HR: 14:10h
AN: H52D-03    [PDF]
TI: Effects of climate and mineral supply rates on long-term chemical weathering rates in granitic landscapes
AU: * Riebe, C S
EM: riebe@seismo.berkeley.edu
AF: Dept. Earth \& Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Kirchner, J W
EM: kirchner@seismo.berkeley.edu
AF: Dept. Earth \& 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
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Dept. Earth Science, University of California, Riverside, CA 92521 United States
AB: We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and physical erosion of granitic terrain. Our 43 study sites encompass widely varying climates and denudation rates; mean annual temperatures vary from 2 to 32\deg C, average annual precipitation spans a 20-fold range (from 22 to 420 cm/yr), and denudation rates vary by 32-fold across our sites. Long-term chemical weathering rates for these 43 sites range from 0 to 173 t km$^{-2}$ yr$^{-1}$, in several cases exceeding the highest granitic weathering rates on record from previous work. Chemical weathering rates are highest at sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from physical erosion of rock. To account for effects of mineral supply in analyzing how climate affects chemical weathering, we introduce the "Weathering Intensity Factor" (WIF), the ratio of chemical weathering rate to physical erosion rate. WIF's increase systematically with average annual precipitation and mean annual temperature, both for the soil as a whole, and for individual component elements including Si, Na, and Ca. Between 59 and 79 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. Moreover, when we couple this Arrhenius relationship with our measurements of long-term erosion rates, we obtain a simple prediction equation that explains between 79 and 93 percent of the variance in long-term chemical weathering rates. The temperature-dependence of WIF is roughly half what one would expect from laboratory measurements of activation energies for feldspar weathering. 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 terrain similar to our study sites. Our results further indicate that chemical weathering rates may often be limited by the rates that fresh minerals are supplied to soils by erosion, implying that tectonic uplift may be an important regulator of long-term chemical weathering rates in mountainous, granitic landscapes.
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting