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1. Modeling glacier extents and equilibrium line altitudes in the Rwenzori Mountains, Uganda, over the last 31,000 yr

   Doughty AM, Kelly MA (April 2021, Special paper Geological Society of America)

   Waitt RB, Thackray GD (Ed.)

   Mountain glacier moraine sequences and their chronologies allow us to evaluate the timing and climate conditions that underpin changes in the equilibrium line altitudes (ELAs), which can provide valuable information on the paleoclimatology of understudied regions such as tropical East Africa. However, moraine sequences are inherently discontinuous, and the precise climate conditions that they represent can be ambiguous due to the sensitivity of mountain glaciers to temperature, precipitation, and other environmental variables. Here, we used a two-dimensional (2-D) iceflow and mass-balance model to simulate glacier extents and ELAs in the Rwenzori Mountains in East Africa over the past 31,000 yr (31 k.y.), including the Last Glacial Maximum (LGM), late glacial period, and the Holocene Epoch. We drove the glacier model with two independent, continuous temperature reconstructions to simulate possible glacier length changes through time. Model input paleoclimate values came from branched glycerol dialkyl glycerol tetraether (brGDGT) temperature reconstructions from alpine lakes on Mount Kenya for the last ~31 k.y., and precipitation reconstructions for the LGM came from various East African locations. We then compared the simulated fluctuations with the positions and ages (where known) of the Rwenzori moraines. The simulated glacier extents reached within 1.1 km of the dated LGM moraines in one valley (93% of the full LGM extent) when forced by the brGDGT temperature reconstructions (maximum cooling of 6.1 °C) and a decrease in precipitation (-10% than modern amounts). These simulations suggest that the Rwenzori glaciers required a cooling of at least 6.1 °C to reach the dated LGM moraines. Based on the model output, we predict an age of 12–11 ka for moraines located halfway between the LGM and modern glacier extents. We also predict ice-free conditions in the Rwenzori Mountains for most of the early to middle Holocene, followed by a late Holocene glacier readvance within the last 2000 yr. 

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2. DEVELOPING A GEOSPATIAL DATABASE OF LATE PLEISTOCENE MOUNTAIN GLACIERS IN THE WESTERN UNITED STATES

   https://doi.org/10.1130/abs/2023RM-387976  

   Laabs, Benjamin; Anderson, Leif; Licciardi, Joseph; Tulenko, Joseph (May 2023, Geological Society of America)

   The glacial geology community has developed digital reconstructions of Pleistocene glaciers in the western U.S. through decades of regionally focused research and interpretations of geologic maps. These paleoglacier reconstructions afford an opportunity to develop paleoclimate reconstructions for the Late Quaternary in the western U.S., especially when combined with cosmogenic chronologies and other paleoclimate proxies and model output. Here, we present a geospatial database of Late Pleistocene mountain glaciers for the conterminous western U.S. based on compilations of paleoglacier reconstructions spanning glaciated mountains in the region. The database consists of paleoglacier outlines as georeferenced polygons drawn at scales ranging from 1:24,000 to 1:100,000, reflecting differences in available mapping data and degrees of confidence in identifying glacial deposits and landforms used to identify paleoglacier limits. The database is available as a web feature service designed to be easily represented in a geographic information system or web mapping application to enable visualization of the pattern of Late Pleistocene mountain glaciation and analysis of paleoglacier outlines and derivative products, such as equilibrium-line altitudes and boundaries of modeled paleoglaciers. We illustrate potential applications of the database for visualization and data assimilation with an example from mountains neighboring the Lake Bonneville basin, where paleoglacier outlines are based on 1:24,000 scale mapping of glacial deposits and landforms and cosmogenic chronologies of moraines are abundant. For this research, the database enables an analysis of the pattern of glaciation in the region and, through assimilation with chronological data, an assessment of the relative timing of glacier maxima and the time when Lake Bonneville overflowed. While the database can be easily shared among users and represented in a geographic information system, development of the database requires community input to maximize its utility for users across disciplines. A goal of this presentation is to encourage interested users to share ideas for developing an accessible, scalable, and community-supported database of paleoglaciers. 

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3. 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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4. Cosmogenic ²¹ Ne exposure ages on late Pleistocene moraines in Lassen Volcanic National Park, California, USA

   https://doi.org/10.5194/gchron-6-639-2024  

   Tulenko, Joseph P; Balco, Greg; Clynne, Michael A; Muffler, L_J Patrick (December 2024, Geochronology)

   Abstract. We report new cosmogenic 21Ne in quartz exposure ages from 18 samples on three distinct moraines deposited in the Lost Creek drainage, approximately 3–7 km down-valley from Lassen Peak in Lassen Volcanic National Park. Although measuring 21Ne in quartz is generally straightforward, accurate 21Ne exposure dating of deposits of late Pleistocene is rarely possible due to the significant quantities of non-cosmogenic 21Ne present in most lithologies. Young quartz-bearing volcanic rocks have been observed to be an exception. We take advantage of moraine boulders sourced from the ∼ 28 ka dacite of Lassen Peak to generate a chronology of alpine deglaciation in Lassen Volcanic National Park. Ages from three distinct moraines are in stratigraphic order at 22.1 ± 3.8, 20.2 ± 2.4, and 15.3 ± 3.8 ka and generally agree with other terminal and some recessional moraine ages across the Cascade Range and Sierra Nevada of the western United States. To date, these are among the youngest surfaces ever dated using cosmogenic 21Ne and provide a cost-effective proof-of-concept approach to dating moraines where applicable. 

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5. Cosmogenic nuclide chronology of pre-last glacial maximum moraines at Lago Buenos Aires, 46°S, Argentina

   https://doi.org/10.1016/j.yqres.2004.12.003  

   Kaplan, Michael R.; Douglass, Daniel C.; Singer, Bradley S.; Ackert, Robert P.; Caffee, Marc W. (May 2005, Quaternary Research)

   At Lago Buenos Aires, Argentina, 10 Be, 26 Al, and 40 Ar/ 39 Ar ages range from 190,000 to 109,000 yr for two moraines deposited prior to the last glaciation, 23,000—16,000 yr ago. Two approaches, maximum boulder ages assuming no erosion, and the average age of all boulders and an erosion rate of 1.4 mm/10 3 yr, both yield a common estimate age of 150,000—140,000 yr for the two moraines. The erosion rate estimate derives from 10 Be and 26 Al concentrations in old erratics, deposited on moraines that are >760,000 yr old on the basis of interbedded 40 Ar/ 39 Ar dated lavas. The new cosmogenic ages indicate that a major glaciation during marine oxygen isotope stage 6 occurred in the mid-latitude Andes. The next five youngest moraines correspond to stage 2. There is no preserved record of a glacial advance during stage 4. The distribution of dated boulders and their ages suggest that at least one major glaciation occurred between 760,000 and >200,000 yr ago. The mid-latitude Patagonian glacial record, which is well preserved because of low erosion rates, indicates that during the last two glacial cycles major glaciations in the southern Andes have been in phase with growth and decay of Northern Hemisphere ice sheets, especially at the 100,000 yr periodicity. Thus, glacial maxima are global in nature and are ultimately paced by small changes in Northern Hemisphere insolation. 

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