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HR: 0800h
AN: C21B-1159
TI: New Cosmogenic Beryllium-10 Exposure-Age Limits on Terminal Moraines of the Last Glaciation in the Bear River Drainage, Uinta Mountains, Utah
AU: * Laabs, B J
EM: blaabs@gustavus.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Munroe, J S
EM: jmunroe@middlebury.edu
AF: Middlebury College, Geology Department, Middlebury, VT 05753 United States
AU: Rosenbaum, J G
EM: jrosenbaum@usgs.gov
AF: U.S. Geological Survey, PO Box 25046, Denver, CO 80225-0046 United States
AU: Refsnider, K A
EM: Kurt.Refsnider@Colorado.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States
AB: The Uinta Mountains were occupied by numerous glaciers during marine oxygen-isotope stage 2 (MIS 2). Reconstructions for the last glaciation reveal that central and eastern valleys in the Uintas contained discrete valley glaciers, many of which advanced beyond the mountain front. Cosmogenic exposure-age limits on two moraines in the south-central part of the range indicate that glaciers retreated from their maximum extents at 16.8 ± 0.7 ka (Munroe et al., 2006), up to 2000 years later than glaciers elsewhere in the Middle and Southern Rockies. These ages suggest that deglaciation of the Uintas was approximately synchronous with the hydrologic fall of Lake Bonneville (at ~16 cal. ka), which implies that glaciers and the lake responded to the same regional climatic forcing. Glacial reconstructions further indicate that glaciers in the western Uintas, nearest to the lake, had equilibrium-line altitudes as much as 600 m lower than glaciers farther east. This evidence suggests that Lake Bonneville may have amplified moisture in the western Uinta Mountains by providing lake-effect precipitation to valleys located immediately downwind. To further investigate this hypothesis, we acquired cosmogenic ¹⁰Be surface-exposure ages from a terminal moraine in the Bear River drainage in the northwestern Uinta Mountains. This valley was occupied by outlet glaciers of the Provo Ice Field (of Refsnider, 2006), which covered an area of about 685 km² and drained via several valleys in the Uintas. Cosmogenic-exposure ages of moraine boulders range from 24.5 ± 2.5 ka to 19.0 ± 3.5 ka (± 2σ) with an error-weighted mean of 21.5 ± 1.4 ka (n = 7), which we interpret to represent a minimum age of deglaciation in the Bear River drainage. Radiocarbon ages of glacial flour in sediment from Bear Lake, a large lake downstream of the glaciated area, are generally consistent with cosmogenic-exposure ages and indicate that deglaciation began at about 24 cal. ka (Rosenbaum et al., 2005). When considered together, these age limits indicate that the Bear River glacier began retreating about 4000 years earlier than glaciers elsewhere in the Uinta Mountains and about 2000 years before the peak of Lake Bonneville (at 19 cal. ka). While this chronology appears inconsistent with the suggestion that the lake enhanced glacier mass balance in the Uintas, it may instead reflect spatial variability of precipitation in the western Uintas or complex responses of the Provo Ice Field to climate change near the end of the last glaciation.
DE: 0720 Glaciers
DE: 0799 General or miscellaneous
DE: 4999 General or miscellaneous
SC: Cryosphere [C]
MN: 2006 Fall Meeting