T76 The Neoproterozoic Earth-Life System

Monday, 11 October 2021
8:00 AM - 12:00 PM
E141/E142 (Hybrid Room)

COUPLED CARBON, SULFUR, AND BARIUM CYCLES DURING THE EDIACARAN SHURAM EXCURSION: A SYNTHESIS

Paper number: T76-8
Program: Technical Sessions
Time: 9:45 AM – 10:00 AM

Authors

Abstract

Compared with the Phanerozoic strata, sulfate minerals are notably rare in the Precambrian record largely due to lower concentrations of sulfate in dominantly anoxic oceans. Here, we present a compilation of sulfate minerals, including diagenetic barite (BaSO4), pseudomorphs of gypsum (CaSO4·2H2O) and anhydrite (CaSO4), and celestine (SrSO4) that are stratigraphically associated with the Ediacaran Shuram excursion (SE) — arguably the largest negative carbon isotope excursion in Earth history. In this study, we investigated 15 SE-equivalent sections including: the upper Doushantuo Formation in South China, the Shuram Formation in Oman, the Wonoka Formation in Australia, the Krol Group in India, the Nama Group in southern Namibia, the Rainstrom Member in western USA, the upper Clemente Formation in Mexico, and the Alyanchskaya Formation in Siberia. All of the studied successions reveal the presence of sulfate minerals and/or concentration enrichment in carbonate-associated sulfate ([CAS]). The sulfur isotope (δ34S) trends of both sulfate and sulfide during the SE show progressively decreasing values, suggesting an increasing sulfur reservoir coincident with independent U isotope evidence for oxygenation of the oceans. To test the hypothesis of an increased oceanic sulfate level, we conducted a petrographic search for evidence of sulfate-bearing minerals, including barite, evaluated stratigraphic trends of barium concentration ([Ba]) in five SE-equivalent sections, with a primary focus on South China. Where [Ba] data are available, it reveals considerable enrichments relative to pre- and post-SE intervals. Based on our compiled observations, we propose that elevated seawater sulfate concentrations during the SE — triggered by enhanced oxidative weathering of terrestial pyrite or evaporite dissolution — faciliated the formation of sulfate minerals as primary precipitates and secondary authigenic cements. At the same time, a larger influx of dissolved Ba to the Ediacaran basins further faciliated barite deposition. Enhanced seawater sulfate concentrations would also stimulate microbial sulfate reduction and anaerobic oxidation of organic matter (including methane) in the sulfate-methane transition zone (SMTZ). Our study highlights the dynamic interplay of biogeochemical C, S, and Ba cycles in increasingly oxygenated Ediacaran surface environments.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-364304

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