Greenland melt drives continuous export of methane from the ice-sheet bed

Guillaume Lamarche-Gagnon (), Jemma L. Wadham, Barbara Sherwood Lollar, Sandra Arndt, Peer Fietzek, Alexander D. Beaton, Andrew J. Tedstone, Jon Telling, Elizabeth A. Bagshaw, Jon R. Hawkings, Tyler J. Kohler, Jakub D. Zarsky, Matthew C. Mowlem, Alexandre M. Anesio and Marek Stibal
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Guillaume Lamarche-Gagnon: University of Bristol
Jemma L. Wadham: University of Bristol
Barbara Sherwood Lollar: University of Toronto
Sandra Arndt: Université Libre de Bruxelles
Peer Fietzek: Kongsberg Maritime Contros GmbH
Alexander D. Beaton: National Oceanography Centre
Andrew J. Tedstone: University of Bristol
Jon Telling: Newcastle University
Elizabeth A. Bagshaw: Cardiff University
Jon R. Hawkings: University of Bristol
Tyler J. Kohler: Faculty of Science, Charles University
Jakub D. Zarsky: Faculty of Science, Charles University
Matthew C. Mowlem: National Oceanography Centre
Alexandre M. Anesio: Aarhus University
Marek Stibal: Faculty of Science, Charles University

Nature, 2019, vol. 565, issue 7737, 73-77

Abstract: Abstract Ice sheets are currently ignored in global methane budgets1,2. Although ice sheets have been proposed to contain large reserves of methane that may contribute to a rise in atmospheric methane concentration if released during periods of rapid ice retreat3,4, no data exist on the current methane footprint of ice sheets. Here we find that subglacially produced methane is rapidly driven to the ice margin by the efficient drainage system of a subglacial catchment of the Greenland ice sheet. We report the continuous export of methane-supersaturated waters (CH4(aq)) from the ice-sheet bed during the melt season. Pulses of high CH4(aq) concentration coincide with supraglacially forced subglacial flushing events, confirming a subglacial source and highlighting the influence of melt on methane export. Sustained methane fluxes over the melt season are indicative of subglacial methane reserves that exceed methane export, with an estimated 6.3 tonnes (discharge-weighted mean; range from 2.4 to 11 tonnes) of CH4(aq) transported laterally from the ice-sheet bed. Stable-isotope analyses reveal a microbial origin for methane, probably from a mixture of inorganic and ancient organic carbon buried beneath the ice. We show that subglacial hydrology is crucial for controlling methane fluxes from the ice sheet, with efficient drainage limiting the extent of methane oxidation5 to about 17 per cent of methane exported. Atmospheric evasion is the main methane sink once runoff reaches the ice margin, with estimated diffusive fluxes (4.4 to 28 millimoles of CH4 per square metre per day) rivalling that of major world rivers6. Overall, our results indicate that ice sheets overlie extensive, biologically active methanogenic wetlands and that high rates of methane export to the atmosphere can occur via efficient subglacial drainage pathways. Our findings suggest that such environments have been previously underappreciated and should be considered in Earth’s methane budget.

Date: 2019
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DOI: 10.1038/s41586-018-0800-0

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