HR: 08:00h
AN: H41H-01    [Abstracts]
TI: Linkages Between Physical Erosion and Chemical Weathering, Measured by Cosmogenic Nuclides and Geochemical Mass Balance
AU: * Riebe, C S
EM: riebe@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720-4767 United States
AU: Kirchner, J W
EM: kirchner@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, 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: We have recently shown how rates of physical erosion and chemical weathering can be measured over 1,000- to 10,000-year time scales, using cosmogenic $^{10}$Be measurements coupled with the bulk elemental composition of regolith and its parent rock. We have 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$^{-2}$). Long-term chemical weathering rates range from 0 to 173 t km$^{-2}$ yr$^{-2}$, in several cases exceeding the highest granitic weathering rates on record from previous work. Chemical weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock. Our measurements show that, for a given precipitation and temperature regime, chemical weathering rates increase proportionally with fresh material supply rates. We refer to this as ``supply-limited'' weathering, in which fresh material is chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. We consider several mechanisms that may be responsible for supply-limited weathering, and for the resulting strong correlation between rates of chemical weathering and physical erosion. To the extent that chemical weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.
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