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Granite Weathering in the Damma Glacier Forefield

Abstract:

In alpine regions weathering processes are of particular interest with respect to the effects of climate change. Large areas of bare rock will be exposed as glaciers retreat. Weathering includes both, mechanical as well as chemical processes. In pristine alpine areas, where soil formation has not yet occurred physical weathering is the main initial process and includes tectonic forcing, thermal expansion and contraction of rock, alternate freezing and thawing of water between the rocks cracks and fissures, and pressure release after erosion of overlaying rock material or glacier retreat. Physical weathering causes the disintegration and breakdown of rock into smaller fractions, or into its constituent minerals. Chemical weathering primarily affects the mineral composition of the parent rock material through alteration and redistribution. In contact with liquid water, rock minerals are subject to dissolution, carbonation, hydration, and oxidation. Dissolution of most primary rock-forming minerals is limited by slow kinetics of the reactions at the mineral-water interfaces. Dissolution rates depends on extrinsic factors (T, pH, Eh, and exudates from microbes and plant roots) and on intrinsic factors (mineral surface properties and weathering state). The presented research is part of an interdisciplinary study of the initial phase of weathering to obtain a detailed picture of the processes occurring at the biosphere-hydrosphere-geosphere interface (http://www.cces.ethz.ch/projects/clench/BigLink). For our studies we established a grid for representative sampling of soil evolution along the chronosequence up to 150 years in the forefield of the retreating Damma glacier in Central Switzerland. To document both, the influence of physical and chemical weathering, we investigated a.o. the grain size distribution and the mineralogical composition of the soil samples in different horizons. As expected, the amount of the clay fraction <2 m increases, but only slightly with increasing soil age. Despite the inhomogeneous metagranitic mineralogy of the Damma granite only little variations in the mineralogical composition of the soil samples up to an age of 130 years could be found. Only the older soils, particularly in the fraction <2 m, show a significantly increased content of 14 Å clay minerals (up to 50 wt%!) and a strong decrease in the feldspar and mica content.

Authors:

Plötze, Michael and Furrer, Gerhard

Index Terms:

granite; glacier; weathering; claygroup; Furrer, Gerhard; Plotze, Michael

Further Information:

Date published: 2009