Pyrite-enhanced chemical weathering in Karkevagge, Swedish Lapland

摘要

Conventional wisdom once held that weathering in cold climates was overwhelmingly due to physical processes. However, that convention was challenged when chemical weathering was identified, but unexplained, as the dominant landscape denudation process in Kärkevagge, an alpine valley in Swedish Lap-land. The research reported here involved scanning electron microscopy and energy dispersive X-ray fluorescence of rock coatings to investigate chemical weathering in Kärkevagge. Analyses revealed that white coatings associated with streams emerging on the valley cliff face are commonly an amorphous aluminum oxyhydroxide sulfate such as basaluminite [Al₄(SO₄)(OH)₁₀·H₂O]. Efflorescence on seasonal vegetation in stream channels demonstrates 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 presumably to the pH of the associated waters, because Fe compounds tend to precipitate only at pH 5, which is the pH of the stream water. Pyrite oxidation may be 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. Kärkevagge demonstrates that subarctic conditions do not preclude intense chemical weathering where conditions are favorable, but it does not establish that strong chemical weathering is a widespread attribute of subarctic conditions.