An ice-free greenhouse interval (ca. 660 similar to 650 Ma) occurred between the global Sturtian (ca. 717Ma similar to 660Ma) and Marinoan glaciation (ca. 650Ma similar to 635Ma) during the Cryogenian. While volcanic CO2 emissions have been suggested as a trigger for this ice-free interval, evidence for volcanism has been lacking. In this study, spikes in both Hg concentration (76 to 366 ppb) and Hg to total organic carbon ratio (Hg/TOC: 114 to 717 ppb/wt.%) were observed in the Wuhe section of South China, suggesting that extensive volcanic emissions of Hg occurred during the Cryogenian interglacial period. Positive Delta Hg-199 (0.02 to 0.18 parts per thousand) values were observed in these Hg-rich shales, indicating that the volcanic Hg was cycling in the atmosphere, deposited in seawater, and sequestered to sediment via organic matter burial. Combined with other geochemical proxies (e.g., organic carbon isotope and chemical index of alteration), our results provide the first direct evidence that volcanism played a key role in forming the brief Cryogenian interglacial period. (C) 2021 Elsevier B.V. All rights reserved.

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