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International audience; The intensity of the current climate change has strong consequences for high mountain tourism activities. Winter activities are currently the most studied (ski industry). However, the consequences of environmental changes are also strong in summer, as geomorphological processes are increased at high altitudes. The Mont Blanc Massif (Western Alps) is particularly favourable terrain for the development of research into these processes. Its emblematic high summits (28 of the 82 peaks > 4000 m in the Alps), its hundreds of glaciers, its particularly developed tourism with summer/winter equivalence, the practice of mountaineering, which is still very present, etc. all contribute to the interest of studying this geographical area. A lot of work has been carried out on glaciological and geomorphological issues. These studies, which concern the "physical" impact of climate change on the high mountains, are also supplemented by studies of their consequences on human societies. Amongst others, we can cite the question of the impacts on practices such as mountaineering or glacier tourism. Risk-related issues are also taken into account, with, for example, question related to the stability of infrastructures (huts, ski lifts) or the impact of glacial retreat on the formation of new, potentially dangerous lakes. Accordingly, the aims of this paper are to show the extent of the research developed on climate change in the Mont Blanc massif and how social and environmental sciences are interlinked to provide a holistic vision of the issues of this territory. As these experiments are not exactly interdisciplinary experiments, this presentation also aims to discuss the points that need to be developed in order to promote inter- and trans-disciplinary research.

International audience; In the high mountain rock walls of the Mont Blanc massif, changes in the granite surface colour are related to its exposure age. The light grey colour of fresh rock surfaces turns orange when is long exposed to weathering. In order to study this colour/age relationship, reflectance spectroscopy was performed on 73 samples, and Terrestrial Cosmogenic Nuclide (TCN) dating was used to obtain their surface exposure age. The standard deviation of the reflectance values was calculated for each wavelength of the visible spectrum to study the behaviour of each spectral region. The aim was to find two colour regions that showed opposite behaviour, and once they are combined, they could provide a representative index of the rock colour changes that are linked to the degree of weathering.As an adaptation of the Green Red Vegetation Index used to measure colour changes in vegetation, the GReen-Infrared GRanite Index (GRIGRI), a normalized difference between the granite 770 nm and 530 nm reflectance values, was developed. The GRIGRI value of a weathered granite surface has a close relationship with its exposure age (R2 = 0.85). The reflectance spectra of seven samples for which TCN dating failed and two samples for which the TCN age was considered to be an outlier were used to calculate their GRIGRI value to assess the colour-based ages, that were plausible according to rock wall morphology and the TCN exposure ages of the surrounding surfaces. We propose a new method of surface dating for the rock walls of the Mont Blanc massif using reflectance spectroscopy.

Rockfalls and rock avalanches are a recurrent process in high mountain areas like the Mont Blanc massif. These processes are surveyed due to the hazard they present for infrastructure and alpinists. While rockfalls and rock avalanches have been documented for the last 150 years, we know very little about their frequency since the Last Glacial Maximum (LGM). In order to improve our understanding, it is imperative to date them on a longer timescale. A pilot campaign using Terrestrial Cosmogenic Nuclide (TCN) dating of five samples was carried out in 2006 at the Aiguille du Midi (3842 m a.s.l.). In 2011, a larger scale study (20 samples) was carried out in five other test sites in the Mont Blanc massif. This paper presents the exposure ages of the 2011 TCN study as well as the updated exposure ages of the 2006 study using newer TCN dating parameters. Most of these exposure ages lie within the Holocene but three ages are Pleistocene (59.87 ± 6.10 ka for the oldest). A comparison of these ages with air temperature and glacier cover proxies explored the possible relationship between the most active rockfall periods and the warmest periods of the Holocene: two clusters of exposure ages have been detected, corresponding to the Middle Holocene (8.2–4.2 ka) and the Roman Warm Period (c. 2 ka) climate periods. Some recent rockfalls have also been dated (

International audience; 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.

International audience; 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.