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Accurate reconstruction of Laurentide Ice Sheet volume changes following the Last Glacial Maximum is critical for understanding ice sheet contribution to sea-level rise, the resulting influence of meltwater on oceanic circulation, and the spatial and temporal patterns of deglaciation. Here, we provide empirical constraints on Laurentide Ice Sheet thinning during the last deglaciation by measuring in situ cosmogenic 10Be in 81 samples collected along vertical transects of nine mountains in the northeastern United States. In conjunction with 107 exposure age samples over five vertical transects from previous studies, we reconstruct ice sheet thinning history. At peripheral sites (within 200 km of the terminal moraine), we find evidence for ∼600 m of thinning between 19.5 ka and 17.5 ka, which is coincident with the slow initial margin retreat indicated by varve records. At locations >400 km north of the terminal moraine, exposure ages above and below 1200 m a.s.l. exhibit different patterns. Ages above this elevation are variable and older, while lower elevation ages are indistinguishable over 800−1000 m elevation ranges, a pattern that suggests a subglacial thermal boundary at ∼1200 m a.s.l. separating erosive, warm-based ice below and polythermal, minimally erosive ice above. Low-elevation ages from up-ice mountains are between 15 ka and 13 ka, which suggests rapid thinning of ∼1000 m coincident with Bølling-Allerød warming. These rates of rapid paleo-ice thinning are comparable to those of other vertical exposure age transects around the world and may have been faster than modern basin-wide thinning rates in Antarctica and Greenland, which suggests that the southeastern Laurentide Ice Sheet was highly sensitive to a warming climate. 

Abstract We review geochronological data relating to the timing and rate of Laurentide Ice Sheet recession in the northeastern United States and model ice margin movements in a Bayesian framework using compilations of previously published organic¹⁴C (n= 133) andin situcosmogenic¹⁰Be (n= 95) ages. We compare the resulting method‐specific chronologies with glacial varve records that serve as independent constraints on the pace of ice recession to: (1) construct a synthesis of deglacial chronology throughout the region; and (2) assess the accuracy of each chronometer for constraining the timing of deglaciation. Near the Last Glacial Maximum terminal moraine zone,¹⁰Be and organic¹⁴C ages disagree by thousands of years and limit determination of the initial recession to a date range of 24–20 ka. We infer that¹⁰Be inherited from pre‐glacial exposure adds 2–6 kyr to many exposure ages near the terminal moraines, whereas macrofossil¹⁴C ages are typically 4–8 kyr too young due to a substantial lag between ice recession and sufficient organic material accumulation for dating in some basins. Age discrepancies between these chronometers decrease with distance from the terminal moraine, due to less¹⁰Be inherited from prior exposure and a reduced lag between ice recession and organic material deposition.¹⁴C and¹⁰Be ages generally agree at locations more than 200 km distal from the terminal moraines and suggest a mostly continuous history of ice recession throughout the region from 18 to 13 ka with a variable pace best documented by varves.