Revised chronology of northwest Laurentide ice-sheet deglaciation from ¹⁰Be exposure ages on boulder erratics

We present new ¹⁰Be surface exposure ages from boulders on bedrock to directly date northwest Laurentide ice-sheet deglaciation through a wide swath of the western Canadian Shield that had no previous reliable temporal constraints on ice-margin retreat. Uplift-corrected boulder ¹⁰Be surface exposure ages are 13.9 ± 0.2 ka (n = 6) at a site on the western edge of the Slave Craton and 12.4 ± 0.2 ka (n = 5, 1 outlier) at a second site ∼110 km up-ice to the east. These direct ¹⁰Be ages for ice-margin retreat are ∼2.4 kyr and ∼1.6 kyr older, respectively, than the canonical deglacial chronology for the northwest Laurentide ice sheet that is based on minimum-limiting ¹⁴C dates. We infer an ice-margin retreat rate of 60–100 m yr⁻¹ between the two sites over an interval spanning the transition from the Allerød warm period into the Younger Dryas cold period. This is significantly slower than the rapid >800 m yr⁻¹ retreat rate for the northwest Laurentide ice sheet inferred from earlier deglacial chronologies, which has been hypothesized as a potential source of meltwater forcing for the Younger Dryas cold period. These first direct ages on spatio-temporal patterns of deglaciation in this data-poor region suggest that additional refinement of the deglacial chronology is needed to test hypotheses on the relation between ice-sheet retreat, associated meltwater discharge, abrupt climate change, and rapid sea-level rise.