Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean

Younger Dryas flood tracked Our current concepts of abrupt climate change are strongly influenced by compelling palaeoclimate evidence for events like the Younger Dryas, in which massive changes in climate occurred essentially instantaneously. It is generally thought that an injection of freshwater from the retreating Laurentide Ice Sheet altered the Atlantic Meridional Overturning Circulation and triggered the Younger Dryas, but convincing geological evidence to support this theory has, to date, proven elusive. Now Julian Murton and colleagues at last identify a major flood event that is chronologically consistent with the Younger Dryas. Gravels found above an erosion surface in north-west Canada are consistent with the major flood path running through the Mackenzie River into the Arctic Ocean, rather than south along the Mississippi River or east through the Great Lakes. Our current concepts of abrupt climate change are influenced by palaeoclimate evidence for events such as the Younger Dryas cold interval, in which massive climate changes occurred essentially instantaneously. It is thought that an injection of fresh water from the retreating Laurentide Ice Sheet altered the Atlantic meridional overturning circulation and triggered the Younger Dryas, but convincing geological evidence has been elusive. Here, a major flood event that is chronologically consistent with the Younger Dryas has been identified—through the MacKenzie River into the Arctic Ocean. 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 change 1 , 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 Dryas 1 , 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 tim