SEDIMENT TRANSPORT CYCLES OF THE LAURENTIDE ICE SHEET I: SOFT TO HARD BED TRANSITION DURING WISCONSIN MARINE ISOTOPE STAGE 5D-2

Progressive removal of unconsolidated sediment from the North American Precambrian Canadian Shield (the Shield) by the Laurentide Ice Sheet (LIS) over the Pleistocene played a critical role in regulating its dynamic relation to climate. Regolith removal switched accumulation zone basal conditions from soft-bed to hard-bed, driving significant LIS thickening in later cycles, and has been implicated in the global transition from 41 ka to 100 ka glacial cycles evident in the marine isotope record. A second mechanism amplifying the evolving LIS dynamics in a similar and complementary fashion is evidenced by erratic dispersal from the eastern Hudson Bay basin (HBB) to the western Great Lakes region and the transitional characteristics of lodgement till along this flowline transect. This evidence records systematic change in LIS Labrador sector basal boundary conditions over the Wisconsin (MIS 5d-2) glaciation. LIS expansion beyond the HBB was accompanied by sediment advection across the Shield to its southern margin, and deposited a continuous subglacial lodgement till (CSLT), imposing soft-sediment basal conditions throughout the LIS interior. The CSLT is remarkably uniform in texture and composition over hundreds of kilometers in flowline length. It is stratigraphically overlain by a complex of texturally and compositionally variable lodgement and meltout tills, spatially and temporally associated with the post-LGM retreating LIS margin. Mean transport length (MTL) of boulder, pebble and matrix components of CSLT and post-LGM marginal tills varied widely temporally over the Wisconsin glaciation, reflecting significant variability in subglacial erosion and entrainment rates. MTLs for early CSLT tills are up to 1500 km contrasted with ~2 km for post-LGM marginal tills, indicating rapid circum-LGM transition in subglacial erosional regimes. Field data indicate early LIS growth was characterized by high sediment flux from the HBB and downflow development of a CSLT over the Shield to its margin. As the LIS approached its LGM limit, down-ice ELA translation decreased sediment flux from the HBB while simultaneously promoting CSLT removal from the underlying Shield. The resulting transition to hard-bed conditions promoted LIS thickening and volume increase, even as its southern margin retreated.