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 (Ravanel and Deline 2013). Rockfall frequency has been studied over the past 150 years by comparison of historical photographs (Ravanel and Deline 2008), 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 nine high-elevated rockwalls in the Mont Blanc massif (Figure 1). 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) the Holocene Warm Period, ii) the Roman Warm Period, and iii) the Little Ice Age and post-LIA. Ages of this last one are generally related to small rockfall volumes (< 15000 m3), considered as the normal erosion. A 4th cluster at 4.2-5.0 ka is not associated with any evident global climate period.
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 is in the range grey to orange/red: the redder a rock surface, the older its age (Böhlert et al, 2008). 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= 0.863; Figure 2). The GRIGRI method has been tested for 8 samples where TCN dating failed, and for two samples where 10Be exposure age are considered outliers. The resulting ages, according to the geomorphology of the scars and their surroundings, are plausible.
TCN dating enabled to understand the rockfall frequency of the MBM over a large timescale. Dating complements historical evidence such as photograph datasets and present-day observations and surveys. The data presented here is probably the biggest dataset that explores the relationship between Holocene rockfall episodes and climate. We demonstrate that a significant part of the occurred rockfalls correlate well with climate variability. A new method of dating surface exposure age of the MBM using spectral data is presented here. We aim to develop surface exposure dating using photographic RGB coordinates, thus to obtain a tool allowing to launch a large scale surface dating campaign.