Strongly 13C-depleted authigenic carbonates (e.g., δ13CVPDB <−30‰; VPDB—Vienna Peedee belemnite) in nature are generally believed to form by sulfate-dependent anaerobic oxidation of methane (AOM). However, we demonstrate using geochemical data and thermodynamic calculation that such calcites are most likely derived from biogenic oxidation of methane in sulfate-poor, nonmarine environments during early diagenesis, as observed in the Triassic sandy conglomerates from the Junggar Basin, northwestern China. This process operated through preferential oxidation of 13C-depleted methane by Mn oxides in closed conditions, producing calcites with higher Mn contents and δ13C values in association with more 13C-enriched residual methane as a result of kinetic isotope fractionation. Thus, the Mn-rich and 13C-depleted carbonates are proposed as tracers of Mn-dependent AOM, which should have served as an important sink of greenhouse methane in low-sulfate early Earth’s oceans.

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