The melting Laurentide Ice Sheet discharged thousands of cubic kilometres of fresh water each year into surrounding oceans, at times suppressing the Atlantic meridional overturning circulation and triggering abrupt climate change1,2,3,4. Understanding the physical mechanisms leading to events such as the Younger Dryas cold interval requires identification of the paths and timing of the freshwater discharges. Although Broecker et al. hypothesized in 1989 that an outburst from glacial Lake Agassiz triggered the Younger Dryas1, specific evidence has so far proved elusive, leading Broecker to conclude in 2006 that “our inability to identify the path taken by the flood is disconcerting”2. Here we identify the missing flood path—evident from gravels and a regional erosion surface—running through the Mackenzie River system in the Canadian Arctic Coastal Plain. Our modelling of the isostatically adjusted surface in the upstream Fort McMurray region, and a slight revision of the ice margin at this time, allows Lake Agassiz to spill into the Mackenzie drainage basin. From optically stimulated luminescence dating we have determined the approximate age of this Mackenzie River flood into the Arctic Ocean to be shortly after 13,000 years ago, near the start of the Younger Dryas. We attribute to this flood a boulder terrace near Fort McMurray with calibrated radiocarbon dates of over 11,500 years ago. A large flood into the Arctic Ocean at the start of the Younger Dryas leads us to reject the widespread view that Agassiz overflow at this time was solely eastward into the North Atlantic Ocean.
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The Mackenzie Delta research was funded by the Royal Society, the Quaternary Research Association, the British Society for Geomorphology and the Geological Survey of Canada. The Aurora Research Institute (Inuvik) and the Polar Continental Shelf Project provided logistical support. J.T.T. and Z.Y. thank the Natural Sciences and Engineering Research Council of Canada for support through the Discovery Grants Program. We thank R. A. Ashurst, P. Coles and D. K. Murton for laboratory and cartographic assistance, J. R. Mackay for drawing our attention to the Fort Good Hope region, and C. D. Clark, M. R. Frogley and E. J. Rhodes for comments. We also thank V. R. Baker and G. A. Duller for reviews that improved the manuscript considerably.
Author Contributions J.B.M. and M.D.B. designed the field research in the Mackenzie Delta region; J.B.M. and S.R.D. performed the stratigraphic analyses and interpreted the palaeogeography; M.D.B. collected samples and performed the luminescence dating; J.T.T. and Z.Y. studied and modelled the Fort McMurray region. J.B.M., J.T.T. and M.D.B. drafted the manuscript, with all authors contributing to it.
This file contains Supplementary Figures 1-6 with legends, Supplementary Methods, Supplementary Tables 1-2, a Supplementary Discussion and Supplementary References.
About this article
Nature Communications (2015)