GLACIAL ISOSTASY AND FLUCTUATING LAKES: A RECONSTRUCTION OF THE GREAT LAKES DURING THE PAST 20,000 YEARS

During the retreat of the Laurentide Ice Sheet, proglacial lakes changed dynamically as the ice margin migrated uncovering new outlets and unloading the earth. A numerical model of the viscoelastic earth response to this unloading gives deformation predictions which, when subtracted from the present day high resolution digital elevation model (DEM), recreates the paleo-topography. As isostatic relaxation progresses, the entire hydrologic system of predicted lakes and rivers evolves without including any assumptions of historic outlet elevations or geologic evidence.

The model couples predicted deformations with GIS analysis to recreate the hydrology of the Great Lakes region during the past 20,000 years. Results are predicted snapshots at 1000-year intervals giving lake extent and locations of controlling outlets. The configuration of these predicted lakes is similar to that determined from more than a century of fieldwork. Because lake bathymetry is included in the DEM it is also possible to determine lake volume through time. Changes in lake volume in turn load the surface of the earth and cause additional deformation of up to 20 meters. Predictions of time-dependent outlet elevations explain the succession of outlets resulting from tilting of the earth's surface. Furthermore, predictions of present rates of vertical motion compare favorably to lake gauge and GPS data showing modern tilting of the Great Lakes region.

Tilted lake shoreline data, tide gauge data, lake level gauges and relative sea level curves can constrain the ice sheet thickness history. Geophysical inverse theory is used to determine the global ice sheet thickness history that best fits these global data during the past 35,000 years. Constraints on ice sheet volume from eustatic sea level curves guide the inversion process and ice sheet surface topography is required everywhere to be decreasing in thickness in the direction of ice flow. The Laurentide Ice Sheet is found to be thin and dynamic but poorly constrained over the Great Lakes.