The influence of soil age on calculated mineral weathering rates

Mineral weathering rates for two chronosequences of soils have been calculated using an empirical method based on mineralogy, the depletion of elements relative to a conservative element and the computer model PROFILE. Weathering rates calculated by the empirical and depletion methods showed a decrease in rates with soil age whilst those calculated using the PROFILE model showed an increase with soil age. The counter intuitive PROFILE prediction is due to the use of surface area-normalised reaction rate coefficients which assume that: 1) mineral reactivity is constant with time and, 2) total mineral surface area is equivalent to reactive surface area. In Europe, mineral weathering rates in soils are an important input in determining levels of acid deposition above which ecosystem damage will occur (critical loads). As soils in Great Britain and much of NW Europe can range in age from < 10(3) to > 10(5) a, it is suggested that, until computer models can take account of soil age and the concomitant changes in mineral reactivity and surface area, modelled weathering rates will be subject to large uncertainties (C) 1999 Elsevier Science Ltd. All rights reserved.