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

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 similar to 21 ka. Ranges located in the northwest US exhibit earlier deglaciation timing between similar to 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 Be-10 in 35 boulders on moraines marking the maximum Late Pleistocene positions of alpine glaciers from three valleys. The Be-10 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.

Automated summary

The study suggests that alpine glaciers in the Pioneer Mountains of Montana had similar initial deglaciation timing as the northwestern Yellowstone glacial system and the northern Montana ice cap, indicating a stronger ability to maintain full ice extent closer to the Laurentide Ice Sheet (LIS) margin.