UNDERSTANDING GLACIER THINNING AND RETREAT DURING THE LAST GLACIAL MAXIMUM IN YOSEMITE TO PREDICT CONTEMPORARY DEGLACIATION

Close analysis of deglaciation following the Last Glacial Maximum (LGM), 22-12 ka can offer insight into how glacier retreat proceeds in a warming climate. The extent and age of the Tioga glaciation in Yosemite National Park (NP), California from the LGM is relatively well-constrained. However, the glaciers’ thinning and retreat rates and geometric changes throughout the deglaciation are less known. We use cosmogenic ¹⁰Be and ¹⁴C exposure dating on previously glaciated bedrock surfaces from Lyell Canyon to pinpoint the glacier surface elevation at different times and locations. We collected 16 granodiorite bedrock samples from the canyon walls along three vertical transects: at the distal end of the canyon, in the middle of the canyon, and near the head of the U-shaped canyon. Sample elevations range from 3388m to 2781m. The samples are being processed for cosmogenic ¹⁰Be and ¹⁴C concentrations (for the lower and higher elevations in the transects, respectively). Some of the data will be available by August 2021, at which point exposure ages of the samples will be calculated. Together with previously acquired ¹⁰Be exposure ages from glacially polished bedrock and erratics on the canyon floor, data from our vertical transects will help to define the relationship between glacier retreat and thinning along the valley. The combination of isotopic measurements has the potential to reveal whether the glacier melted rapidly or went through multiple thinning and thickening cycles. We will combine these age constraints with paleoclimate records to quantify the glacier’s geometry and mass balance as the climate warmed. Knowledge of the timing, rates, and patterns of Tioga glacier retreat and thinning will constitute a useful test case that aids mountain glacier melting prediction and water budget planning under contemporary climate change in analogous environments.