Kilometer-scale Δ13CH3D profiles distinguish end-member mixing from methane production in deep marine sediments

•Δ13CH3D values constrain marine sedimentary CH4 sources beyond two-component mixing.•Methane isotopologues re-equilibrate along geothermal gradients up to 100 ± 15 °C.•Disequilibrium is expected for methane in sediments between ca. 100–150 °C.•Microbial methanogenesis may occur 5–35 °C above what was previously accepted.