The influence of mineralogy on weathering rates and processes in an acid-sensitive granitic catchment

Weathering in an upland catchment on granitic parent material has been stud ied by chemical and mineralogical analyses of soils. Long-term weathering r ates for base cations, calculated from chemical analyses of the mineral hor izons from soil profiles using Zr as an internal, immobile, index element, are among the smallest recorded for Scottish soils (1.7-3.1 meg m(-2) a(-1) ), indicating that these soils are susceptible to acid deposition. Sodium i s the base cation lost to the greatest extent from the soils, due to weathe ring of plagioclase feldspar, mainly in the coarse size-fractions. Calcium is lost not only from plagioclase feldspar, but also from hornblende, grain s of which show dissolution etch pits and denticulate surface features when examined by scanning electron microscopy. Weathering of hornblende, presen t in basic inclusions in the granite, is a significant weathering process i n these soils. A range of values for Sr-87/Sr-86 ratios in stream-waters co nfirms the spatial variability of the material supplying Ca to the streams. The current weathering rate, calculated from input-output budgets to be 28 .9 meg m(-2) a(-1), is much greater than the long-term weathering rate, but small compared to other catchments on similar parent material. (C) 2001 El sevier Science Ltd. All rights reserved.