USING LICHENOMETRY AND BE-10 DATING ON NEOGLACIAL MORAINES IN THE ALASKA RANGE, ALASKA

Much of the terrestrial record throughout Alaska indicates that Little Ice Age (LIA) glacier advances represent maximum Holocene ice limits. Due to this, only broad periods of pre-LIA, Neoglacial glacier advance have been reconstructed. Overrun trees in glacier fore-fields have served to construct a glacial chronology in coastal Alaska that place ice advances around 2700 yr B.P., 1350 yr BP, and the subsequent LIA period (~ AD 1450-1900). Lichenometry has been used extensively in the Brooks Range to map a history of Neoglaciation that may have begun as early as 4500 yr BP. Here, lichenometry and Be-10 surface exposure dating are used on two Neoglacial moraine sequences located in the Fish Lake valley, east-central Alaska Range. In one sequence seven small moraine crests mark ice positions with lichenometry-based ages between ~AD 920 and AD 1935, when rapid ice retreat commenced. A longer, second sequence yields much larger lichen diameters and suggests that a pre-LIA Holocene record has been preserved. Lichen diameters greater than 100 mm over three moraine crests indicate that LIA glacier extent was not the Holocene maximum position in the study valley. In addition to lichenometry, six boulders were sampled for Be-10 surface exposure dating from these Holocene moraine crests. Samples are currently being processed and exposure ages are expected shortly.