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1. Laurentide ice sheet thinning and erosive regimes at Mount Washington, New Hampshire, inferred from multiple cosmogenic nuclides

   Koester, A. J. (April 2021, GSA Special Paper)

   Richard B. Waitt; Glenn D. Thackray; Alan R. Gillespie (Ed.)

   The northward retreat history of the Laurentide ice sheet through the lowlands of the northeastern United States during the last deglaciation is well constrained, but its vertical thinning history is less well known because of the lack of direct constraints on ice thickness through time and space. In addition, the highest elevations in New England are characterized by gently sloping upland surfaces and weathered block fields, features with an uncertain history. To better constrain ice-sheet history in this area and its relationship to alpine geomorphology, we present 20 new 10Be and seven in situ 14C cosmogenic nuclide measurements along an elevation transect at Mount Washington, New Hampshire, the highest mountain in the northeastern United States (1917 m above sea level [a.s.l.]). Our results suggest substantially different exposure and erosion histories on the upper and lower parts of the mountain. Above 1600 m a.s.l., 10Be and in situ 14C measurements are consistent with upper reaches of the mountain deglaciating by 18 ka. However, some 10Be ages are up to several times greater than the age of the last deglaciation, consistent with weakly erosive, cold-based ice that did not deeply erode preglacial surfaces. Below 1600 m a.s.l., 10Be ages are indistinguishable over a nearly 900 m range in elevation and imply rapid ice-surface lowering ca. 14.1 ± 1.1 ka (1 standard deviation; n = 9). This shift from slow thinning early in the deglaciation on the upper part of the mountain to abrupt thinning across the lower elevations coincided with accelerated ice-margin retreat through the region recorded by Connecticut River valley varve records during the Bølling interstadial. The Mount Washington cosmogenic nuclide vertical transect and the Connecticut River valley varve record, along with other New England cosmogenic nuclide records, suggest rapid ice-volume loss in the interior northeastern United States in response to Bølling warming. 

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2. Reconstructing the late Pleistocene glacial history of the Cairngorm Mountains, Scotland, using paired cosmogenic ¹⁰ Be and ¹⁴ C

   https://doi.org/10.1002/jqs.3721  

   Kelley, Samuel E; Doughty, Alice M; Lifton, Nathaniel; Pendleton, Simon; O'Brien, Cormac; Warren, Graeme (August 2025, Journal of Quaternary Science)

   ABSTRACT Chronologic and geomorphic evidence of past glaciations is largely recorded in areas at relatively low elevation and/or landscapes defined by high glacial erosion. As a result, data from areas that did not experience significant glacial erosion and the highest elevation locations are lacking from reconstructions of past ice masses. Here, we use paired¹⁰Be and¹⁴C exposure dating on quartz derived from glacial erratics and bedrock to resolve past ice mass fluctuations across an elevation and geomorphic gradient in the Cairngorm Mountains of Scotland. We find that the highest elevation samples in our dataset, at 1140 m above sea level, yield¹⁰Be ages indicating 23 000 years of exposure and are at secular equilibrium for¹⁴C, indicative of a single period of exposure starting as a nunatak during the Last Glacial Maximum. Other samples from above 1000 m elevation produce¹⁴C exposure ages indicating deglaciation during the Bølling–Allerød Interstadial, while¹⁰Be concentrations display varying levels of inheritance due to local variations in glacial and non‐glacial erosion. Samples from lower elevations indicate a Lateglacial moraine building event 15 000 years ago, coincident with the Wester Ross readvance, followed by a period of glacier retreat interrupted by moraine deposition at ~14 000 and ~11 000 years ago. This dataset reconstructs both thinning and marginal recession of paleo‐ice masses within the Cairngorm Mountains across a >600 m vertical gradient during the most recent glacial–interglacial transition, demonstrating the utility of paired¹⁰Be/¹⁴C exposure dating in ice mass reconstructions in areas of low glacial erosion, increasing the geomorphic diversity of chronologic data available for glacial reconstructions. 

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3. New ¹⁰ Be exposure ages improve Holocene ice sheet thinning history near the grounding line of Pope Glacier, Antarctica

   https://doi.org/10.5194/tc-16-4887-2022  

   Adams, Jonathan R.; Johnson, Joanne S.; Roberts, Stephen J.; Mason, Philippa J.; Nichols, Keir A.; Venturelli, Ryan A.; Wilcken, Klaus; Balco, Greg; Goehring, Brent; Hall, Brenda; et al (January 2022, The Cryosphere)

   Abstract. Evidence for the timing and pace of past grounding lineretreat of the Thwaites Glacier system in the Amundsen Sea embayment (ASE)of Antarctica provides constraints for models that are used to predict thefuture trajectory of the West Antarctic Ice Sheet (WAIS). Existingcosmogenic nuclide surface exposure ages suggest that Pope Glacier, a formertributary of Thwaites Glacier, experienced rapid thinning in the early tomid-Holocene. There are relatively few exposure ages from the lower ice-freesections of Mt. Murphy (<300 m a.s.l.; metres above sea level) that are uncomplicated byeither nuclide inheritance or scatter due to localised topographiccomplexities; this makes the trajectory for the latter stages ofdeglaciation uncertain. This paper presents 12 new 10Be exposure agesfrom erratic cobbles collected from the western flank of Mt. Murphy, within160 m of the modern ice surface and 1 km from the present grounding line.The ages comprise two tightly clustered populations with mean deglaciationages of 7.1 ± 0.1 and 6.4 ± 0.1 ka (1 SE). Linear regressionanalysis applied to the age–elevation array of all available exposure agesfrom Mt. Murphy indicates that the median rate of thinning of Pope Glacierwas 0.27 m yr−1 between 8.1–6.3 ka, occurring 1.5 times faster thanpreviously thought. Furthermore, this analysis better constrains theuncertainty (95 % confidence interval) in the timing of deglaciation atthe base of the Mt. Murphy vertical profile (∼ 80 m above themodern ice surface), shifting it to earlier in the Holocene (from 5.2 ± 0.7 to 6.3 ± 0.4 ka). Taken together, the results presentedhere suggest that early- to mid-Holocene thinning of Pope Glacier occurredover a shorter interval than previously assumed and permit a longer durationover which subsequent late Holocene re-thickening could have occurred. 

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4. Timing of Cordilleran-Laurentide ice-sheet separation: Implications for sea-level rise

   https://doi.org/10.1016/j.quascirev.2024.108554  

   Reyes, Alberto V; Carlson, Anders E; Clark, Jorie; Guillaume, Louise; Milne, Glenn A; Tarasov, Lev; Carlson, Elizabeth CB; He, Feng; Caffee, Marc W; Wilcken, Klaus M; et al (March 2024, Quaternary Science Reviews)

   During the last deglaciation, collapse of the saddle between the North American Cordilleran and Laurentide ice sheets led to rapid ice-sheet mass loss and separation, with meltwater discharge contributing to deglacial sea level rise. We directly date ice-sheet separation at the end of the saddle collapse using 64 10Be exposure ages along an ~1200-km transect of the ice-sheet suture zone. Collapse began in the south by 15.4 ± 0.4 ka and ended by 13.8 ± 0.1 ka at ~56◦N. Ice-sheet model simulations consistent with the 10Be ages find that the saddle collapse contributed 6.2–7.2 m to global mean sea-level rise from ~15.5 ka to ~14.0 ka, or approximately one third of global mean sea-level rise over this period. We determine 3.1–3.6 m of the saddle collapse meltwater was released during Meltwater Pulse 1A ~14.6-14.3 ka, constituting 20–40% of this meltwater pulse’s volume. Because the separation of the Cordilleran and Laurentide ice sheets occurred over 1–2 millennia, the associated release of meltwater during the saddle collapse supplied a smaller contribution to the magnitude of Meltwater Pulse 1A than has been recently proposed. 

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5. A critical re‐analysis of constraints on the timing and rate of Laurentide Ice Sheet recession in the northeastern United States

   https://doi.org/10.1002/jqs.3563  

   Halsted, Christopher T.; Bierman, Paul R.; Shakun, Jeremy D.; Davis, P. Thompson; Corbett, Lee B.; Drebber, Jason S.; Ridge, John C. (January 2024, Journal of Quaternary Science)

   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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