Ice flow dynamics of the northwestern Laurentide Ice Sheet during the last deglaciation

Abstract. Reconstructions of palaeo-ice-stream activity provide insight into the processes governing ice stream evolution over millennial timescales. The northwestern sector of the Laurentide Ice Sheet experienced a period of rapid retreat driven by warming during the Bølling–Allerød (14.7–12.9 ka) that may have contributed significantly to global mean sea level rise during this time. Therefore, the northwestern Laurentide Ice Sheet provides an opportunity to investigate ice sheet dynamics during a phase of rapid ice sheet retreat. Here, we classify coherent groups of ice-flow-parallel lineations into 326 flowsets and then categorise them as ice stream, deglacial, inferred deglacial or event flowsets. Combined with ice-marginal landforms and a new ice margin chronology (Dalton et al., 2023), we present the first reconstruction of ice flow dynamics of the northwestern Laurentide Ice Sheet at 500-year time steps through the last deglaciation (17.5–10.5 ka). At the local Last Glacial Maximum (17.5 ka), the ice stream network was dominated by large, marine-terminating ice streams (> 1000 km long) that were fed by the Cordilleran–Laurentide ice saddle to the south and the Keewatin Dome to the east. As the ice margin retreated onshore, the drainage network was characterised by shorter, land-terminating ice streams (< 200 km long), with the exception of the Bear Lake and Great Slave Lake ice streams (∼ 600 km long) that terminated in large glacial lakes. Rapid reorganisation of the ice drainage network, from predominantly northerly ice flow to westerly ice flow, occurred over ∼ 2000 years, coinciding with a period of rapid ice sheet surface lowering in the ice saddle region. We note a peak in ice stream activity during the Bølling–Allerød that we suggest is a result of increased ablation and a steepening of the ice surface slope in ice stream onset zones and the increase in driving stresses that contributed to rapid ice drawdown. The subsequent cessation of ice stream activity by the end of the Bølling–Allerød was a result of ice drawdown lowering the ice surface profile, reducing driving stresses and leading to widespread ice stream shutdown.