Organic Carbon Isotope Evidence for Neoproterozoic Postglacial Methane Hydrate Destabilization

The association of seep-like sedimentary features, carbon and sulfur isotope anomaly, and unusual oxygen isotope variability from the late Neoproterozoic postglacial cap carbonates suggests a methane hydrate destabilization event at ca. 635 Ma, following the Earth's most severe glaciation. This event may have triggered the termination of the “Snowball Earth” and had fundamental impact on the Ediacaran earth system and the evolution of early animals. A detailed organic carbon isotope analysis of the Doushantuo cap carbonate and its overlying strata in multiple sections across a platform-to-basin transect in south China reveals a strongly negative δ¹³C_org excursion. Organic carbon isotope values start at -25‰ in the basal cap carbonate; reach a minimum of -35‰ in the organic- and pyrite-rich shales (up to 5% TOC) immediately overlying the cap carbonate, and return back to -27‰ ~ -28‰ above the 3-m-thick shale interval. The temporal isotopic trend and its spatial persistence across the Doushantuo basin strongly support a massive injection to the atmosphere of isotopically-light methane (or CO₂) in the late stage of a methane hydrate destabilization event. The regionally persistent organic- and pyrite-rich shale overlying the cap carbonate may record an ocean anoxic event at the end of the methane release event. The data also suggest that the stratified and anoxic/euxinic Ediacaran ocean, if existed, may have been initiated after the cap carbonate deposition rather than inherited from the Mesoproterozoic or early Neoproterozoic.