Dynamical response of the southwestern Laurentide Ice Sheet to rapid Bølling-Allerød warming

Abstract. The climatic transition from the Last Glacial Maximum (LGM) to the early Holocene (ca. 18–12 ka BP) involved rates of temperature change comparable with present-day warming trends. The most rapid recorded changes in temperature occurred during the abrupt climate oscillations known as the Bølling-Allerød interstadial (14.7–12.9 ka BP) and the Younger Dryas stadial (12.9–11.7 ka BP). Accurate reconstructions of ice sheet behaviour during these climate oscillations provide the opportunity to assess long-term ice sheet evolution in reaction to a rapidly changing climate. Here, we use glacial geomorphological inverse methods (flowsets) to reconstruct the ice flow dynamics associated with changes in ice stream catchments (ice divides and domes) and the marginal retreat pattern of the southwestern sector of the Laurentide Ice Sheet (SWLIS). We combine this ice dynamic reconstruction with a recently compiled regional deglaciation chronology to present a model of ice sheet behaviour spanning pre-LGM to the early Holocene. Our reconstruction depicts rapid ice geometry changes, including three macroscale reorganizations of the ice drainage network followed by regional deglaciation synchronous with abrupt warming during the Bølling-Allerød interstadial. Initial westward flow is documented, most probably associated with an evolving ice stream network during the advance to the LGM. Ice streaming at the LGM was marked by southward flows, unconstrained by topography. Following this, a significant switch in the ice sheets dynamics occurred at ~15 ka BP to topographically controlled south-eastward flow constrained by preglacial-valley systems. This was replaced by a second switch in ice flow orientation to the southwest at ~14.5 ka BP. Rates of ice sheet retreat then slowed considerably during the Younger Dryas stadial; at this time, the ice margin was situated north of the Canadian Shield boundary and ice flow continued to be sourced from the northeast. Resulting from these changes in ice sheet dynamics, we recognize a pattern of deglacial landform zonation within the SWLIS characterized by active ice margin recession and ice sheet stagnation and downwasting punctuated by local surging (terrestrial ice sheet collapse): (1) the outer deglacial zone is characterized by large recessional moraines aligned with the direction of active ice margin retreat; (2) the intermediate deglacial zone contains large regions of hummocky and stagnation terrain, in some areas overprinted by the signature of local surges, reflecting punctuated stagnation and downwasting and; (3) the inner deglacial zone comprises inset recessional moraines demarcating progressive regional ice margin retreat. We attribute these macroscale changes in ice flow geometry and the associated deglacial behaviour to external climatic forcing during the Bølling-Allerød and Younger Dryas but also recognize the role of internal (glaciological, lithological and topographic) controls.