic sulfate levels during the early Mesoproterozoic

Multiple-sulfur isotope compositions (S-32, S-33, S-34 and S-36) were analyzed for paired carbonate-associated sulfate (CAS) and disseminated pyrite (PY) from the similar to 1.6-Ga Gaoyuzhuang Formation of the North China Craton to reconstruct the history of sulfate levels in Proterozoic oceans. The 200-m-thick study interval yielded relatively constant values for delta S-34(CAS) (13.0 +/- 1.8 parts per thousand), delta S-34(PY) (8.0 +/- 2.3 parts per thousand), and Delta S-34(CAS-PY) (similar to 5 parts per thousand), as well as relatively constant Delta S-33 (0 +/- 0.05 parts per thousand) and Delta S-36 (0.35 +/- 0.15 parts per thousand) for both CAS and pyrite. Limited variation in delta S-34(PY) and slightly lower 6,33S of pyrite relative to CAS suggest water-column precipitation of pyrite. Limited fractionation of sulfur during microbial sulfate reduction (as documented by Delta S-36(CAS-PY)) implies low seawater sulfate concentrations in the early Mesoproterozoic ocean. We quantitatively constrained paleo-seawater [SO42-] using a novel modeling approach based on measured values of Delta S-36(CAS-PY) and partial derivative delta S-34(CAS)/partial derivative t(max). For the study unit, Delta S-34(CAS-PY) is 5.4 +/- 1.4 parts per thousand (n = 17), and partial derivative delta S-34(CAS)/partial derivative t(max) is 6.8-34 parts per thousand Myr(-1) based on sedimentation rates of 30-150 m Myr(-1). These data indicate early Mesoproterozoic seawater [SO42-] of similar to<0.1 to 0.35 mM (with a maximum possible concentration of 1.8 mM), a range that is lower and more tightly constrained than earlier estimates for the Mesoproterozoic. Compilation of published data suggests that low seawater sulfate concentrations began about similar to 1.7 Ga and persisted until at least the mid-Mesoproterozoic (similar to 1.4 Ga), documenting a distinct early Mesoproterozoic perturbation in ocean chemistry that may have been related to a decline in atmospheric pO(2) after Great Oxidation Event I. (C) 2014 Elsevier B.V. All rights reserved.