RAPID RETREAT OF THE SOUTHWESTERN LAURENTIDE ICE-SHEET DURING THE BØLLING-ALLERØD INTERVAL (Invited Presentation)

The timing of Laurentide Ice Sheet deglaciation, along its southwestern margin, controls the evolution and drainage of large glacial lakes and has implications for the migration of early humans into the Americas. Accurate reconstruction of the ice sheet’s retreat also constrains glacial isostatic adjustment models and is imperative for our understanding of ice sheet sensitivity to climate forcings. Despite its importance, much of the retreat history of the southwestern Laurentide Ice Sheet is still poorly constrained by minimum limiting ¹⁴C data. Here, we present a database of 26 ¹⁰Be surface exposure ages from glacial erratics spanning southwestern Alberta to northwestern Saskatchewan, Canada. Using a Bayesian framework, we combine these data with geomorphic mapping, 10Be, and high-quality minimum-limiting 14C ages to provide an updated chronology. This dataset presents an internally consistent retreat record and indicates that the initial detachment of the SWLIS from its convergence with the Cordilleran Ice Sheet began by ca. 15.0 ka, concurrent with or slightly prior to the onset of the Bølling-Allerød interval (14.7–12.9 ka) and retreated >1200 km to its Younger Dryas (YD) position in ∼2500 yr. Ice-sheet stabilization at the Cree Lake Moraine facilitated a meltwater drainage route to the Arctic from glacial Lake Agassiz within the YD, but not necessarily at the beginning. Our record of deglaciation and new YD constraints demonstrate deglaciation of the Interior Plains was ∼60% faster than suggested by minimum 14C constraints alone. Numerical modelling of this rapid retreat estimates a loss of ∼3.7 m of sea-level equivalent from the SWLIS during the Bølling-Allerød interval.