5Th Swiss Geoscience Meeting, Geneva 2007

5th Swiss Geoscience Meeting, Geneva 2007

Comparison of exposure ages and spectral properties of rock surfaces in steep, high alpine rock walls –

a field study at Aiguille du Midi, France

Ralph Böhlert*, Stephan Gruber*, Markus Egli*, Max Maisch*, Dagmar Brandová*, Susan Ivy-Ochs*,**, Peter W. Kubik***, Philip Deline**** & Wilfried Haeberli*

*Department of Geography, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich ()

**Institute for Particle Physics, ETH Zürich, Schafmattstrasse 20, CH-8093 Zurich

*** Paul Scherrer Institut, c/o Institute for Particle Physics, ETH Zürich, Schafmattstrasse 20, CH-8093 Zurich

****Laboratoire EDYTEM, Université de Savoie, F-73370 Le Bourget-du-Lac

Permafrost thaw affects the stability of steep rock slopes. To put into perspective modern observations of rock fall, information on past rock fall activity is required. The combination of cosmogenic nuclide dating and imaging spectrometry seems to be a useful approach for providing data of surface age via the spectral signature of weathered surfaces in homogeneous lithology. In this study we present dating results for near vertical granite rock walls together with a first analysis of spectral signatures and their relation to the age of the surfaces dated.

We analysed four samples from different sites near the summit of Aiguille du Midi in the Mt. Blanc area (F) using in situ produced 10Be. Aiguille du Midi (3842 m a.s.l.) is geologically situated in the Mt. Blanc massif and consists of the quartz-rich “Mt. Blanc granite”. Assuming that intensively red coloured parts in the rock wall were exposed to weathering over a longer time period and consequently have a higher age than fresh, grey ones, we chose the sampling sites on the basis of visual identifiable differences in colour. This assumption is based also on field observations made at the adjacent Dru region, where the colour of the young rock surface in the region of a recent rockfall clearly contrasts to the surrounding, reddish weathered areas. Results of 10Be dating were corrected for topographic shielding. We made no correction for erosion and snow cover (surface angles of the slope were ≥ 79°). Spectral reflectance of each dated rock sample and of an additional piece of fresh rock was measured using an ASD FieldSpec Pro Fr spectroradiometer and a Thermo Oriel light source in order to relate surface age and colour. Furthermore, the chemical composition of the samples was determined by X-ray fluorescence (XRF).

The rock surface ages varied between less than 2.000 years and more than 40.000 years. We found a clear correlation between age and surface reflectance behaviour in the range of approx. 415-650 nm which corresponds roughly to the visible spectrum. These findings may be a first step towards a possibility of generating age distribution data in steep rock walls. The strong correlation between the two methods means that at least in Late-Quaternary-aged high alpine surfaces (with a similar geology) the redness is a direct indication of exposure time to weathering. Dating of rock walls therefore helps to reconstruct rockfall activity history, which also may provide deeper insights into the spatial and temporal permafrost development in high alpine environments.