The poisonous rocks of Kنrkevagge

The black schist in Kärkevagge, Swedish Lapland has been reported to weather easily and produce a poisonous effect on vegetation. This research was an investigation of that phenomenon as part of a larger study of weathering in Kärkevagge. In July 1999, soil and plant samples were collected downslope of a “poisonous” boulder. Samples from the adjacent unaffected slope served as references. Biomass, plant elemental composition, and soil fertility were determined. Most plants within 1.4 m downslope of the poison boulder were dead, and effects on plant growth could be seen to about 6 m. Plants near the boulder had elevated levels of K, B, Al, Cd, Se, and Fe, and lower levels of Ca, Mn, and Ba compared to reference plants. In the soil near the poison boulder, extractable S, Cd, and Fe and salt contents were greater, while pH and extractable Cl, P, Ca, Mg, K, Ba, Ni, and Cr were lower than in the reference soil. At 10 m downslope of the boulder, soil and plant chemistry was more similar to the reference materials, but some effects were still noted. Elemental analyses of the poison boulder and soil revealed no particular plant toxins, although contents of Fe and S were higher and Ca lower than in reference materials. We believe the poison is sulfuric acid, which forms as a consequence of pyrite oxidation. The dark color of the boulder is consistent with a pyrite-bearing lithology and other field observations and laboratory analyses support the hypothesis. Coatings on local rocks include jarosite, gypsum, and copiapite, secondary minerals associated with pyrite oxidation and weathering accelerated by sulfuric acid. Radiocarbon dating indicates that the rock fell off the cliff some time before 245 14C years BP.