HR: 08:45h
AN: H41H-04    [Abstracts]
TI: Chemical Weathering Processes and Mass Losses on an Actively Eroding Hill Slope
AU: * Green, E G
EM: lis@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720-4767
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720-4767
AU: Banfield, J F
EM: jill@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, CA 94720-4767
AB: Chemical weathering rates for landscapes are difficult to quantify because the timescales over which weathering occurs are often unknown. In this study, we use timescales defined by prior cosmogenic nuclide analyses and a suite of geochemical measurements to calculate weathering rates in saprolite and soil and to determine how these rates vary across an eroding hill slope. We also estimate the relative contributions of solute and erosional mass loss to landscape lowering. Analyses were conducted on a soil-mantled hill slope developed on a granodiorite pluton in southern NSW, Australia. Between $\sim$ 35% and 55% of total mass loss from the hill slope occurs in solution. Saprolite at the soil-saprolite boundary is less weathered at the ridge than at distance from the ridge. The calculated flux of silica from the saprolite is 5 tons km$^{-2}$ yr$^{-1}$ and does not vary with distance from the ridge or overlying soil thickness. With the exception of Ca and Na, rates of loss of most major elements in solution are lower in the saprolite (prior to bioturbation) than in the soil. Ca and Na are preferentially lost from the saprolite. Within the soil, major element fluxes initially increase as soils are transported down slope. Most elemental fluxes reach a maximum between 20 and 40 m of transport, and then decrease slightly. For example, soil silica weathering fluxes increase from 6 tons km$^{-2}$ yr$^{-1}$ at the ridge to a maximum of ~9 tons km$^{-2}$ yr$^{-1}$ at transport distances between 20 and 40 m, and then decline to 8 tons km$^{-2}$ yr$^{-1}$ in soils that have been transported greater than 40 m. The soils are geochemically stratified. Soil regions close to the soil-saprolite boundary are less weathered than those at distance from the soil-saprolite boundary. This suggests that the soil column never becomes thoroughly vertically mixed as soils move down slope. By the time soils have been transported from the convex ridge crest to the concave swale, they have lost more than 50% of their original mass to chemical weathering. The findings of this study indicate that erosional transport and solute processes make comparable contributions to landscape lowering at this site, and that transport-distance dependent solute loss may partially explain deviations between observations of hillslope curvature and predictions of curvature based on linear creep transport.
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
DE: 1045 Low-temperature geochemistry
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting