Correction to: Nature https://doi.org/10.1038/s41586-018-0610-4, published online 17 October 2018.
In Extended Data Fig. 1 of this Letter, the map showed the location of our field work incorrectly. The location is: 49° 48′ 15′′ W, 65° 10′ 45′′ N, and the figure has been corrected online. The original, incorrect Extended Data Fig. 1 is shown as Fig. 1 to this Amendment for transparency.