THE FATE OF PREGLACIAL REGOLITH BENEATH THE LAURENTIDE ICE SHEET

Glaciological theory, observations of the West Antarctic Ice Sheet, and models for the Pleistocene evolution of the Laurentide Ice Sheet all suggest that the initial distribution and eventual export of easily deformable subglacial sediment influence the dynamics and evolution of large ice sheets. In particular, Clark and Pollard (Paleoceanography 13, 1, 1998) suggest subglacial sediment dynamics as a cause for the mid-Pleistocene discontiuity in the repeat time of major glaciations. We explore this idea using the geochemical tracer beryllium-10, which is produced in the atmosphere by cosmic-ray bombardment and delivered to the surface by precipitation. It bonds strongly to sediment particles and thus accumulates to high levels in stable, deeply weathered terrains. The thick regolith present throughout North America prior to Pleistocene glaciation contained Be-10 concentrations 10 to 100 times higher than would be expected to accumulate during an interglaciation: thus the Be-10 concentration in tills near the former margins of the Laurentide Ice Sheet is a record of the subglacial erosion and export of this regolith. Early Pleistocene tills contain Be-10 concentrations similar to those observed for ancient cratonic soils elsewhere, entirely excluding erosion of fresh bedrock as a source for these tills and indicating that early ice sheets remobilized only a thin layer of very old surficial material. Late Pleistocene tills, on the other hand, contain low Be-10 concentrations, suggesting that the ancient regolith had been largely removed from glaciated areas and that these tills were formed either by deeper erosion or remobilization of relatively young surface materials. Our efforts to determine the age of the tills in question suggest that the supply of Be-10 rich regolith was exhausted between 1.2 and 0.8 Myr ago, which is consistent with the idea that the depletion of easily deformable regolith affected the dynamics of the ice sheet at this time. Supported by NSF EAR-0207844.