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 apparent¹⁴C-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 a¹⁴C-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.