THE LAST DEGLACIATION OF THE YELLOWSTONE PLATEAU: TIMING AND IMPLICATIONS

This presentation honors the legacy of Ken Pierce’s influential body of work on the history of the greater Yellowstone glacial system, and reviews ongoing research that builds on the vast foundation of knowledge that Ken developed in this region throughout his career. We focus on new results that refine the age of deglaciation on the Yellowstone Plateau during the late Pleistocene. The precise timing of plateau deglaciation and attendant unloading of the >1 km-thick ice cap has important implications for a wide range of multidisciplinary questions, including the initiation of Yellowstone’s hydrothermal systems and features, potential triggering of hydrothermal explosions, impacts on volcanic-magmatic processes, influences on tectonic activity, paleoclimatic controls on glacial retreat, and the development of postglacial ecosystems and environmental conditions.

The deglaciation of the plateau was previously constrained indirectly by cosmogenic nuclide exposure dating of moraine sequences around the periphery of the plateau, as well as basal radiocarbon ages and tephra layers in plateau lake sediments. In particular, the Deckard Flats moraines in the northern sector of the greater Yellowstone glacial system were interpreted by Ken as ice-marginal features that formed during a dynamic “readjustment” to the demise of the plateau ice cap. In prior work, ¹⁰Be exposure dating of the Deckard Flats moraines yielded an age of 15.1 ± 1.2 ka, reflecting the time when the Yellowstone Plateau ice source ceased to exist. Here we report new exposure ages from erratics in the Upper Geyser Basin which provide a more direct constraint on the timing of plateau deglaciation. Three ³⁶Cl ages from these erratics average 14.8 ± 0.4 ka, supporting the inference of rapid deglaciation of the Yellowstone Plateau during the time of the Deckard Flats readjustment. Exposure ages from additional erratics on the plateau are being developed to further refine the timing of deglaciation. These ages will be combined with geochemical analyses, U-Pb dating of boulder lithologies, and new LiDAR data collected in 2021 to provide information on erratic provenance and former ice flow directions in order to reconstruct a more complete picture of the timing and patterns of ice withdrawal on the Yellowstone Plateau during the last deglaciation.