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The timing of the last deglaciation of the Laurentide Ice Sheet in western New York is poorly constrained. The lack of direct chronology in the region has led to a hypothesis that the Laurentide Ice Sheet re-advanced to near its Last Glacial Maximum terminal position in western New York at ∼ 13 ka, which challenges long-standing datasets. To address this hypothesis, we obtained new chronology from the Kent (terminal) and Lake Escarpment (first major recessional) moraines using radiocarbon ages in sediment cores from moraine kettles supplemented with two optically stimulated luminescence ages from topset beds in an ice-contact delta. The two optically stimulated luminescence ages date the Kent (terminal) position to 19.8 ± 2.6 and 20.6 ± 2.9 ka. Within the sediment cores, there is sedimentological evidence of an unstable landscape during basin formation; radiocarbon ages from the lowest sediments in our cores are not in stratigraphic order and date from 19 350–19 600 to 14 050–14 850 cal BP. We interpret these ages as loosely minimum-limiting constraints on ice sheet retreat. Our oldest radiocarbon age of 19 350–19 600 cal BP – from a rip-up clast – suggests ice-free conditions at that time. Above the lowest sediments there is organic-rich silt and radiocarbon ages in stratigraphic order. We interpret the lowest ages in these organic-rich sediments as minimum-limiting constraints on kettle basin formation. The lowest radiocarbon ages from organic-rich sediments from sites on both Kent and Lake Escarpment moraines range from 15 000–15 400 to 13 600–14 000 cal BP. We interpret the 5 kyr lag between the optically stimulated luminescence ages and kettle basin formation as the result of persistent buried ice in ice-cored moraines until ∼ 15 to 14 ka. The cold conditions associated with Heinrich Stadial 1 may have enabled the survival of ice-cored moraines until after 15 ka, and, in turn, climate amelioration during the Bølling period (14.7–14.1 ka) may have initiated landscape stabilization. This model potentially reconciles the sedimentological and chronological evidence underpinning the re-advance hypothesis, which instead could be the result of moraine instability and sediment mobilization during the Bølling–Allerød periods (14.7–13 ka). Age control for future work should focus on features that are not dependent on local climate. [ABSTRACT FROM AUTHOR]

The timing of the last deglaciation of the Laurentide Ice Sheet in western New York is poorly constrained. The lack of direct chronology in the region has led to a provocative hypothesis that the Laurentide Ice Sheet re-advanced to near its Last Glacial Maximum terminal position in western New York at ~13 ka, which challenges long-standing datasets. To address this hypothesis, we obtained new chronology from the Kent (terminal) and Lake Escarpment (first major recessional) moraines using radiocarbon ages in basal sediments from moraine kettles supplemented with two optically stimulated luminescence ages. The two optically stimulated luminescence ages date the Kent (terminal) position to 19.8 ± 2.6 and 20.6 ± 2.9 ka. Within the sediment cores from both moraines, the lowest reliable radiocarbon ages range from 15,000–15,400 to 13,600–14,000 cal yr BP. Below these dated levels is sedimentologic evidence of an unstable landscape during basin formation; radiocarbon ages from these lowest sediments are not in stratigraphic order and date from 19,350–19,600 to 14,050–14,850 cal yr BP. The oldest radiocarbon age of 19,350–19,600 cal yr BP – from a rip-up clast – suggests ice-free conditions at that time. We interpret that the 5 kyr lag between the optically stimulated luminescence ages and the lowest reliable radiocarbon ages is the result of persistent buried ice in ice-cored moraines until ~15 to 14 ka. The cold conditions associated with Heinrich Stadial 1 may have enabled the survival of ice-cored moraines in permafrost until after 15 ka, and in turn, climate amelioration during the Bølling Period (14.7– 14.1 ka) may have initiated landscape stabilization. This model potentially reconciles the sedimentological and chronological evidence underpinning the provocative re-advance hypothesis, which instead could be the result of moraine instability during the Bølling-Allerød periods (14.7–13 ka). Age control for future work should focus on features that are not dependent on local climate. [ABSTRACT FROM AUTHOR]

The lack of continental ice sheets in Alaska during the Last Glacial Maximum (LGM; 26–19 ka) has long been attributed to extensive aridity in the western Arctic. More 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 limited summer temperature depressions in controlling the relatively limited glacier growth during the LGM. To explore this further, we present a new statewide map of LGM alpine glacier equilibrium line altitudes (ELAs), LGM Δ ELAs (LGM ELA anomalies relative to the Little Ice Age, LIA), and Δ ELA-based estimates of temperature depressions across Alaska to assess paleoclimate conditions. We 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) in glaciated massifs in the state, excluding areas in southern Alaska that were covered by the Cordilleran Ice Sheet. The data show a 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 LGM–LIA Δ ELAs from the Alaska Range, supported with limited LGM–LIA Δ ELAs from the Brooks Range and the Kigluaik Mountains, average to - 355 ± 176 m. This value is much greater than the global LGM average of ca. - 1000 m. Using a range of atmospheric lapse rates, LGM–LIA Δ ELAs in Alaska translate to summer cooling of < 2–5 ∘ C. Our results are consistent with a growing number of local climate proxy reconstructions and global data assimilation syntheses that indicate mild summer temperature across Beringia during the LGM. Limited LGM summer temperature depressions could be explained by the influence of Northern Hemisphere ice sheets on atmospheric circulation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 which areas of the present ice sheet are the most sensitive to warming, where a retreating ice sheet would 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. [ABSTRACT FROM AUTHOR]