Heterogeneous CO2and CH4content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes

Abstract

Accelerated melting of the Greenland Ice Sheet has increased freshwater delivery to the Arctic Ocean and amplified the need to understand the impact of Greenland Ice Sheet meltwater on Arctic greenhouse gas budgets. We evaluate subglacial discharge from the Greenland Ice Sheet for carbon dioxide (CO2) and methane (CH4) concentrations and δ13C values and use geochemical models to evaluate subglacial CH4and CO2sources and sinks. We compare discharge from southwest (a sub-catchment of the Isunnguata Glacier, sub-Isunnguata, and the Russell Glacier) and southern Greenland (Kiattut Sermiat). Meltwater CH4concentrations vary by orders of magnitude between sites and are saturated with respect to atmospheric concentrations at Kiattut Sermiat. In contrast, meltwaters from southwest sites are supersaturated, even though oxidation reduces CH4concentrations by up to 50% during periods of low discharge. CO2concentrations range from supersaturated at sub-Isunnguata to undersaturated at Kiattut Sermiat. CO2is consumed by mineral weathering throughout the melt season at all sites; however, differences in the magnitude of subglacial CO2sources result in meltwaters that are either sources or sinks of atmospheric CO2. At the sub-Isunnguata site, the predominant source of CO2is organic matter (OM) remineralization. However, multiple or heterogeneous subglacial CO2sources maintain atmospheric CO2concentrations at Russell but not at Kiattut Sermiat, where CO2is undersaturated. These results highlight a previously unrecognized degree of heterogeneity in greenhouse gas dynamics under the Greenland Ice Sheet. Future work should constrain the extent and controls of heterogeneity to improve our understanding of the impact of Greenland Ice Sheet melt on Arctic greenhouse gas budgets, as well as the role of continental ice sheets in greenhouse gas variations over glacial interglacial timescales.