Thermal characteristics of permafrost in the steep alpine rock walls of the Aiguille du Midi (Mont Blanc Massif, 3842 m a.s.l.)

Abstract

International audiencePermafrost and related thermo-hydro-mechanical processes are regarded as crucial factors in rock wall stability in high alpine areas, but a lack of field measurements means that the characteristics of such locations and the processes to which they are subjected are poorly understood. To help remedy this situation, in 2005 work began to install a monitoring system at the Aiguille du Midi (3842 m a.s.l.). This paper presents temperature records from nine surface sensors (eight years of records) and three 10 m-deep boreholes (four years of records), installed at locations with different surface and bedrock characteristics. Annual and seasonal offsets between mean surface temperatures and air temperatures suggest that snow cover and slope aspect are also important factors governing bedrock surface temperatures in steep terrain. Snow-free sensors revealed additional effects of microtopography and micrometeorology. Active layer thicknesses ranged from < 2 m to nearly 6 m, depending on sun-exposure and interannual variations in atmospheric conditions. Warm and cold permafrost (about −1.5 °C to −4.5 °C at 10 m-depth) coexists within the Aiguille du Midi, resulting in high lateral heat fluxes. A temperature inflection associated with a fracture provided evidence of non-conductive processes, most notably cooling due to air ventilation and some intermittent and local warming. Our field data, the first to be obtained from an Alpine permafrost site where temperatures are below −4 °C, confirm the results of previous studies of permafrost in steep bedrock slopes and highlight the importance of factors such as snow cover and fracturing

Snow control on active layer thickness in steep alpine rock walls (Aiguille du Midi, 3842ma.s.l., Mont Blanc massif)

“…Nearly 700 events have been documented in 10 years (in 2003and from 2007Ravanel and Deline, 2013) among which 96% occurred between June and September, i.e. during the warmest months of the year during which the active layer develops as shown by Magnin et al (2015a). Rockfalls are generally superficial, i.e.…”

“…AdM towers the Chamonix valley by 2800 m on the North side and the Glacier du Géant by 250 m on its South side. Warm (c. −1.5°C at 10 m-depth in the S face) and cold (c. −4.5°C at 10 m-depth in the NW face) permafrost coexist within the Piton Central, leading to lateral heat fluxes from the warm S face to the colder N face (Magnin et al, 2015a). The site is accessible all the year-long by a cable-car from Chamonix, which facilitates the maintenance of the monitoring system.…”

“…The AdM South face is influenced by the cold opposite N faces which generates a negative heat flux at the bottom of BH_S (Magnin et al, 2015a). The heat flux at the lower boundary of the model domain was therefore (Williams and Smith, 1989), a heat flux of − 0.25 Wm −2 is obtained (using thermal properties as defined below).…”

“…A detailed description of the temperature measurement is given by Magnin et al (2015a). In this study, we use one of the nine surface miniloggers ( Fig.…”

“…Nonetheless, micro-topography (e.g. surface roughness and concavity) allows for thick snow deposits over rock walls, which in the European Alps warm shaded faces by insulating them from air temperature, and cool sun-facing slopes because of their albedo (Haberkorn et al, 2015a(Haberkorn et al, , 2015bMagnin et al, 2015a).…”