HR: 1340h
AN: PP33B-1275    [Abstracts]
TI: Be-10 Ages From Northern Alaska Range Moraines Help Constrain the Timing of the Penultimate Glaciation in Eastern Beringia
AU: * Young, N E
EM: nyoung2@buffalo.edu
AF: Department of Geology University at Buffalo, 876 Natural Sciences Complex, Buffalo, NY 14260, United States
AU: Briner, J P
EM: jbriner@buffalo.edu
AF: Department of Geology University at Buffalo, 876 Natural Sciences Complex, Buffalo, NY 14260, United States
AU: Kaufman, D S
EM: darrell.kaufman@NAU.edu
AF: Department of Geology Northern Arizona University, 306 Geology Annex (Building 13), Flagstaff, AZ 86011-4099, United States
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Center for Accelerator Mass Spectrometry Lawrence Livermore National Laboratory, P.O. Box 808, L-206, Livermore, CA 94550, United States
AB: Alaska and the adjoining Yukon Territory is currently the site of uncertainty concerning the timing of the penultimate glaciation. While the timing of the Last Glacial Maximum across this region is generally synchronous (~18-25 ka), several ages for the penultimate glaciation have been proposed, including 50-60 ka, 140 ka, and 250 ka, correlating with marine isotope stage (MIS) 4/early MIS 3, MIS 6, and MIS 8, respectively. Furthermore, an out-of-phase, late MIS 5 age has also been suggested in western Beringia. Here, 10 new ¹⁰Be ages are presented from a key reference locality in the Delta River valley, Alaska, situated in the north- central Alaska Range. Because boulders are absent at this moraine sequence, quartzose pebble collections from moraine crest surfaces are used. Five pebble samples taken from the Delta (penultimate glaciation) moraine range between 24.7 and 68.0 ka and average 53.5 ± 11.0 ka after excluding a young outlier (24.7 ± 0.6 ka). Because previous research has shown that exposure ages typically date moraine stabilization, emphasis should be placed on older ages within a suite of exposure ages where inheritance is of minimal concern. Five pebble samples taken from the up-valley, Donnelly moraine (late Wisconsin glaciation) range between 11.5 and 65.3 ka, and average 13.3 ± 2.0 ka after removing one sample due to obvious inheritance (65.3 ± 2.1 ka). A nearby site in the north-central Alaska Range also shows promise for exposure dating late Quaternary moraines. The Fish Lake valley, located approximately 70 km southeast of the Delta River valley, is host to an extensive sequence of moraines that spans between the Little Ice Age and the penultimate glaciation; 17 boulder samples from the penultimate and late Wisconsin moraines collected in the summer of 2006 will also be presented. ¹⁰Be ages from these boulders will serve as an important comparison to the pebble-based ages from the Delta River valley. Although ¹⁰Be dating of pebble collections is untested, the average ages are in broad agreement with recent exposure dating studies elsewhere in Alaska and the Yukon supporting a MIS 4/early MIS 3 age for the penultimate glaciation in eastern Beringia.
DE: 1105 Quaternary geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1824 Geomorphology: general (1625)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting