HR: 0800h
AN: H51C-1157    [Abstracts]
TI: A Numerical Simulation of the Effects of Mass-Wasting on Cosmogenically Determined Erosion Rates
AU: * Niemi, N
EM: niemi@crustal.ucsb.edu
AF: Institute for Crustal Studies, 1140 Girvetz Hall University of California, Santa Barbara, CA 93106 United States
AU: Oskin, M
EM: oskin@email.unc.edu
AF: Department of Geological Sciences, University of North Carolina at Chapel Hill Campus Box #3315 Mitchell Hall, Chapel Hill, NC 27599 United States
AU: Burbank, D
EM: burbank@crustal.ucsb.edu
AF: Institute for Crustal Studies, 1140 Girvetz Hall University of California, Santa Barbara, CA 93106 United States
AB: The successful quantification of long-term erosion rates underpins out understanding of landscape formation, the topographic evolution of mountain ranges, and the mass balance within active orogens. The measurement of in situ-produced cosmogenic radionuclides (CRNs) in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide long-term erosion rates over a wide variety of landscapes and erosional processes. Recent studies have provided encouragement that measurement of in-situ CRNs does, in fact, frequently yield consistent long-term average erosion rates. Cosmogenically derived erosion rates from small catchments within the Sierra Nevada are highly consistent (e.g. Riebe et al., 2000), while CRN samples collected along European rivers yield similar results at catchment scales of 10$^2$-10$^4$ km$^2$ (Schaller et al., 2001). On the contrary, samples from low-order catchments in the Nepalese Himalaya yield cosmogenically derived erosion rates that range from hundredths to tens of mm/yr (A. Heimsath, unpub. data), while results reported from the San Bernardino Mountains in southern California yield erosion rates that vary from $<$0.01 to $>$2.7 mm/yr (Binnie et al., 2003). We propose that stochastic mass-wasting processes are responsible for the wide range of cosmogenically derived erosion rates observed in active tectonic settings, and have developed a numerical simulation of cosmogenic nuclide production and distribution in landslide-dominated catchments to address the effect of these mass-wasting processes on cosmogenic erosion rates in active landscapes. This model simulates the production of cosmogenic nuclides on the landscape, and their removal from the landscape by erosional processes, such as grain-by-grain attrition and mass wasting. CRN concentrations in the `removed material' are analyzed at different catchment scales to yield simulated cosmogenic erosion rates. Results of such simulations indicate that the temporal stability of erosion rates determined from CRN concentrations in alluvial sediment decreases with increased ratios of mass-wasting to grussification rates within a given catchment area, and that increasingly large catchment areas must be sampled when mass wasting rtaes are high in order to accuately evaluate long-term erosion rates. In addition, results of this simulation suggest that sediment sampling for CRNs is the appropriate method for determining long-term erosion rates in regions dominated by mass-wasting processes, while bedrock surface sampling for CRNs is generally an ineffective means of deriving long-term erosion rates.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1869 Stochastic processes
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting