VARIABILITY IN POST-GLACIAL WEATHERING BY MICROENVIRONMENT, STORBREEN FORELAND, JOTUNHEIMEN MOUNTAINS, NORWAY

Using rock porosity as a surrogate measure for chemical weathering, we have investigated the impact of age and microenvironment on the post-glacial weathering of cobbles across the glacial foreland of Storbreen, a cirque glacier in the Jotunheimen Mountains of central Norway. A Little Ice Age (~1750 AD) terminal moraine demarcates the foreland within which are recessional moraines and beyond which is a glaciate surface dating to ~ 9 000 years. Bare and lichen-covered surficial granulite pyroxene gneiss cobbles were sampled from each surface category and matched by cobbles buried within the C horizon at the same sites. 100-micron interval transects beneath each cobble surface were determined using a scanning electron microprobe to determine the depth of weathering. While freshly deglaciated surfaces and tills are commonly taken to be unweathered, the presence of reworked material must be accepted. In both cases it must be recognized that materials are subsequently exposed to a wide range of ground microenvironments, or ‘climates', that drive weathering. A well-dated foreland such as the one at Storbreen provides an excellent opportunity to identify variability in both the nature and rates of chemical weathering. Analysis of percentage porosity data subjected to a rank-ordered transformation determined statistically significant at-a-site differences between bare and lichen-covered cobble surfaces, and between surface and buried cobbles. At-a-site weathering increases in the order: bare < lichen-covered < buried cobbles. Statistically significant differences were determined between the 9 000 year-old and 1750 surfaces, but were not detectable among moraines materials younger than 1750.