GLACIAL RETREAT RATES AND TEMPERATURE DEPRESSIONS IN THE PIONEER MOUNTAINS, MT, FROM THE PINEDALE TO EARLY HOLOCENE

The timing of glacier retreat and the rates of ice loss at the end of the last glaciation are important benchmarks for comparison to today’s rapid glacier decline. Here we present glacier retreat rates from the Pioneer Mountains of Montana based on 10 in situ cosmogenic ¹⁰Be samples collected from four cirque basins. Based on the location of the samples from the cirques, we interpret their exposure ages as the timing of when glaciers had retreated up the valleys following the last glacial maximum (LGM). The ¹⁰Be samples produced a range of ages from 14.9 to 11.1 ka, with an average exposure age of 13.1 ± 1.3 ka (1 SD), marking the end of the deglacial transition.

Based on ages derived from moraines marking the local LGM maximum (18.2 ± 0.9 ka) from our previous work in the Pioneer Mountains, most glaciers had retreated into their cirques 3 to 5 kyr after leaving the valleys. The retreat rates range from 1.6 to 2.6 km ka^-1 depending on the elevation of the cirque basin, with a range-wide average of 2.1 km ka^-1.

To reconstruct the past dimensions of the cirque and valley glaciers, we used several methods for determining the equilibrium line altitude (ELA); including THAR (Toe-to-Headwall Altitude Ratio), AABR (area-altitude balance ratio), AAR (accumulation area ratio), along with glacio-geomorphic landforms. We made comparisons between the reconstructed modern, early Holocene, and LGM ELAs to estimate temperature depressions using multiple approaches. For the modern to LGM, using similar methods to Leonard (1989), we estimate a temperature depression of 10 ± 2˚C, assuming no change in precipitation. We produced an LGM temperature depression of 9.7˚C using the 2-D python-based numerical glacier model of Anderson et al (2018), by matching the model output with maximum glacier extent and ELA. The model uses a positive degree-day approach to represent snow and ice melt, as well as monthly precipitation and temperature fields from PRISM data. Using this model we also produce transient estimates of past glacier size from the LGM through the early Holocene.

The findings from the new cirque ¹⁰Be ages provide insight into the rate of glacial retreat during the last deglaciation in southwestern Montana and constraints on temperature and precipitation changes between the local LGM and early Holocene when glaciers rapidly retreated up-valley.