TIMING OF LATE PLEISTOCENE GLACIER CULMINATIONS AND RETREAT IN GRAND TETON NATIONAL PARK

Records of glaciation in the greater Yellowstone region have long served as a cornerstone for understanding the timing of late Pleistocene glaciations in the central Rocky Mountains. In previous investigations, we applied cosmogenic ³He and ¹⁰Be surface exposure dating to establish high-resolution moraine chronologies in selected drainages of the Yellowstone-Teton mountain glacier complex. Here, we expand on the geographic and temporal coverage of glacial events in the greater Yellowstone and Grand Teton regions by developing >20 new ¹⁰Be exposure ages on glacial features in several drainages on the eastern and western sides of the Teton Range. Our new investigations are focused on moraines and scoured bedrock surfaces at Glacier Gulch, Avalanche Canyon, Taggart Lake, Phelps Lake, Granite Canyon, Teton Canyon, and adjacent areas. All new and previously obtained ¹⁰Be ages are calculated using recently published calibrations of ¹⁰Be production rates, which are lower than the globally averaged production rate used in our prior work. Mean ages of outermost Pinedale moraine loops in the Tetons range from 18.6 ka at Granite Canyon to 16.4 ka at Taggart Lake, suggesting asynchrony of glacier culminations among adjacent Teton mountain glaciers. Results also distinguish an age progression from proximal to distal portions of the Taggart Lake end moraine sequence, implying that the glacier persisted or oscillated for ~2 ka in the terminal area before retreating upvalley. A mean age of 17.3 ka is obtained from moraine boulders along an ice limit corresponding to the Pinedale 2 phase of the southern margin of the greater Yellowstone glacial system. Direct dating of the Pinedale 2 ice limit confirms our prior indirect dating of this ice position via outwash relationships with the Jenny Lake outer moraines, which are recalculated to 17.1 ka using recently calibrated ¹⁰Be production rates. This expanded chronology provides a refined understanding of the timing of late Pleistocene glacier events in the central Rocky Mountains, and enables a more critical examination of climatic and non-climatic influences on deglaciation.