WIDE SPAN OF EXPOSURE AGES ON HOLOCENE MORAINES IN THE SIERRA NEVADA: PROCESS OR CLIMATIC CONTROLS?

New high-precision ¹⁰Be CRN exposure analyses of boulders from distal cirque moraines in the Sierra Nevada present a conundrum: although all moraines are geomorphically fresh and unstable (mapped as Matthes age), some show young ages (last 150-300 yrs; late Little Ice Age - LIA) consistent with previous studies of Neoglaciation in the range, whereas other moraines show a majority of ages thousands of years older than LIA. Matthes moraines below the Lyell Glacier (Yosemite NP) show exposure ages consistently between 150-300 yr before 2015, whereas similar moraines below the adjacent Maclure Glacier have exposure ages ranging from 1900-3900 yr old. Boulders on correlative Matthes moraines to the north below Price Peak (west Lake Tahoe) have exposure ages ranging from 1900-9900 yr old; no boulders analyzed so far on these northern moraines show LIA ages.

The abundance of pre-LIA exposure ages on many of these moraines seemingly contradicts both historical photographic evidence as well as geomorphic and lake-sediment evidence, which indicate that the most extensive Holocene glaciation in the Sierra Nevada was during the late LIA. Some ages, such as those below the Maclure Glacier, may reflect slow-flow dynamics associated with debris-covered glaciers (rock glaciers). The older ages below Price Peak, however, are more difficult to reconcile with previous records, particularly those indicating absence of glaciers in the Sierra before ~3500 cal yr BP. Consistency of the exposure ages implies that inheritance from prior exposure is unlikely to explain all the ages. Other possible explanations include deposition related to protalus processes or to discrete events (e.g., earthquake-induced rockfall); conversely, the ages may indicate that Holocene glaciation in the range was more spatially and temporally heterogeneous than previously thought. Additional exposure ages from these moraines and others farther south may help resolve this conundrum.