10BE EXPOSURE DATING OF HOLOCENE MORAINES IN THE SIERRA NEVADA, CALIFORNIA

Constraining the extent and timing of Holocene glaciations is critical to addressing standing hypotheses that ascribe climatic fluctuations to changes in atmospheric and oceanic circulation patterns, or anthropogenic forcing. In the terrestrial record, such constraint typically relies on chronologies obtained from ¹⁰Be exposure dating of moraine deposits. However, the short exposure time of Holocene moraines, particularly those formed during the Little Ice Age (LIA), makes obtaining precise chronologies extremely challenging. To date, only a handful of LIA deposits in three locations (New Zealand, Swiss Alps, and Peruvian Andes) have been dated with ¹⁰Be.

Here, we report ¹⁰Be (and supporting ²⁶Al) exposure ages from LIA moraines from multiple sites in the Sierra Nevada (Lyell, Maclure, Palisade, and Conness glaciers). Our Sierran LIA record is comparable to published results from other locations and supports a globally synchronous LIA deglaciation. This result is consistent with the contention that the LIA was terminated by anthropogenically-driven warming. We also report preliminary results from several older neoglacial moraines present at these sites. Chronology from the neoglacial deposits will be used to test whether the timing of the return to glacial conditions in the Sierras correlates to a southward shift in the Intertropical Convergence Zone, which has been hypothesized to increase El Nino-like conditions in the Pacific Ocean. This record should be ideal for testing this hypothesis since precipitation in the Sierras is highly sensitive to El Nino conditions.