HR: 0830h
AN: H51E-1129    [PDF]
TI: Regolith Composition Gradients as a Means to Quantify Chemical Weatheirng Rates
AU: * White, A F
EM: afwhite@usgs.gov
AF: U. S. Geologic Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: The measurement of changes in solid-state and solute compositions as functions of depth in a regolith is the most direct method of characterizing chemical weathering. Such composition trends can be conceptualized as solid and solute weathering gradients. Under simple steady-state conditions, such gradients are defined in terms the respective weathering rates and weathering velocities. The solid state weathering velocity is the rate at which the weathering front propagates through the regolith and can be estimated from the depth and the age or denudation rate of the land surface. The solute weathering velocity is the rate of solute transport or fluid flux. Increasing the weathering rate produces a shallower weathering gradient in which a measured solid or solid component increases more rapidly with depth in the regolith. Increasing the weathering velocity, steepens the weathering gradient. The slope of these gradients, measured under field conditions can be converted to quantitative weathering rates in combination with additional physical and chemical parameters. Rates based on solute gradients reflect fluid residence times and are measurements of present-day weathering impacted by contemporary environmental conditions including hydrology, climate and vegetation. Weathering rates based on solid-state gradients reflect average rates integrated over geologic time. Under steady state conditions, the two weathering rates must be equal and proportional to the respective mass changes with time. Differences in the rates may reflect changes in the weathering environment. Weathering gradients and rates are discussed in terms of saprolite formation from granite rocks from various weathering environments including Puerto Rico, Georgia USA and Australia. Strong trends are shown to exist between weathering gradients and climate, principally precipitation and temperature. Quantitative results, produced from a coupled fluid flow-kinetic reaction model, show that thermodynamic saturation of pore water solutes, rather than solid state-reaction kinetics is the principal control on weathering gradients and rates.
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
DE: 1099 General or miscellaneous
SC: Hydrology [H]
MN: 2003 Fall Meeting