HR: 0830h
AN: V51C-0310    [PDF]
TI: Vadose Zone Weathering Rates Inferred from U-Series Disequilibrium
AU: * Maher, K
EM: kmaher@uclink4.berkeley.edu
AF: University of California, Berkeley, Department of Earth and Planetary Science, Berkeley, CA 94720-4746 United States
AU: * Maher, K
EM: kmaher@uclink4.berkeley.edu
AF: Geosciences and Environmental Technology Division, Lawrence Livermore National Laboratory, Livermore, CA 94550 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Berkeley, Department of Earth and Planetary Science, Berkeley, CA 94720-4746 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, Ca 94720 United States
AU: Steefel, C
EM: steefel1@llnl.gov
AF: Geosciences and Environmental Technology Division, Lawrence Livermore National Laboratory, Livermore, CA 94550 United States
AU: Christensen, J N
EM: jnchristensen@lbl.gov
AF: Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, Ca 94720 United States
AB: The U-series isotope system can be used to quantify reaction rates in aquifers and thick vadose zone environments. The approach is based on the $\alpha$-recoil of $^{234}$Th atoms across grain boundaries, which enriches the pore fluid in $^{234}$U. Dissolution of the solid phase releases mainly 238U to the pore fluid, so that the $^{234}$U/$^{238}$U activity ratio of the pore fluid is a measure of the local ratio of the dissolution U flux to the $\alpha$-recoil flux. The interpretation of U-series disequilibrium in weathering environments depends on the $\alpha$-recoil flux, which is highly dependent on both the distribution of U and the grain size distribution of the soil. The recoil length of the $^{234}$Th atom in a silicate is approximately 700 $\AA$, therefore the fine-grained material, with a greater surface area to volume ratio, contributes a significant amount of the $^{234}$U to the porewater. As a result, estimates of the recoil flux based on the mean grain size may under predict the supply of $^{234}$U by over an order of magnitude. In order to quantify the reaction rate, this recoil flux must be accurately measured by consideration of the recoil contribution of the fine-grained material, despite the fact that it may only comprise a small fraction of the total mass of the sediment. U concentrations and high precision isotope measurements ($^{234}$U/$^{238}$U) of pore fluids, mineral separates, and grain size intervals are used to constrain reaction rates for a 70 m vadose zone core from eastern Washington State. The pore water $^{234}$U/$^{238}$U activity ratios range from 1.04 to 1.20 in the pore fluids, and from 0.94 to 1.0 in the various size fractions. Data from the solid phases is used to construct a predictive model for $\alpha$-recoil flux in heterogeneous soil. The calculated recoil loss, which considers each grain size interval, is up to 7 times greater than for a geometric prediction based on the mean grain size. The measured $^{234}$U/$^{238}$U ratios for the vadose zone, in conjunction with the estimates for the a-recoil flux, yield weathering rates of approximately 10$^{-6.4}$ yr$^{-1}$, which agree with estimates derived from measurements in granitic soils that are based on mineral abundances and soil age. The data suggest that recoil loss from the fine-grained size fractions in a heterogeneous soil can have a significant impact on the interpretation of U isotopic data.
DE: 1015 Composition of the core
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1094 Instruments and techniques
DE: 1099 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting