HR: 14:40h
AN: V13F-05    [Abstracts]
TI: The Impact of Climate and Boundary Conditions on Hillslope Erosion Rates in Northern Chile
AU: * Owen, J J
EM: jowen@nature.berkeley.edu
AF: Dept. of Environmental Science, Policy, and Management, University of California, Berkeley, 137 Mulford Hall-3114, Berkeley, CA 94720-3114,
AU: Dietrich, W
EM: bill@eps.berkeley.edu
AF: Dept. of Earth and Planetary Sciences, University of California, Berkeley, 313 McCone Hall, Berkeley, CA 94720-4767,
AU: Nishiizumi, K
EM: kuni@ssl.berkeley.edu
AF: Space Sciences Lab, University of California, Berkeley, 7 Gauss Way, Berkeley, CA 94720-7450,
AU: Chong Diaz, G
EM: gchong@ucn.cl
AF: Dept. de Ciencias Geologicas, Universidad Catolica del Norte, Avenida Angamos 0610, Antofagasta, 1270709, Chile
AU: Amundson, R
EM: earthy@nature.berkeley.edu
AF: Dept. of Environmental Science, Policy, and Management, University of California, Berkeley, 137 Mulford Hall-3114, Berkeley, CA 94720-3114,
AB: Soil production rates on hillslopes (~equivalent to bedrock erosion) are often modeled as a function of curvature and soil depth at the hillslope-scale. However, erosion rates are also dependent on climate and tectonics (both of which affect geomorphology and the soil mass balance), and questions remain regarding the role of climate on rates of soil production. In northern Chile, we measured bedrock erosion rates (using cosmogenic radionuclides ¹⁰Be and ²⁶Al) on granitic hillslopes along a climatic gradient (from hyperarid to semiarid) with differing boundary conditions (actively incising channels and stable landforms) to determine how rainfall and hillslope base levels differentially control erosion rates. We found that there is a strong positive correlation between rainfall and average erosion rates due primarily to the shift from slow, abiotic erosive processes on the hyperarid hillslopes to faster biotic processes on the semiarid hillslopes. In the hyperarid region, thick (10-10² cm) soils formed through the accumulation of dust and salt decrease the effectiveness of near-surface processes on the underlying bedrock. In the semiarid region, roots and animal bioturbation drive erosion rates nearly two orders of magnitude higher than in the hyperarid region. The landforms surrounding hillslopes serve as boundary conditions, setting the pace of hillslope processes. As expected, we found that hillslopes bounded by actively incising channels ("active slopes") have higher erosion rates than those bounded by inactive landforms ("stable slopes"). Active slopes appear to be maintaining their grade with channel incision, and thus reflect climatically-driven, regional erosion. In contrast, stable slopes are relaxing towards their stable base levels and away from dynamic steady-state. The climate of northern Chile may have been relatively constant (arid-hyperarid) since the late Pliocene. Given the extremely slow rates of bedrock erosion in the hyperarid and arid regions, most of the hillslope topography is a relict feature from the past, and present day processes are slowly modifying this topography. However, our results suggest that soil thicknesses and erosion rates on the active hillslopes have adjusted to the present climate and there is a positive correlation. This is different from some previous assessments of the impact of climate on denudation, and, when extrapolated to regional landscape evolution, supports the work by others on the potential role of climate in the variation of the Andean landscape.
DE: 0486 Soils/pedology (1865)
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1815 Erosion
DE: 1826 Geomorphology: hillslope (1625)
DE: 1862 Sediment transport (4558)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting