DETERMINING RATES OF CHEMICAL-WEATHERING IN SOILS - SOLUTE TRANSPORT VERSUS PROFILE EVOLUTION

SiO2 fluxes associated with contemporary solute transport in three dee ply weathered granitoid profiles are compared to bulk SiO2 losses that have occurred during regolith development. Climates at the three prof iles range from Mediterranean to humid to tropical. Due to shallow imp eding alluvial layers at two of the profiles, and seasonally uniform r ainfall at the third, temporal variations in hydraulic and chemical st ate variables are largely attenuated below depths of 1-2 m. This allow s current SiO2 fluxes below the zone of seasonal variations to be esti mated from pore-water concentrations and average hydraulic flux densit ies. Mean-annual SiO2 concentrations were 0.1-1.5 mM. Hydraulic conduc tivities for the investigated range of soil-moisture saturations range d from <10(-9) to >10(-6) m s(-1). Estimated hydraulic flux densities for quasi-steady portions of the profiles varied from 6 x 10(-9) to 14 x 10(-9) m s(-1) based on Darcy's law and field measurements of moist ure saturations and pressure heads. Corresponding fluid-residence time s in the profiles ranged from 10 to 44 years. Total SiO2 losses, based on chemical and volumetric changes in the respective profiles, ranged from 19 to 110 kmoles SiO2 m(-2) of land surface as a result of 0.2-0 .4 Ma of chemical weathering. Extrapolation of contemporary solute flu xes to comparable time periods reproduced these SiO2 losses to about a n order of magnitude. Despite the large range and non-linearity of mea sured hydraulic conductivities, solute transport rates in weathering r egoliths can be estimated from characterization of hydrologic conditio ns at sufficiently large depths. The agreement suggests that current w eathering rates are representative of long-term average weathering rat es in the regoliths. Published by Elsevier Science B.V.