TESTING 10BE EXPOSURE DATING OF HOLOCENE CIRQUE MORAINES USING GLACIOLACUSTRINE SEDIMENTS IN THE SIERRA NEVADA, CALIFORNIA

Cosmogenic radionuclide (CRN) exposure dating of very young Holocene moraines has recently become technically feasible and has large potential for future glacial studies worldwide. However, possible uncertainties with such dating have not been rigorously evaluated. An ongoing study with Lawrence Livermore National Laboratory has yielded a surprisingly wide range of ¹⁰Be CRN ages of boulder emplacement on Holocene cirque moraines in the Sierra Nevada (Lyell and Maclure in Yosemite National Park, Mt. Price near Lake Tahoe). The CRN data indicate possible glacier maxima at 2-3 ka, 4 ka, and early Holocene, in addition to the late Little Ice Age (LIA). These results contradict previous studies of Neoglaciation in the range that indicate the Holocene maxima were exclusively during the LIA. In our study, we collected glaciolacustrine sediment records from lakes directly downstream of each of the dated moraine sequences in order to test the accuracy of these CRN ages compared to the glacial rock flour record. Despite being from adjacent basins, the lake cores below Lyell and Maclure glaciers show markedly different records. A fine-grained core from below Lyell Glacier records two maxima in rock flour centered at ~1760 and ~460 cal yr BP. In contrast, the Maclure Lake core is predominantly composed of coarse sandy slopewash from which we interpret two intervals of fine-grained, high-MS sediment as likely rock-flour maxima centered at ~1550 and ~770 cal yr BP, as well as a possible older interval from ~4600–2900 cal yr BP. Cores from below the cirque moraines at Mt Price record no apparent rock flour. Together, the cores and clustered CRN ages suggest considerable temporal variability in maximum extent of small cirque glaciers in the Sierra Nevada. However, asynchrony between CRN ages and rock flour records indicate that other factors (e.g., inheritance, seismically induced rockfall, paraglacial processes) may introduce significant uncertainty in the CRN ages.