中国科学院南京地质古生物研究所机构知识库

In the aftermath of Marinoan global glaciation (similar to 650-635 Ma), the deglacial intense continental weathering tended to dramatically lower the atmospheric pCO(2) level, potentially driving the Earth back to the glacial climatic condition. However, the resultant global cooling and glaciation did not occur. The CO2 drawdown by continental weathering might have been compensated by additional CO2 release via reverse weathering in the ocean, as evidenced by abundant precipitations of authigenic clay minerals, for example, Mg-rich saponite and clinochlore in the lower part of Doushantuo Formation (635-551 Ma) in South China. Massive precipitations of Mg-rich authigenic clays imply a distinct marine Mg cycle in the early Ediacaran ocean, but their origins have been debated. Here, we measured the Mg isotopic compositions of authigenic clays from the Doushantuo Formation. Both saponite and clinochlore are extremely Mg-26-enriched, with Mg isotopes of saponite (delta Mg-26(sap)) up to +0.39 parts per thousand and clinochlore up to +1.26 parts per thousand. Considering the Mg isotopic fractionation in saponite precipitation ranging from -0.95 parts per thousand to -1.45 parts per thousand, high delta Mg-26(sap) values requires extremely high seawater Mg isotopic composition (delta Mg-26(sw)), which cannot be resolved by direct precipitation with seawater Mg supply. Instead, it requires other diagenetic processes that elevated the porewater Mg isotopic composition (delta Mg-26(pw)). A numerical model was applied to quantify the Mg isotopic fractionation in diagenesis. The modeling result indicates an earlier dolomitization relative to saponite precipitation might have elevated the porewater Mg isotopes, followed by the precipitation of extremely Mg-26-enriched saponite. The coupled carbonate-silicate diagenesis is a key mechanism sustaining stable seawater Mg concentration and delta Mg-26, highlighting authigenic clay mineral as a buffer for Mg and C cycles counterbalancing massive weathering input after Marinoan glaciation.