DEGLACIATION OF A SOFT-BEDDED LAURENTIDE ICE-SHEET

We present a series of numerical reconstructions of the Laurentide Ice Sheet during the last deglaciation (18-7 C-14 ka) that evaluates the sensitivity of ice-sheet geometry to subglacial sediment deformation. These reconstructions assume that the Laurentide Ice Sheet flowed over extensive areas of water-saturated, deforming sediment (soft beds) co rresponding to the St. Lawrence lowland, the Great Lakes region, the w estern prairies of the U.S. and Canada, and the Hudson Bay and Hudson Strait regions. Sediment rheology is based on a constitutive law that incorporates experimental results from late Wisconsin till deposited b y the Laurentide Ice Sheet which suggest only mildly nonlinear viscopl astic behavior. By varying the effective viscosity of till, we produce d four reconstructions for the ice sheet during the last glacial maxim um 18 C-14 ka, and two reconstructions each of the ice sheet at 14, 13 , 12, 11 and 10 C-14 ka. We also produced one reconstruction for 9, 8. 4, 8, and 7 C-14 ka. Reconstructions that assume a low effective visco sity for all areas of deforming sediment show a multidomed ice sheet w ith a large bowl-shaped depression over Hudson Bay and thin ice ( < 10 00 m above modern sea-level) over the western and southern margins. Th ose reconstructions that assume a higher effective viscosity of till i n the Hudson Bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over Hu dson Bay and a more northerly position of the central ice divide. Thes e two different geometries may represent ice-sheet orographic changes associated with a Heinrich event. Further increases in effective visco sity of till, approaching the effective viscosity of ice, would result in a high, monolithic ice dome centered over Hudson Bay, reinforcing the notion that a multidomed ice sheet reflects the distribution of su bstrate geology. Modeled ice-surface geometry at the last glacial maxi mum shows many of the same general features as previous reconstruction s that incorporate deformable beds. Our reconstructions with higher ef fective till viscosities in Hudson Bay also agree with the ICE-4G reco nstructions (Peltier, 1994), which are based on inversion of relative sea-level data, for the early part of the last deglaciation(18-13 C-14 ka), but then depart significantly from ICE-4G beginning at about 12 C-14 ka due to differing assumptions of the history of deglaciation. M odeled ice volume for the last glacial maximum suggests a glacioeustat ic change of 50-55 m by a soft-bedded Laurentide Ice Sheet; this would increase as the effective viscosity of till increases. Subsequent ice -volume changes through the last deglaciation generally parallel the t rend of eustatic rise recorded at Barbados, New Guinea, and Tahiti, bu t suggest that the Laurentide Ice Sheet was not the source of meltwate r pulse 1A. (C) 1998 Elsevier Science Ltd. All rights reserved.