Decline in oceanic sulfate levels during the early Mesoproterozoic
Author(s)Luo, Genming; Ono, Shuhei; Huang, Junhua; Algeo, Thomas J.; Li, Chao; Zhou, Lian; Robinson, Andrew; Lyons, Timothy W.; Xie, Shucheng; ... Show more Show less
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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.
DepartmentMassachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
Luo, Genming et al. “Decline in Oceanic Sulfate Levels during the Early Mesoproterozoic.” Precambrian Research 258 (2015): 36–47.
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