ICE SHEET MODEL SIMULATIONS REVEAL POLYTHERMAL ICE CONDITIONS EXISTED ACROSS THE NORTHEAST UNITED STATES DURING THE LAST GLACIAL MAXIMUM

Dipstick studies have become useful tools to assess the vertical thinning history of the Laurentide Ice Sheet (LIS) during the last deglaciation across the Northeastern United States. However, geochronological constraints on past ice sheet thinning across high elevation summits often shows signs of inheritance, indicative of weakly erosive ice flow conditions and the presence of cold-based ice. While warm based ice and erosive conditions likely existed on the flanks of these summits and throughout neighboring valleys, summit inheritance issues have hampered efforts to constrain the timing of the emergence of ice-free conditions at high elevation summits. These geomorphic reconstructions indicate that a complex erosional and thermal regime likely existed across the southeasternmost extent of the LIS sometime during the LGM. While ice sheet models have been successful in simulating the response of the LIS to deglacial warming, current models simulate warm based ice conditions across this region, and therefore do not capture this detailed transition from cold based ice at high elevation summits to warm based ice at lower elevations. Because this has implications both for our understanding of the geomorphology and ice history of this region, we use a newer generation ice sheet model, the Ice Sheet and Sea-level System Model (ISSM), to simulate the LGM conditions of the LIS across the Northeast United States. Our model uses higher order physics, a small ensemble of LGM climate boundary conditions, and a high-resolution horizontal mesh that resolves bedrock features down to 30 meters to reconstruct LGM ice flow, ice thickness, and thermal conditions. The simulations agree with existing geomorphic data of ice flow and thermal conditions at the LGM. Across the Adirondack Mountains, Mt. Washington, and Mount Katahdin (and more locations), warm based ice covers the valleys and flanks surrounding high peaks, while cold based ice is simulated at the summits. The elevation of this thermal boundary varies depending on the geographic location and climate boundary conditions used, but ranges between 800-1200 meters, and is defined by low ice velocities (<15 m/yr). In total, our simulations support that polythermal ice conditions existed across Northeast United States during the LGM.