The appearance of elevated aqueous methane concentrations that are commensurate with the location and onset of subglacial drainage, suggests the environment of methane production must be beneath the glacier. Fluctuating discharge and changing subglacial methane concentrations on a seasonal basis preclude straightforward calculation of an annual methane flux from beneath the glacier. However, a typical summer season discharge of 50 m3 s−1 from the meltwater outlet stream Jökulsá á Sólheimasandi20, and the corresponding average aqueous methane concentration of 11.2 mg l−1 (Table 1) can be used to estimate a flux of 48 Tonnes per day of methane transported away from the ice margin. This high flux occurs as meltwater exits the ice-marginal proglacial lake after the onset of discharge from the subglacial drainage system. When calculated as a day-rate per m2 ice-covered area (maximum 78 km2 glaciated catchment area cf.21) this equates to a subglacial production capacity of 39 mM CH 4 m−2 day−1. Using an upstream – downstream mass balance along the 4 km length (20 m width) of proglacial meltwater channel (taken as the difference in methane concentration between the meltwater outlet sampling site and the catchment outlet at the N1 road bridge, supplementary Fig. S1), evasion to the atmosphere was calculated as 86%. This equated to an evasive flux of 41 tonnes of methane to the atmosphere per day (32 M CH 4 m−2 day−1 as an area-weighted flux from stream to atmosphere). This mass balance approach to calculating an evasive methane flux along the stream assumes minimal dilution, and no in-stream methanotrophy. Both are valid assumptions given the minimal input of additional meltwater between upstream and downstream sampling points, the minimal production/consumption of methane in the proglacial sediments (Table S1), and the limited change in isotopic composition of aqueous methane (Table 1). The onset of upwelling subglacial water varies on an annual basis at all glaciers, dependent on antecedent conditions. Prior to the upwelling of subglacial meltwaters during the 2014 sampling season (day of year 128), a more conservative flux of methane transported away from the ice margin is estimated as 0.6 tonnes per day (equivalent to 0.5 mM CH 4 m−2 day−1, based on an average winter discharge of 10 m3 s−1 and mean methane concentration of 0.65 mg l−1). Evasion to the atmosphere was calculated as 54% along the 4 km stream reach, equating to 0.25 M CH 4 m−2 day−1. Methane evasion from the Sólheimajökull sub-aerial stream network greatly exceeds mean flux values between river to atmosphere reported in the literature (4.23+\−8.41 mM CH 4 m−2 day−1)22 indicating the potential significance of the subglacial methane source, if similar processes are also occurring at other glaciers.

The origin of the methane can be inferred through stable isotopic analysis of δ13Cch 4 and Dch 4 . Isotopic fractionation during biogenic methanogenesis typically leads to δ13C values between −50 to −110‰, and δD values between −170 to −531‰23. Geogenic methane produced at high geothermal temperatures undergoes exchange with the surrounding water and mantle carbon, producing deuterium and carbon contents enriched in 2H and 13C respectively24. Signatures of mixed geogenic/microbial origin should therefore lie on an end member mixing trajectory as depicted in Fig. 1, with microbially-sourced methane clearly emanating from the point of subglacial upwelling. However, possible alteration to methane signatures by methanotrophic activity (methane oxidation) will enrich the remaining pool of methane reactants in 13C and deuterium. As the most enriched values exceed the geogenic range, the observed isotopic signatures cannot be explained by a mixture of biogenic and geogenic methane (Fig. 1). Extensive potential for methanotrophic activity, as evidenced through the incubation of sediments under oxidizing conditions (see Supplementary Fig. S2), likely explains the isotopic fractionation trajectory away from the microbial end member signature. Fractionation between the starting methane isotopic composition (CH 4(i) ) and composition of residual methane (CH 4(t) ) is quantified following25 as α = 1.019 for 13C/12C, and for fractionation of D/H as α = 1.197. These incubation determined values of C and H enrichment during methanotrophy are encompassed within the published range of experimental values23, and result in relative changes to isotopic signatures during reaction progress that lie on a similar gradient to field data from this study (Fig. 1 and methods). This isotope signature confirms that methane emanating from the subglacial environment of Sólheimajökull is predominantly regulated by microbial activity.

The important role played by microbial activity in determining this remarkably high methane flux from beneath Sólheimajökull is surprising given the extensive geothermal activity beneath the Mýrdalsjökull icecap26. However, based on isotopic evidence, subglacial geothermal activity appears not to contribute to the methane flux. Instead, we consider the subglacial geothermal activity to be instrumental only in driving the summer subglacial discharge to low redox status, allowing preservation and transport of microbially-generated, dissolved methane to the point of upwelling without oxidation to CO 2 . Most temperate glacial drainage systems which do not overlie volcanic and/or geothermal systems are characterised by a slow flow winter component in which subglacial water is confined to linked cavities, basal film flow and/or water saturated till, dependent upon the state of the glacier bed (hard- or soft-based). Under these conditions of distributed drainage (the ‘closed’ system), connectivity to the atmosphere is poor and dissolved gases can be depleted to produce meltwaters of low redox status. During the summer season, a discrete well-connected subglacial drainage system, characterised by well-defined conduits, expands up-glacier dependent upon the flux of surface run-off to the glacier bed, and typically follows the supraglacial snowline. Within this ‘open’ configuration, oxygen saturated meltwaters can drain rapidly from the surface of the glacier and through the subglacial system27,28. At polar glaciers of a polythermal nature, the drainage system displays similar characteristics, albeit with the winter slow flow component of the drainage system remaining sealed beneath the glacier until basal water pressures force a pressurised outflow, either shortly after the onset of the summer season29, or intermittently throughout the winter to produce characteristic proglacial icings e.g.30. However, at Sólheimajökull, the presence of the Katla geothermal area beneath the head of the glacier imparts profoundly different characteristics to meltwater discharge (Fig. 2). During the summer season (Fig. 2a), headward expansion of the conduit drainage system proceeds in the conventional fashion based on an enhanced flux of meltwater between glacier surface and bed. When the conduit drainage system connects with the zone of geothermal activity, release of reduced gases into the drainage system produces the characteristic volatile-rich, oxygen-depleted chemical composition of the discharge, as evidenced by the hydrogen sulphide content and sulphur isotopic composition of the meltwaters19. The summer season sub-oxic meltwater arguably inhibits methanotrophic activity beneath the glacier, allowing the preservation of dissolved biogenic methane until the point of upwelling and contact with the atmosphere. The transported methane comprises methane formed during the winter ‘closed’ system phase (zero-flux scenario3), together with methane produced during the summer season. During the winter season (Fig. 2b), the conduit drainage system is restricted to the lower elevations of the glacier, where year-round ablation maintains a conduit configuration connected to the atmosphere and isolated from the Katla geothermal zone. Under this configuration, methane production is limited and methanotrophic activity minimises the methane flux.

Figure 2 Schematic model of hydrological evolution at Sólheimajökull, Iceland. The headward expansion of the conduit drainage system intersects with the geothermal area, where release of reduced gases determines the sub-oxic meltwater status essential for preserving aqueous methane until the point of emergence from beneath the glacier. (a) Summer season snowpack ablation delivers meltwater to expand the conduit drainage system headwards into the Katla geothermal field. This results in a drainage system well-connected to deep within the geothermal field, delivering water of reducing status. Methane generated within the basal sediments through microbial methanogenesis is preserved during export. (b) Winter season limited surface ablation restricts the conduit drainage system to the lower reaches of the glacier. This results in a drainage system operating close to atmospheric conditions within the vicinity of the glacier snout and poorly connected to deeper beneath the ice mass. Much of the methane generated within the surrounding sediments is oxidised proximal to the channelized drainage system before being exported from beneath the glacier. Full size image

The geothermally-influenced nature of the Sólheimajökull system is unusual in its ability to present a low redox status window which inhibits methane oxidation and preserves aqueous methane until the point of release from beneath the glacier. The dominance of biogenic methane production beneath an Icelandic temperate ice mass nonetheless raises the distinct possibility that methane generation could be proceeding undetected in other subglacial environments where the cocktail of temperate ice, low oxygen concentration, organic carbon and methanogenic communities coincide to promote methanogenesis. Increasing evidence for zones of strong geothermal activity beneath the West Antarctic Ice Sheet suggests that subglacial microbial communities with methanogenic potential may be more significant and extensive than previously anticipated31,32. However, cold ice barriers and the length of meltwater pathways to ice termini means methane is typically trapped beneath ice masses, or oxidised during subglacial transit away from its zone of production. This prevents confident extrapolation of the subglacial methane production at Sólheimajökull to other regions, or to a global scale. Thus, the relative contribution of subglacial methane to global atmospheric fluxes critically depends on the extent of sub-oxic ‘windows’ at temperate and polythermal basal ice systems. We suggest that, in order to identify subglacial methane fluxes from temperate and polythermal glacial systems, and better constrain any associated climatic impact, the quest for quantifying methane release dynamics should focus on these sub-oxic windows of meltwater discharge. This may include studying other ice masses with elevated geothermal heat fluxes; characterising baseflow seepage and initial fractions of summer subglacial discharge at both temperate and polythermal glaciers; and analysing gases trapped within proglacial icings. Under a changing climate with accelerated ice thinning33, enhanced overburden pressure release on subglacial volcanic and geothermal systems is likely to drive an increase in eruptive activity34. Anomalous heat fluxes are known to both precede and follow volcanic activity, likely due to pressure-induced boiling in geothermal systems35. Release of reduced gases during this enhanced geothermal activity would determine the prevalence of sub-oxic windows (and methane content) of meltwater discharge. Greater headward expansion of drainage systems towards geothermal areas currently isolated beneath ice mass overburden would also ensure transport of meltwater and associated methane content to a position of sub-aerial degassing into the atmosphere. Pressure driven sub-oxia would likely become more prevalent with ice thinning until overwhelmed by the flux of oxygenated surface melt reaching the glacier bed or until ice disappearance. In this manner, this poorly quantified flux of sub-ice cap methane is likely an indirect, albeit self-reinforcing consequence of climatic change.