HR: 1340h
AN: H43C-0385    [Abstracts]
TI: Chemical weathering rates in deep-sea sediments: Comparison of multicomponent reactive transport models and estimates based on $^{234}$U
AU: * Maher, K
EM: katem@eps.berkeley.edu
AF: University of California, Berkeley, Center for Isotope Geochemistry, 301 McCone Hall, Berkeley, CA 94720-4746 United States
AU: Steefel, C I
EM: CISteefel@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Drive, Berkeley, CA 94720 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Berkeley, Center for Isotope Geochemistry, 301 McCone Hall, Berkeley, CA 94720-4746 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Drive, Berkeley, CA 94720 United States
AB: Chemical weathering rates in natural systems are typically much slower than expected based on experiments and theory. There are several possible explanations. However, because it has been difficult to determine what effects in particular reduce the rates in specific settings, natural rates remain difficult to predict. Silicate-rich deep-sea sediments provide an ideal in-situ laboratory for investigating weathering rates because certain potentially important factors, such as advective transport through heterogeneous media, limitations on the availability of reactive surface area due to low porosity and/or cementation, unsaturated flow conditions, and seasonal variations in fluid flux and temperature, do not occur in this setting. Geochemical profiles from Site 984 in the North Atlantic are modeled using a multi-component reactive transport model (CRUNCH) to determine in-situ rates of plagioclase dissolution and other diagenetic processes, including sulfate reduction and anaerobic methane oxidation. Various possible processes which might contribute to slower rates in the field are considered, including the effect of mineral saturation state, secondary precipitation of clays, inhibition by dissolved aluminum, and the availability of reactive surface area. The reactive transport model includes an isotopic solid-solution formulation that tracks the isotopic composition of precipitating (calcite) and dissolving (plagioclase and calcite) phases, thus allowing the determination of plagioclase dissolution rates. The rate constants for plagioclase determined by geochemical transport modeling of major element profiles are within the same range determined from U-series calculations and suggest that natural weathering rates for this system are on the order of 10$^{-17.5}$ to 10$^{-17.7}$ mol/m$^{2}$/sec assuming estimates of reactive surface area are correct, several orders of magnitude slower than laboratory-derived rates. The slow plagioclase rates are most likely due to the fact that dissolution takes place close to equilibrium, but the close to equilibrium conditions require either slow clay precipitation or precipitation of soluble clays. Unavailability of reactive surface area could also explain the slow rates, but this is considered less likely because of the very high porosity (about 80%) and the low cementation.
DE: 4825 Geochemistry
DE: 1886 Weathering (1625)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting