Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet

Clumped isotopes of methane (CH4), specifically Δ13CH3D and Δ12CH2D2, provide additional information to constrain its sources and sink processes. These isotopes complement interpretations of CH4 provenance based on bulk isotopes. However, interpreting the origin of CH4 using isotopes becomes challenging when the isotopic signature is altered by post-generation processes. In this study, we measured, for the first time, the bulk and clumped isotopic composition of sub-glacial CH4 samples. These samples were collected from the air-filled headspace of the glacier portal (ice cave) at the edge of the Isunnguata Sermia glacier (ISG), located at the western margin of the Greenland ice sheet (GrIS). Our goal was to identify the processes underlying the sub-glacial production and potential processing of CH4. The Δ13CH3D and Δ12CH2D2 values of the samples measured in this study are 3.7 ± 0.3‰ and 39.7 ± 2.0‰, respectively (95% confidence interval). The Δ12CH2D2 values are close to those of atmospheric CH4. The elevated Δ12CH2D2 values can be attributed to the alteration of the source’s isotope signal by aerobic oxidation. This conclusion is supported by previous studies at this site, which reported the presence of methanotrophic bacteria and dissolved oxygen close to saturation in the meltwater. Our results confirm that the correlation between Δ13CH3D and Δ12CH2D2 is a useful tool for deciphering oxidation pathways. Our results support the inference that aerobic CH4 oxidation can strongly modify the Δ12CH2D2 isotope signal, which must be considered when determining the source signatures of environmental samples.