HR: 0800h
AN: H51C-0363    [Abstracts]
TI: Long-term field measurements of mineral-specific chemical weathering rates at Rio Icacos, Puerto Rico, determined from cosmogenic nuclides and geochemical mass balance
AU: * Ferrier, K L
EM: ferrier@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, 449 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Kirchner, J W
EM: kirchner@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, 479 McCone Hall, Berkeley, CA 94720-4767 United States
AU: Riebe, C S
EM: cliff@stillwatersci.com
AF: Stillwater Sciences, 2855 Telegraph Avenue, Suite 400, Berkeley, CA 94705 United States
AU: Finkel, R C
EM: rfinkel@llnl.gov
AF: Center for Accelerator Mass Spectrometery, Lawrence Livermore National Laboratory, P.O. Box 808, L-206, Livermore, CA 94550 United States
AB: Chemical weathering regulates rates of landscape evolution, determines soil mineral composition, and produces solutes that are vital to stream ecology. Chemical weathering of silicate minerals also draws down atmospheric CO₂ and so buffers Earth's greenhouse effect over million-year timescales. Here we show that field weathering rates of individual mineral phases can be measured by combining estimates of long-term denudation rates (inferred from concentrations of cosmogenic nuclides in actively eroding soils) with mass-balance calculations (based on chemical and mineralogical compositions of soils and their parent bedrock). These measurements are intrinsically averaged over long timescales, similar to those of soil formation and denudation in actively eroding terrain. We analyzed powder X-ray diffraction (XRD) patterns from 67 samples collected from the Rio Icacos watershed, Puerto Rico, to obtain quantitative estimates of major mineral abundance in bedrock, saprolite and soil. These mineral abundance data, combined with estimates of denudation rate (from in-situ cosmogenic ¹⁰Be measurements) and immobile element enrichment (from X-ray fluorescence measurements), yield field-based mineral-specific weathering rates averaged over the past 13,000-16,000 years. Our Rio Icacos data imply a weathering rate indistinguishable from zero (50 ± 520 mol ha^-1 yr^-1) for relatively insoluble quartz, as expected, and weathering rates of 1990 ± 470 mol ha^-1 yr^-1 for plagioclase feldspar and 220 ± 70 mol ha^-1 yr^-1 for hornblende, suggesting that weathering rates of relatively soluble minerals can be determined to an accuracy of 25-30%. These results demonstrate that this technique yields robust field-based mineral-specific weathering rates averaged over ~10,000-year timescales.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1815 Erosion
DE: 1826 Geomorphology: hillslope (1625)
DE: 1865 Soils (0486)
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: Fall Meeting 2005