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

Abstract

Ice sheets are currently ignored in global methane budgets 1,2. 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 3,4 , 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 5 to about 17% of methane exported. Atmospheric evasion is the main methane sink once runoff reaches the ice margin, with estimated diffusive Author contribution J.L.W. and G.L.G. designed the study. B.S.L. supervised stable-isotope analyses. S.A. performed the reaction-transport hydrate model calculations. P.F. assisted in the interpretation and analysis of the CONTROS HydroC ®-CH4 raw results. G.

Glacier Outflow Dissolved Organic Matter as a Window Into Seasonally Changing Carbon Sources: Leverett Glacier, Greenland

“…The second potential source of C 416 is subglacially derived DOM either reworked or produced in situ by microbial communities. Microbial production and transformation of DOM can be through aerobic and anaerobic microbial processes (Cameron et al, ; Lamarche‐Gagnon et al, ; Stibal et al, ). Although the LG subglacial outflow is supersaturated in oxygen, it is also supersaturated in methane (Lamarche‐Gagnon et al, ); thus, the spatial extent of GrIS subglacial anaerobic environments is potentially vast.…”

Patterns in Microbial Assemblages Exported From the Meltwater of Arctic and Sub-Arctic Glaciers

“…One potentially useful, yet under-utilized, tool for investigating hydrological and biogeochemical weathering processes are the diverse microbial cells collected and exported by meltwater from the glacial ecosystem. For example, subglacial microbes are found at the intersection of the glacier and the underlying bedrock, and are functionally diverse, having been shown to utilize a myriad of metabolic pathways operating over a spectrum of redox conditions (Boyd et al, 2010(Boyd et al, , 2011(Boyd et al, , 2014Stibal et al, 2012a,c;Hamilton et al, 2013;Dieser et al, 2014), which may enable them to influence a host of weathering reactions and biogeochemical transformations (Sharp et al, 1999;Mitchell et al, 2013;Montross et al, 2013;Lamarche-Gagnon et al, 2019). Yet, due to their physical inaccessibility, these habitats are notoriously difficult to investigate, and much of our knowledge of these habitats at present comes from discrete samples taken from marginal areas (e.g., Boyd et al, 2011;Žárský et al, 2018).…”

A Mechanistic Model for Relative Permeability of Gas and Water Flow in Hydrate‐Bearing Porous Media With Capillarity

“…Besides its application to investigate production from gas hydrate reservoirs as a function of rock properties and hydrate morphology, the proposed model can be used to study relative permeability of methane trapped under the ice sheets and glaciers (Archer, 2007;Wadham et al, 2012), which is critical to quantify the formation and release of such methane (Andrews, 2019;Lamarche-Gagnon et al, 2019) from large amounts of hydrates present in thick sedimentary basins beneath the Antarctic Ice Sheet (Burns et al, 2018;Christiansen & Jørgensen, 2018;Lamarche-Gagnon et al, 2019). The need to procure core samples from such environmentally sensitive locations to investigate relative permeability can be partly overcome with the physics-enforced prediction capability of the proposed model.…”