Abstract
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 CH4 and CO2 sources 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 CH4 concentrations 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 CH4 concentrations by up to 50 % during periods of low discharge. CO2 concentrations range from supersaturated at sub-Isunnguata to undersaturated at Kiattut Sermiat. CO2 is consumed by mineral weathering throughout the melt season at all sites; however, differences in the magnitude of subglacial CO2 sources result in meltwaters that are either sources or sinks of atmospheric CO2. At the sub-Isunnguata site, the predominant source of CO2 is organic matter (OM) remineralization. However, multiple or heterogeneous subglacial CO2 sources maintain atmospheric CO2 concentrations at Russell but not at Kiattut Sermiat, where CO2 is 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.
Highlights
Glaciers play an important role in global chemical cycles due to the production of fine-grained sediments that participate in carbonate and silicate mineral weathering reactions (Table 1), which are the principal sink of atmospheric CO2 over geologic timescales (Berner et al, 1983; Walker et al, 1981)
Sites differ in pH, and average values at Kiattut Sermiat (8.2 ± 0.4) are higher than both Russell (7.2 ± 0.2) and sub-Isunnguata (6.6 ± 0.6; Fig. 3b), and while values vary over time, no consistent trend is identified between sites
CH4 concentrations differ by orders of magnitude between sites (Fig. 3e) and are consistently supersaturated with respect to atmospheric concentrations at sub-Isunnguata (648 ± 411 ppm or 1575 ± 997 nM) and Russell (58 ± 33 ppm or 110 ± 78 nM) sites, while they are close to atmospheric equilibrium at Kiattut Sermiat (4 ± 2 ppm or 9 ± 5 nM)
Summary
Glaciers play an important role in global chemical cycles due to the production of fine-grained sediments that participate in carbonate and silicate mineral weathering reactions (Table 1), which are the principal sink of atmospheric CO2 over geologic timescales (Berner et al, 1983; Walker et al, 1981). The importance of CO2 consumption by mineral weathering is poorly understood, including effects from the advance and retreat of continental ice sheets (Ludwig et al, 1999). Recent evaluations of carbon budgets in proglacial environments indicate that mineral weathering results in net sequestration of atmospheric CO2, suggesting that proglacial systems are underrecognized as Arctic CO2 sinks (St Pierre et al, 2019); alternate processes could lead to the production of greenhouse gases in glacial systems. Combined inorganic and organic subglacial processes may produce glacial meltwater that is a source or sink of greenhouse gas. While the net impact of these processes on modern carbon fluxes is poorly constrained, determining these impacts will improve modern carbon budgets as well as depictions of how fluxes may have evolved during the advance and retreat of continental ice sheets
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