CARBON ISOTOPE EVIDENCE FOR METHANE SEEPS IN THE 635 MA DOUSHANTUO CAP CARBONATE AT MULTIPLE GEOGRAPHIC LOCALITIES IN SOUTH CHINA

The 4–5-m-thick Doushantuo cap carbonate (~ 635 Ma) overlying the Cryogenian Nantuo glaciogenic diamictite contains a distinctive suite of sedimentary and chemostratigraphic features. The origin of the Doushantuo cap carbonate and other equivalent cap carbonates has been a matter of debate. Hypotheses about alkalinity for cap carbonate precipitation vary from post-glacial chemical weathering of silicate and carbonate rocks, upwelling of a bicarbonate-laden deep ocean, to anaerobic oxidation of methane released during deglaciation. Previous studies reported carbonate carbon isotope values as low as -41‰ (PDB) from micro-drilled samples of the Doushantuo cap carbonate, suggesting a role of methane seeps in the origin of the structures, textures, and geochemical features of the Doushantuo cap. However, existing data for methane-derived carbon isotope values were from a single locality (the Huajipo section) in the Yangtze Gorges area of South China, limiting interpretation of the role of methane in the formation of the Doushantuo cap carbonate. Here we report extremely negative carbon isotope values (as low as -48‰ PDB) from two additional sections, the Jiulongwan and Wangzishi sections about 4.5 km to the NE and 55 km to the SE of the Huajipo section. Such negative carbon isotope values were measured from both micro-drilled and bulk samples from the basal (0.8 m above base of cap dolostone at Jiulongwan) and upper (4.4 m above base of cap dolostone at Wangzishi) Doushantuo cap carbonate. The new isotopic data support the stratigraphic consistency of extremely negative carbon isotopes in the Doushantuo cap carbonate and suggest that post-glacial gas hydrate dissociation and methane release in South China may have been geographically widespread and geochronologically may have extended over the time span of cap carbonate deposition.