During 2012, the activities of the French permafrost community are going on Spitsbergen, Central Norway 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 chambers and numerical modelling of water/permafrost interactions.
French Icelandic research team
In 2012, the French-Icelandic research team, with Denis Mercier, Etienne Cossart, Armelle Decaulne, Thierry Feuillet, Julien Coquin, Helgi Páll Jónsson and Þorsteinn Sæmundsson (figure 1), has pursued its researches on the deglaciation in the Skagafjördur area, in the North of Iceland.
A multivariate statistical method (factor analysis of mixed data and hierarchical classification) was used to classify the environmental settings where sorted patterned ground develops. A total of 750 periglacial features, distributed over 75sites, were studied. Nine explanatory variables were assessed by field work and using a digital elevation model, the variables were subdivided into three groups (latitude, topography and soil characteristics) and then integrated into a GIS. Furthermore, a correlation between the environmental variables and anintrinsic variable (patterned ground mesh diameter) was determined by a bivariate test. The results show that sorted patterned ground are spread over three homogenous areas, mostly differentiated by altitude, insolation, grain size characteristics and type of drift. In addition, feature diameters differ significantly from one group to another. Finally, it appears that patterned ground diameters are positively correlated with (i) the proportion of clay to medium silt content (r=0.35), (ii) altitude (r=0.51), and especially with (iii) clast length (r=0.97). This strong relationship with clast length is observed in each homogenous patterned ground area at both site and feature scales.
Figure 1: Helgi Páll Jónsson, Denis Mercier, Armelle Decaulne, Etienne Cossart, Þorsteinn Sæmundsson, Julien Coquin, Thierry Feuillet) on the field in the Skagafjördur area, in the North of Iceland in July 2012.
An another research program concerns the Höfðahólar rock avalanche. This landslide was investigated on the basis of a geomorphological analysis of its landforms and close surrounding environment. Thanks to sound chronological constraints (14C dating from birch remnants in peat areas that developed within depressions over the chaotic rock-avalanche deposit, tephrochronological sequences resulting from subsequent ash fallouts over the deposit, calibration of an age–depth model of peats and previously dated raised beaches), we define the rock-avalanche implementation with a wider timeframe between 10,200 and 7975 cal. yr BP and with a narrower frame between 9000 and 8195 ± 45 cal. yr BP. Such a well constrained timing proposes one of the most precise datings of an early-Holocene major slope failure in Iceland. This result fits well in the known chronology of the deglaciation in this area and in the prevailing Icelandic theory of a generalized phase of landsliding that occurred shortly after the deglaciation of the area. The main driver for the rock avalanche occurrence is associated to a paraglacial origin; glacio-isostatic rebound, associated to rockwall debuttressing, is thought to be the main factor in the genesis of this Boreal major disequilibrium.
Periglacial dynamics in central Norway - Geolittomer, Nantes.
R. Kerguillec, UMR 6554 Geolittomer, University of Nantes, France, purses researches on actual periglacial dynamics in central Norway, within a thesis of physical geography. Since 2008, four field missions took place in Dovrefjell and Rondane national parks.
The first aim of this topic is the study of the actual functional periglacial belts in these two high mountain areas, and first to locate the exact position of the lower limit of periglacial activity because of its fundamental morphologic and geographic significance. The method used is a morphologic one which consists of the identification of three authentic periglacial belts. For comparison, researches were also carried out on the Norwegian coast, at similar latitude of Dovrefjell/Rondane, to bring periglacial belts altitudinal tendency towards the west into light: L. King (1986) found that permafrost increases in altitude towards the west and, on the other hand, that forest belt decreases in the same direction.
Figure 2: The recent researches in the area of Gamlemsveten, northeast of Ålesund, prove that the active periglacial belt follows the same tendency of decreasing towards the west, and shows a difference of altitude of 300 m in comparison with Dovre-Rondane.
Researches were also carried out on the actual mobility of periglacial belts, especially in Dovrefjell, in relation with the contemporary climatic modifications in mountains environments. Observations took place on the margins of Dovrefjell’s glaciers and at snowpatches sites from Little Ice Age, especially to identify periglacial conquest dynamics on recently deglaciated terrains. Field work was done on these topics during summer 2011 in Rondane and Dovrefjell in collaboration with D. Sellier, UMR 6554 Geolittomer, University of Nantes, France. Fresh periglacial features appear on névé sites, in relation with thawing permafrost since the end of Little Ice Age. Because of their characteristics, they give evidence in favor of contemporary periglacial conquest dynamics.
Permafrost investigations in Central Yakutia - IDES,Orsay.
A detailed analysis of the hydrology of the Lena has been performed by E. Gautier at the Laboratoire de Géographie Physique (Meudon), F. Costard, A. Sejourne, L. Dupeyrat (IDES laboratory, Orsay University) in cooperation with A. Fedorov (Permafrost Institute, Yakutsk). The impact of the breakup on the erosional process on the head of several fluvial islands was analyzed from one of the largest Arctic fluvial systems – the Lena River (Yakutia). The purpose of this work was to reevaluate the role of the thermal erosion during ice breakup 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. In the case of high water levels, the flood, in permanent contact with the frozen river bank, 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. This program was funded by ANR CLIMAFLU and by GDR Mutations polaires.
In the same area, a detailed study of significant melting of ice-wedges on hillside slopes of thermokarst lakes in Central Yakutia (eastern Siberia) was performed by A. Séjourné (Institute of Geological Sciences, Wroclaw Poland) in collaboration with F. Costard and J. Gargani (IDES laboratory, Orsay University) and A. Fedorov (Permafrost Institute of Yakutsk). The important melting of ice-wedges could lead to formation of amphitheatrical hollows referred as thermocirques (figure 3). The melting of ground-ice on the scarp of thermocirque triggers falls and small mud-flows that induce the retreat of the scarp parallel to itself. The evolution of thermocirques in Central Yakutia has been little studied and analyzing their formation could help to understand the recent thermokarst in relation to climate change in Central Yakutia. We studied the thermocirques with field surveys in July 2009-2010 and October 2012 to examine the processes and origin of melting of ice-wedges and; photo-interpretation of time series of satellite GeoEye images (50 cm/pixel) to study the temporal evolution of thermocirques.
Figure 3: Thermocirques in central Yakutia. From A. Sejourne, 2012.
The CRYO-SENSORS program in western Spitsbergen - IDES, ORSAY;
The laboratory IDES (Univ Paris-Sud / CNRS) in collaboration the laboratories THEMA and FEMTO (Univ. de Franche Comté/CNRS) have continued their research on the Austrelovenbreen catchment close to Ny Alesund (western Spitsbergen) in the framework of the CRYO-SENSORS program (Funded by ANR and by GDR Mutations polaires). The catchment (10 km2) is optimal for hydrological studies because the drainage system forms a well-defined outlet going downstream. The project aims to study both hydrological and glaciological mass-balances of the catchment. In 2012, specific investigations were conducted in order to study the exchanges between river water and the supra-permafrost water-table. For this purpose, several monitoring have been undertaken in rivers as well as in 2 lines of piezometers (physicochemical characteristics of surface and ground-water, potentiometric level, soil temperature, geophysical investigations). The results show a seasonal evolution of the hydrographs closely linked to climatic factors. Although the meltwater from snow and glacier ice strongly contributes to the outlet flows, the discharge of subglacial river and that of the suprapermafrost water-table also controls the fluxes by constituting a river base flow. The groundwater water-table reaches a thickness up to 1.50 m for an active layer thickness of ca. 2.50 m at maximum. The contribution of the water-table towards the rivers might be consequent as this process proceeds all along during the hydrological season. The combination of hydrochemical and isotopic tracers are helpful to separate all different end-members of the runoff (figure 4).
Figure 4: Evolution of the isotopic composition of the river water at the outlet of the catchment (summer 2009)
Numerical modeling of a glacier/permafrost system - LSCE/IDES
During past years, LSCE has been developing activities in numerical modeling for permafrost issues involving coupled thermal transfer with water flow in the Cast3M code. The main activities of the present year correspond to two application studies.
- The first deals with the evolution of a river talik during a glaciation cycle within the framework of nuclear wastes storage in the Paris sedimentary basin (Grenier et al. 2012, Hydrogeology Journal). The question behind is how the recharge of the aquifer is modified when the cold climate phase starts. Results obtained on a plain-river sub-unit of the landscape show that the recharge area reduces to the river, provided that the size of the river is larger than a few hundreds of meters, or if water flow from the river to the underground dominates, allowing a vertical channel to remain open due to warmer surface water inputs.
- The second study is a 2D numerical modeling of the evolution of a glacier / permafrost system (2D section along the flowline) to assess the thermal state at the base of the glacier and to understand the hydrology of the water catchment (Roux et al., 2012, TICOP Proceedings, Salekhard, Siberia, June 2012). The case of the Austre Lovenbreen glacier is studied in collaboration with IDES (Univ. Paris Sud, Orsay). A glacier (Thermo-Mechanical) model and a ground thermal model are chained to simulate glacier evolution (retreat of 15 m/y on average) and permafrost development at the glacier front, and to estimate the probability of the glacier to be warm-based. Sensitivity to the input parameter set shows that the glacier bed is most probably still warm but that it should become cold within decades to a century for the scenarios considered (figure 5).
Figure 5: Simulated evolution of glacier ice thickness (positive values) and -0.5°C isotherm depth (negative values) along a flowline profile from mountain top to seashore. Dotted line is the initial ice-thickness (1925). Simulation times are plotted for, 1: 1975, 2: 2025, 3: 2075, and 4: 2125. Dashed is the hypothetical current state (2009). Two extreme initial ground temperature conditions were considered leading to full black and grey lines (with or without initial permafrost close to seashore). From Roux et al. 2012
Coupled groundwater-heat transport model - Sisyphe, Paris VI
2012 showed the completion of both physical experiments and associated numerical modelling carried out at Sisyphe (A. Rivière and A. Jost, UMR CNRS 7619, University Pierre et Marie Curie Paris VI), in collaboration with Cerege (J. Gonçalvès, UMR CNRS 7330, University Aix-Marseille) and M2C (M. Font-Ertlen, UMR CNRS 6143, University Caen Basse-Normandie). It allowed the comparison between observed and simulated temperature and pressure over a freeze-thaw cycle at the sand box scale. It focused in particular on the generated overpressures in the subpermafrost aquifer (figure 6).
Figure 6: Results from laboratory experiments and numerical modelling: recorded and simulated subpermafrost hydraulic head response during a freeze-thaw cycle.
The coupled groundwater-heat transport model is now applied on various time and space scales to analyse groundwater-permafrost interactions: (i) at the catchment scale and in the present warming context, to quantify groundwater-river exchanges in periglacial areas subject to thawing permafrost, (ii) at the basin scale, to assess the long-term impact of past permafrost on hydrodynamics in large aquifer system such as the Paris basin (figure 7).
Figure 7: Model results on a cross-section in the Paris basin, France: (a) temperature distribution at 20 ka BP and (b) hydraulic head anomalies (20 ka BP minus initial head difference).