CHEMICAL AND MINERALOGICAL WEATHERING RATES AND PROCESSES IN AN UPLAND GRANITIC TILL CATCHMENT IN SCOTLAND

Weathering in an upland granitic till catchment receiving an intermediate level of acidic deposition has been studied by chemical and mineralogical analyses of soil profiles and chemical analysis of precipitation and streamwater. Long-term weathering rates for base cations calculated from analyses of soil profile horizons using Zr as an internal, immobile, index element are similar for alpine podzols and peaty podzols and are 2-3 meq.m-2.a-1 for Ca and Mg, and 10-11 meq.m-2.a-1 for K and Na. The high loss of Na is associated with the weathering of oligoclase, particularly in the coarse sand fraction. Loss of K is related to weathering of K-feldspar and micas. Current weathering rates for base cations calculated from input-output budgets are higher than long-term rates by factors of 12, 8 and 3 for Ca, Mg and Na, but lower by a factor of 7 for K probably due to biomass uptake. The higher current overall loss of base cations may be due to increased rates of weathering in recent times but this is not conclusive as there are large uncertainties inherent in both methods of estimation.