Decline in oceanic sulfate levels during the early Mesoproterozoic

Abstract

Multiple-sulfur isotope compositions ([superscript 32]S, [superscript 33]S, [superscript 34]S and [superscript 36]S) were analyzed for paired carbonate-associated sulfate (CAS) and disseminated pyrite (PY) from the ∼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 δ[superscript 34]S[subscript CAS] (13.0 ± 1.8‰), δ[superscript 34]S[subscript PY] (8.0 ± 2.3‰), and Δ[superscript 34]S[subscript CAS-PY] (∼5‰), as well as relatively constant Δ[superscript 33]S (0 ± 0.05‰) and Δ[superscript 36]S (0.35 ± 0.15‰) for both CAS and pyrite. Limited variation in δ[superscript 34]S[subscript PY] and slightly lower Δ[superscript 33]S of pyrite relative to CAS suggest water-column precipitation of pyrite. Limited fractionation of sulfur during microbial sulfate reduction (as documented by Δ[superscript 34]S[subscript CAS-PY]) implies low seawater sulfate concentrations in the early Mesoproterozoic ocean. We quantitatively constrained paleo-seawater [SO[subscript 4][superscript 2−]] using a novel modeling approach based on measured values of Δ[superscript 34]S[subscript CAS-PY] and ∂δ[superscript 34]S[subscript CAS]/∂t(max). For the study unit, Δ[superscript 34]S[subscript CAS-PY] is 5.4 ± 1.4‰ (n = 17), and ∂δ[superscript 34]S[subscript CAS]/∂t(max) is 6.8–34‰ Myr[superscript −1] based on sedimentation rates of 30–150 m Myr[superscript −1]. These data indicate early Mesoproterozoic seawater [SO[subscript 4][superscript 2−]] of ∼<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 ∼1.7 Ga and persisted until at least the mid-Mesoproterozoic (∼1.4 Ga), documenting a distinct early Mesoproterozoic perturbation in ocean chemistry that may have been related to a decline in atmospheric pO[subscript 2] after Great Oxidation Event I.