TCN dating of high-elevated rockfalls in the Mont Blanc massif. A new method of dating rockfalls in the Mont Blanc massif using reflectance spectroscopy
Résumé
Rockfalls and rock avalanches are active processes in the Mont Blanc massif, with infrastructure and alpinists at risk. Thanks to a network of observers (hut keepers, mountain guides, alpinists) set up in 2007 current rockfalls are well surveyed and documented. Rockfall frequency has been studied over the past 150 years by comparison of historical photographs, showing that it strongly increased during the three last decades, likely due to permafrost degradation caused by the climate change. In order to understand the possible relationship between rockfall frequency and the warmest periods of the Lateglacial and the Holocene, we study the morphodynamics of some selected high-elevated (> 3000 m a.s.l.) rockwalls of the massif on a long timescale.
Since rockfall deposits in glacial areas are evacuated by the glaciers, our study focuses on the rockfall scars. 10Be TCN dating of a rockwall surface gives us the rock surface exposure age, interpreted as a rockfall age. Here we present a dating dataset of 80 samples carried out between 2006 and 2016 at six high-elevated rockwalls in the Mont Blanc massif. The resulting ages vary from present (0.04 ± 0.02 ka) to far beyond the Last Glacial Maximum (c. 100 ka). Three clusters of exposure ages are correlated to i) two Holocene Warm Periods (7.50 - 5.70 ka), ii) the Bronze Age Optimum (3.35 - 2.80 ka) and iii) the Roman Warm Period (2.35 - 1.75 ka). A fourth age cluster has been detected with ages ranging 4.91 – 4.32 ka. The biggest cluster, ranging 1.09 ka – recent, shows rather small volumes (< 15,000 m3). This is interpreted as the normal erosion activity corresponding to the current climate.
Furthermore, a relationship between the colour of the Mont Blanc granite and its exposure age has been established: fresh rock surface is light grey (e.g. in recent rockfall scars) whereas weathered rock surface shows a colour in the range grey to orange/red: the redder a rock surface, the older its age. Reflectance spectroscopy is used to quantify the granite surface colour. We explored the spectral data in order to find an index to measure the rock weathering evolution along time, thus allowing to date the rock surface exposure age using reflectance spectroscopy: the GReen Infrared GRanite Index (GRIGRI), based on the Remote Sensing-used GRVI Vegetation Index. GRIGRI uses the ratio between Green (530 nm) and Photographic Infrared (770 nm) reflectance to obtain the index, directly related to the granite exposure age (r= 0.861). The GRIGRI method has been tested for 8 samples where TCN dating failed, and for two samples where 10Be exposure age is considered outlier. The resulting ages, according to the geomorphology of the scars and their surroundings, are plausible.