MARINE BIOGEOCHEMICAL CYCLE TRIGGERED THE MARINOAN SNOWBALL EARTH GLACIATION

The Cryogenian (720–635 million years ago, Ma) snowball Earth glaciations, the Sturtian (717–660 Ma) and Marinoan (650–635 Ma), represent the most severe climatic condition in Earth’s history. Existing models cannot explain the repeated occurrences of global glaciations, nor the existence of a non-glacial interval of ~ 10 million years within a prolonged global ice-age (~85 million years). Here, we use sulfur isotopes (δ³⁴S) to decipher biogeochemical processes that may have triggered the Marinoan glaciation. Our data show that, in the non-glacial period, sedimentary pyrite in continental margin sediments is characterized by unusually high δ³⁴S values, whereas pyrite in basinal sediments has much lower δ³⁴S values. This spatial pattern is attributed to the emission and offshore transportation of ³⁴S-depleted methyl sulfide and other volatile organosulfur compounds (VOSC) that were produced in sulfidic (H₂S-enriched) continental margins via H₂S methylation. Because methyl sulfides can serve as the non-competitive substrates for methanogens and anaerobic CH₄ oxidation could be inhibited by methanethiol, a strong marine CH₄ flux would accompany the VOSC emission. Large CH₄ fluxes would have sufficed to keep the Earth warm for 10⁷ years. However, this climate system is metastable. As a feedback of sustained sulfidic condition in continental margins, atmospheric oxygenation and oceanic ventilation may have significantly reduced marine CH₄ fluxes and led to the initiation of the Marinoan glaciation. Upon recovering from the Marinoan glaciation, renewed oxygenation permanently reduced marine CH₄ fluxes, and hence no more snowball Earth events occurred.