9405334 Clark Unstable ice-sheet behavior is now recognized as an important characteristic of the Laurentide Ice Sheet (LIS) during the last glaciation, and it played a crucial role in forcing abrupt climate change in the circum-North Atlantic region and perhaps globally. In this collaborative work, we propose to evaluate mechanisms of unstable ice-sheet behavior based on the late-Pleistocene sedimentary record of the Des Moines and Lake Michigan lobes along the southern margin of the LIS. This behavior is far better documented by well-dated records of these lobes than for any other sector of the ice sheet, thus providing the best opportunity to explore mechanisms of such behavior and it relationship to abrupt climate change. Results from this research will have significant application to other areas where unstable behavior of the LIS occurred, but which are less well-constrained by the geologic record (for example, Hudson Strait). Models of ice-sheet instability thus far have treated the sources of the instability as arising from internal ice-sheet dynamics involving saturated, deforming subglacial sediment. The Des Moines and Lake Michigan lobes advanced across fine-grained sediments that, when water saturated, would have deformed under the shear stress applied by the ice. Additional studies, however, suggest external (climate) forcing mechanisms for unstable ice- sheet behavior. We will continue our collaborative work of integrating field, experimental, and modeling studies to investigate potential forcing mechanisms. Our field studies will center on those aspects of the sediment record that offer the most information on subglacial processes with respect to mechanisms of ice-sheet behavior as well as for comparison to modeling studies. Our experimental work will involve using geotechnical analyses of fine-grained diamictons deposited by the lobes to define the range in rheological parameters of different till sheets that are needed in the con stitutive sediment flow law we use in modeling studies. We will use a one-dimensional coupled ice-sediment numerical model that integrates results of experimental work to investigate mechanisms of unstable ice-sheet behavior due to internal ice-sheet dynamics or external climate forcing. Finally, we will focus on intercomparison of results from field and modeling studies of subglacial hydrology, subglacial sediment processes, and sediment transport fluxes.
