IN SITU COSMOGENIC BE-10 AND C-14 EXPOSURE AGE EVIDENCE FOR THE TIMING OF CIRQUE GLACIERS FORMATION IN LYELL CANYON, YOSEMITE DURING THE POST-LAST GLACIAL MAXIMUM DEGLACIATION

We have obtained 16 new in situ cosmogenic ¹⁰Be and ¹⁴C bedrock exposure ages from Lyell Canyon to uncover more details of the timing of glacier thinning and retreat after the Last Glacial Maximum (LGM) in Yosemite National Park. Analyzing post-LGM deglaciation (22-12 ka) can offer insight into how glaciers retreat in a warming climate. The Tioga glaciation’s age and extent in Yosemite National Park, California are relatively well-constrained. Our new exposure ages from Yosemite can quantify the change of the glaciation after the LGM. This is important because the rate and timing of glacier retreat after the LGM allows us to learn about the LGM-Holocene climate transition. We sampled granodiorite bedrock on the Lyell Canyon walls in three vertical transects: at the end, in the middle, and near the head of Lyell Canyon. These samples range from 2781m to 3388m in elevation. The samples from the lower and higher elevations in the transects are being processed for cosmogenic ¹⁰Be and ¹⁴C concentrations, respectively. The lower elevation samples’ ¹⁰Be exposure ages range from around 7 to 14 ka. The higher elevation samples’ ¹⁴C exposure ages range from around 3 to 20 ka. All three transects show a similar exposure age pattern where the oldest exposure age is in the middle and the exposure ages decrease towards both the highest elevation samples and the lowest elevation samples. Matching with our forward model, this exposure age pattern shows the formation of cirque glaciers during the glacier thinning in Lyell Canyon after the glacial maximum. We can use the exposure age data to reconstruct Lyell glacier’s geometry change and compare that to the local paleoclimate records to estimate the mass balance change during the climate warming period. Our data-supported past cirque glacier and valley glacier melting timing after the LGM provides valuable insight for paleoclimate models to reconstruct glacier mass balance in more detail. Our new findings on the timing, rates, and patterns of post-LGM Tioga glacier thinning constitutes a useful test case that aids mountain glacier melting predictions under contemporary climate change in analogous environments.