10BE AGE CONTROL OF GLACIATION IN THE BEARTOOTH MOUNTAINS, USA FROM THE LATEST PLEISTOCENE THROUGH THE HOLOCENE

Abstract. Alpine glaciers in the western United States are often associated
with late Holocene Little Ice Age (LIA) advances. Yet, recent studies have
shown many of these glacial landforms are remnants of latest Pleistocene
retreat with only the most cirque-proximal moraines preserving LIA activity. Additionally, the timing and magnitude of glacial advances during the Neoglacial–LIA interval remains uncertain, with presumed maximum extents
occurring during the LIA driven by lower Northern Hemisphere insolation
levels. Here we present 10Be surface exposure ages from a glacial
valley in the Beartooth Mountains of Montana and Wyoming, United States.
These new data constrain the presence of the glacier within 2–3 km of the
cirque headwalls by the end of the Pleistocene with implications for
large-scale retreat after the Last Glacial Maximum. Cirque moraines from two glaciers within the valley preserve a late Holocene readvance, with one
reaching its maximum prior to 2.1±0.2 ka and the other 0.2±0.1 ka. Age variability among the moraines demonstrates that not all
glaciers were at their largest during the LIA and presents the possibility of
regional climate dynamics controlling glacial mass balance.
