The effect of acid deposition on cation fluxes in artificially acidified catchments in western Norway

Abstract

Chemical weathering has been studied at two artificially acidified and two control catchments at a clean pristine area in western Norway. The gabbroic gneiss bedrock in the catchment area contains a Na-rich plagioclase which is a major constituent and the most important mineral undergoing weathering in the soils. Estimates for rates of chemical weathering combine several independent methods including input-output budgets, runoff chemistry under short-term acidic episodes, and laboratory weathering studies. The two manipulated catchments have a net flux of Ca ++, Mg ++, and silica which is about 50% higher than that of the control catchments, but the net flux of Na + is the same in all catchments. The increased flux of base cations in runoff waters at the two acidified catchments is the result of increased removal of base cations from exchange sites and increased chemical weathering of plagioclase and mafic minerals. The exchangeable Na-pool is small and its contribution to the fluxes in runoff waters is less than the fluxes related to precipitation and chemical weathering of plagioclase and mafic minerals. The flux of non-marine Na + in runoff gives an estimate of the plagioclase weathering rate at the catchments of about 27 kg Plag. ha −1 a −1. Given the assumption that the dissolution of soil minerals releases silica and base cations in amounts equivalent to the rates measured in mineral dissolution experiments and their weight fraction in the soil, the estimated total weathering rate of base cations at Sogndal is about 20 meq/m 2/a. The calculated total weathering rate agrees well with rates estimated by mass-balance studies ( Wright et al., 1988a) but is considerably lower than the total weathering rates reported from other ecosystems.