Dynamics of the alpine permafrost as a possible response to global warming: Results from a 1986-2006 survey and interpretation of recent field observations (French alps)
|Location||International Geological Congress,oslo 2008|
|Author||Bodin, Xavier۱; Fort, Monique۱; Thibert, Emmanuel۲; Fabre, Denis۳; Schoeneich, Philippe۴|
|Holding Date||03 September 2008|
The alpine permafrost has traditionally been thought as a geosystem mainly sensitive to long-term climate fluctuations. Indeed, thermal evidence of the Little Ice Age cold period has been found in high altitude boreholes and attest of an increase of the deep permafrost temperature ranging from 0.5 to 1°C since 100 years. Nevertheless, more and more data tend to prove that the permafrost dynamics may also react to annual, and even shorter, changes in the climatic parameters.
The gravity-driven creeping activity of rockglaciers is well adapted to represent the climate/permafrost relationship, especially through the monitoring of the surface displacements, since ground temperature and liquid water content are crucial controls on the deformation of such ice-rich accumulations. In France, the longest surveys of surface velocity takes place on the Laurichard rockglacier, which is annually monitored by geodetic methods since 1986. Moreover, geoelectrical soundings were performed on the rockglacier in 1986, 1998, 2004 and 2006, whereas ground thermal state is measured in sub-surface since 2003. Several results emerge from the analysis of our datasets: 1) as other rockglaciers in the European Alps, the Laurichard rockglacier experienced a speed-up during the 90’s, during which a clear rupture in the regional air temperature series has also taken place; 2) in parallel, the geoelectrical soundings suggest that the probable increase of the ground temperature may have resulted in a decrease of the ice content, and in a probable higher liquid water content; 3) at an interannual scale, the ground surface temperature is well correlated with the surface velocity of the rockglacier, but precipitation also influences the permafrost creeping, either by the thermal insulating effect of the snow cover or by the release of melt water. Similar results have also been found on other rockglaciers of the Alpine Range.
Therefore, the recent behaviour of the Laurichard rockglacier, which is located at the lower limits of the alpine permafrost, has probably to be interpreted as a typical response of the high mountain periglacial geosystems to the global warming. As the permafrost is coming closer to the melting point, greater sensitivity to annual nivo-meteorological conditions as well as higher deformation rate of ice-rich debris accumulations have to be expected. Recent field observations in the French Alps, such as the collapse of a rockglacier, the formation of thermokarst lakes or the destabilisation of human facilities at altitudes between 2400 and 3200 m asl., may be partly related to the same phenomena as those that are occurring on the Laurichard rockglacier.