Pyrite-enhanced chemical weathering in Karkevagge, Swedish Lapland

Conventional wisdom once held that weadering in cold climates was overwhelmingly due to physical process. However, that convention was challenged when chemical weadering was identified, but unexplained, as the dominant landscape denudation process in Karkevagge, an alpine valley in Swedish Lapland. The research reported here involved scanning electrone microscopy and energy dispersive X-ray flourescence of rock coatings to investigate chemical weathering in Karkevagge. Analyses revealed that white coatings associated with streams emerging on the valley cliff face are commonly an amorphous aluminium oxyhydroxide sulfate such as basaluminite [Al-4(SO4)(OH)(10)center dot H2O]. Efflorescence on seasonal vegetation in stream channels demonstrated that this is an active process. The white coating chemistry exhibited no systematic spatial patterns along the valley axis or with position on the cliff face. Although the white coatings were not crystalline and did not contain appreciable amounts of Fe or Ca, in sheltered overhangs among boulders on the valley floor there were other well-crystallized secondary sulfate minerals commonly associated with pyrite oxidation, including gypsum, jarosite, and amorphous Fe compounds. This difference is due to presumably to the pH of the associated waters, because Fe compounds tend to precipitate only at pH < 5, and Al compounds at pH > 5, which is the pH of the stream water. Pyrite oxidation maybe an important early weathering process in this and in many other environments. However, it largely goes unrecognized because it occurs relatively rapidly in a geological sense and typically is only identified in recently disturbed landscapes associated with mining or other large-scale earth-moving activities. Karkevagge demonstrates that subarctic conditions do not preclude intense chemical weathering where conditions are favorable, but it does not established that strong chemical weathering is a widespread attribute of subarctic conditions.