LGM AND YOUNGER DRYAS GLACIATIONS IN THE KLAMATH MOUNTAINS, CA, EVIDENCE FROM 10BE EXPOSURE DATING AND NUMERICAL MODELING

In the Western United States, alpine glaciers responded differently to global climate changes following the Last Glacial Maximum (LGM, ~20 ka), reflecting regional disparities in climatic trends. In order to better understand this variation, we focus our study in the southern Klamath Mountains of Northern California where a well-preserved record of late Pleistocene glaciation fills a spatial gap in published glacial chronologies between the Sierra Nevada and the Cascades. We present new moraine maps from 5 glacially sculpted valleys with 30 new cosmogenic 10Be exposure ages from moraine boulders and scoured bedrock to constrain the timing of glacial advances. Mapping shows evidence of a least 3 sets of moraines, based on weathering and relative position. 10Be data defines 2 distinct advances ending at 16.83 ± 1.85 ka and 12.29 ± 1.23 ka; the older moraines yielded a complex set of 10Be ages between 110-20 ka, and may represent multiple early events. The penultimate advance correlates well with data that suggests regional deglaciation by ~17 ka, possibly associated with Heinrich event 1. The latest event may be related to the global Younger Dryas (~12 ka) cooling event. To estimate the climate during these latter two glaciations, we applied a 2D MATLAB numerical mass-and-energy balance model coupled with a Fortran ice flow model. Based on modern climate and topography, we determined sets of precipitation and temperature changes that would generate glaciers of the extent implied by the remnant moraines. Upon comparison with local proxies and global climate models, Our results suggest that at ~17 ka, temperatures were 5-6° cooler and precipitation was 0-25% greater than today, and that at ~12 ka, conditions were only slightly warmer (1-2°) and/or drier (0-15%) than at ~17 ka. The timing of these glacial maxima appears out-of-phase with glaciers in the Sierra Nevada to the south, which receded at ~18 and ~14 ka. This inconsistency may be due to the apparent dependence of Klamath glaciation on temperature change rather than precipitation, whereas glaciers in the Sierra Nevada required significantly increased precipitation in addition to decreased temperature. We infer that the presence of Younger Dryas glaciers in the Klamath Mountains reflects early drying of the Sierra Nevada rather than asynchronous changes in temperature.