Cosmogenic 36Cl surface exposure constraints on late Pleistocene coastal refugia in the eastern Mount Edgecumbe volcanic field, Alaska

The extent to which parts of Southeast Alaska remained ice-free during the Last Glacial Maximum (LGM) has implications for models of early human migration to the Americas. The Mount Edgecumbe Volcanic Field (MEVF) on Kruzof Island has been proposed as one such ice-free refugium, but evaluating this hypothesis depends critically on the age of its lava flows. We use geologic mapping, in situ cosmogenic 36 Cl surface exposure dating, terrain analysis, and forward modeling to determine the emplacement age of a basalt lava flow in the eastern MEVF. We sampled four different surfaces of the lava flow along an elevation transect spanning from above the marine limit to the modern coastline, including surfaces that experienced prolonged marine inundation during the Holocene. Measured and modeled 36 Cl concentrations are consistent with postglacial emplacement of the lava flow at ~ 14.8 ka and incompatible with surface exposure in an ice-free refugium during the LGM. The exposure histories document a coastal landscape undergoing rapid reorganization during the late Pleistocene, with changing ice extent, sea level, and volcanism that would have limited the development of persistent refugia for human migration.