Last glacial maximum climate inferences from cosmogenic dating and glacier modeling of the western Uinta ice field, Uinta Mountains, Utah Kurt A. Refsnider a, ⁎ , Benjamin J.C. Laabs b , Mitchell A. Plummer c , David M. Mickelson a , Bradley S. Singer a , Marc W. Caffee d a Department of Geology and Geophysics, University of Wisconsin, 1215 W Dayton St., Madison, WI 53706, USA b Department of Geological Sciences, SUNY Geneseo, 1 College Circle, Geneseo, NY 14454, USA c Idaho National Laboratory, Idaho Falls, ID 83415-2107, USA d Department of Physics, Purdue University, 1296 Physics Building, W. Lafayette, IN 47907, USA Received 30 January 2007 Available online 4 January 2008 Abstract During the last glacial maximum (LGM), the western Uinta Mountains of northeastern Utah were occupied by the Western Uinta Ice Field. Cosmogenic 10 Be surface-exposure ages from the terminal moraine in the North Fork Provo Valley and paired 26 Al and 10 Be ages from striated bedrock at Bald Mountain Pass set limits on the timing of the local LGM. Moraine boulder ages suggest that ice reached its maximum extent by 17.4±0.5 ka (±2σ). 10 Be and 26 Al measurements on striated bedrock from Bald Mountain Pass, situated near the former center of the ice field, yield a mean 26 Al/ 10 Be ratio of 5.7 ± 0.8 and a mean exposure age of 14.0 ± 0.5 ka, which places a minimum-limiting age on when the ice field melted completely. We also applied a mass/energy-balance and ice-flow model to investigate the LGM climate of the western Uinta Mountains. Results suggest that temperatures were likely 5 to 7°C cooler than present and precipitation was 2 to 3.5 times greater than modern, and the western-most glaciers in the range generally received more precipitation when expanding to their maximum extent than glaciers farther east. This scenario is consistent with the hypothesis that precipitation in the western Uintas was enhanced by pluvial Lake Bonneville during the last glaciation. © 2007 University of Washington. All rights reserved. Keywords: Uinta Mountains; Last glacial maximum; Cosmogenic nuclide; CRN; Glacier modeling; Ice field; Lake Bonneville; Glacial geology; Paleoclimate Introduction Alpine glaciers are sensitive indicators of changes in climate (Oerlemans et al., 1998) and respond on sub-millennial timescales (Paterson, 1994). Thus, paleoclimatic inferences are commonly made based on the extent of past mountain glaciers. Throughout the western U.S., reconstructions of alpine paleoglaciers and associated equilibrium line altitudes (ELAs) have been used to determine possible precipitation and temperature combinations that supported glaciers during the last glacial maximum (LGM; e.g. Leonard, 1984, 1989; Munroe and Mickelson, 2002; Brugger, 2006). In order to put these glaciological reconstructions into a temporal context, a firm chronology of past glacial advances must be established. With the advent of cosmogenic nuclide surface- exposure dating, broad patterns in the timing of mountain glacier advances across the western U.S. and related patterns of regional and temporal variations in paleoclimate conditions are emerging (e.g., Gosse et al., 1995; Phillips et al., 1997; Licciardi et al., 2001, 2004; Owen et al., 2003; Benson et al., 2004, 2005; Brugger, 2006; Munroe et al., 2006). However, dated glacial features in most ranges of the Rocky Mountains are still scarce, and im- proving the knowledge of regional climate during the LGM re- quires a more comprehensive temporal record of glacial advances and retreat and accurate techniques for estimating paleoclimatic conditions based on the glacial record. The Uinta Mountains of northeastern Utah (Figs. 1a and b) were extensively glaciated during the LGM (Atwood, 1909; Oviatt, 1994; Laabs and Carson, 2005; Munroe, 2005). The first Available online at www.sciencedirect.com Quaternary Research 69 (2008) 130 – 144 www.elsevier.com/locate/yqres ⁎ Corresponding author. Current address: Institute of Arctic and Alpine Research, 1560 30th St. 450 UCB, Boulder, CO 80303, USA. E-mail address: kurt.refsnider@colorado.edu (K.A. Refsnider). 0033-5894/$ - see front matter © 2007 University of Washington. All rights reserved. doi:10.1016/j.yqres.2007.10.014