Radiocarbon and bulk isotope composition of subglacial methane and carbon dioxide emitted at the western margin of the Greenland ice sheet

Subglacial meltwater from the western margin of the Greenland Ice Sheet (GrIS) is a net source of methane (CH 4 ) and carbon dioxide (CO 2 ). Estimating CH 4 and CO 2 emissions from GrIS meltwater and their relevance to the global carbon budget requires understanding their sources and controlling mechanisms. In this study, we measured the stable isotope composition and radiocarbon content of CH 4 and CO 2 , as well as the Bernard ratio, for gas samples extracted from subglacial meltwater and air samples collected from an ice cave at the edge of Isunnguata Sermia glacier (ISG). In June 2022, the concentration of CH 4 in the subglacial meltwater at ISG was 100 times higher than in the supraglacial meltwater. Stable isotope data and Bernard ratios indicated that CH 4 dynamics in subglacial meltwater were mainly controlled by microbial oxidation and hydrological conditions. In peak melt season, the bulk isotope composition of CH 4 was enriched compared to early melt season, likely due to enhanced microbial CH 4 oxidation. The apparent 14 C-age of subglacial CH 4 at ISG is about 1.6 kyr BP, in good agreement with Neoglacial ice sheet advance. This implies that the source of CH 4 is neither thermogenic nor from hydrates, as such sources would have a 14 C-free signal. CO 2 is older than CH 4 (about 6 kyr BP). The different apparent ages of CH 4 and CO 2 indicate that the source of CH 4 is not CO 2 reduction. The most probably source of subglacial CH 4 at ISG is acetoclastic methanogensis of organic matter buried under the ice sheet during the Neoglacial ice sheet advance. CO 2 appears to originate from the remineralization of older carbon in sediments or soils, as well as from CH 4 oxidation.