By Lauren C. Andrews
2 January 2019
(Nature) – Sediments beneath glaciers and ice sheets harbour carbon reserves that, under certain conditions, can be converted to methane, a potent greenhouse gas. However, the formation and release of such methane is an unquantified component of the Arctic methane budget. Writing in Nature, Lamarche-Gagnon et al.1 present direct measurements of dissolved methane in water discharged from a land-terminating glacier of the Greenland Ice Sheet during the summer. This water, which is known as proglacial discharge, was supersaturated with methane, and the amount of methane released to the atmosphere from this discharge rivals that from other terrestrial rivers. The findings suggest that the form and evolution of subglacial hydrological systems contribute to the control of the Arctic methane cycle.
Atmospheric methane concentrations varied substantially in the past, and it has been hypothesized2 that large reserves of methane can form and be trapped under ice sheets and glaciers when there is a favourable combination of carbon-rich sediments, high subglacial pressures, oxygen-poor conditions and low temperatures. Rapid release of this methane during glacial retreat might trigger rapid warming3, but whether large-scale release of such glacial methane could occur in the future is disputed4.
Field observations provide equivocal evidence for whether subglacial sediments act as a source or sink of methane. Ice-core drilling operations in West Antarctica detected methane-producing microbes in proglacial and subglacial sediments5, but analyses of Antarctic subglacial lake sediments6 and proglacial sediments7 indicate that bacterial oxidation consumes almost all the methane produced, preventing its release to the atmosphere. This bacterial methane cycling suggests that the subglacial hydrological system could serve as a methane sink.
Lamarche-Gagnon et al. show that subglacial microbial oxidation of methane is not sufficient to mitigate the gas’s release to the atmosphere within a well-characterized subglacial catchment (an area within which subglacial water collects and drains out of a common outlet) in Greenland. Subglacial sediments can therefore act as a local source of methane, corroborating the results of other recent studies of subglacial methane8,9. Lamarche-Gagnon et al. go further by demonstrating that the continuous flux of methane from the Greenland subglacial environment varies with the efficiency of subglacial meltwater drainage.
Near the margins of the Greenland Ice Sheet, the glacial hydrological system seems to be well suited to exporting methane. During winter, some meltwater from previous summers is stored in the inactive subglacial hydrological system10. Lamarche-Gagnon et al. hypothesize that this winter storage allows meltwater to become enriched with methane through interaction with sediments in an oxygen-free environment. [more]
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.
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