Cosmogenic 10Be surface-exposure dating and numerical glacier modeling are used to reconstruct glacial chronology and climate in the Colorado Sangre de Cristo Mountains during the local last glacial maximum (LLGM) and the subsequent deglaciation. Twenty-two surface-exposure ages on moraine boulders and polished-bedrock outcrops in the Willow Creek valley and ten in two adjacent valleys indicate that glaciers were at or near their maxima from ∼21 ka until 17–16 ka, and then retreated rapidly, nearly deglaciating the Willow Creek valley entirely by ∼14 ka. Coupled energy/mass-balance and flow modeling of two of the glaciers indicates that, if changing ice extent was driven only by temperature and insolation changes, temperature depressions of 5.0 and 5.1 °C from modern conditions, with an uncertainty of approximately +1.5/−1.0 °C, would have sustained the glaciers in mass-balance equilibrium at their LLGM extents. Doubling or halving of modern precipitation during the LLGM would have been associated with 2.7–3.0 °C and 6.9–7.0 °C temperature depression respectively. Approximately half of the subsequent LLGM–to-modern climate change was accomplished by ∼14 ka. If the rapid main phase of deglaciation between about 16 ka and 14 ka was driven solely by temperature and insolation changes, it would have been associated with a temperature rise of about 2.5 °C, at a mean rate of approximately 1.1 °C/ky. This new chronology of the last glaciation is generally consistent with others developed recently in the Colorado Rocky Mountains. The numerical modeling, however, suggests a lesser LLGM temperature depression from modern conditions than have most previous studies in Colorado.
Bibliographical noteFunding Information:
We thank Zach Snyder, Alex Robertson, Ben Mackall, and John Collis for CRN-sampling help in the field, and Laura Best, Elizabeth Huss, Olivia Kaplan, Brendon Quirk, Alec Spears, Doug Steen and Katherine Truong for assistance processing samples in the lab at SUNY Geneseo. Joe Licciardi and Jason Briner provided insight and wisdom to our discussion of 10Be production rates and scaling. We also thank Ann Rowan and an anonymous reviewer for their insightful comments and suggestions. San Isabel and San Juan National Forests allowed us to work and sample in some of their most popular, and spectacular, backcountry areas. Financial support was provided to EML, BJCL, and MAP by the U.S. National Science Foundation [NSF/GLD 1024838 and NSF/GLD 1024852], and to KAB by the University of Minnesota, Morris FREF program.
- Cosmogenic surface-exposure dating
- Glacier modeling
- Last glacial maximum
- Sangre de Cristo Mountains