Extremely 26Mg-enriched authigenic clays from the Ediacaran Doushantuo Formation (South China) indicating the coupled carbonate-silicate diagenesis

Abstract

In the aftermath of Marinoan global glaciation (∼650–635 Ma), the deglacial intense continental weathering tended to dramatically lower the atmospheric pCO₂ 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 ²⁶Mg-enriched, with Mg isotopes of saponite (δ²⁶Mg_sap) up to +0.39‰ and clinochlore up to +1.26‰. Considering the Mg isotopic fractionation in saponite precipitation ranging from −0.95‰ to −1.45‰, high δ²⁶Mg_sap values requires extremely high seawater Mg isotopic composition (δ²⁶Mg_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 (δ²⁶Mg_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 ²⁶Mg-enriched saponite. The coupled carbonate-silicate diagenesis is a key mechanism sustaining stable seawater Mg concentration and δ²⁶Mg, highlighting authigenic clay mineral as a buffer for Mg and C cycles counterbalancing massive weathering input after Marinoan glaciation.