Carbon Emissions From the Edge of the Greenland Ice Sheet Reveal Subglacial Processes of Methane and Carbon Dioxide Turnover

Abstract

Direct gaseous emissions of methane (CH4) and carbon dioxide (CO2) from the subglacial environment under Greenland Ice Sheet (GrIS) were only recently discovered and it is yet to be determined how important it is for the panarctic carbon budget. We measured in situ net gaseous emissions of subglacial CH4 and CO2, dissolved concentrations and isotopic composition of gases (13C and 2H) at the onset, near maximum, and at the end of the melt season in 2018 and 2019. We found a tight relation between gaseous and dissolved CH4 and CO2, respectively, indicating that degassing from the subglacial meltwater is the main source of these gases in the subglacial air. The diurnal variability of in situ mole fractions of CH4 and CO2 in subglacial air was related to meltwater runoff showing that the net emission magnitude is directly related to glacial hydrology. We observed that maximum in situ mole fractions of CH4 and CO2 appeared at the onset of the melt season and decreased over the melt season. The isotopic signature of CH4 in the subglacial air indicated that it likely originated from microbial methanogenesis which remained constant during the season. Isotopic signatures of subglacial CO2 indicate mixed sources from microbial oxidation of CH4, remineralization of sedimentary organic carbon, and possibly influenced by removal of CO2 by weathering. Our study indicate large emissions of both CO2 and CH4, but continuous studies over entire melt seasons are needed to determine the origin and emission magnitudes and their relation to the glacial dynamics.