Ice sheet model simulations reveal polythermal ice conditions existed across the NE USA during the Last Glacial Maximum

Abstract. Geologic evidence of the Laurentide Ice Sheet (LIS) provides abundant constraints on the areal extent of the ice sheet during the Last Glacial Maximum (LGM). Direct observations of LGM LIS thickness are non-existent, however, with most geologic data across high elevation summits in the Northeastern United States (NE USA) often showing signs of inheritance, indicative of weakly erosive ice flow 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, although this has not been confirmed by ice sheet models. Instead, current ice sheet models simulate warm-based ice conditions across this region, with disagreement likely arising from the use of low resolution meshes (e.g., >20 km) which are unable to resolve the high bedrock relief across this region that strongly influenced overall ice flow and the complex LIS thermal state. Here 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 NE USA and at 3 localities with high bedrock relief (Adirondack Mountains, White Mountains, and Mount Katahdin), with results confirming the existence of a complex thermal regime as interpreted by the geologic data. The 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. These results indicate that across the NE USA, polythermal conditions existed during the LGM. While the majority of this domain is simulated to be warm-based, cold-based ice persists where ice velocities are slow (<15 m/yr) particularly across regional ice divides (e.g., Adirondacks). Additionally, sharp thermal boundaries are simulated where cold-based ice across high elevation summits (White Mountains and Mount Katahdin) flank warm-based ice in adjacent valleys. Because geologic data is geographically limited, these high-resolution simulations can help fill gaps in our understanding of the geographical distribution of the polythermal ice during the LGM. We find that the elevation of this simulated thermal boundary ranges between 800–1500 meters, largely supporting geologic interpretations that polythermal ice conditions existed across NE USA during the LGM, however this boundary varies geographically. In general, we show that a model with finer spatial resolution and higher order physics is able to simulate the polythermal conditions captured in the geologic data, with model output being of potential utility for site selection in future geologic studies and geomorphic interpretation of landscape evolution.