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ABSTRACT Be dating of moraines has greatly improved our ability to constrain the timing of past glaciations and thus past cold events. However, the spread in ages from a single moraine is often greater than would be expected from measurement uncertainty, making paleoclimatic interpretations equivocal. Here we present 28 new10Be ages from ice‐cored Neoglacial moraines on Baffin Island, Arctic Canada, and explore the processes at play in moraine formation and evolution through field observations and a numerical debris‐covered glacier model. The insulating effect of debris cover modifies glacier lengths and results in the development of ice‐cored moraines over multiple advances and thousands of years. Although ice cores can persist for several millennia, spatially variable ice core melt‐out contributes to moraine degradation and boulder destabilization, making it likely that the10Be clock is reset on moraine boulders in these settings. Thus, exposure ages from ice‐cored moraines must be interpreted with caution. The oldest ages, after excluding samples with inheritance, provide the best estimates of initial moraine formation. Three Baffin Island moraines yield10Be ages suggesting formation at 5.2, 4.6 and 3.5 ka, respectively, adding to a growing body of evidence for significant summer cooling millennia before the Little Ice Age.
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A critical re‐analysis of constraints on the timing and rate of Laurentide Ice Sheet recession in the northeastern United States
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.
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- PAR ID:
- 10485290
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Journal of Quaternary Science
- Volume:
- 39
- Issue:
- 1
- ISSN:
- 0267-8179
- Format(s):
- Medium: X Size: p. 54-69
- Size(s):
- p. 54-69
- Sponsoring Org:
- National Science Foundation
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