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  1. Abstract In Southeast Greenland, summer melt and high winter snowfall rates give rise to firn aquifers: vast stores of meltwater buried beneath the ice-sheet surface. Previous detailed studies of a single Greenland firn aquifer site suggest that the water drains into crevasses, but this is not known at a regional scale. We develop and use a tool in Ghub, an online gateway of shared datasets, tools and supercomputing resources for glaciology, to identify crevasses from elevation data collected by NASA's Airborne Topographic Mapper across 29000 km 2 of Southeast Greenland. We find crevasses within 3 km of the previously mapped downglacier boundary of the firn aquifer at 20 of 25 flightline crossings. Our data suggest that crevasses widen until they reach the downglacier boundary of the firn aquifer, implying that crevasses collect firn-aquifer water, but we did not find this trend with statistical significance. The median crevasse width, 27 meters, implies an aspect ratio consistent with the crevasses reaching the bed. Our results support the idea that most water in Southeast Greenland firn aquifers drains through crevasses. Less common fates are discharge at the ice-sheet surface (3 of 25 sites) and refreezing at the aquifer bottom (1 of 25 sites). 
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  2. Abstract. Numerical simulations of the Greenland Ice Sheet (GrIS) over geologictimescales can greatly improve our knowledge of the critical factors drivingGrIS demise during climatically warm periods, which has clear relevance forbetter predicting GrIS behavior over the upcoming centuries. To assess thefidelity of these modeling efforts, however, observational constraints ofpast ice sheet change are needed. Across southwestern Greenland, geologicrecords detail Holocene ice retreat across both terrestrial-based and marine-terminating environments, providing an ideal opportunity to rigorouslybenchmark model simulations against geologic reconstructions of ice sheetchange. Here, we present regional ice sheet modeling results using theIce-sheet and Sea-level System Model (ISSM) of Holocene ice sheet historyacross an extensive fjord region in southwestern Greenland covering thelandscape around the Kangiata Nunaata Sermia (KNS) glacier and extendingoutward along the 200 km Nuup Kangerula (Godthåbsfjord). Oursimulations, forced by reconstructions of Holocene climate and recentlyimplemented calving laws, assess the sensitivity of ice retreat across theKNS region to atmospheric and oceanic forcing. Our simulations reveal thatthe geologically reconstructed ice retreat across the terrestrial landscapein the study area was likely driven by fluctuations in surface mass balancein response to Early Holocene warming – and was likely not influencedsignificantly by the response of adjacent outlet glaciers to calving andocean-induced melting. The impact of ice calving within fjords, however,plays a significant role by enhancing ice discharge at the terminus, leadingto interior thinning up to the ice divide that is consistent withreconstructed magnitudes of Early Holocene ice thinning. Our results,benchmarked against geologic constraints of past ice-margin change, suggestthat while calving did not strongly influence Holocene ice-margin migrationacross terrestrial portions of the KNS forefield, it strongly impactedregional mass loss. While these results imply that the implementation andresolution of ice calving in paleo-ice-flow models is important towardsmaking more robust estimations of past ice mass change, they also illustratethe importance these processes have on contemporary and future long-term icemass change across similar fjord-dominated regions of the GrIS. 
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  3. Abstract We leverage a data set of >720 shell-bearing marine deposits throughout southeastern Alaska (USA) to develop updated relative sea-level curves that span the past ∼14,000 yr. This data set includes site location, elevation, description when available, and 436 14C ages, 45 of which are published here for the first time. Our sea-level curves suggest a peripheral forebulge developed west of the retreating Cordilleran Ice Sheet (CIS) margin between ca. 17,000 and 10,800 calibrated yr B.P. By 14,870 ± 630 to 12,820 ± 340 cal. yr B.P., CIS margins had retreated from all of southeastern Alaska's fjords, channels, and passages. At this time, isolated or stranded ice caps existed on the islands, with alpine or tidewater glaciers in many valleys. Paleoshorelines up to 25 m above sea level mark the maximum elevation of transgression in the southern portion of the study region, which was achieved by 11,000 ± 390 to 10,500 ± 420 cal. yr B.P. The presence of Pacific sardine (Sardinops sagax) and the abundance of charcoal in sediments that date between 11,000 ± 390 and 7630 ± 90 cal. yr B.P. suggest that both ocean and air temperatures in southeastern Alaska were relatively warm in the early Holocene. The sea-level and paleoenvironmental reconstruction presented here can inform future investigations into the glacial, volcanic, and archaeological history of southeastern Alaska. 
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  4. Abstract. 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 cosmogenicnuclides in bedrock from beneath the Greenland Ice Sheet collected duringpast deep-drilling campaigns reveal that the ice sheet was significantlysmaller, and perhaps largely absent, sometime during the past 1.1 millionyears. 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 IceDrilling Program require < 700 m ice thickness and a frozen bed,while quartz-bearing bedrock lithologies are required for measuring a largesuite of cosmogenic nuclides. We find that these and other requirementsyield only ∼ 3.4 % of the Greenland Ice Sheet bed as asuitable drilling target using presently available technology. Additionalfactors related to scientific questions of interest are the following: which areas of thepresent ice sheet are the most sensitive to warming, where would a retreating icesheet expose bare ground rather than leave a remnant ice cap, andwhich areas are most likely to remain frozen bedded throughout glacialcycles and thus best preserve cosmogenic nuclides? Here we identifylocations beneath the Greenland Ice Sheet that are best suited for potentialfuture drilling and analysis. These include sites bordering Inglefield Landin northwestern Greenland, near Victoria Fjord and Mylius-Erichsen Land innorthern Greenland, and inland from the alpine topography along the icemargin 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. 
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  5. null (Ed.)
    Abstract Understanding marine-terminating ice sheet response to past climate transitions provides valuable long-term context for observations of modern ice sheet change. Here, we reconstruct the last deglaciation of marine-terminating Cordilleran Ice Sheet (CIS) margins in Southeast Alaska and explore potential forcings of western CIS retreat. We combine 27 new cosmogenic 10 Be exposure ages, 13 recently published 10 Be ages, and 25 new 14 C ages from raised marine sediments to constrain CIS recession. Retreat from the outer coast was underway by 17 ka, and the inner fjords and sounds were ice-free by 15 ka. After 15 ka, the western margin of the CIS became primarily land-terminating and alpine glaciers disappeared from the outer coast. Isolated alpine glaciers may have persisted in high inland peaks until the early Holocene. Our results suggest that the most rapid phase of CIS retreat along the Pacific coast occurred between ~17 and 15 ka. This retreat was likely driven by processes operating at the ice-ocean interface, including sea level rise and ocean warming. CIS recession after ~15 ka occurred during a time of climatic amelioration in this region, when both ocean and air temperatures increased. These data highlight the sensitivity of marine-terminating CIS regions to deglacial climate change. 
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  6. Abstract

    Changes in ice‐sheet size impact atmospheric circulation, a phenomenon documented by models but constrained by few paleoclimate records. We present sub‐centennial‐scale records of summer temperature and summer precipitation hydrogen isotope ratios (δ2H) spanning 12–7 ka from a lake on Baffin Island. In a transient model simulation, winds in this region were controlled by the relative strength of the high‐pressure systems and associated anticyclonic circulation over the retreating Greenland and Laurentide ice sheets. The correlation between summer temperature and precipitation δ2H proxy records changed from negative to positive at 9.8 ka. This correlation structure indicates a shift from alternating local and remote moisture, governed by the two ice‐sheet high‐pressure systems, to only remote moisture after 9.8 ka, governed by the strong Greenland high‐pressure system after the Laurentide Ice Sheet retreated. Such rapid atmospheric circulation changes may also occur in response to future, gradual ice‐sheet retreat.

     
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  7. The North Atlantic was a key locus for circulation-driven abrupt climate change in the past and could play a similar role in the future. Abrupt cold reversals, including the 8.2 ka event, punctuated the otherwise warm early Holocene in the North Atlantic region and serve as useful paleo examples of rapid climate change. In this work, we assess the cryospheric response to early Holocene climate history on Baffin Island, Arctic Canada, using cosmogenic radionuclide dating of moraines. We present 39 new 10Be ages from four sets of multi-crested early Holocene moraines deposited by cirque glaciers and ice cap outlet glaciers, as well as erratic boulders along adjacent fiords to constrain the timing of regional deglaciation. The age of one moraine is additionally constrained by in situ 14C measurements, which confirm 10Be inheritance in some samples. All four moraines were deposited between ~9.2 and 8.0 ka, and their average ages coincide with abrupt coolings at 9.3 and 8.2 ka that are recorded in Greenland ice cores. Freshwater delivery to the North Atlantic that reduced the flux of warm Atlantic water into Baffin Bay may explain brief intervals of glacier advance, although moraine formation cannot be definitively tied to centennial-scale cold reversals. We thus explore other possible contributing factors, including ice dynamics related to retreat of Laurentide Ice Sheet outlet glaciers. Using a numerical glacier model, we show that the debuttressing effect of trunk valley deglaciation may have contributed to these morainebuilding events. These new age constraints and process insights highlight the complex behavior of the cryosphere during regional deglaciation and suggest that multiple abrupt cold reversalsdas well as deglacial ice dynamicsdlikely played a role in early Holocene moraine formation on Baffin Island. 
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  8. Abstract. Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE), the Greenland Ice Sheet (GrIS) was smaller than its currentconfiguration. Determining the exact dimensions of the Holocene ice-sheetminimum and the duration that the ice margin rested inboard of its currentposition remains challenging. Contemporary retreat of the GrIS from itshistorical maximum extent in southwestern Greenland is exposing a landscapethat holds clues regarding the configuration and timing of past ice-sheetminima. To quantify the duration of the time the GrIS margin was near itsmodern extent we develop a new technique for Greenland that utilizes in situcosmogenic 10Be–14C–26Al in bedrock samples that have becomeice-free only in the last few decades due to the retreating ice-sheet margin atKangiata Nunaata Sermia (n=12 sites, 36 measurements; KNS), southwest Greenland. To maximizethe utility of this approach, we refine the deglaciation history of the regionwith stand-alone 10Be measurements (n=49) and traditional 14C agesfrom sedimentary deposits contained in proglacial–threshold lakes. We combineour reconstructed ice-margin history in the KNS region with additionalgeologic records from southwestern Greenland and recent model simulations ofGrIS change to constrain the timing of the GrIS minimum in southwestGreenland and the magnitude of Holocene inland GrIS retreat, as well as to explore theregional climate history influencing Holocene ice-sheet behavior. Our10Be–14C–26Al measurements reveal that (1) KNS retreated behindits modern margin just before 10 ka, but it likely stabilized near thepresent GrIS margin for several thousand years before retreating fartherinland, and (2) pre-Holocene 10Be detected in several of our sample sitesis most easily explained by several thousand years of surface exposure duringthe last interglaciation. Moreover, our new results indicate that the minimumextent of the GrIS likely occurred after ∼5 ka, and the GrISmargin may have approached its eventual historical maximum extent as early as∼2 ka. Recent simulations of GrIS change are able to match thegeologic record of ice-sheet change in regions dominated by surface massbalance, but they produce a poorer model–data fit in areas influenced by oceanicand dynamic processes. Simulations that achieve the best model–data fitsuggest that inland retreat of the ice margin driven by early to middleHolocene warmth may have been mitigated by increased precipitation. Triple10Be–14C–26Al measurements in recently deglaciated bedrockprovide a new tool to help decipher the duration of smaller-than-present iceover multiple timescales. Modern retreat of the GrIS margin in southwestGreenland is revealing a bedrock landscape that was also exposed during themigration of the GrIS margin towards its Holocene minimum extent, but it has yetto tap into a landscape that remained ice-covered throughout the entireHolocene. 
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