Chemical Weathering Rates, Erosion Rates and Mobility of Major and Trace Elements in a Boreal Granitic Till

Chemical weathering rates and erosionrates of granitic till in northern Sweden have beenestimated. The present-day chemical weathering rate iscompared with the long-term average weathering ratesince the last deglaciation approximately 8,700 yearsago. Also, the present-day release rates of major andtrace elements due to chemical weathering are comparedwith the mobility of these elements in a spodosolprofile as shown by soil water samples from the vadozezone. The estimation of the past weathering rate isbased on elemental depletion trends in a soil profile(typic haplocryod), whereas the present weatheringrate is based on elemental input/output budgets in asmall catchment (9.4 km²). The long-term averagechemical erosion rate, expressed as the sum of majorelement oxides (SiO₂, Al₂O₃, CaO,Fe₂O₃, K₂O, MgO, MnO, Na₂O,P₂O₅, TiO₂), was estimated to be4.9 gm^-2 yr^-1. The long-term base cation(Ca²⁺, Mg²⁺, Na⁺, K⁺) depletionwas 0.325 keq ha^-1 yr^-1. The currentchemical erosion rate was estimated to be2.43.0 g ^-2 yr^-1, which is at least an order ofmagnitude higher than the rate of physical erosion,and the base cation flux due to chemical weathering is0.356–0.553~keq ha^-1 yr^-1. However,0.074 keq ha^-1 yr^-1 of this flux may be related tocation exchange processes induced by atmospheric inputof acid rain. There is no evidence for any recentlyincreased weathering rate of silicates in this area.The inputs of Cd, Cu, Ni and Zn exceed the outputs,and hence, these elements are currently accumulatingin the soil.There is a distinct seasonal variation in thechemical composition of the soil water. Results fromthe soil water samples show that Na, followed by Siand S (Cl was not measured), are the most mobileelements in the spodosol profile between the E-horizonand the C-horizon, and that Al and Fe were the leastmobile elements. However, there is no simplerelationship between the mobility of an element in thespodosol profile and the current release rate due toweathering of that element. This fact may haveimplications for the validity of the comparisonbetween the past and present weathering rates asperformed in this study.