10 Be age control of glaciation in the Beartooth Mountains, USA, from the latest Pleistocene through the Holocene

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.