During 2014, the activities of the French permafrost community are going on Western Alps, Iceland and Central Yakutia (Russia). Permafrost studies in France are covering a wide range of different activities: e.g. geomorphological field study, field monitoring, laboratory simulation in cold rooms and numerical modelling of water/permafrost interactions.
These past years, researches conducted by EDYTEM Lab on the rock fall activity in the Mont Blanc Massif have raised hypotheses on the role of permafrost as triggering factor in such hazards. The resulting research questions address permafrost distribution in the steep rock walls of Mont Blanc Massif, and the understanding of its evolution under climate change, from seasonal to long-term responses.
In the framework of the European PERMAdataROC (2006-2008) and PERMANET (2008-2011) projects, followed by the CIBLE program funded by the Region Rhône-Alpes, the monitoring system at the Aiguille du Midi (Mont Blanc massif, 3842 m a.s.l) has begun in 2005 with the installation of rock surface and borehole temperature sensors, further complemented with diverse measurements such as a crack-meters network. The alpine-wide statistical model calibrated by L. Boeckli (University of Zurich) with temperature data homogenised in the PERMANET framework, has been initialized and verified with local input data by F. Magnin (EDYTEM Lab, Université de Savoie). The model has been implemented on a high-resolution DEM (4 m). A permafrost index map over steep and non glaciated slopes is then built on the basis of this statistical model. Eight electrical resistivity surveys have been conducted on six selected in order to validate the lower limits of permafrost occurrence suggested by this map. To calibrate the temperature and resistivity pathways from frozen to unfrozen ranges, laboratory experiments have been conducted (collaboration with M. Krautblatter, Technical University of Munich).
Temperature data from the Aiguille du Midi encompass four years of active layer freeze-thaw cycles, with thicknesses ranging from of 40° (Figure 1). Permafrost possibly occupies rock walls from 2300 m a.s.l. in north faces and from 2700 m a.s.l. in south faces. It is probably continuous from 3600 m a.s.l. Warm and discontinuous permafrost areas, increasingly regarded as the most unstable, are suggested to occur between 2000 to 2600 m a.s.l. in north faces and 2400 to 3200 m a.s.l. in south faces. ERT surveys on six sites between 2810 m and 3350 m a.s.l. (top of the transect), two of which measured repeatedly in 2012 and 2013, have detected discontinuous permafrost. The inverted model from the ERT transect acquired at the south face of the Gros Rognon site ( 3350 m a.s.l) is presented in Figure 1 and shows warm permafrost conditions. Permafrost occurrence indicated by resistivity values >80 kΩm at the top and steepest part, is possibly induced by the cold effect of the opposite north face. This is suggested by the high resistivity gradient along depth, possibly reflecting a high negative temperature gradient towards the north face. This interpretation is coherent with results from numerical experiments in steep bedrock permafrost that enhanced the dominant topoclimatic control on the distribution permafrost in rock walls. Close to 0°C temperature conditions are detected at the bottom and least steep part. This interpretation of permafrost conditions is based on results from laboratory temperature-resistivity calibration performed on a rock sample boulder which frozen range took place between 40 and >100 kΩm.
Figure 1. Permafrost index map of Gros Rognon glaciers from electrical resistivity surveys.
In 2014, Charles Le Cœur from LGP (UMR 8591, Meudon, France) has pursued its researches on some synchronous rockglaciers in western Vanoise (Westesn Alps). In the Gebroulaz area, a set of rock glaciers (figure 2) offers evidence asynchronous discontinuous permafrost feature generated during post Late Laglacial maximum cold sequences. Theses debris accumulations are located on the eastern side of the Gebroulaz glacier. Pre and post Younger Dryas rockglaciers were developed either from local glacier tongues or from large scree and rockfalls. Periglacial feature were emplaced on deglaciated areas as local glaciers retreated from Older Dryas to the end of Yonger Dryas. One first set of coarse debris tongue corresponds to rock covered glacier changed into rock glacier: Chanrouge (prior than Youger Dryas), Infernet 1 and 2 (after Late Glacial). In another set, it is possible to differentiate thick rock debris tongues expanded on non-glaciated areas during Younger Dryas (Fond1 and 2) and short rockglaciers accumulated after melt of local Younger Dryas glaciers (Eaux Noires, Coua 1 and 2) comparison of debris accumulations reported to headwalls supports this differenciation. More evidence can be found on roches moutonnées, below the rockglaciers, where uneven weathering surfaces result from different durations. Therefore, these rock glaciers offer an indication for a non-synchronous periglacial evolution during post glacial cold sequences.
Figure 2. Gebroulaz rock glaciers (Credit photo: @ Charles Le Coeur).
Romain Perrier from University Paris Denis Diderot/UMR CNRS Prodig 8586, intended to characterize the distribution, the state and the functioning of permafrost at various spatial scales and in various topoclimatic contexts. It also intends determine permafrost response(s) to actual climate change. His investigations have been carried out in two valleys (Clarée and Ubaye) of the French Southern Alps. Firstly, permafrost spatial distribution has been studied at regional scale by means of a statistico-empirical model. Results show that permafrost may be found between 2600 and 3000 m and its distribution is influenced by altitude and solar radiation. Secondly and at the local scale, the implementation of a geophysical, thermal and geodetic monitoring has helped to qualify the regional spatial model as well as to characterize actual permafrost functioning. At rockglacier scale, geophysical investigations reveal a patchy permafrost distribution and a high heterogeneity of ground ice that both may be explained by local geodynamics and recent glacial (LIA) history. Thermal monitoring has revealed the existence of 4 main thermal regimes that mainly depend on snow cover specificities and permafrost occurrence. During the two years (2010-2012) period of monitoring some sites have shown some permafrost thermal disequilibrium with current climate conditions. Geodetic monitoring of rockglaciers shows an annual velocity as well as vertical displacements that range from few centimeters up to a meter. More generally surface displacements are mainly conditioned by local topography and ground ice type. Thirdly, permafrost degradation assessment through rockglacier morphological changes is difficult to determine. Significant morphological changes have only been observed on rockglacier areas that contain ground massive ice inherited from LIA advance. However, the use of permafrost thermal disequilibrium proxies has enabled to build up a regional topo-climatic model together with a map of areas susceptible to thermal disequilibrium.
In 2014, the Icelandic slopes of coastal mountains were surveyed to locate and better understand the extent of paraglacial readjustment processes. Especially, ridge-top splitting events have been highlighted following the rapid late Weichselian deglaciation in Skagafjörður by Julien Coquin, Denis Mercier, Olivier Bourgeois and Armelle Decaulne (University of Nantes, CNRS UMR 6554 and 6112, France) and Etienne Cossart (University Paris 1, CNRS UMR 8586, FRance). It is thought that ridge-top splitting influences large-scale glacial patterns by facilitating glacial erosion along ridge-top grabens (figure 3). To pursue the work on the large postglacial landslides, to identify the main collapse factors and decipher the timing events occurred, the group also investigated the Westfjord peninsula, locating over 100 landforms.
Figure 3. A series of landslides in the Vatnadalur valley, north-western Iceland (Photo A. Decaulne).
Francois Costard (GEOPS laboratory, Orsay University, France) and Emmanuèle Gautier (LGP laboratory, Meudon, France) in collaboration with F A. Fedorov and P. Konstantinov (Permafrost Institute of Yakutsk) have continued their research on the impact of the breakup on the erosional process on the head of several fluvial islands from one of the largest Arctic fluvial systems – the Lena River (Yakutia). The purpose of this work was to evaluate the role of the thermal erosion during ice breakups of the Lena River. In 2008-2011, a 4-years observation program was initiated to quantify the relative influence of fluvial thermal erosion during the ice breakup of the Lena River. During the initial stage of the ice breakup, ice pushes into riverbanks and produces huge accumulations of sediment that protect the island head against the mechanical and thermal effects of the river flow. That initial stage is relatively short, and occurs within a few days period. In a second phase after the fluvial ice thawing, the island heads are ice-free. In the case of high water levels, the flood, in permanent contact with the frozen riverbank, undergoes efficient thermal and mechanical erosion, sometime through the fall season during a secondary discharge peak. The careful analysis of the annual data shows a high variability of the erosion rate, mostly due to the variability of the duration and timing of the flood season. The heads of islands undergo strong erosion with mean values of 12 m per year and maximal values reaching 40 m. All these results were recently published in PPP Vol. 25, Issue 3, pp. 162-171.
As observed in most regions in the Arctic, the thawing of ice-rich permafrost (thermokarst) is increasing in Central Yakutia (Eastern Siberia). However, the relationship between thermokarst development and climate variations is not well understood in this area. In order to understand the current thermokarst dynamics in Central Yakutia, Antoine Sejourné (GEOPS laboratory, Orsay, France) in collaboration with A. Fedorov (Permafrost Institute of Yakutsk) studied the bank degradation of thermokarst lakes in Central Yakutia due to permafrost thaw. The retrogressive thaw slumps (figure 4) and highly degraded ice-wedge polygons (baydjarakhs) were analyzed, using 2011-2013 high resolution satellite image time series and field studies. The retrogressive thaw slump activity results in the formation of thermocirque with an average headwall retreat ranging from 0.5 to 3.16 m/year. The thermocirques and the baydjarakhs are statistically more concentrated on the south- to southwest-facing banks of thermokarst lakes. Their development and this statistical distribution indicate a control of the current permafrost thaw on the banks of thermokarst lakes by insolation. In the context of recent air temperature increase in Central Yakutia, the rate of thermocirque development may increase in the future.
Figure 4. Thermocirques in Central Yakutia (Photo A. Sejourne, 2014).
During past years, Christophe Grenier, Nicolas Roux, Emmanuel Mouche from LSCE Gif sur Yvette, France) has been developing activities in numerical modeling for permafrost issues involving coupled thermal transfer with water flow in the Cast3M code (www-cast3m.cea.fr). This modeling activity was complemented by laboratory experiments and field work involving collaborations with François Costard at GEOPS (Univ. Paris Sud, Orsay, formerly IDES Lab.) and the Permafrost Institute in Yakutsk (Yakutia, Russia). The topic studied in the PhD work of Nicolas Roux concerns the evolution of the river's taliks in the context of climate change with an approach combining numerical simulation, analogical experiments in cold room and field study in Yakutia. The field study focuses on the evolution of the soil - river continuum in an Alas valley in Yakutia. The site was equipped in 2012 with thermal, hydrological & hydrogeological sensors and the water properties and isotopic signatures were monitored. The second year of data was obtained during September 2014 field study. The inter-annual variability appeared strong, mainly due to a warmer winter for the second year of monitoring. The second river transect of measurements was reinforced and will be completed in April 2015. The next campaign in September 2015 will then provide a third year and hopefully a full transect monitoring of the river and ground thermal field evolution (from below the river to the sides of the valley). A paper with the main results associated with the present knowledge is under preparation. The experimental study in cold room at GEOPS addresses the same issue of river – soil interaction considering a channel with a "river" flowing on a frozen porous medium. The first objective is to identify the main controlling parameters for the progression of the 0°C isotherm into the frozen material based on thermal monitoring of the system. The second objective is to simulate the experiment with Cast3M code and identify the appropriate boundary conditions, parameters and finally validate the code for such purposes.
Another line of action concerns the development of coupled Thermo-Hydrological codes. While a larger amount of publications appears on such issues, the resolution of such a coupled non-linear system with phase change still remains a difficult issue. LSCE has proposed and launched a TH code inter-comparison exercise to 1°) evaluate and validate codes by means of inter-comparison on test cases and experimental studies, and 2°) create a research community around such issues to exchange and improve codes in view of more realistic system simulations. The kick off meeting of the INTERFROST benchmark took place in Paris on the 18th and 19th of November (figure 5) with researchers from the US, Canada, Sweden, Germany, France involving 13 simulation codes (see Picture below). The associated experiments at GEOPS were also visited. The group agreed on running the T1 and TH1 test cases as base cases (analytical solutions) and run to the TH2 and TH3 cases for inter-comparison. The next meeting is planned for the first half of April 2014 where the results will be provided by the participants for comparison. More information on test cases, actions, milestones and participants can be found on the INTERFROST web site (https://wiki.lsce.ipsl.fr/interfrost).
Figure 5. INTERFROST benchmark meeting in Paris on the 18th and 19th of November 2014.
Report prepared by François Costard ( firstname.lastname@example.org)