INVESTIGATING ARID ALPINE PLEISTOCENE GLACIATION IN THE PIONEER MOUNTAINS OF MONTANA USING COSMOGENIC 10-BERYLLIUM

This project uses cosmogenic isotope dating and field methods to reconstruct the glacial retreat history of the Pioneer Mountains in Montana. Using ¹⁰Be exposure ages to constrain the age of alpine glacier moraines, we test the hypothesis that the location of the Pioneer Mountains proximal to the paleo Laurentide Ice Sheet (LIS) and leeward of the Continental Divide strongly influenced the timing of maximum glacial expansion due to limits in precipitation as the growth of the LIS altered regional atmospheric circulation patterns. Here we compare our alpine glacial retreat ages with those defining retreat of the LIS, and other northern Rocky Mountain alpine glacial ages.

We first assessed glacial valleys in the Pioneer Mountains considering aspect, size, elevation, shading, and rock type. Using these criteria, we then selected three alpine glacial valleys for sampling: Birch Creek, Dingley Creek, and Canyon Creek. Six initial boulder samples were taken from Birch Creek in November, 2020, and another 36 samples were collected in May, 2021. Our first ages (n = 6, 16.4 +/− 0.8 ka), gathered from a single outer lateral moraine in Birch Creek Valley, suggest that the timing of deglaciation in the Pioneer Mountains is younger than most ages of terminal moraines in other parts of the northern Rocky Mountains, particularly the Greater Yellowstone Glacial System and Wind River Range.

With today's warming climate and resulting decrease in snowpack thickness and duration, it is vital to understand alpine cryosphere response to climate warming, as seasonal snowpack and glaciers play a key role in modulating streamflow. Located in southwest Montana, the Pioneer Mountains hold a unique record of the responses of these dry, continental, glaciated mountain ranges in the presence of the anti-cyclonic katabatic winds that descended off the LIS. The new ages from the Pioneer Mountains help us test whether the differences in maximum-ice times found in eastern Idaho and northwestern Wyoming are due to regional climatic contrasts or non-climatic factors (e.g., hypsometry and response time). Results from this work will advance the understanding of glaciers under extremely dry and very cold conditions, providing insight about how current glaciers in the coldest, driest regions of the world may respond to modern climate warming.