Comparisons of short-term and long-term chemical weathering rates in granitoid regoliths

Chemical weathering rates in three deeply weathered granitoid profiles are evaluated based on SiO2 fluxes. Aqueous fluxes below the zone of seasonal variations are estimated from mean-annual pore-water SiO2 concentrations of 0.1 to 1.5 mM and average hydraulic-flux densities of 6 x 10(-9) to 14 x 10(-9) m s(-1) for quasi-steady state portions of the profiles. Corresponding fluid-residence times in the profiles ranged from 10 to 44 years. Total solid state SiO2 losses, based on chemical and volumetric changes, ranged from 19 to 110 kmoles SiO2 m(-2) as a result of 200 to 375 ka of chemical weathering. Extrapolation of contemporary solute fluxes to comparable time periods reproduced SiO2 losses to about an order of magnitude. A strong positive correlation exists between moisture content and rates of chemical weathering.