Thermogenic methane release after Marinoan glaciation induced extremely 13C-depleted calcite precipitation in cap dolostones

The Marinoan glaciation was one of the most severe known Snowball Earth climate events, terminating abruptly at ca. 635 Ma with the deposition of globally distributed cap carbonate sediments overlying glacial deposits. This extreme global warming event has been attributed to methane release from the destabilization of gas hydrates, as supported by the abnormally negative δ13C signatures (δ13C = −48‰ to −41‰) of calcite cements that occur in the cap dolostones. However, the hypothesis of gas hydrate destabilization is controversial as the timing of methane release and the methane origin are not clear. Here, we demonstrate that extremely 13C-depleted (δ13C = −51.9‰ to −50.9‰) calcite precipitation occurred much later (618 ± 5.8 Ma) than the termination of Marinoan glaciation in the cap dolostones in South China, and it was associated with thermogenic methane release, as supported by a fluid inclusion gas δ13C1 value (−38.7‰) and the C1/(C2 + C3) ratio measured in the most 13C-depleted calcites. Our findings prove that extremely negative δ13C signatures of calcite cements in cap dolostones are unrelated to clathrate destabilization, thus excluding this mechanism as a cause for the termination of the Marinoan Snowball Earth. This study not only refines our understanding of Ediacaran carbon cycling but also has implications for evaluating hydrocarbon generation in Precambrian sedimentary basins.