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 (CH4) and carbon dioxide (CO2). Estimating CH4 and CO2 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 CH4 and CO2, 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 CH4 in the subglacial meltwater at ISG was 100 times higher than in the supraglacial meltwater. Stable isotope data and Bernard ratios indicated that CH4 dynamics in subglacial meltwater were mainly controlled by microbial oxidation and hydrological conditions. In peak melt season, the bulk isotope composition of CH4 was enriched compared to early melt season, likely due to enhanced microbial CH4 oxidation. The apparent 14C-age of subglacial CH4 at ISG is about 1.6 kyr BP, in good agreement with Neoglacial ice sheet advance. This implies that the source of CH4 is neither thermogenic nor from hydrates, as such sources would have a 14C-free signal. CO2 is older than CH4 (about 6 kyr BP). The different apparent ages of CH4 and CO2 indicate that the source of CH4 is not CO2 reduction. The most probably source of subglacial CH4 at ISG is acetoclastic methanogensis of organic matter buried under the ice sheet during the Neoglacial ice sheet advance. CO2 appears to originate from the remineralization of older carbon in sediments or soils, as well as from CH4 oxidation.