A LATE HOLOCENE 10BE BASED GLACIAL CHRONOLOGY FOR THE BEARTOOTH MOUNTAINS, SOUTHWESTERN MONTANA

As climate continues to rapidly change, it becomes increasingly important to understand how glaciers responded to past climate changes in order to better forecast their future responses. Currently, the late Holocene alpine glacial history in many locations in the western United States is not precisely understood and needs to be further constrained so that climate modelers can use this information to better predict future responses of existing glaciers to climate change. Here, we precisely date three moraines in a cirque in the Beartooth Mountains of southern Montana using ¹⁰Be cosmogenic surface exposure dating. Previous studies estimated that these moraines were formed during the so-called Neoglaciation (Graf, 1971), with the younger moraines likely associated with the Little Ice Age (LIA). However, the exact age of these glacial landforms is not known due to a lack of numerical dating control. Three ¹⁰Be-dated boulders collected along a moraine crest near Little Glacier Lake produced a mean age and standard deviation of 1,110 ± 190 years before present and five other boulders collected along an older, distal moraine near Emerald Lake produced a median age of 12,810 ± 500 years before present. Both ages challenge the previous interpretations that these moraines formed during the LIA and the mid Holocene, respectively. Additional boulder samples from our Little Glacier Lake moraine (n=8) and a second late Holocene moraine near Triangle Lake (n=9) will also be dated to further constrain the glacial history in this area and presented at the meeting.