Methane and carbon dioxide dynamics beneath the Greenland Ice Sheet: Insights from ice core basal materials

Abstract. Ice sheets and their subglacial environments may represent an important source of methane (CH4) and carbon dioxide (CO2), potentially contributing to the atmospheric burden of these greenhouse gases. In this study, we investigate CH4 and CO2 production, transport, and consumption at and near the sediment-ice interface beneath the Greenland Ice Sheet, utilizing basal materials from two deep-drilling projects. At the Camp Century site, on the northwestern margin of the Greenland Ice Sheet, vertical gas profiles (N2, O2, Ar, CO2, and CH4), combined with microbial DNA analyses, indicate that CH4 accumulates in subglacial environments either from the release of gases from buried soils and sediments beneath the ice sheet or via in situ methanogenesis. The CH4 then diffuses across the sediment-ice interface and undergoes partial oxidation to CO2 within debris-rich ice layers, contributing to the pronounced CO2 accumulation near the interface. CH4 and CO2 remain at higher concentrations than atmospheric levels for several tens of meters in the upper section of the basal ice sequence, suggesting mechanical mixing possibly during advection from inland. At the GRIP site, located at the summit of the Greenland Ice Sheet, biologically derived CH4 and CO2 in basal materials is transported into the ice predominantly through mechanical mixing processes such as shearing and folding, although diffusion across the sediment-ice interface could also occur in the unsampled bottom section. There is no evidence of CH4 consumption by methanotrophs at GRIP, suggesting that variations of bed conditions, ice dynamics and the nature of the organic material control the fate of CH4 produced in the subglacial environments.