SURFACE EXPOSURE DATING AND CLIMATIC INTERPRETATION OF PLEISTOCENE GLACIERS ALONG THE CONTINENTAL DIVIDE, MONTANA, USA

Alpine glaciers occupied low mountains along the Continental Divide of western Montana during the last glacial maximum (LGM). A 500 km² ice cap system formed over the northern Boulder Mountains (BM; 2,688 m maximum elevation), while 30 km to north-northwest smaller cirque glaciers developed in the Nevada Mountain area (NM; 2,539 m maximum elevation). Here we present the results of new mapping, paleo equilibrium-line altitude (ELA) reconstructions, degree-day modeling, and surface exposure age dating of these systems.

Field mapping, geographic information system (GIS) software, and spreadsheet models of the basal shear stress of glacier ice were used to reconstruct Pleistocene glacier hypsometries. Paleo ELAs were then calculated with the accumulation area ratio (AAR) method using a ratio of 0.65. Preliminary paleo-ELAs of NM glaciers average 1670 m for two glaciers east of the Continental Divide. NM paleo-ELAs are well constrained due to limited ice extent and relative ease of field mapping, though efforts to produce paleo-ELAs for the larger, less easily constrained BM system are also underway. Climatic interpretation of NM paleo ELAs was made using a degree-day model. Without significant changes in precipitation, mean summer temperatures in the study area during the LGM were on the order of 12 to 13 °C cooler than at present. Even if, as global circulation models suggest, the region was significantly drier at that time, mean summer temperatures need only have been on the order of 14-15 °C cooler to support the glaciers in the study area.

Samples for ¹⁰Be surface exposure dating were collected from boulders in a terminal moraine of a BM outlet glacier in the Little Blackfoot River valley near Elliston, Montana. ¹⁰Be samples were prepared and analyzed at the PRIME Lab, and preliminary exposure ages calculated using the CHRONUS-Earth constant rate production model. Of the six boulders sampled, two outliers suggest inherited ¹⁰Be. The remaining four boulders produced a mean age of 16.7 ka cal with a maximum internal (analytical) uncertainty of 989 years. Representing the northernmost extent of small-scale alpine glaciation along the Continental Divide during the LGM, these results may constrain ongoing efforts to model late-Quaternary climate change in the northern Rocky Mountains.