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| Titel |
Quantifying Landscape Response to Past (Last Glacial) and Present Day Erosion with Detrital Thermochronology |
| VerfasserIn |
Todd A. Ehlers, Greg M. Stock, Kenneth A. Farley, Brian Yanites |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038757
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| Zusammenfassung |
| Quantifying landscape response to climate change is limited by insufficient knowledge of
spatial and temporal variations in catchment erosion. Detrital cooling ages collected from
Quaternary glacial moraines and modern river sediments provide a tool to address these
problems. We use detrital thermochronology to quantify spatial variations in alpine glacial
erosion during the Last Glacial Maximum (LGM). Results are compared to the distribution of
present-day erosion recorded in samples from modern river sediments, and predicted patterns
in glacial erosion from a plan-form (shallow-ice approximation) glacial erosion
model.
The elevation dependence of detrital apatite (U-Th)/He (AHe) ages is used as a
sediment tracer to track the elevations where glacially eroded sediment is produced
from bedrock. We measured ~204 AHe single grain ages from three moraines
located between 2.3 and 3.7 km elevation in the Lone Pine catchment, Sierra Nevada,
California. Measured AHe age probability density functions (PDFs) were compared
with predicted PDFs, calculated by convolving bedrock age-elevation relationships
with catchment hypsometries clipped at different altitudes to reflect variable source
elevations of sediment. Statistical comparison of the PDFs using a Monte Carlo
approach and Kuiper test are used to evaluate the spatial distribution of erosion in the
catchments.
Results from the lowest elevation moraine indicate sediment is produced from the lower
 50-70% of catchment elevations at the 95% confidence level, suggesting erosion near the
base and sides of the glacier outweigh erosion from higher elevation head wall retreat
and rock fall onto the glacier. Furthermore, grain-age distributions from different
sediment size fractions are virtually indistinguishable, suggesting either both size
fractions are sourced from similar elevations, and/or a significant disaggregation of
coarse-grained material into finer material during transport. Finally, the intermediate to
high-elevation moraines within the cirque indicate glacial erosion is possibly uniform and
occurs over 70-100% of the elevations above the sample location. These spatial
variations in glacial erosion are in stark contrast to previously published results
from the neighboring fluvial dominated Inyo Creek where a uniform distribution of
erosion is observed from detrital AHe analysis of modern river sediments. Taken
together, these results demonstrate: (1) a high sensitivity of detrital thermochronology
to spatial variations in glacial and fluvial erosion processes, and (2) an increase
in topographic relief and/or significant down-valley widening during glaciation
as supported by the abundance of sediment sourced from lower elevations in the
catchment.
Observed patterns of glacial erosion are in general agreement with initial predictions from
a plan-form glacial erosion model calibrated to the study area. Model predicted glacial
erosion rates during the Last Glacial Maximum are highest over the same range of elevations
that detrital thermochronometer data indicated moraine sediment was sourced from. Work in
progress is evaluating the sensitivity of model predicted patterns in glacial erosion to different
climate histories. |
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