The contribution of the Greenland Ice Sheet (GrIS) to sea-level rise is accelerating and there is an urgent need to characterize which sectors of the ice sheet are the most vulnerable. Estimating the volume of Greenland ice that was lost during past warm periods can support efforts to constrain the ice sheet's response to future warming. Sub-ice sediment and bedrock, retrieved from deep ice core campaigns or targeted drilling efforts, yield critical and direct information about past ice-free conditions. However, it is challenging to scale the few available sub-ice point measurements to the geometry of the entire ice sheet. Here, we provide a framework for assessing sea-level potential, which we define within an ensemble of ice-sheet model simulations as the amount the GrIS has contributed to sea level when a particular location in Greenland is ice free. An assessment of dominant sources of uncertainty in our paleo ice-sheet modelling, including climate forcing, ice-sheet initialization, and solid-Earth properties, reveals spatial patterns in the sensitivity of the ice sheet to these processes and related feedbacks. We find that the sea-level potential of central Greenland is most sensitive to lithospheric feedbacks and ice-sheet initialization, whereas the ice-sheet margins are most sensitive to climate forcing parameters. We map the GrIS response to warming, in order to (1) estimate of the region(s) of GrIS that likely contributed to the first meter(s) of global sea-level change across a range of plausible deglaciation scenarios, (2) guide future sub-glacial access efforts that can provide targeted information about the response of the ice sheet to past warming, and (3) contextualize existing and future datasets within a glaciologically coherent, full-geometry framework to establish the minimum GrIS contribution to past sea level when a particular location is ice-free. Through our ensemble approach, we can assign a plausible range of GrIS contributions to global sea level for deglaciated conditions at any site. Our results identify primarily areas in southwest Greenland, and secondarily north Greenland, as best-suited for subglacial access drilling that seeks to constrain the response of the ice sheet to past and future warming.

Projections of future sea-level rise benefit from understanding the response of past ice sheets to warming during past Quaternary interglacials. Constraints on the extent of inland Greenland Ice Sheet retreat during the Middle Holocene (~8–4 thousand years before present) are limited because geological records of a smaller-than-modern phase largely remain beneath the modern ice sheet. We drilled through 509 metres of firn and ice at Prudhoe Dome, northwestern Greenland, to obtain sub-ice material yielding direct evidence for the response of the northwest Greenland ice sheet to Holocene warmth. Here we present infrared stimulated luminescence measurements from sub-ice sediments that indicate that the ground below the summit was exposed to sunlight 7.1 ± 1.1 thousand years ago. This proposed complete deglaciation of Prudhoe Dome, coeval to reduced extent at other ice caps across northern Greenland, is consistent with interglacial-only δ18O values from the Prudhoe Dome ice column and ice depth–age modelling. Our results point to a substantial response of the northwest Greenland ice sheet to early Holocene warming, estimated to be +3–5 °C from palaeoclimate data. This range of summer temperatures is similar to projections of warming by 2100 CE. The ~500-metre-thick Prudhoe Dome in northwestern Greenland completely deglaciated 7,000 years ago, highlighting the sensitivity of the ice sheet to mid-Holocene warming, according to luminescence and geochemical data from sub-ice sediments and ice cores.

The lack of geological constraints on past ice-sheet change in marine-based sectors of the Greenland Ice Sheet (GrIS) following the Last Glacial Maximum limits our ability to assess (1) the drivers of ice-sheet change, and (2) the performance of ice-sheet models that are benchmarked against the paleo-record of GrIS change. Here, we provide new in situ 10Be surface exposure chronologies of ice-sheet margin retreat from the outer Scoresby Sund and Storstrømmen Glacier regions in eastern and northeastern Greenland, respectively. Ice retreated from Rathbone Island, east of Scoresby Sund, by ∼ 14.1 ka, recording some of the earliest documentations of terrestrial deglaciation in Greenland. The mouth of Scoresby Sund deglaciated by ∼ 13.2 ka, and retreated at an average rate of ∼ 43 m yr−1 between 13.2 and 9.7 ka. Storstrømmen Glacier retreated from the outer coast to within ∼ 3 km of the modern ice margin between ∼ 12.7 and 8.6 ka at an average rate of ∼ 28 m yr−1. Retreat then slowed or reached a stillstand as ice retreated ∼ 3 km between ∼ 8.6 ka to the modern ice margin at ∼ 8.0 ka. These retreat rates are consistent with late glacial and Holocene estimates for marine-terminating outlet glaciers across East Greenland, and comparable to modern retreat rates observed at the largest ice streams in northeastern, and northwestern Greenland.

We used mapping of bedrock lithology, bedrock fractures, and lake density in Inglefield Land, northwestern Greenland, combined with cosmogenic nuclide (10Be and 26Al) measurements in bedrock surfaces, to investigate glacial erosion and the ice sheet history of the northwestern Greenland Ice Sheet. The pattern of eroded versus weathered bedrock surfaces and other glacial erosion indicators reveal temporally and spatially varying erosion under cold- and warm-based ice. All of the bedrock surfaces that we measured in Inglefield Land contain cosmogenic nuclide inheritance with apparent 10Be ages ranging from 24.9 ± 0.5 to 215.8 ± 7.4 ka. The 26Al/10Be ratios require minimum combined surface burial and exposure histories of ∼ 150 to 2000 kyr. Because our sample sites span a relatively small area that experienced a similar ice sheet history, we attribute differences in nuclide concentrations and ratios to varying erosion during the Quaternary. We show that an ice sheet history with ∼ 900 kyr of exposure and ∼ 1800 kyr of ice cover throughout the Quaternary is consistent with the measured nuclide concentrations in most samples when sample-specific subaerial erosion rates are between 0 and 2 × 10−2 mm yr−1 and subglacial erosion rates are between 0 and 2 × 10−3 mm yr−1. These erosion rates help to characterize Arctic landscape evolution in crystalline bedrock terrains in areas away from focused ice flow.

Projections of future sea-level rise benefit from understanding the response of past ice sheets to interglacial warmth. Constraints on the extent of inland Greenland Ice Sheet (GrIS) recession during the Middle Holocene (~8 – 4 ka) are limited because geological records of a smaller-than-modern phase largely remain beneath the modern ice sheet. We drilled through 509 m of firn and ice at Prudhoe Dome (PD), northwest Greenland to obtain sub-ice material yielding direct evidence for the response of the NW GrIS to Holocene warmth. Our infrared stimulated luminescence measurements from sub-ice sediments indicates that the ground below the summit was exposed to sunlight at 7.1 ± 1.1 ka. This complete deglaciation of PD, coeval to reduced extent at other ice caps across Northern Greenland, is further supported by interglacial-only δ18O values from the PD ice column as well as ice depth-age modeling. Our results point to a significant response of the NW GrIS to early Holocene warming, estimated to be +3–5 ℃ from paleoclimate data. This range of summer temperatures is similar to projections of warming by 2100 CE.

We present 40 new 10Be exposure ages of moraines and other glacial deposits left behind by the southeastern sector of the Laurentide Ice Sheet (LIS) in southern New England and New York, summarize the regional moraine record, and interpret the dataset in the context of previously published deglaciation chronologies. The regional moraine record spans the Last Glacial Maximum (LGM), with the outermost ridge of the terminal complex dating to similar to 26-25 ka, the innermost ridge of the terminal complex dating to similar to 22 ka, and a series of smaller recessional limits within similar to 50 km of the terminal complex dating to similar to 21-20.5 ka. The chronology generally agrees with independent age constraints from radiocarbon and glacial varves. A few inconsistencies between ages from cosmogenic-nuclide measurements and those from other dating methods are explained by geological scatter, where several bedrock samples and boulders from the outer terminal moraine exhibit nuclide inheritance, while some exposure ages of large moraines are likely affected by postdepositional disturbance. The exposure age chronology places the southeastern sector of the LIS at or near its maximum extent, from similar to 26 to 21 ka, which is broadly consistent with the LGM sea-level lowstand, local and regional temperature indicators, and local summer insolation. The net change in LIS extent, represented by this chronology, occurred more slowly (< 5 to 25 m yr(-1)) than the subsequent retreat through the rest of New England, consistent with a slow general rise in insolation and modeled summer temperature. We conclude that the major pulse of LIS deglaciation and accelerated recession, recorded by dated glacial deposits north of the moraines discussed here, did not begin until after atmospheric CO2 increased around 18 ka, marking the onset of Termination I.

The melting of the Greenland ice sheet (GrIS) is a major contributor to past and future global sea-level rise. Understanding the response of the GrIS to times in the past when temperatures were as warm or warmer than today offers insights into its current and future response to climate change. In the southwest sector, the GrIS retreated inland beyond its current margin during the (at least regionally) warmer-than-present mid-Holocene, before it readvanced to the historical maximum position during the Little Ice Age. This was then followed by a slight retreat to its current position. To investigate the timing and magnitude of southwest GrIS retreat and readvance in response to Holocene warmth, we model the response of the solid Earth and local relative sea level (RSL) to past ice sheet change. We test a suite of eleven ice sheet scenarios that are based on ICE-6G_C but are modified in the timing and magnitude of ice retreat and readvance and pair them with four different viscoelastic Earth structures. We compare model predictions to observations of paleo sea level, present-day sea-level change, and present-day vertical land motion (VLM) around Nuuk, Greenland. We find that the modeled timing and magnitude of the Holocene retreat and readvance have a significant impact on modern sea-level change and VLM in Nuuk. Models that assume a readvance approaching the southwest GrIS’ historical maximum between 2 and 1 ka are most consistent with observations. The RSL response, however, is less sensitive to the timing of the minimum GrIS extent. Nonetheless, better data-model fits are generally obtained when the minimum ice sheet extent is reached between 5 and 3 ka, within the tested range of 6-3 ka. Comparing this timing to local and regional records of temperature and ice-sheet change suggest that the evolution of the southwestern GrIS presented here was in-phase with the likely evolution of southwestern GrIS mass balance through the Holocene. Our results have implications for future ice sheet modeling studies targeting southwestern Greenland by providing additional constraints and strengthening existing ones. Moreover, this work provides a deeper understanding of the interactions between the climate and the cryosphere and thus of future ice sheet change.

Alpine glacier-based temperature reconstructions spanning the last deglaciation provide critical constraints on local to regional climate change and have been reported from several formerly glaciated regions around the world yet remain sparse from high-northern-latitude regions. Using newly and previously 10Be-dated moraines, we report paleo-glacier equilibrium line altitudes (ELAs) for 15 time slices spanning the Last Glacial Maximum (LGM) to the Little Ice Age (LIA) for a valley in the western Alaska Range. We translate our ELA reconstructions into a proxy for summer temperature by applying a dry adiabatic lapse rate at each reconstructed ELA relative to the outermost LIA moraine. We observe ∼4 °C warming through the last deglaciation at our site that took place in two steps following initial gradual warming: ∼1.5 °C abrupt warming at 16 ka, ∼2 kyr after the onset of global CO2 rise, and ∼2 °C warming at ∼15 ka, near the start of the Bølling. Moraine deposition and modest summer cooling during Heinrich Stadial 1 and the early Younger Dryas (YD) suggest that despite these events being expressed more strongly in wintertime, the classic blueprint of North Atlantic climate variability extends to the western Arctic region.

Abstract. The contribution of the Greenland Ice Sheet (GIS) to sea level rise (SLR) is accelerating and there is an urgent need to improve predictions of when and from what parts of the ice sheet Greenland will contribute its first meter. Estimating the volume of Greenland ice that was lost during past warm periods offers a way to constrain the ice sheet’s response to future warming. Sub-ice sediment and bedrock, retrieved from deep ice core campaigns or targeted drilling efforts, yield critical and direct information about past ice-free conditions. However, it is challenging to scale the few available sub-ice point measurements to the geometry of the entire ice sheet. Here, we provide a framework for assessing sea-level potential, which we define as the amount the GIS has contributed to sea level when a particular location in Greenland is ice-free, from an ensemble of ice-sheet model simulations representing a wide range of plausible deglaciation scenarios. An assessment of dominant sources of uncertainty in our paleo ice sheet modelling, including climate forcing, ice-sheet initialization, and solid-Earth properties, reveals spatial patterns in the sensitivity of the ice sheet to these processes and related feedbacks. We find that the sea-level potential of central Greenland is most sensitive to lithospheric feedbacks and ice-sheet initialization, whereas the ice-sheet margins are most sensitive to climate forcing parameters. Our framework allows us to quantify the local and regional uncertainty in sea-level potential, which we use to evaluate the GIS bedrock according to the usefulness of information sub-ice sediments and bedrock provide about past ice-sheet geometry. Through our ensemble approach, we can assign a plausible range of GIS contributions to global sea level for deglaciated conditions at any site. Our results identify primarily areas in southwest Greenland, and secondarily north Greenland, as best-suited for subglacial access drilling that seeks to constrain the response of the ice sheet to past and future warming.

Glacial erosion creates diagnostic landscapes and vast amounts of sediment. However, knowledge about the rate at which glaciers erode and sculpt bedrock and the proportion of quarried (plucked) versus abraded material is limited. To address this, we quantify subglacial erosion rates and constrain the ratio of quarrying to abrasion during a recent, ∼ 200-year long overriding of a bedrock surface fronting, Sermeq Kujalleq (Jakobshavn Isbræ), Greenland, by combining 10Be analyses, a digital terrain model, and field observations. Cosmogenic 10Be measurements along a 1.2 m tall quarried bedrock step reveal a triangular wedge of quarried rock. Using individual 10Be measurements from abraded surfaces across the study area, we derive an average abrasion rate of 0.13 ± 0.08 mm yr−1. By applying this analysis across a ∼ 1.33 km2 study area, we estimate that the Greenland Ice Sheet quarried 378 ± 45 m3 and abraded 322 ± 204 m3 of material at this site. These values result in an average total erosion rate of 0.26 ± 0.16 mm yr−1, with abrasion and quarrying contributing in roughly equal proportions within uncertainty. Additional cosmogenic 10Be analysis and surface texture mapping indicate that many lee steps are relicts from the prior glaciation and were not re-quarried during the recent overriding event. These new observations of glacier erosion in a recently exposed landscape provide one of the first direct measurements of quarrying rates and indicate that quarrying accounts for roughly half of the total glacial erosion in representative continental shield lithologies.

Numerical simulations of the Greenland Ice Sheet (GrIS) over geologic timescales can greatly improve our knowledge of the critical factors driving GrIS demise during climatically warm periods, which has clear relevance for better predicting GrIS behavior over the upcoming centuries. To assess the fidelity of these modeling efforts, however, observational constraints of past ice sheet change are needed. Across southwestern Greenland, geologic records detail Holocene ice retreat across both terrestrial-based and marine-terminating environments, providing an ideal opportunity to rigorously benchmark model simulations against geologic reconstructions of ice sheet change. Here, we present regional ice sheet modeling results using the Ice-sheet and Sea-level System Model (ISSM) of Holocene ice sheet history across an extensive fjord region in southwestern Greenland covering the landscape around the Kangiata Nunaata Sermia (KNS) glacier and extending outward along the 200 km Nuup Kangerula (Godthåbsfjord). Our simulations, forced by reconstructions of Holocene climate and recently implemented calving laws, assess the sensitivity of ice retreat across the KNS region to atmospheric and oceanic forcing. Our simulations reveal that the geologically reconstructed ice retreat across the terrestrial landscape in the study area was likely driven by fluctuations in surface mass balance in response to Early Holocene warming – and was likely not influenced significantly by the response of adjacent outlet glaciers to calving and ocean-induced melting. The impact of ice calving within fjords, however, plays a significant role by enhancing ice discharge at the terminus, leading to interior thinning up to the ice divide that is consistent with reconstructed magnitudes of Early Holocene ice thinning. Our results, benchmarked against geologic constraints of past ice-margin change, suggest that while calving did not strongly influence Holocene ice-margin migration across terrestrial portions of the KNS forefield, it strongly impacted regional mass loss. While these results imply that the implementation and resolution of ice calving in paleo-ice-flow models is important towards making more robust estimations of past ice mass change, they also illustrate the importance these processes have on contemporary and future long-term ice mass change across similar fjord-dominated regions of the GrIS.

Sea-level rise of even one meter will have drastic global impacts. Melting the Greenland Ice Sheet (GIS) would raise sea level by 7.4 meters. There is an urgent need to improve predictions of how quickly the GIS will contribute its first meter of sea-level rise, and from where on the ice sheet that water will come. Estimating the volume of Greenland ice that was lost during past warm periods offers a way to constrain the ice sheet’s likely response to future warming. Here, we assess the sea-level potential across Greenland, based on an ensemble of ice-sheet model simulations that represent a wide range of plausible deglaciation styles. The most vulnerable region of the ice sheet is in West Greenland between approximately 64ºN and 76ºN, ranging from ~10 to ~150 km behind the present-day ice margin. The ensemble spread for the most stable regions of the GIS is sensitive to lithospheric feedbacks, while the most vulnerable GIS region is predominantly sensitive to spatial climatology and precipitation lapse rate. These results can guide future subglacial sampling by identifying regions and locations where such data will have the greatest impact on our understanding of ice-sheet vulnerability/contribution to sea-level rise in a warming world.

We report 50 new and 22 previously published Be-10 ages from 15 distinct moraine crests in the western Alaska Range spanning from the Last Glacial Maximum (LGM) terminal moraine to a latest Pleistocene moraine immediately down valley from late Holocene moraines. Moraines were deposited between 21.3 +/- 0.8 ka and 12.8 +/- 0.6 ka and conform morphostratigraphically, giving us high confidence in the chronology. Our record, and the key records reviewed in our compilation from across Alaska indicate a culmination of the regional LGM between ca. 21-19 ka. Our chronology, unmatched in resolution from a single valley in Alaska, indicates that steady glacier recession from ca. 21-18 ka was punctuated by accelerated retreat from ca. 17-16 ka, followed by a period of prolonged moraine deposition between ca. 16 and 15 ka. After ca. 15 ka rapid glacier retreat was punctuated by a re-advance ca. 12.8 ka. Other chronologies across Alaska show further evidence of moraine deposition between ca. 16-15 ka and ca. 13-12 ka. The emerging pattern of glacier retreat through the last deglaciation in Alaska is compared to several global, regional, and local climate proxies to assess what climate factors controlled the timing and pace of glacier retreat in Alaska. We hypothesize that warming caused by rising boreal summer insolation drove initial and steady deglaciation from the LGM terminal moraine position until ca. 18 ka, after which time global warming from rising CO2 concentrations accelerated retreat. Subsequent periods of moraine deposition in Alaska coincide with decreasing trends in the NGRIP ice core record, at the culmination of Heinrich Stadial 1 (ca. 16-15 ka), and during the early Younger Dryas between 13 and 12.5 ka. While comparisons are made between alpine glacier records and the timing of other geologic events that may have impacted local-to-regional climate (e.g., the opening of the Bering Strait, the saddle collapse between the Laurentide and Cordilleran Ice sheets, and post-LGM Bering Sea dynamics), the relationship between our record and these geologic events are ambiguous. We suggest glaciers across Alaska were possibly more sensitive to other regional and global climate forcing mechanisms, mainly rising insolation, global CO2 rise, and Northern Hemisphere Ocean circulation forcing through deglaciation. (C) 2022 Elsevier Ltd. All rights reserved.

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 maximum extent and elevation of the Greenland Ice Sheet in southwestern Greenland during the Last Glacial Maximum (LGM, 26-19.5 ka) is poorly constrained. Yet, the size of the Greenland Ice Sheet during the LGM helps to inform estimates of past ice-sheet sensitivity to climate change and provides benchmarks for ice-sheet modeling. Reconstructions of LGM ice extents vary between an inner continental shelf minimum, a mid-shelf position, and a maximum extent at the shelf break. We use three approaches to resolve LGM ice extent in the Sisimiut sector of southwestern Greenland. First, we explore the likelihood of minimum versus maximum Greenland Ice Sheet reconstructions using existing relative sea-level data. We use an empirical relationship between marine limit elevation and distance to LGM terminus established from other Northern Hemisphere Pleistocene ice sheets as context for interpreting marine limit data in southwestern Greenland. Our analysis supports a maximum regional Greenland Ice Sheet extent to the shelf break during the LGM. Second, we apply a simple 1-D crustal rebound model to simulate relative sea-level curves for contrasting ice-sheet sizes and compare these simulated curves with existing relative sea-level data. The only realistic ice-sheet configuration resulting in relative sea-level model-data fit suggests that the Greenland Ice Sheet terminated at the shelf break during the LGM. Lastly, we constrain the LGM ice-sheet thickness using cosmogenic Be-10, Al-26, and C-14 exposure dating from two summit areas, one at 381 m above sea level at the coast, and another at 798 m asl 32 km inland. Twenty-four cosmogenic radionuclide measurements, combined with results of our first two approaches, reveal that our targeted summits were likely ice-covered during the LGM and became deglaciated at ca. 11.6 ka. Inventories of in situ C-14 in bedrock at one summit point to a small degree of inherited C-14 and suggest that the Greenland Ice Sheet advanced to its maximum late Pleistocene extent at 17.1 +/- 2.5 ka. Our results point to a configuration where the southwestern part of the Greenland Ice Sheet reached its maximum LGM extent at the continental shelf break.

Beryllium-10 has become the premiere cosmogenic nuclide for quantifying Earth-surface process. Routine measurement of 10Be at the ≤ 2–3% precision level, coupled with precise 10Be production-rate calibrations, now allow for 10Be-based records of glacier and ice-sheet change to be reliably compared to independent records of climate variability. Here, we review efforts over the last 10+ years to characterize the Holocene behavior of ice sheets and glaciers fringing Baffin Bay using in situ 10Be. Hundreds of 10Be measurements present a detailed picture of ice-margin migration through the early Holocene. Widespread net deglaciation was interrupted by ice-margin readvances or stillstands, marked by modes of moraine deposition, near the end of the Younger Dryas (12.9–11.7 ka BP), 10.4–10.2 ka BP, 9.3 ka BP, and 8.2 ka BP, with perhaps additional widespread moraine deposition occurring ca. 9.7 ka BP and 7.3 ka BP. Modes of moraine deposition encompass independent and glaciologically distinct ice masses – the Greenland and Laurentide ice sheets and local alpine glaciers situated on Baffin Island and western Greenland – providing a robust, albeit discontinuous, record of widespread climatic changes in the Baffin Bay region in the early Holocene. Periods of glacier advance coincide with abrupt cooling events documented in Summit Greenland ice cores indicating that i) Baffin Bay ice masses largely followed the pattern of temperature change displayed in Greenland ice cores, ii) abrupt cooling events were of sufficient magnitude and duration to briefly synchronize the behavior of independent and glaciologically distinct ice masses across Baffin Bay despite varying degrees of dynamical influence, and iii) centennial-scale synchronization of ice masses requires that abrupt temperature changes recorded at Summit Greenland also occurred during the summertime within glacier ablation zones. Advancements in 10Be methodology combined with an environment conducive towards developing 10Be-based records of ice-margin change has resulted in ice-margin reconstructions that identify a potentially fundamental negative feedback mechanism inherent to melting ice sheets in the Baffin Bay region – elevated and episodic meltwater delivery into the Labrador Sea results in a decrease in the Atlantic meridional overturning circulation and regional cooling, which, in turn, drives a brief reversal of deglaciation. Under the right conditions 10Be can be used to develop centennial-scale, climatically relevant records of glacier and ice-sheet change.

Abstract. Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE), the Greenland Ice Sheet (GrIS) was smaller than its current configuration. Determining the exact dimensions of the Holocene ice-sheet minimum and the duration that the ice margin rested inboard of its current position remains challenging. Contemporary retreat of the GrIS from its historical maximum extent in southwestern Greenland is exposing a landscape that holds clues regarding the configuration and timing of past ice-sheet minima. To quantify the duration of the time the GrIS margin was near its modern extent we develop a new technique for Greenland that utilizes in situ cosmogenic 10Be–14C–26Al in bedrock samples that have become ice-free only in the last few decades due to the retreating ice-sheet margin at Kangiata Nunaata Sermia (n=12 sites, 36 measurements; KNS), southwest Greenland. To maximize the utility of this approach, we refine the deglaciation history of the region with stand-alone 10Be measurements (n=49) and traditional 14C ages from sedimentary deposits contained in proglacial–threshold lakes. We combine our reconstructed ice-margin history in the KNS region with additional geologic records from southwestern Greenland and recent model simulations of GrIS change to constrain the timing of the GrIS minimum in southwest Greenland and the magnitude of Holocene inland GrIS retreat, as well as to explore the regional climate history influencing Holocene ice-sheet behavior. Our 10Be–14C–26Al measurements reveal that (1) KNS retreated behind its modern margin just before 10 ka, but it likely stabilized near the present GrIS margin for several thousand years before retreating farther inland, and (2) pre-Holocene 10Be detected in several of our sample sites is most easily explained by several thousand years of surface exposure during the last interglaciation. Moreover, our new results indicate that the minimum extent of the GrIS likely occurred after ∼5 ka, and the GrIS margin may have approached its eventual historical maximum extent as early as ∼2 ka. Recent simulations of GrIS change are able to match the geologic record of ice-sheet change in regions dominated by surface mass balance, but they produce a poorer model–data fit in areas influenced by oceanic and dynamic processes. Simulations that achieve the best model–data fit suggest that inland retreat of the ice margin driven by early to middle Holocene warmth may have been mitigated by increased precipitation. Triple 10Be–14C–26Al measurements in recently deglaciated bedrock provide a new tool to help decipher the duration of smaller-than-present ice over multiple timescales. Modern retreat of the GrIS margin in southwest Greenland is revealing a bedrock landscape that was also exposed during the migration of the GrIS margin towards its Holocene minimum extent, but it has yet to tap into a landscape that remained ice-covered throughout the entire Holocene.