GSA Connects 2021 in Portland, Oregon

Paper No. 151-7
Presentation Time: 9:00 AM-1:00 PM


LARSON, Will B.1, BRYANT, Mana M.1, SCHOENEMANN, Spruce W.1, CORBETT, Lee2 and BIERMAN, Paul R.3, (1)Environmental Sciences Department, University of Montana Western, 710 S. Atlantic St, Dillon, MT 59725, (2)Department of Geology, University of Vermont, Burlington, VT 05405, (3)Geology Department and Rubenstein School of the Environment and Natural Resources, University of Vermont, Burlington, VT 05405

Although temperature and precipitation both control ice behavior, it is often challenging to untangle their respective roles in now-vanished glaciers and ice sheets. Here, we study alpine glaciation in a dry region of western North America in order to assess glacier behavior in a precipitation-starved mountain range once proximal to the Laurentide Ice Sheet. This project focuses on using field mapping and equilibrium line altitude calculations in the Pioneer Mountains of southwest Montana to understand the dynamics of alpine glaciers in a dry, continental locale.

In order to better visualize glacial features remotely, we carried out a series of drone surveys to create DEM (digital elevation model) maps and orthophotos of our sample locations. The high-resolution aerial imagery greatly assisted in identifying subtle glacial landforms (e.g., debris flows, recessional moraines, and stagnant ice areas) and the relative position of boulders we sampled for dating. The detailed geomorphic mapping work will provide insight for interpreting ongoing and future cosmogenic exposure dating of the moraine boulders and improved interpretation of the maximum glacial extent and retreat timing.

The semi-arid climate of southwest Montana has preserved many geomorphic features of the last glaciation. Our work focuses on three valleys: Birch Creek, Canyon Creek, and Dingley Creek. They were selected based on different factors such as aspect, shading, valley size, precipitation, and weather patterns that could influence the growth and/or decay of the glaciers as well as their response to regional climatic changes.

In order to reconstruct the past dimensions of these glaciers, we used several methods of determining the equilibrium line altitude (ELA); including the AABR (area-altitude balance ratio) and the AAR (accumulation area ratio). For example, the paleo ELA reconstructed using AABR for the Birch Creek valley is 2577 m, for Canyon Creek 2651 m, and for Dingley Creek 2608 m. The average paleo ELA was 2612 m (AABR) and 2580 m (AAR), respectively. The difference between the AABR and AAR methods was minimal (~30m), with the AABR method generally higher.

Results from this work will advance the understanding of how current glaciers in the coldest, driest regions of the world may respond to temperature increases and precipitation changes.