The recession of the Laurentide Ice Sheet in central Northwest Territories during the Pleistocene-Holocene transition
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(1,2)Sam Kelley, (2)Jason Briner, (3)Brent Ward
(1)University College Dublin, (2)University at Buffalo, (3)Simon Frasier University
The Laurentide Ice Sheet (LIS) during the Pleistocene-Holocene transition provides a useful natural laboratory for examining the behavior of a mid- to high-latitude ice sheet during a period of large-scale downwasting. The LIS is well-studied, with much effort focused on constraining the recession of the southern and eastern margins from the Last Glacial Maximum positions. However, a paucity of chronologic constraints exists for the northwestern sector of the Laurentide Ice Sheet (LIS), especially during the Pleistocene-Holocene transition, which limits our understanding of past ice sheet behavior.
The objective for our work is to use 10Be exposure dating to constrain the timing and rate of retreat for the western margin of the LIS in the Great Slave Lake area, and use our new chronology to assess the validity of numerically modeled ice sheet reconstructions for this poorly-constrained region. To accomplish our objectives, we plan to analyze samples from glacially-transported crystalline boulders and prominent, glacially eroded, bedrock outcrops for 10Be exposure dating; these samples comprise three E-W trending transects located between Great Slave Lake in the south and Lac de Gras in the north.
Our new 10Be cosmogenic exposure ages from the southeastern Northwest Territories demonstrate that regional deglaciation occurred around 11,000 years ago. The population of ages broadly overlaps, indicating that either the retreat occurred within the resolution of our chronology or that the ice sheet experienced widespread stagnation and rapid down-wasting. These ages, not corrected for changes in atmospheric depth due to isostatic rebound, are older than minimum limiting radiocarbon constraints by ~1000 years, indicating that existing LIS reconstructions may underestimate the timing and pace of ice margin recession for this sector. Constraining the timing of the recession of the northwest sector of the LIS has the potential to inform our understanding about the damming of large proglacial lakes, such as Glacial Lake McConnell. The ages from our southern transect, collected from elevated bedrock hills, indicate LIS retreat from through the McConnell basin occurred after 12,000 years ago, and thus constitute maximum limiting constraints on the expansion of Glacial Lake McConnell southeastward into the present-day Great Slave Lake basin.
Ice Sheet; Quaternary Geochronology; Climate Change