Late Pleistocene cosmogenic Cl-36 glacial chronology of the southwestern Ahklun Mountains, Alaska

Thirty-two cosmogenic Cl-36 surface exposure ages constrain the timing of t wo late Pleistocene glacial advances in the western Ahklun Mountains, south western Alaska. Boulders were sampled from one early Wisconsin (sensu lato) and six late Wisconsin moraines deposited by ice-cap outlet glaciers and l ocal alpine glaciers. Four moraine boulders deposited during an extensive e arly Wisconsin ice-cap outlet glacier advance have a mean surface exposure age of 60,300 +/- 3200 yr. A moraine deposited by an ice-cap outlet glacier during the restricted late Wisconsin advance has a mean surface exposure a ge of 19,600 +/- 1400 yr. Five moraines deposited by late Wisconsin alpine glaciers have mean ages that range between 30,000 and 17,000 yr. The Cl-36 ages are consistent with limiting C-14 and thermoluminescence ages from rel ated deposits and indicate that Ahklun Mountains glaciers reached their mos t extensive position of the last glaciation early during the late Pleistoce ne, in contrast to the deep-sea isotopic record of global ice volume.(1) (C ) 2001 University of Washington.