Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hawkings, J. R., Linhoff, B. S., Wadham, J. L., Stibal, M., Lamborg, C. H., Carling, G. T., Lamarche-Gagnon, G., Kohler, T. J., Ward, R., Hendry, K. R., Falteisek, L., Kellerman, A. M., Cameron, K. A., Hatton, J. E., Tingey, S., Holt, A. D., Vinsova, P., Hofer, S., Bulinova, M., Větrovský, T., Meire, L., Spencer, R. G. M. Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet. Nature Geoscience, 14, (2021): 496-502, https://doi.org/10.1038/s41561-021-00753-w.

Description: The Greenland Ice Sheet is currently not accounted for in Arctic mercury budgets, despite large and increasing annual runoff to the ocean and the socio-economic concerns of high mercury levels in Arctic organisms. Here we present concentrations of mercury in meltwaters from three glacial catchments on the southwestern margin of the Greenland Ice Sheet and evaluate the export of mercury to downstream fjords based on samples collected during summer ablation seasons. We show that concentrations of dissolved mercury are among the highest recorded in natural waters and mercury yields from these glacial catchments (521–3,300 mmol km−2 year−1) are two orders of magnitude higher than from Arctic rivers (4–20 mmol km−2 year−1). Fluxes of dissolved mercury from the southwestern region of Greenland are estimated to be globally significant (15.4–212 kmol year−1), accounting for about 10% of the estimated global riverine flux, and include export of bioaccumulating methylmercury (0.31–1.97 kmol year−1). High dissolved mercury concentrations (~20 pM inorganic mercury and ~2 pM methylmercury) were found to persist across salinity gradients of fjords. Mean particulate mercury concentrations were among the highest recorded in the literature (~51,000 pM), and dissolved mercury concentrations in runoff exceed reported surface snow and ice values. These results suggest a geological source of mercury at the ice sheet bed. The high concentrations of mercury and its large export to the downstream fjords have important implications for Arctic ecosystems, highlighting an urgent need to better understand mercury dynamics in ice sheet runoff under global warming.

Description: This research is part of a European Commission Horizon 2020 Marie Skłodowska-Curie Actions fellowship ICICLES (grant agreement #793962) to J.R.H. Greenland terrestrial research campaigns were funded by a UK NERC standard grant (NE/I008845/1) and a Leverhulme Trust Research Grant (RPG-2016-439) to J.L.W., with additional support provided by a Royal Society Wolfson Merit Award to J.L.W. Additional funding came from Czech Science Foundation grants (GACR; 15-17346Y and 18-12630S) to M.S. Fjord fieldwork was supported by European Research Council grant ICY-LAB (grant agreement 678371) and Royal Society Enhancement Award (grant RGF\EA\181036) to K.R.H. L.M. was funded by research programme VENI (0.16.Veni.192.150, NWO). T.J.K. was supported by Charles University Research Centre program no. 204069. The authors thank the captain and crew of the RV Kisaq and staff at the Greenland Institute of Natural Resources for assistance during fjord fieldwork, and all those involved with fieldwork at Leverett Camp during the 2012 and 2015 field campaigns. M. Cooper is thanked for providing the geological overview file for Extended Data Fig. 1a, and K. Mankoff for help in generating the modelled GrIS discharge datasets. The authors also thank G. White in the geochemistry group at the National High Magnetic Field Geochemistry Laboratory, which is supported by NSF DMR-1644779 and the State of Florida, for analytical support.

Description: 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.