Climatic and ice dynamic controls on the deglaciation of the Laurentide Ice Sheet

Ice sheets respond to both external climatic forcings and dynamic processes related to ice flow, basal sliding, marine instabilities, and solid-Earth interactions, yet the relative importance of these controls is unclear. Here, we investigate the role of these drivers during the last deglaciation of the Laurentide Ice Sheet (LIS). To overcome an incomplete chronology of the deglaciation, we use Bayesian data assimilation to reconstruct retreat of the LIS since the Last Glacial Maximum (26 to 19 ka) as a continuous spatiotemporal field with quantified uncertainty by integrating geochronologic and geomorphic constraints with an ensemble of physics-based ice-sheet models. Our reconstruction shows a complex and asynchronous pattern of deglaciation. At continental scale, average retreat rates align with hemispheric climate change, however, on a regional scale we find that the rate of deglaciation was spatially and temporally variable. Our reconstruction allows for a novel comparison to proxies of mechanical controls on deglaciation that reveals regional retreat was closely aligned with the extent of soft sediment underlying ice-sheet margins, the degree of ice streaming, and the presence of proglacial lakes. These mechanical controls explain rapid retreat in places that appear out of sync with climate changes. Based on these results, we identify basal sliding and dynamic discharge as critical controls on large ice-sheet collapse and propose that internal feedbacks will strongly influence the response of the world's modern ice sheets to atmospheric and ocean warming.