Understanding Glacier Thinning and Retreating During the Last Glacial
Maximum in Yosemite to Predict Contemporary Deglaciation
Abstract
We describe new cosmogenic Be-10 and C-14 exposure age dating on
previously glaciated bedrock samples from Lyell Canyon as constraints to
model the glacier’s rate and timing of thinning and retreat after the
Last Glacial Maximum (LGM). Close analysis of deglaciation following the
LGM (22-12 ka) can offer insight into how glacier retreat proceeds in a
warming climate. The extent and age of the LGM glaciation in Yosemite
National Park, California are relatively well-constrained. Our new
exposure ages from Yosemite can quantify the change of the glaciation
after the LGM. This is important because the rate and timing of glacier
retreat after the LGM allows us to learn about the LGM-Holocene climate
transition. We collected 16 granodiorite bedrock samples from the Lyell
Canyon walls in three vertical transects: at the end, in the middle, and
near the head of Lyell Canyon. Sample elevations range from 2781m to
3388m. The samples are being processed for cosmogenic Be-10 and C-14
concentrations (for the lower and higher elevations in the transects,
respectively). Together with previously acquired Be-10 exposure ages
from glacial polished bedrock and boulders at the canyon floor, our
vertical transects will help to define the relationship between glacier
retreat and thinning along the valley. The combination of different
nuclide measurements has the potential to reveal whether the glacier
melted rapidly or went through multiple thinning and thickening cycles.
We created several simple forward models of cosmogenic Be-10 and C-14
exposure ages on the valley wall for different glacier thinning
patterns: (i) rapid thinning, (ii) thinning and thickening cycles during
the melting, (iii) thickening first, followed by thinning, and (iv)
breaking an upper small cirque glacier from the main glacier during the
thinning. After we have obtained all our data, we will compare the
exposure age data to our modeled scenarios, as well as local
paleoclimate records, to quantify the glacier’s geometry and mass
balance during the climate warming period. Understanding the timing,
rates, and patterns of LGM retreat and thinning constitute a useful test
case that aids mountain glacier melting predictions and water budget
planning under contemporary climate change in analogous environments.