New isotope data obtained from relatively conformable, carbonate-rich strata of the Ediacaran Yangtze platform in South China reveal substantial δ 13 C variability. In platform sections, four negative δ 13 C anomalies with a nadir down to ≤− 8‰ (PDB) are present in the interval between the cap carbonate level (∼ 635 Ma) and the Precambrian/Cambrian boundary (∼ 542 Ma), while in slope and basinal sections, δ 13 C values are negative through the entire Doushantuo Formation (∼635-551 Ma). If these δ 13 C values are close to their primary seawater signature, they imply a strong (≥ 10‰) surface-to-deep ocean δ 13 C gradient that is consistent with long-term deep ocean anoxia and the presence of a large dissolved organic carbon (DOC) reservoir. The two prominent negative δ 13 C excursions within the Doushantuo Formation above the cap carbonate level are associated with shoaling and local exposure of the platform. The anomalies may thus record remineralization of a large oceanic DOC pool via sulfate reduction that transferred 13 C-depleted carbon from the oceanic DOC reservoir to the surface ocean during regression. Inconsistencies in Ediacaran δ 13 C profiles globally and variations in South China in particular highlight the need for further evaluation of local departures in δ 13 C from an inferred average seawater signature.

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The 3-to 5-m-thick Doushantuo cap carbonate in south China overlies the glaciogenic Nantuo Formation (ca. 635 Ma) and consists of laterally persistent, thinly laminated and normally graded dolomite and limestone indicative of relatively deep-water deposition, most likely below storm wave base. The basal portion of this carbonate contains a distinctive suite of closely associated tepee-like structures, stromatactis-like cavities, layer-parallel sheet cracks, and cemented breccias. The cores of tepees are composed of stacked cavities lined by cements and brecciated host dolomicrite. Onlap by laminated sediment indicates synsedimentary disruption of bedding that resulted in a positive seafloor expression. Cavities and sheet cracks contain internal sediments, and they are lined by originally aragonitic isopachous botryoidal cements with acicular radiating needles, now replaced by dolomite and silica. Pyrite and barite are common, and calcite is locally retained as a primary mineral. These features share morphological and petrographic attributes with modern and ancient methane seeps in which methane gas and fluids provide both a force for physical disruption from buoyancy and a source of alkalinity for significant cementation. The presence of d 13 C values as low as 241% in well preserved limestone crusts and cements within and immediately above the tepee-like structures provides unequivocal evidence for methane influence, and the widespread distribution of identical sedimentary structures and paragenetic cement sequences across the entire basin at the same basal cap carbonate level is consistent with gas hydrate destabilization and the development of methane seeps as a result of postglacial warming of the ocean. Considering the broad distribution of similar features at the same stratigraphic level in other cap carbonates globally, we suggest that the late Neoproterozoic postglacial methane release may have influenced the oceanic oxygen level as well as contributed to postglacial warming via the greenhouse effects of methane.

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