Pleistocene glacial geology, Ahklun Mountains, southwestern Alaska
The Ahlkun Mountains contain a showcase of Pleistocene glacial geologic features that record multiple advances of ice cap and valley glaciers. Research between 1996 and 2000 included glacial-geologic mapping, relative-weathering studies, and surface exposure dating. The paleoclimatic context was analyzed using the paleosnowline altitudes of the reconstructed glaciers.
Glacial-geologic mapping was completed over much of the southern and northern Ahklun Mountains. We mapped moraines and associated glacial-lacustrine and glacial-fluvial features, measured relative-weathering features (soil development and moraine morphology), and studied stratigraphic exposures along the principal drainages. Paleo-equilibrium line altitudes (ELA) were estimated for 211 valley glaciers, and for 97 modern glaciers that occupy the highest reaches of the Ahklun Mountains using a, raster-based Geographic Information System (GIS) approach. The estimated ELA lowering of 300-550 m is less than half the lowering recorded over much of the globe, and suggests that drier-than-present conditions prevailed during the LGM. A reduction in winter snowfall is consistent with retreat of a moisture source to the west as the Bering Sea shelf emerged with falling sea level. The southwestward decrease in ELA's suggests that the Bering Sea was a significant source of moisture, despite extensive sea ice and emergence of the continental shelf.
Surface exposure ages based on cosmogenic isotopes provide a chronology for Wisconsin glaciation in the western Ahklun Mountains. Cosmogenic exposure (36Cl) ages were obtained from six late-, and two early-Wisconsin moraines. The ages agree with 14C analyses on other Wisconsin moraines in the Ahklun Mountains. They indicate restricted late Wisconsin glacier advances ~24-26 and ~17-20 36Cl ka, and a more extensive early Wisconsin advance ~60 36Cl ka. The ages suggest a maximum late Wisconsin advance late during the LGM.
An expansion of alpine glaciers during the latest Pleistocene produced a series of extraordinarily well-defined end moraines in the Ahklun Mountains. The moraines are several kilometers beyond modern glacier termini, and ~80 km upvalley of the late Wisconsin terminal moraines. Eleven cosmogenic 10Be and 26Al exposure ages on moraine boulders, combined with radiocarbon ages from a lake core upvalley suggest that the advance culminated between 12.4 and 11.0 ka, sometime during, or shortly following, the Younger Dryas event (ca. 12.9-11.6 ka). These data further strengthen emerging evidence for Younger Dryas-age cooling of the North Pacific Region.
References
Briner, J.P. Kaufman, D.S., Werner, A., Caffee, M., Levy, L., Kaplan, M.R., Finkel, R.C., 2002, Glacier readvance during the late glacial (Younger Dryas?) in the Ahklun Mountains, southwestern Alaska: Geology 30, 679-682.
Briner, J P., Swanson, T.W., and Caffee, M., 2001, Late Pleistocene cosmogenic 36Cl glacial chronology of the southwestern Ahklun Mountains, Alaska. Quaternary Research, 56, 148-154.
Manley, W.F., Kaufman, D.S., and Briner, J.P., 2001, Late Quaternary glacial history of the southern Ahklun Mountains, southeast Beringia—Soil development, morphometric, and radiocarbon constraints: Quaternary Science Reviews 20, 353-370.
Kaufman, D.S., Manley, W.F., Forman, S.L., Hu, F.S., Preece, S.J., Westgate, J.A., and Wolfe, A.P., 2001, Paleoenvironment of the last interglacial-to-glacial transition, Togiak Bay, southwestern Alaska: Quaternary Research 55, 190-202.
Kaufman, D.S., Manley, W.F., Forman, S.L., Layer, P., 2001, Pre-late-Wisconsin glacial history, coastal Ahklun Mountains, southwestern Alaska—New amino acid, thermoluminescence, and 40Ar/39Ar results: Quaternary Science Reviews 20, 337-352.
Briner, J.P. and Kaufman, D.S., 2000, Late Pleistocene glacial history of the southwestern Ahklun Mountains, Alaska: Quaternary Research 53, 13-22.
