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Direct observations of the size of the Greenland Ice Sheet during Quaternary interglaciations are sparse yet valuable for testing numerical models of ice-sheet history and sea level contribution. Recent measurements of cosmogenic nuclides in bedrock from beneath the Greenland Ice Sheet collected during past deep-drilling campaigns reveal that the ice sheet was significantly smaller, and perhaps largely absent, sometime during the past 1.1 million years. These discoveries from decades-old basal samples motivate new, targeted sampling for cosmogenic-nuclide analysis beneath the ice sheet. Current drills available for retrieving bed material from the US Ice Drilling Program require < 700 m ice thickness and a frozen bed, while quartz-bearing bedrock lithologies are required for measuring a large suite of cosmogenic nuclides. We find that these and other requirements yield only ∼ 3.4 % of the Greenland Ice Sheet bed as a suitable drilling target using presently available technology. Additional factors related to scientific questions of interest are the following: which areas of the present ice sheet are the most sensitive to warming, where would a retreating ice sheet expose bare ground rather than leave a remnant ice cap, and which areas are most likely to remain frozen bedded throughout glacial cycles and thus best preserve cosmogenic nuclides? Here we identify locations beneath the Greenland Ice Sheet that are best suited for potential future drilling and analysis. These include sites bordering Inglefield Land in northwestern Greenland, near Victoria Fjord and Mylius-Erichsen Land in northern Greenland, and inland from the alpine topography along the ice margin in eastern and northeastern Greenland. Results from cosmogenic-nuclide analysis in new sub-ice bedrock cores from these areas would help to constrain dimensions of the Greenland Ice Sheet in the past.

The lack of continental ice sheets in Alaska during the Last Glacial Maximum (LGM; 26–19 ka) has long been attributed to arid and relatively warm summer conditions. Records of this aridity across Alaska are relatively abundant, yet quantitative temperature reconstructions have been comparatively lacking until recently. Climate model outputs, a few isolated paleoclimate studies, and global paleoclimate synthesis products show mild summer temperature depressions in Alaska compared to much of the high northern latitudes. This suggests the importance of summer temperature in controlling the relatively limited glacier growth during the LGM. We present a new statewide map of LGM alpine glacier equilibrium line altitudes (ELAs), LGM ∆ELAs (LGM ELA anomalies relative to the Little Ice Age [LIA]), and ∆ELA-based estimates of temperature depressions across Alaska to assess paleo-precipitation and -temperature conditions. We mapped glacier extents and reconstructed paleoglacier surfaces in ArcGIS to calculate ELAs using an accumulation area ratio (AAR) of 0.58 and an area-altitude balance ratio (AABR) of 1.56. We calculated LGM ELAs (n = 480) across every glaciated massif in the state, excluding areas in southern Alaska that were covered by the Cordilleran Ice Sheet. We see a similar trend of increasing ELAs from the southwest to the northeast during both the LGM and the LIA indicating a consistent southern Bering Sea and northernmost Pacific Ocean precipitation source. Our ∆ELAs from the Alaska and Brooks ranges, and the Kigluaik Mountains, average to −355 ± 176 m, well above the global LGM average of ca. −1000 m. Using atmospheric lapse rates, we calculate minimum summer cooling of −3.5 ± 1.7 ºC and maximum summer temperature depressions of −1.9 ± 0.9 ºC. Our results are consistent with a growing number of local proxy reconstructions and global data assimilation syntheses that indicate mild summer temperature across Beringia. Limited summer temperature depressions could be explained by increased incoming solar radiation across Alaska during the LGM. Limited summer temperature depressions – and general aridity – in Alaska during the LGM have been previously hypothesized as resulting from the complex influence of North American ice sheets on atmospheric circulation.

Direct observations of the size of the Greenland Ice Sheet during Quaternary interglaciations are sparse yet valuable for testing numerical models of ice-sheet history and sea level contribution. Recent measurements of cosmogenic nuclides in bedrock from beneath the Greenland Ice Sheet collected during past deep-drilling campaigns reveal that the ice sheet was significantly smaller, and perhaps largely absent, sometime during the past 1.1 million years. These discoveries from decades-old basal samples motivate new, targeted sampling for cosmogenic-nuclide analysis beneath the ice sheet. Current drills available for retrieving bed material from the US Ice Drilling Program require < 700 m ice thickness and a frozen bed, while quartz-bearing bedrock lithologies are required for measuring a large suite of cosmogenic nuclides. We find that these and other requirements yield only ∼ 3.4 % of the Greenland Ice Sheet bed as a suitable drilling target using presently available technology. Additional factors related to scientific questions of interest are the following: which areas of the present ice sheet are the most sensitive to warming, where would a retreating ice sheet expose bare ground rather than leave a remnant ice cap, and which areas are most likely to remain frozen bedded throughout glacial cycles and thus best preserve cosmogenic nuclides? Here we identify locations beneath the Greenland Ice Sheet that are best suited for potential future drilling and analysis. These include sites bordering Inglefield Land in northwestern Greenland, near Victoria Fjord and Mylius-Erichsen Land in northern Greenland, and inland from the alpine topography along the ice margin in eastern and northeastern Greenland. Results from cosmogenic-nuclide analysis in new sub-ice bedrock cores from these areas would help to constrain dimensions of the Greenland Ice Sheet in the past.