Methane-derived authigenic carbonates of mid-Cretaceous age in southern Tibet: Types of carbonate concretions, carbon sources, and formation processes

The tabular concretions are characterized by δ13C values of −21.87‰ to −6.67‰ in the Xiege Sections. These depleted δ13C values suggest that the carbon of the tabular concretions was derived at least in part, from AOM. The tabular concretions are characterized by δ13C values of −28.81‰ to −12.99‰ in the Gamba Section. According to the δ13C values and field observation, we infer that their carbon source was more likely to be a mixture of the oxidized methane formed by released hydrate and seawater-dissolved inorganic carbon.