We thank all of those who assisted with fieldwork at LG, especially J. Hatton, as well as F. Sgouridis and J. Williams at the LOWTEX laboratory of the University of Bristol. This research is part of the UK NERC-funded DELVE programme (NERC grant NE/I008845/1 to J.L.W.). G.L.-G. was funded by the University of Bristol Scholarship Programme and a FRQNT Scholarship (number 185136). The work was also supported by a Leverhulme research fellowship to J.L.W., a UK NERC grant (NE/J02399X/1) to A.M.A. for DNA analyses, as well as Czech Science Foundation grants (GACR; 15-17346Y and 18-12630S) to M.S. Isotopic analyses were conducted by G. Lacrampe-Couloume at the University of Toronto with support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) to B.S.L. We also thank the Kangerlussuaq International Science Station, especially R. Møller, for support with field logistics, as well as M. A. Cooper, M. Macdonald and S. Hoffer for comments.

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Nature thanks J. Crawford and the other anonymous reviewer(s) for their contribution to the peer review of this work.