During 2015, 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.
Antoine Séjourné (GEOPS laboratory, Orsay, France) in collaboration with A. Fedorov (Permafrost Institute of Yakutsk) has continued their research on the dynamic retrogressive thaw slumps in Yakutia (figure 1). As observed in most regions in the Arctic, the thawing of ice-rich permafrost (thermokarst) is increasing in Central Yakutia (Eastern Siberia). In order to understand the current thermokarst dynamics, we studied retrogressive thaw slumps on the banks of thermokarst lakes in Yakutia, using high resolution satellite images taken in 2011–2013 and conducting field studies (Séjourné et al., 2015). However, meteorological parameters initiating each summer the thaw slumping remain unconstrained. In 2014, we sampled several larch (Larix sibirica) inside retrogressive thaw slumps. The aims are to reconstruct the development of this slump, his expansion rate and controlling meteorological parameters in CY during the last 100 years. The patterns of tree rings highlight growth disturbances and date uprooting, i.e. the tilting of trees and their death, demonstrating climatic changes and environmental transformations in ecosystems related to changes in air temperature, precipitation, and the dynamics of cryogenic processes. Tree-ring analyses have been commonly used to reconstruct landslides and snow avalanches in mountains but it has been little used for thermokarst lake development and not for RTS. Tree-ring disturbed by RTS can help studying the permafrost dynamics over a longer continuous period of time instead of snapshots with aerial and satellite images or field studies.
Figure 1. Retrogressive thaw slumping induces the destabilization and tilting of surrounding trees (tree of ~7 m high for scale).
Pursuing researches on the occurrence of paraglacial landslides in the fjords of Northern Iceland, a geophysical survey took place in September 2015 to highlight the part of landslides that is nowadays covered by the sea in the coastal area of Skagafjördur. Agnès Baltzer (University of Nantes and CNRS LETG UMR 6554) and Armelle Decaulne (CNRS LETG UMR 6554) took part in the research. The StrataBox used for the survey offered a high resolution marine sediment imaging of two landslides (figure 2) thanks to numerous survey lines, showing over 7 m thick landslide sediment with specific structures, tongue-like shaped.
Figure 2. The Hofdaholar landslide has the distal part eroded by the sea. The landslide extents in the sea more than 300 m offshore, with a thickness over 7 m (photo A. Decaulne, 2010).
Armelle Decaulne (CNRS LETG UMR 6554, Nantes, France) and Najat Bhiry (Centre d’Etudes Nordiques, University Laval, Canada) shared fieldwork in August 2015 to recognize active and inherited mass movements on slopes of the inner islands of Clearwater Lake (figure 3), in southern Nunavik, impulsing researches within the Tursujuq National Park. The study is part of Labex DRIIHM and OHMi NUNAVIK, and aims in the coming years at studying the postglacial evolution of slopes and potential risks for tourists and users of the Park.
Figure 3. The slopes of the inner islands of Clearwater Lake are dominated by screes, in some instances reworked by snow avalanches and debris flows; landslides are also common. Several generations of landforms are observed, with some very active slopes at present time (photo A. Decaulne, 2015).
These past years, researches conducted by EDY TEM Lab (X. Bodin, F. Magnin, L. Ravanel, Ph. Deline) and Institut de Géographie Alpine (Ph. Schoeneich) have been focus on the thermal regime of permafrost and on the rock fall activity in the Mont Blanc Massif.
The years 2014 and 2015 have shown extreme permafrost features in the Mont Blanc massif. The Summer 2014 was remarkably cold and directly impacted the active layers at the Aiguille du Midi (3842 m a.s.l) that experienced their minimum thickness since the beginning of the records in December 2009: 4.8-m-depth on the S face, 2-m-depth on the NE face and 1.3-m-depth on the NW face. The NW active layer was restricted to short-term (weeks) thawing cycles and no continuous seasonal thaw was observed contrary to the other years. In contrast, the summer 2015 was extremely hot and two boreholes have exceeded their previous active layer thickness by far: 3.6-m-depth at the NE and 6.3 m-depth at the S, whereas it reached 2.4-m-depth at the NW, similar than in 2012. Despite the cold summer of 2014, the permafrost temperature registered a warmer signal than the previous years corresponding to the warm season of 2014 (Figure 4) at 10-m-depth in December 2014. A warming trend is clearly visible in the NE borehole at 10-m-depth since the beginning of the records, and less clear in the NW borehole. However, the past two years registered remarkably warm winter signals at 10-m-depth. In the S-exposed borehole, no clear trend is detected. Although the bedrock is a relatively dry ground, some wet-detritic material was found when the borehole was drilled, suggesting that water-phase change may play a significant role in the thermal dynamics of this borehole. In consequence, the S borehole trend is possibly damped by the latent heat consumption since it is located on a warm permafrost area.
Figure 4. Daily temperature recorded at 10-m-depth in the three boreholes of the Aiguille du Midi since 2010.
Interestingly, only 17 rock falls were inventoried in 2014 by the network of observers (hut keepers and mountain guides) established since 2007 on the Mont Blanc massif, whereas > 160 rock falls were registered during the Summer 2015 (Figure 5). The comparison of the active layer dynamics and the rock fall inventory hints at a predominant role of the active layer thickening in the rock fall triggering; considerable efforts are lead today to clarify this relationship.
Figure 5. Rock fall at the “Tour Ronde” (Mont Blanc massif) that occurred the 27th August 2015 (picture: Gianluca Marra).
PermaFrance, the French monitoring network for mountain permafrost, continued its activities. It was recognized in 2014 as “Atelier d‘observation” by the Observatoire des Sciences de l‘Univers of Grenoble (OSUG), hopefully a first step towards the labelling as Observation System. A further step was the submission in spring 2015 of an application for an integrated national cryosphere observation system (including glaciers, snow and permafrost), which will be integrated in the Oscar research infrastructure on critical zone. The six boreholes of the network are integrated in the GTN-P database and a webpage has been created (permafrance.osug.fr).
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) and the Permafrost Institute in Yakutsk (Yakutia, Russia). Nicolas Roux defended his PhD work this autumn on the topic of the evolution of the river’s taliks in the context of climate change. The approach combines numerical simulation, analogical experiments in cold room at GEOPS lab and field study in Yakutia. So far the paper concerning the experiment in cold room has been accepted and will soon be published in PPP. The field study focuses on the evolution of the soil - river continuum in an Alas valley in Yakutia (figure 6). The site was equipped in 2012 with thermal, hydrological & hydrogeological sensors and the water properties and isotopic signatures were monitored.
The monitoring results already provided interesting results analyzed in terms of river and lake thermal and hydrological influence regions. The next phase on the site consisted in gathering the main monitoring equipment along a river transect. This was started in September 2014. Unfortunately some temperature monitoring devices were destroyed due to a large spring breakup event in 2015 strongly modifying the river shores. Thus the monitoring data remains uncomplete so far but the replacement of the devices should lead to full transect monitoring results by Sept. 2016 providing a unique monitoring case of a river transect thermal evolution.
Figure 6. Syrdakh site in Yakuita. Credit Photo: Nicolas Roux.
The other main activity concerns the development of coupled Thermo-Hydrological (TH) codes that face difficulties in the simulations of such a non-linear system of coupled equations with phase change effects providing steep fronts. Launched initially in 2014, April 2015 has seen the second meeting in Paris of the InterFrost TH code inter-comparison exercise. 13 codes competed corresponding to laboratories in America (USA, Canada) and Europe (France, UK, Sweden, The Netherlands, Germany). Some preliminary inter-comparison results were provided in (Rühaak et al. 2015, Energy Procedia) but the full exercise was closed and compiled recently with a publication in preparation for 2016. The main conclusion is that the inter-comparison results suggests that all codes modelling the exact equations and characteristic laws provide identical qualitative trends and even converge on a close quantitative basis. More information on test cases, actions, milestones and participants can be found on the InterFrost web site (https://wiki.lsce.ipsl.fr/interfrost).
Report prepared by François Costard (firstname.lastname@example.org)