Chemical weathering and boulder mantles, Kärkevagge, Swedish Lapland

A deposit of giant boulders covers much of the floor of Kärkevagge, an alpine valley in the Swedish Arctic. These mica schist and garnet-mica schist boulders apparently have been there since the waning stages of glaciation in the valley, about 13,000 to 9000 years ago according to cosmogenic dating. Boulder elevations range from 665 to 825 m asl, with the higher and possibly younger boulders situated farther up the valley. We used total chemical analyses of rock and incipient soils forming on the boulders to determine relative weathering intensity on the tops of 20 boulders arrayed across lower, middle, and upper valley positions. An additional three boulders from a deposit of giant boulders damming Lake Rissajaure at the valley head were also analyzed. The dam may be a later deposit, as indicated by preliminary cosmogenic dates, or may be part of the larger valley deposit as other interpretations surmise. Total chemistry was not significantly different between the mica schist and garnet-mica schist boulders. However, total chemistry did vary by position. Boulders at the lowest positions were chemically more similar to those at the dam than to the other positions. This difference was removed when calculating the overall weathering indices by taking the ratio of weathering indices of the C horizon to exposed rock. A clear trend in the weathering was revealed by the analyses: lower valley boulders show significantly more weathering than the upper group and the middle group is intermediate. This indicates that weathering rates differ because of elevation or age of the boulders strewn down the valley. However, boulders on the Lake Rissajaure dam complicate those simple interpretations because they show as much weathering evidence as the lower boulders. All of the weathering indices exhibited this same trend. Elevated levels of Fe in the two more weathered positions indicate influence of pyrite, a known weathering accelerant, and undoubtedly the factor controlling boulder weathering in Kärkevagge.