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Active shelf H2S oxidation after the Marinoan glaciation

∆17O anomaly; Pyrite sulfur isotopes; Shelf H2S reoxidation; Doushantuo cap carbonate; Snowball Earth

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

A significantly non-mass-dependent ¹⁷O depletion was recorded in ~635Ma post-Marinoan barites within cap carbonates, which was proposed to be critical evidence for elevated atmospheric CO₂ levels during the termination of the Marinoan glaciation. Most studies attribute the incorporation of ∆¹⁷O anomaly-bearing O₂ into seawater sulfate to sulfide oxidation, however, direct evidence of this hypothesis remains elusive. To address this gap, we conducted a detailed analysis of pyrite crystal morphology, content, and sulfur isotopes (δ³⁴S_py) in the Doushantuo cap carbonate from the Yangtze Block, South China. Our results demonstrate that pyrites exhibit euhedral to subhedral morphologies and are distributed within dolomite crystal pores, suggesting an early diagenetic origin. Notably, the pyrite content and isotopic compositions of the Doushantuo cap carbonate display a pronounced spatial gradient: δ³⁴S_py values from deep-water facies (basin facies: 11.02‰; slope facies: 11.94‰) are isotopically lighter than those from shallow-water facies (33.08‰), while pyrite content in shallow (0.08 wt.%) and slope facies (0.06 wt.%) is markedly lower than in basin facies (1.89 wt.%). Using a one-dimensional diffusion-advection-reaction model, we propose that these spatial gradients in pyrite sulfur systematics result from variations in the proportion of H₂S reoxidation. Model results indicate that ~90% of H₂S was oxidized in shallow environments, compared to ~50% in deep-water facies. This active H₂S reoxidation process can transfer ∆¹⁷O-depleted O₂ to sulfate, which would ultimately record in barite. Our study thus provides direct evidence for shelf H₂S reoxidation as a mechanism for preserving ∆¹⁷O anomalies in barites and further supports a rapid oceanic oxidation event following the Snowball Earth glaciation.