C53C-0322:
Cosmogenic noble gas paleothermometry provides new constraints on LGM temperature estimates inferred from glacier extents in the Sierra Nevada, California
Friday, 19 December 2014
Marissa M Tremblay1,2, Curtis W Baden3, Greg Balco2 and David L Shuster1,2, (1)University of California Berkeley, Department of Earth and Planetary Science, Berkeley, CA, United States, (2)Berkeley Geochronology Center, Berkeley, CA, United States, (3)Stanford University, Department of Geological and Environmental Sciences, Stanford, CA, United States
Abstract:
Numerical models successfully simulate LGM glacier extents in the Sierra Nevada, California, over a large range of precipitation and temperature combinations (Kessler et al. 2006, J. Geophys. Res., 111, F02002, doi:10.1029/2005JF000365). We use cosmogenic noble gas paleothermometry on samples from summit flats in the Sierra Nevada to estimate temperatures during the last glacial period in this region and further constrain the climatological predictions of these models. Cosmogenic noble gas paleothermometry utilizes the open-system behavior of cosmogenic noble gases at surface temperatures in common minerals like quartz to quantify the thermal histories of rocks during exposure to cosmic ray particles at the Earth’s surface. We sampled boulders and bedrock tors atop summit flats inferred to be exposed throughout the last glacial period and measured cosmogenic 3He and 21Ne concentrations in quartz from these samples. We use cosmogenic 21Ne, which is quantitatively retained at Earth surface temperatures in quartz, to constrain exposure durations and erosion rates, and cosmogenic 3He, which exhibits open-system behavior in quartz, to quantify time-integrated temperatures during surface exposure. Data from samples collected at the summit of Mt. Langley in the southern Sierra Nevada indicate that 30–45% of the cosmogenic 3He produced in quartz has been retained at surface exposure temperatures; the rest has been diffusively lost. Preliminary models using these data and published diffusion kinetics indicate that the difference between modern and LGM summit temperatures may be significantly greater than the 5.6°C difference predicted by Kessler et al. (2006). We would expect significantly less 3He to be retained if average temperatures were 5.6°C lower at the LGM. Additional cosmogenic noble gas measurements on samples from this site and other sites in the Sierra Nevada and White Mountains as well as sample-specific diffusion kinetics will enable us to quantify this difference more precisely.
