Thermogenic methane beneath the North Greenland Ice Sheet revealed by isotopic and geological evidence

Glacial meltwater has been increasingly recognised as a potential source of atmospheric methane, yet its origin and variability in the High Arctic remain poorly constrained. In this study, we present measurements of methane concentration and carbon‑isotope composition in meltwater draining the northern Greenland Ice Sheet. Here we show that methane concentrations (12-20 nM) are significantly lower than those reported from other Greenland catchments, despite clear evidence of subglacial input. Isotopic signatures and regional geological context indicate that this methane is predominantly thermogenic, reflecting a geological source that is largely independent of subglacial microbial activity. These findings advance current understanding of methane sources beneath the Greenland Ice Sheet, revealing a thermogenic contribution alongside microbial methane in Arctic methane cycling. In this work we highlight the need to account for geological methane reservoirs when assessing present and future cryosphere-carbon feedbacks.