Evidence for methane isotopic bond re-ordering in gas reservoirs sourcing cold seeps from the Sea of Marmara

•Δ13CH3D and Δ12CH2D2 investigated in marine cold seeps from the Sea of Marmara.•Microbial/thermogenic samples show equilibrium temperatures up to 130°C.•Non-enzymatic mechanism for isotope bond ordering to reservoirs temperatures. The measurement of methane clumped isotopologues (Δ13CH3D and Δ12CH2D2) allows exploring isotope bond ordering within methane molecules, and may reveal equilibrium temperatures. Whether such temperature reflects the formation or re-equilibration temperature of the methane is not well understood, but would have critical implications for the use of methane clumped isotopologues as geo-thermometers. Here we investigate gas bubbles from vigorous emissions at cold seeps (n = 14) in the Sea of Marmara, Turkey. These cold seeps are sourced from deeper sedimentary reservoirs. Conventional geochemical tracers such as carbon and hydrogen bulk isotopic ratios (13C/12C and D/H) or n-alkane molecular ratios, suggest these gases reflect various degrees of mixing between thermogenic and microbial sources. Some samples would generally be considered purely microbial in origin (C1/C2+>1500; δ13C