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

Abstract

Clumped isotopes of methane (CH₄), specifically ${\Delta }^{13}$CH₃D and ${\Delta }^{12}$CH₂D₂, 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 ${\Delta }^{13}$CH₃D and ${\Delta }^{12}$CH₂D₂ values of the samples measured in this study are 3.7 ± 0.3‰ and 39.7 ± 2.0‰, respectively (95% confidence interval). The ${\Delta }^{12}$CH₂D₂ values are close to those of atmospheric CH₄. The elevated ${\Delta }^{12}$CH₂D₂ 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 ${\Delta }^{13}$CH₃D and ${\Delta }^{12}$CH₂D₂ is a useful tool for deciphering oxidation pathways. Our results support the inference that aerobic CH₄ oxidation can strongly modify the ${\Delta }^{12}$CH₂D₂ isotope signal, which must be considered when determining the source signatures of environmental samples.