Determining rates of chemical weathering in soils—solute transport versus profile evolution

Abstract

SiO 2 fluxes associated with contemporary solute transport in three deeply weathered granitoid profiles are compared to bulk SiO 2 losses that have occurred during regolith development. Climates at the three profiles range from Mediterranean to humid to tropical. Due to shallow impeding alluvial layers at two of the profiles, and seasonally uniform rainfall at the third, temporal variations in hydraulic and chemical state variables are largely attenuated below depths of 1–2 m. This allows current SiO 2 fluxes below the zone of seasonal variations to be estimated from pore-water concentrations and average hydraulic flux densities. Mean-annual SiO 2 concentrations were 0.1–1.5 mM. Hydraulic conductivities for the investigated range of soil-moisture saturations ranged from <10 −9 to >10 −6 m s −1. Estimated hydraulic flux densities for quasi-steady portions of the profiles varied from 6×10 −9 to 14×10 −9 m s −1 based on Darcy's law and field measurements of moisture saturations and pressure heads. Corresponding fluid-residence times in the profiles ranged from 10 to 44 years. Total SiO 2 losses, based on chemical and volumetric changes in the respective profiles, ranged from 19 to 110 kmoles SiO 2 m −2 of land surface as a result of 0.2–0.4 Ma of chemical weathering. Extrapolation of contemporary solute fluxes to comparable time periods reproduced these SiO 2 losses to about an order of magnitude. Despite the large range and non-linearity of measured hydraulic conductivities, solute transport rates in weathering regoliths can be estimated from characterization of hydrologic conditions at sufficiently large depths. The agreement suggests that current weathering rates are representative of long-term average weathering rates in the regoliths.