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. Accelerated melting of the Greenland Ice Sheet hasincreased freshwater delivery to the Arctic Ocean and amplified the need tounderstand the impact of Greenland Ice Sheet meltwater on Arctic greenhousegas budgets. We evaluate subglacial discharge from the Greenland Ice Sheetfor carbon dioxide (CO 2 ) and methane (CH 4 ) concentrations and δ13 C values and use geochemical models to evaluate subglacialCH 4 and CO 2 sources and sinks. We compare discharge from southwest(a sub-catchment of the Isunnguata Glacier, sub-Isunnguata, and the RussellGlacier) and southern Greenland (Kiattut Sermiat). Meltwater CH 4 concentrations vary by orders of magnitude between sites and are saturatedwith respect to atmospheric concentrations at Kiattut Sermiat. In contrast,meltwaters from southwest sites are supersaturated, even though oxidationreduces CH 4 concentrations by up to 50 % during periods of lowdischarge. CO 2 concentrations range from supersaturated atsub-Isunnguata to undersaturated at Kiattut Sermiat. CO 2 is consumed bymineral weathering throughout the melt season at all sites; however,differences in the magnitude of subglacial CO 2 sources result inmeltwaters that are either sources or sinks of atmospheric CO 2 . At thesub-Isunnguata site, the predominant source of CO 2 is organic matter(OM) remineralization. However, multiple or heterogeneous subglacialCO 2 sources maintain atmospheric CO 2 concentrations at Russell butnot at Kiattut Sermiat, where CO 2 is undersaturated. These resultshighlight a previously unrecognized degree of heterogeneity in greenhousegas dynamics under the Greenland Ice Sheet. Future work should constrain theextent and controls of heterogeneity to improve our understanding of theimpact of Greenland Ice Sheet melt on Arctic greenhouse gas budgets, as wellas the role of continental ice sheets in greenhouse gas variations overglacial–interglacial timescales.

the cryosphere

Heterogeneous CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; content of glacial meltwater from the Greenland Ice Sheet and implications for subglacial carbon processes