A NEW PATHWAY OF SULFUR CYCLING IN THE CRYOGENIAN OCEAN

Dissimilatory microbial sulfate reduction (MSR), reducing sulfate to sulfide, is the most important microbial pathway in the biogeochemical sulfur cycling. The sulfide product are depleted in heavy sulfur isotopes (³⁶S, ³⁴S, ³³S) relative to the sulfate precursor. The sulfur isotopes fractionation (δ³⁴S_{sulfate-sulfide}) occurred in MSR are used to trace the sulfur fluxes in biogeochemical sulfur cycle. However, our current understanding of biogeochemical sulfur cycle is challenged by the precipitation of pyrites with extremely high δ³⁴S (mainly above +30‰, with the extreme values up to +70‰). Superheavy pyrites are most abundantly discovered from the interglacial intervals (~663Ma to 654Ma) between the Marinoan and Sturtian global glaciations of the Cryogenian Period (720 Ma to 635 Ma), including the Datangpo Formation in the Yangtze Platform, Court Formation in Namibia, Tapley Hill and Aralka Formation in Australia and Twitya Formation in Canada. In order to uncover the origin of superheavy pyrites, we measured δ³⁴S_CAS of the carbonate samples from the Member V of the Fulu Formation deposited in the basin environment, which can be correlated with the Datangpo Formation in the slope of the Yangtze Platform. δ³⁴S_CAS of the Member V carbonate ranges from 10.5‰ to 18.9‰, significantly lower than δ³⁴S_py of the equivalent Datangpo Formation (25.3‰ to 39.7‰), suggesting that δ³⁴S_sw in the basin environment was low. In the slope environment where seawater was sulfidic, volatile organic sulfur compounds (VOSCs) can be generated by the methylation of H₂S with the presence of dissolved free sulfide (H₂S_(aq), HS^- and S^2-), zero-valent sulfur (S⁰) and polysulfide (S_x^2-). We speculate that the spatial sulfur isotope gradient in the Cryogenian ocean was generated by delivering ³²S from the slope to the basin environment via efficient formation and emission of VOSCs. Our study demonstrates that VOSCs cycle might be an important component in Proterozoic marine sulfur cycle, and might have more prominent effect on δ³⁴S_sw. Thus, interpretation of Proterozoic sulfur isotope data needs to fully appreciate the significance of organo-sulfur cycles.