Evidence for Early Pleistocene glaciation from borecore stratigraphy in north-central Alberta

Open accessBorecores collected from Quaternary sediments in north-central Alberta, Canada, were sampled and studied for paleomagnetic remanence characteristics. A magnetostratigraphy has been established for sediments previously assumed to represent multiple continental (Laurentide) glaciations but for which no geochronology was available. Based on the Quaternary record elsewhere in Alberta and Saskatchewan, it was thought that some of these sediments were deposited during pre-late Wisconsinan glaciations. The Quaternary sedimentary succession of north-central Alberta attains thicknesses up to 300 m within buried valleys and is composed of diamict interbedded with glaciolacustrine and outwash sediments. Most of the sampled units are not accessible from outcrop and their sedimentology and stratigraphy is derived from core data only. In 4 of 16 borecores sampled to date, diamict that correlates with the Bronson Lake Formation till is reversely magnetized, indicating an Early Pleistocene age. Depending on location, this formation is underlain by either Empress Formation sediments or Colorado Group shales, and is overlain by one or more normally magnetized glacigenic sedimentary units of the Bonnyville, Marie Creek, and Grand Centre formations, respectively. This new record of Early Pleistocene glaciation in north-central Alberta places the westernmost extent of earliest Laurentide ice at least 300 km farther west than its previously established limit in the Saskatoon and Regina regions of the Canadian Interior Plains, but still to the east of the maximum extent of the late Wisconsinan (Late Pleistocene) Laurentide Ice Sheet, which extended into the foothills of the Alberta and Montana Rocky Mountains

Identification of Younger Dryas outburst flood path from Lake Agassiz to 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 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 disconcerting2. Here we identify the missing flood pathevident from gravels and a regional erosion surfacerunning 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 13,000years 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 11,500years 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