This research is part of a European Commission Horizon 2020 Marie Sk��odowska-Curie Actions fellowship ICICLES (grant agreement #793962) to JRH. Greenland terrestrial research campaigns were funded by a U.K. NERC Standard Grant (NE/I008845/1) and a Leverhulme Trust Research Grant (RPG-2016-439) to JLW, with additional support provided by a Royal Society Wolfson Merit Award to JLW. Additional funding came from Czech Science Foundation grants (GACR; 15-17346Y and 18-12630S) to MS. Fjord fieldwork was supported by European Research Council grant ICY-LAB (grant agreement 678371) and Royal Society Enhancement Award (Grant RGF\EA\181036) to KRH. LM was funded by research programme VENI (0.16.Veni.192.150, NWO). TK was supported by Charles University Research Centre Program No. 204069. The captain and the crew of the R/V Kisaq and staff at Greenland Institute of Natural Resources are also thanked for their assistance during fjord fieldwork. The authors thank all those involved with fieldwork at Leverett camp during the 2012 and 2015 field campaigns. We also thank the analytical support from 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.

Geochemical measurements and hydrochemical datasets linked to the publication "Large subglacial source of mercury from the southwestern margin of the Greenland Ice Sheet" in Nature Geoscience. Presented are (1) data for mercury concentrations in glacial meltwater outflows from the Greenland Ice Sheet taken in 2012, 2015 and 2018, (2) data for mercury concentrations in fjord waters from Nuup Kangerlua, Ameralik Fjord and S��ndre Str��mfjord, and (3) all associated hydrochemical data presented in the manuscript. For additional details (analytical techniques, precision, accuracy and limits of detection) please refer to the methodology in the publication. This third version has additional riverine data added the the 2012 dataset.