LAST GLACIAL TEMPERATURES IN THE SIERRA NEVADA INFERRED FROM COSMOGENIC NOBLE GAS PALEOTHERMOMETRY

Understanding the climate of the Sierra Nevada, California, and greater western North America during the last glacial period is important for calibrating climate models in this drought-susceptible area. However, deconvolving effects of temperature and precipitation on paleoclimate proxies can be difficult and often qualitative. Here, we apply cosmogenic noble gas paleothermometry (CNGP) to samples from Mt. Langley in the Sierra Nevada to quantitatively reconstruct temperatures during the last glacial period. Helium and neon experience diffusive loss at Earth surface temperatures in common minerals like quartz and feldspars. CNGP utilizes the diffusive behavior of cosmogenic ³He and ²¹Ne in these minerals to quantify the thermal histories of rocks during exposure to cosmic ray particles at Earth’s surface. We collected samples at the summit of Mt. Langley, inferred to be ice free during the last glacial, and from a nearby cirque floor, inferred to be shielded from cosmic rays by ice until 13 ka, and measured cosmogenic ³He and ²¹Ne concentrations in quartz from these samples. We will use cosmogenic ²¹Ne, which is retained at surface temperatures, to constrain steady-state erosion rates of summit samples, and cosmogenic ³He, which exhibits open-system behavior, to quantify time-integrated temperatures during surface exposure. We will also quantify the kinetics of helium diffusion in these samples through step-degassing experiments on proton-irradiated quartz aliquots.