HR: 14:55h
AN: H53E-06 INVITED     [Abstracts]
TI: Effects of Climate on Long-term Rates of Physical Erosion and Chemical Weathering: Evidence from Cosmogenic Nuclides and Geochemical Mass Balance
AU: * Kirchner, J W
EM: kirchner@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Riebe, C S
EM: riebe@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Ferrier, K L
EM: ferrier@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94551 United States
AB: Cosmogenic nuclides such as $^{10}$Be and $^{26}$Al have recently become important tools for measuring long-term denudation rates. We have recently shown how cosmogenic nuclide measurements of denudation fluxes can be partitioned into their physical and chemical components, using the enrichment of insoluble tracers in regolith relative to its parent rock. We used these methods to measure long-term rates of physical erosion and chemical weathering for 42 sites, encompassing widely varying climates and denudation rates. Across these sites, mean annual temperatures vary from 2 to 25 $\deg$C, average annual precipitation spans a 20-fold range (from 22 to 420 cm/yr), and denudation rates vary by 32-fold (from 23 to 755 t km$^{-2}$ yr$^{-2}$). Our measurements show that chemical weathering rates are tightly coupled with physical erosion rates, such that the relationship between climate and chemical weathering rates may be obscured by site-to-site differences in the rate that minerals are supplied to soil by physical erosion of rock. The relative importance of chemical weathering can be quantified using the "Weathering Intensity Factor" (WIF), the ratio of the chemical weathering rate to the physical erosion rate. Over 60 percent of the variance in WIF's can be explained by a simple Arrhenius-like relationship based on mean annual temperature and average annual precipitation. The temperature-dependence of WIF is roughly half of what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter-comparisons of short-term average weathering rates from the field. Our results imply that the strength of climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated granitic terrain similar to our study sites.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
DE: 1030 Geochemical cycles (0330)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting