| Summary: | During the Illinois Episode glaciation (~190,000 to 130,000 years ago), the Laurentide Ice Sheet of North America reached its southernmost extent in southern Illinois, covering approximately 90% of Illinois at the time. In this study, I seek to understand the factors that controlled the shape and extent of the Laurentide Ice Sheet in southern Illinois by using a numerical model of glacial dynamics, ICE CASCADE. My model area includes mainly southern Illinois and south-eastern Indiana, with topography mapped at a 1 km horizontal resolution. Multiple model experiments were run to test the effect of varied surface temperature, ice flux from northern Illinois, bed topography, enhanced sliding in bedrock lowlands, and calving speed in the Wabash and modern and ancestral Mississippi river systems. I compare the extent of modeled ice and the mapped location of the Illinois Episode glacial margin along five flow lines. Provided that my assumptions are corrects, and without accounting for regional scale ice streaming, models which minimize the misfit between modeled and mapped Illinois Episode ice extent require ice thicknesses in central Illinois of approximately 1 km, comparable to independent estimates. However, these best-fitting models also require temperatures of -2°C to -3°C in the St. Louis area, near the cold end of estimates based on regional paleoclimate proxies. All models reached their smallest misfits within 15,000 years, consistent with independent estimates of advance rates during the Wisconsin Episode glaciation. Based on this study, bed topography and increased sliding in lowlands have only minor effects on the overall extent and shape of the model ice sheet during its advance to the penultimate glacial maximum. Calving of ice into the Mississippi and Wabash river valleys, however, strongly influenced modeled ice sheet shape and extent. Significant calving rates, generally of at least 30 to 60 ma-1, are required to minimize misfit between modeled and mapped ice extent. These results indicate that calving may have played a major role in controlling the shape and extent of the Laurentide Ice Sheet in Illinois during the Illinois Episode glaciation, suggesting that the interaction between major river systems and glacier fronts should be considered more generally.
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