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1. A cosmogenic 10Be moraine chronology of arid, alpine Late Pleistocene glaciation in the Pioneer Mountains of Montana, USA

   https://doi.org/10.1016/j.quascirev.2023.108283  

   Schoenemann, Spruce W. ; Bryant, Mana M. ; Larson, Will B. ; Corbett, Lee B. ; Bierman, Paul R. ( October 2023 , Quaternary Science Reviews)

   We test the hypothesis that glacier systems, located in continental regions proximal to the Laurentide Ice Sheet (LIS), had local ice maxima considerably earlier than the LIS maximum and thus before the insolation minima at ~21 ka. Ranges located in the northwest US exhibit earlier deglaciation timing between ~23 and 22 ka, except for the Yellowstone region where younger time-transgressive ages complicate regional interpretations and the northern Montana ice cap where late glacial ages have recently been produced. Constraining the glacial history of more ice sheet-proximal alpine glaciers provides insight into whether the contrasting maximum-ice times in the northern Rocky Mountains were caused by regional climatic differences, such as anticyclonic wind patterns driven by the presence of the LIS. In the Pioneer Mountains of Montana, we measured in situ cosmogenic 10Be in 35 boulders on moraines marking the maximum Late Pleistocene positions of alpine glaciers from three valleys. The 10Be samples produced a range of ages, spanning pre Bull Lake to the last glaciation (i.e., Pinedale/Marine Isotope Stage (MIS) 2). We find an average exposure age for initial deglaciation of 18.2 ±0.9 during the local Last Glacial Maximum, indicative of synchronous retreat in the Pioneer Mountains. The similarity of initial deglaciation timing of the Pioneer Mountain glaciers with the northwestern Yellowstone glacial system and northern MT ice cap suggests that topography more proximal to the LIS margin maintained full ice extent longer. Our findings, in context of previous work, suggest that in the case of the Pioneer Mountains their more proximal location to the ice margin may have delayed onset of deglaciation by greater exposure to local cooling from katabatic winds and/or additional moisture sourced from large ice-marginal glacial lakes, hence the lack of earlier deglacial ages like those found further to the west and east of the northern Rocky Mountain cordillera. 

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2. Glacier expansion on Baffin Island during early Holocene cold reversals

   https://doi.org/10.1016/j.quascirev.2020.106419  

   Crump, Sarah E. ; Young, Nicolás E. ; Miller, Gifford H. ; Pendleton, Simon L. ; Tulenko, Joseph P. ; Anderson, Robert S. ; Briner, Jason P. ( July 2020 , Quaternary Science Reviews)

   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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3. Minimum limiting deglacial ages for the out-of-phase Saginaw Lobe of the Laurentide Ice Sheet using optically stimulated luminescence (OSL) and radiocarbon methods

   https://doi.org/10.1017/qua.2020.12  

   Fisher, Timothy G. ; Dziekan, Mitchell R. ; McDonald, Jennifer ; Lepper, Kenneth ; Loope, Henry M. ; McCarthy, Francine M.G. ; Curry, B. Brandon ( September 2020 , Quaternary Research)

   null (Ed.)

   Abstract Twenty-four new optically stimulated luminescence (OSL) and radiocarbon ages from sediment cores in nine lakes associated with the Shipshewana and Sturgis moraines in northern Indiana and southern Michigan estimate when recession of the Saginaw Lobe of the Laurentide Ice Sheet was underway in the southern Great Lakes region, USA. Average OSL ages of 23.4 ± 2.2 ka for the Shipshewana Moraine and 19.7 ± 2.2 ka for the Sturgis Moraine are considered minimum limiting deglacial ages for these recessional moraines. The much younger radiocarbon ages are consistent with other regional radiocarbon ages from lakes, and record climate amelioration around ~16.5 cal ka BP. Early recession of the interlobate Saginaw Lobe was well underway by 23.4 ± 2.2 ka, when the adjacent Lake Michigan and Huron-Erie lobes were a few hundred kilometers farther south and near their maximum southerly limits. The results provide the first time constraints when sediment from the Lake Michigan and Huron-Erie lobes began filling the accommodation space left by the Saginaw Lobe. The difference between the oldest radiocarbon and OSL age is 7400 yr for the Shipshewana Moraine and 3400 yr for the Sturgis Moraine. 

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4. Glacial Isostatic Adjustment Shapes Proglacial Lakes Over Glacial Cycles

   https://doi.org/10.1029/2022GL101191  

   Austermann, J. ; Wickert, A. D. ; Pico, T. ; Kingslake, J. ; Callaghan, K. L. ; Creel, R. C. ( December 2022 , Geophysical Research Letters)

   As ice sheets load Earth's surface, they produce ice‐marginal depressions which, when filled with meltwater, become proglacial lakes. We include self‐consistently evolving proglacial lakes in a glacial isostatic adjustment (GIA) model and apply it to the Laurentide ice sheet over the last glacial cycle. We find that the locations of modeled lakes and the timing of their disappearance is consistent with the geological record. Lake loads can deflect topography by >10 m, and volumes collectively approach 30–45 cm global mean sea‐level equivalent. GIA increases deglaciation‐phase lake volume up to five‐fold and average along‐ice‐margin depth ≤90 m compared to glaciation‐phase ice volume analogs—differences driven by changes in the position and size of the peripheral bulge. Since ice‐marginal lake depth affects grounding‐line outflow, GIA‐modulated proglacial lake depths could affect ice‐sheet mass loss. Indeed, we find that Laurentide ice‐margin retreat rate sometimes correlates with proglacial lake presence, indicating that proglacial lakes aid glacial collapse.

    

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5. In Situ Cosmogenic 10Be Dating of Laurentide Ice Sheet Retreat from Central New England, USA

   https://doi.org/10.3390/geosciences13070213  

   Drebber, Jason S. ; Halsted, Christopher T. ; Corbett, Lee B. ; Bierman, Paul R. ; Caffee, Marc W. ( July 2023 , Geosciences)

   Constraining the timing and rate of Laurentide Ice Sheet (LIS) retreat through the northeastern United States is important for understanding the co-evolution of complex climatic and glaciologic events that characterized the end of the Pleistocene epoch. However, no in situ cosmogenic 10Be exposure age estimates for LIS retreat exist through large parts of Connecticut or Massachusetts. Due to the large disagreement between radiocarbon and 10Be ages constraining LIS retreat at the maximum southern margin and the paucity of data in central New England, the timing of LIS retreat through this region is uncertain. Here, we date LIS retreat through south-central New England using 14 new in situ cosmogenic 10Be exposure ages measured in samples collected from bedrock and boulders. Our results suggest ice retreated entirely from Connecticut by 18.3 ± 0.3 ka (n = 3). In Massachusetts, exposure ages from similar latitudes suggest ice may have occupied the Hudson River Valley up to 2 kyr longer (15.2 ± 0.3 ka, average, n = 2) than the Connecticut River Valley (17.4 ± 1.0 ka, average, n = 5). We use these new ages to provide insight about LIS retreat timing during the early deglacial period and to explore the mismatch between radiocarbon and cosmogenic deglacial age chronologies in this region.

    

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