COMPLEX MORAINE EXPOSURE AGES ON HOLOCENE MORAINES IN THE SIERRA NEVADA: A NEOGLACIAL CONUNDRUM

In an effort to test the temporal and spatial variability of Holocene glaciation in the Sierra Nevada, CA, we have collected samples for ¹⁰Be CRN exposure dating from moraines and rock glaciers below six different cirques between Lake Tahoe and Big Pine. Our preliminary results present a conundrum: although all deposits are geomorphically young (fresh unweathered boulders, unstable and unvegetated slopes) and were previously mapped as Matthes age (Little Ice Age (LIA) equivalent), only some moraines show dominantly young ages consistent with the mapping and other previous studies of Neoglaciation in the range. In contrast, some of the moraines show a majority of ages thousands of years older than LIA. For example, nearly all ¹⁰Be ages on Matthes moraines below the Lyell Glacier (Yosemite NP) lie between 150­300 yr before 2015, whereas similar moraines below the adjacent Maclure Glacier have exposure ages ranging from 1900­3900 yr old. Boulders on correlative Matthes moraines to the north below Price Peak (west Lake Tahoe) have exposure ages ranging from 1900­9900 yr old, with no ages within the LIA window.

The abundance of pre­LIA exposure ages on many of these moraines seemingly contradicts both historical photographic evidence as well as geomorphic and lake­sediment evidence, which indicate that the most extensive Holocene glaciation in the Sierra Nevada was during the late LIA. Although inheritance in moraines of such small glaciers likely explains some of the discrepancy, internal consistency of ages at some sites belies that as the sole reason. Some ages, such as those below the Maclure Glacier, may reflect slow­flow dynamics associated with debris­covered glaciers (rock glaciers). The older ages below Price Peak, however, are more difficult to reconcile with previous studies, particularly those indicating absence of glaciers in the Sierra before ~3500 cal yr BP. Other possible explanations include deposition related to protalus processes or to discrete events (e.g., earthquake­induced rockfall); conversely, the ages may indicate that Holocene glaciation in the range was more spatially and temporally heterogeneous than previously thought. CRN analyses of the remainder of the samples, as well as planned lake-sediment coring below select sites, may help resolve this conundrum.