HR: 17:20h
AN: H42K-06 INVITED     [PDF]
TI: Measuring long-term Rates of Physical Erosion and Chemical Weathering Using Cosmogenic Radionuclides
AU: * Kirchner, J W
EM: kirchner@seismo.berkeley.edu
AF: Dept. of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767
AU: Riebe, C S
EM: riebe@seismo.berkeley.edu
AF: Dept. of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767
AU: Ferrier, K L
EM: freedom@eps.berkeley.edu
AF: Dept. of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94551
AB: Physical erosion and chemical weathering are first-order controls on sediment loads and solute chemistry, two important components of surface water quality. Recent developments in cosmogenic radionuclide geochemistry permit rates of physical erosion and chemical weathering to be measured over 10,000-year timescales. We have used cosmogenic $^{10}$Be to measure long-term rates of physical erosion and chemical weathering at dozens of sites, spanning a 20-fold range in precipitation, a 30 \deg C range in temperature, and a 30-fold range in denudation rates. Across this network of sites, rates of physical erosion and chemical weathering are tightly coupled with one another: sites with higher rates of physical erosion also have higher rates of chemical weathering, all else equal. Chemical weathering intensity varies systematically with climate; the ratio of chemical weathering to physical erosion increases linearly with precipitation, and increases as an Arrhenius function of temperature. These measurements provide a long-term perspective for interpreting present-day sediment and solute fluxes in streams. In the mountains of central Idaho, long-term erosion rates are nearly 20 times higher than sediment yields measured over the past several decades, indicating that sediment delivery is extremely episodic. But in the northern California Coast Range, long-term erosion rates are within a factor of two of modern-day sediment yield measurements, suggesting that sediment delivery over decadal timescales is in equilibrium with long-term rates of sediment production in that rapidly-eroding landscape. At Silver Creek, Idaho, cosmogenic estimates of chemical weathering rates are roughly four times higher than 11-year weathering fluxes calculated from catchment input-output mass balances. But at Rio Icacos, Puerto Rico, cosmogenic measurements of long-term chemical weathering rates are consistent with stream solute fluxes, indicating that present-day solute fluxes are in equilibrium with long-term rates of solute generation in that intensive weathering regime.
DE: 1045 Low-temperature geochemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1806 Chemistry of fresh water
DE: 1815 Erosion and sedimentation
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2003 Fall Meeting