RAPID RETREAT OF THE SOUTHWESTERN LAURENTIDE ICE SHEET DRIVEN BY BØLLING-ALLERØD WARMING

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 24 ¹⁰Be surface exposure ages from glacial erratics spanning southwestern Alberta to northwestern Saskatchewan, Canada. We combine these data with regional geomorphic mapping and already existing luminescence, ¹⁰Be surface exposure ages and ‘high quality’ minimum radiocarbon chronologies, exclusive of dates on bulk sediments, terrestrial shells or mixed assemblages, to provide an updated chronology for the retreat of the southwestern Laurentide Ice Sheet. Our compiled dataset of cosmogenic, luminescence and minimum radiocarbon dates present a consistent retreat record. These data suggest that initial detachment of the southwestern Laurentide Ice Sheet from its coalescence with the Cordilleran Ice Sheet occurred at ~15.0 ka BP, concurrent with, or possibly somewhat before the abrupt warming at the onset of the Bølling-Allerød, and retreated >1200 km to its Younger Dryas position in ~2000 years or less. . This new chronology, coupled with ice stream behavior reconstructions implies the southwestern sector of the Laurentide Ice Sheet was highly dynamic, retreating earlier and at a much-accelerated rate than previously thought.