Microvariability in the early stages of cobble weathering by microenvironment on a glacier foreland, Storbreen, Jotunheimen, Norway

摘要

The initial stages of cobble weathering, measured as increasing percentage porosity, were calculated for sets of cobbles taken from the foreland of Storbreen, a cirque glacier in the Jotunheimen, Norway. Cobbles were taken from in front of the 1998 glacier snout, from the 1928, 1870, 1810 and 1750 moraine crests and from the similar to 10 000-year-old land surface beyond the Neoglacial foreland limit. Three microenvironments were examined within each site: (1) lichen-free surfaces from exposed cobbles, (2) lichen-covered surfaces from exposed cobbles and (3) buried cobbles taken from a soil depth of similar to 40 cm. Percentage porosity within plagioclase minerals was calculated using backscatter electron images of prepared thin sections. Porosity was calculated from five depth profiles into each cobble. Depth profiles were subdivided into a sequence of 50 mu m X 50 mu m rectangles running to at least a nominal depth of 500 gm within each cobble. Three cobbles from each dated land surface were sampled, except for the 1750 surface, which had five cobbles. Statistical analysis was by analysis of variance of rank-order transformed data. Findings indicate that cobbles close to the glacier snout are largely unweathered; also, weathering is generally weak in the 1928, 1870 and 1810 positions, but statistically significantly higher in the 1750- and 10 000-year-old positions. Weathering of buried cobbles always exceeded weathering of exposed cobbles and may possibly reach a value beyond which it cannot progress while retaining surface cohesion. The degree of weathering on lichen-free and lichen-covered cobble surfaces is not initially distinguishable, but diverges sharply after similar to 250 years, when lichen-covered surfaces experience significantly higher totals. Overall, the weathering trend in cobbles matches that found in soils at the same sites. Copyright (c) 2007 John Wiley & Sons, Ltd.