Mid-Holocene retreat of the Greenland Ice Sheet indicated by subglacial methane release

Methane (CH4) emissions have been detected at glacier margins globally, with subglacial CH4 production identified beneath the Greenland Ice Sheet. Despite its potential role in carbon cycling, an assessment of the sources, production pathways and prevalence of subglacial CH4 export is lacking. Here we report on extensive sampling of 26 meltwater streams across the entire western margin of the Greenland Ice Sheet, revealing a radiocarbon age of 1.5–4.4 thousand years before present for pervasive, biogenic CH4 laterally transported by emerging subglacial supersaturated meltwater. These ages corroborate a smaller-than-present Greenland Ice Sheet during the Holocene Thermal Maximum (11–5 thousand years ago before present), stimulating proglacial organic matter accumulation, which was then overridden by subsequent glacial advance. Applying a continuum degradation model, we demonstrate that western Greenland’s subglacial organic matter can support CH4 release for another 200 years, with a lateral flux of 715 (481–1,020) tonnes per year from its land-terminating sectors. We highlight the pertinence of subglacial carbon cycling to the release of CH4 from all glacial environments globally, and a dynamic sensitivity of the Greenland Ice Sheet not yet fully realized in ice sheet models, via the isotopic assessment of subglacial CH4. Methane in modern subglacial meltwater coming from the western Greenland Ice Sheet largely dates back to the period following the Holocene Thermal Maximum, when a smaller ice sheet allowed organic matter accumulation and biological methane production after ice readvance.