Variations in weathering processes and rates with time in a chronosequence of soils from Glen Feshie, Scotland

Chemical and mineralogical characteristics have been determined for a chronosequence of six soil profiles ranging in age from 80–13,000 years BP developed on river terraces in the western Cairngorms of Scotland. The C horizons are similar chemically and mineralogically, and the soils have similar pedogenetic histories. Exchangeable Ca and Mg decrease with time and base saturation decreases exponentially from 24.6% in the Ah horizon of the youngest profile to 2.8% in the comparable horizon of the 10,000 year old profile according to the chronofunction y=3.372+22.612 exp(−0.0007365t). Long-term weathering rates of base cations, calculated from the loss of these cations relative to Zr, appear to decrease exponentially with time but this may be due to the method of calculation. The magnitude of loss of base cations decreases in the sequence Na>K>Mg>Ca but when the relative mobilities of these elements are considered, the loss is in the order Mg>Na>Ca>K; this reflects the dissolution of chlorite and loss of Mg, and the more rapid weathering of plagioclase feldspar and loss of Na and Ca (particularly in the coarse sand fraction) than K-feldspar. The clay fractions, although <2% of all horizons, also show distinct patterns with age in that chlorite and mica are less abundant in older soils and vermiculite is more abundant, the latter phase often having hydroxyaluminium polymers in the interlayer region. The chemical and mineralogical trends in the soil sequence are closely associated and are induced by pedogenic weathering.