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Publication - Professor Jemma Wadham

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

    Citation

    Lamarche-Gagnon, G, Wadham, J, Lollar, BS, Arndt, S, Fietzek, P, Beaton, A, Tedstone, A, Telling, J, Bagshaw, E, Hawkings, J, Kohler, T, Žárský, JD, Mowlem, M, Anesio, A & Stibal, M, 2019, ‘Greenland melt drives continuous export of methane from the ice-sheet bed’. Nature, vol 565., pp. 73-77

    Abstract

    Ice sheets are currently ignored in global methane budgets. They have been proposed to cap large reserves of methane that may contribute to a rise in atmospheric methane concentrations if released during periods of rapid ice retreat, but no data on the current methane footprint of ice sheets currently exist. Here we find that subglacially-produced methane is rapidly flushed 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) concentrations coincided 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 were indicative of subglacial methane reserves in excess of export, with an estimated 6.3 (2.4 – 11) tonnes of CH4(aq) laterally transported from the ice sheet bed. Stable isotope analyses revealed a microbial origin for methane; most likely derived 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 oxidation to about 17% of methane exported. Atmospheric evasion is the main methane sink once runoff reaches the ice margin, with estimated diffusive fluxes (4.4 – 28 mmol-CH4 m-2 d-1) rivalling that of other world rivers. Overall, our results provide evidence that ice sheets overlay extensive, biologically active methanogenic wetlands, and that high rates of methane export to the atmosphere can occur where efficient subglacial drainage pathways exist. Our findings suggest that such environments should be considered a previously underappreciated component of Earth’s methane budget.

    Full details in the University publications repository