PYRITE-ENHANCED CHEMICAL WEATHERING IN KÄRKEVAGGE SWEDISH LAPLAND

Conventional wisdom once held that weathering in cold climates was overwhelmingly due to physical processes. Rapp (1960) challenged that convention with his publication on landscape denudation in Kärkevagge, Swedish Lapland where he made the startling discovery that chemical weathering exceeded any measured physical denudation process. His interpretation was based on limited analyses of water chemistry where he found total flux of dissolved solids accounted for most mass loss from the watershed. The dominant anion he observed was sulfate. He also observed other features of chemical weathering in the valley including white streaks of lime in stream channels on the valley flanks. While insightful, Rapp offered no mechanism to explain these findings. Our subsequent work in Kärkevagge has revealed the driving mechanism of chemical weathering to be acid production from pyrite oxidation. We have determined that pyrite occurs in some of the rock units in the valley and its oxidation produces sulfuric acid, H2SO4, which accelerates weathering. The “lime coats” are primarily an amorphous aluminum oxyhydroxide which paints surfaces it contacts. That this is an active feature is demonstrated by efflorescence on actively growing seasonal vegetation in stream channels. In sheltered overhangs, protected from wind and running water, we found other secondary minerals associated with pyrite oxidation, including white crusts of gypsum, yellowish coatings of jarosite KFe3(SO4)2(OH)6, and rust-colored ferrihydrite, Fe+32O3•0.5(H2O). We believe that pyrite oxidation may be an important first step in weathering in many environments. It largely goes unrecognized because it occurs rapidly and typically is only identified in highly disturbed landscapes associated with mining and other large-scale earth-moving activities. An additional implication of our findings is that Rapp may have chosen a poor location to do his work where the particular geochemistry evokes accelerated chemical weathering.