Chronology and Paleoclimate of Late Pleistocene Glaciation in the Klamath Mountains, CA

Glaciers are sensitive to local and global climate variations, especially to changes in precipitation and temperature over sub-millennial timescales. Therefore, glacial deposits are excellent tools for reconstructing past climates. The western United States exhibits an excellent record of glaciation, but ongoing work across the region shows complex and yetunexplained variation in timing and extent of deglaciation between different mountain ranges at the end of the Last Glacial Maximum (LGM). The Trinity Alps of the southern Klamath Mountains in Northern California contain an excellent record of Pleistocene glaciation which I use to fill a significant spatial gap in published glacial chronologies and to provide a bridge between the Sierra Nevada and the Cascades. The Trinity Alps Wilderness is a 2,130 sq. km. federally designated area located at 41.00° N, 123.00°W, approximately 60 km southwest of Mt. Shasta in Northern California. Glacial deposits in the Trinity Alps were located using Google Earth and previously published maps, and were confirmed in the field. Building on a series of previous expeditions, in the summer of 2015 twenty-four samples from five moraines were taken for 10Be exposure dating, as well as three samples from striated bedrock. Of these, six samples were selected for exposure age analysis: five from two early LGM moraines and one from an older moraine. These ages, in addition to twenty-four ages determined in a previous study, provide evidence for at least two stages of post-LGM glaciation of similar extent throughout the Trinity Alps: the first ending at 16.83 ± 1.85 ka, the second at 12.29 ± 1.23 ka. These ages correlate with the regional LGM (~17 ka) and the global Younger Dryas (~12 ka) cooling event, respectively. The moraine maps were then used to constrain results from a climate-driven 2D numerical model of glacier mass balance and flow. This model was used to determine the potential precipitation and temperature difference from modern climate that would generate the mapped glaciers. Comparison of the resulting paleoclimate curves with nearby proxies and global climate models suggest that an approximate 5.5°C decrease in temperature and 0 to 25% increase in precipitation drove LGM glaciation in the region. Additionally, these results suggest that a similarly wet but slightly warmer-than-LGM climate drove a regionally asynchronous re-advance in the Trinity Alps linked with the Younger Dryas cooling event.