Numerical modeling investigations of the subglacial conditions of the southern Laurentide ice sheet Andreas BAUDER, 1,2 David M. MICKELSON, 1 Shawn J. MARSHALL 3 1 Department of Geology and Geophysics, University of Wisconsin–Madison, 1225 West Dayton Street, Madison, WI 53706-1490, USA E-mail: bauder@vaw.baug.ethz.ch 2 Versuchsanstalt fu ¨r Wasserbau, Hydrologie und Glaziologie, Eidgeno ¨ssische Technische Hochschule, ETH-Zentrum, CH-8092 Zu ¨ rich, Switzerland 3 Department of Geography, University of Calgary, Calgary, Alberta T2N 1N4, Canada ABSTRACT. Sub- and proglacial bed conditions influence advance and retreat of an ice sheet. The existence and distribution of frozen ground is of major importance for better understanding of ice-flow dynamics and landform formation. The southern margin of the Laurentide ice sheet (LIS) was dominated by the presence of relatively thin ice lobes that seem to have been very sensitive to external and internal physical conditions. Their extent and dynamics were highly influenced by the interaction of subglacial and proglacial conditions. A three-dimensional thermomechanical ice-sheet model was coupled with a model for the thermal regime in the upper Earth crust. The model has been applied to the LIS in order to investigate the spatial distribution of thermal conditions at the bed. The evolution of the whole LIS was modeled for the last glacial cycle, with primary attention on correct reconstruction of the southern margin. Our results show extensive temporal and spatial frozen ground conditions. Only a slow degradation of permafrost under the ice was found. We conclude that there are significant interactions between the ice sheet and the underlying frozen ground and that these influence both ice dynamics and landform development. 1. INTRODUCTION The influence of water on basal processes is a critical issue in understanding the dynamics of glaciers and ice sheets. Subglacial conditions influence not only the flow regime of ice masses, but also the formation of glacial landforms. Our knowledge of the details of physical processes at the base of the ice is rather limited. In particular, the existence and interaction of water and permanently frozen ground over- ridden by a glacial advance is of major importance for further understanding these processes and the past history of landscapes. The response of frozen ground to changes in surface temperature ranges from a few years to thousands of years. If there is a significant amount of unfrozen water in the soil, a much longer response time results, due to latent heat associated with change in phase. The area of the southern margin of the Laurentide ice sheet (LIS) has been studied and mapped extensively (Colgan and others, 2003; Mickelson and Colgan, 2003). Much of the outer 500 km of the glacierized area is covered with a sediment layer. Based on our present understanding, the conditions necessary for many basal processes have a major impact on the dynamics of an ice sheet. The southern margin of the LIS was dominated by the presence of relatively thin ice lobes that seem to have been very sensitive to external and internal physical conditions, particularly whether the bed was frozen or not. Model results from Marshall and Clark (2002) suggested that 60–80% of the LIS was cold- based at the Last Glacial Maximum (LGM). Extensive investigations with time-dependent, transient ice-sheet models have been carried out on the LIS. Three- dimensional modeling of growth and decay has been used to reconstruct the areal extent and ice volume for the LIS (Marshall and others, 2000, 2002). The impact of ice-stream dynamics in the Hudson Strait, Canada, has been investigated (Marshall and Clarke, 1997). A coupled ice-sheet model with bed thermodynamics for continental scales was introduced by Tarasov and Peltier (1999). Reconstruction of the ice lobes at the southern margin of the ice sheet has been addressed with two-dimensional flowline models (Cutler and others, 2000, 2001). The effects of permafrost on the advancing ice sheet are especially important (Winguth and others, 2004). In this paper, we present a modeling approach to investigate the spatial distribution of subglacial thermal conditions around the southern margin of the LIS. We couple the three-dimensional thermomechanical ice-sheet model (Marshall and others, 2002) with a model for the thermal regime of the upper crust. Our numerical experi- ments are designed to investigate the thermal evolution at the base of the ice sheet at the LGM. 2. MODEL A standard ice-sheet model approach is coupled with a model of the thermodynamics of the upper Earth crust. A detailed description of the UBC ice-sheet model used in this study is given in Marshall and others (2002). The model treats thermomechanical ice flow, a parameterization of the paleoclimatic forcing and the geodynamic calculation of isostatic rebound. The approach has been extended in this analysis by coupling with a thermodynamic treatment of a water–soil mixture based on techniques developed by Osterkamp (1987). Model simulations use a finite-difference scheme on a spherical grid over North America. The vertical coordinate z is defined to be positive upwards, with z ¼ 0 representing the present-day sea level. The grid resolution is 1  in the longitudinal and 0.5  in the latitudinal direction. There are 20 evenly spaced vertical layers for the ice-sheet thickness, and 30 unevenly spaced layers for the upper Annals of Glaciology 40 2005 219