Réf. Bodin & al. 2009 - A

Référence bibliographique complète

BODIN, X., THIBERT, E., FABRE, D., RIBOLINI, A., SCHOENEICH, P., FRANCOU, B., REYNAUD, L., FORT, M. 2009. Two decades of responses (1986-2006) to climate by the Laurichard Rock Glacier, French Alps. Permafrost and Periglacial Processes, 20 (4), p. 331-344.

Abstract: The Laurichard active rock glacier is the permafrost-related landform with the longest record of monitoring in France, including an annual geodetic survey, repeated geoelectrical campaigns from 1979 onwards and continuous recording of ground temperature since 2003. These data were used to examine changes in creep rates and internal structure from 1986 to 2006. The control that climatic variables exert on rock glacier kinematics was investigated over three time scales. Between the 1980s and the early 2000s, the main observed changes were a general increase in surface velocity and a decrease in internal resistivity. At a multi-year scale, the high correlation between surface movement and snow thickness in the preceding December appears to confirm the importance of snow cover conditions in early winter through their influence on the ground thermal regime. A comparison of surface velocities, regional climatic datasets and ground sub-surface temperatures over six years suggests a strong relation between rock glacier deformation and ground temperature, as well as a role for liquid water due to melt of thick snow cover. Finally, unusual surface lowering that accompanied peak velocities in 2004 may be due to a general thaw of the top of the permafrost, probably caused both by two successive snowy winters and by high energy inputs during the warm summer of 2003.

Mots-clés
Rock glacier; Surface kinematics; DC resistivity; Climate controls; French Alps

Organismes / Contact

• UMR 8586 PRODIG, University of Paris-Diderot Paris 7, Paris, France
• Institut de Géographie Alpine, Joseph Fourier University, Grenoble, France - xbodin@gmail.com
• Cemagref, UR ETGR, Grenoble, France
• Parc National des Ecrins, Gap, France
• Conservatoire National des Arts et Métiers, Paris, France
• Dipartimento di Scienze della Terra, University of Pisa, Pisa, Italy
• LTHE, CNRS/ Joseph Fourier University, Grenoble, France
• IRD, Quito, Ecuador
• LGGE, CNRS/ Joseph Fourier University, Grenoble, France


(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Temperature Permafrost (rock glacier)    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
France Combeynot massif (Hautes-Alpes) Combe de Laurichard catchment / rock glacier North-facing slope 2450-2650m 1986- 2006.

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations

The meteorological station data show that air temperatures in the region increased by 1.3°C over the 20th century, following the general trend in the Alps (Casty et al., 2005). A major break occurred after 1984, when the mean annual temperature increased by 1 to 2°C, with the spring and summer seasons experiencing the strongest increase. Total precipitation recorded at the meteorological stations increased by 24–48mm between 1960 and 2006, but no clear trend was present.

Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations

An increase in the ablation flux at the Sarennes glacier (Thibert et al., 2008), starting in the mid-1980s is highly correlated with the regional increase in temperature and was estimated to be 11–20 W/m² (Vincent et al., 2004). Winter accumulation on the Sarennes glacier increased suddenly in 1977. December snow depths measured at the closest high elevation site (La Toura, 2590m asl) also indicate an increase over the period 1983–2000, and snow in this month can significantly influence the ground thermal regime.

Temporal Change in Laurichard rock glacier (RGL1) Surface Movement:
Mean annual velocities measured longitudinally range from 0.39 to 1.44 m/yr, and are typical for surface movements induced by deep-seated creep of ice-rich debris (Barsch, 1996; Haeberli et al., 2006). [...] The mean velocity of the RGL1 increased from 0.48 m/yr (σ = 0.12) in 1986 to a maximum of 1.2 m/yr (σ = 0.32) in 2004. Four main phases are recognised: (1) low velocities of about 0.5 m/yr from 1986–91; (2) a marked acceleration up to 1.0 m/yr from 1991–97; (3) fluctuating high velocities up to 1.2 m/yr from 1997–2004; and (4) a deceleration to 0.7 m/yr from 2004–06. These phases appear synchronous with regional air temperature variations. Although geodetic measurements were not carried out annually prior to 1999, it appears that interannual variability observed after this year is significantly lower than the multi-year change beforehand. Hence, the increase in rock glacier surface velocities between 1986 and 1999 may be linked to increasing air temperatures, the greatest velocities of 1999–2004 are synchronous with the highest air temperatures, and the decline in velocity after 2004 is associated with falling air temperatures. Similar observations have been made at rock glaciers elsewhere in the European Alps (Roer et al., 2005; Kääb et al., 2007; Delaloye et al., 2008).

Changes in Internal Structure:
There was a general decrease in the maximum apparent resistivity (ρamax) at the VES sounding sites located at the root and on the tongue from 1986 to 2004–06. At the root, ρamax decreased by 35–50% from 8.105 Ω.m in 1986 to 3–4.105 Ω.m 18 years later. At the tongue, ρamax decreased by 20–50% from 5.104 Ω.m in 1986 to 1.5–4.104 Ω.m in 2004. The relatively large variations in resistivity over the period 1998–2006 suggest that it may be difficult to distinguish resistivity changes due to long-term modification of the permafrost (e.g. a thickening of the active layer, an increase in ground temperature, a decrease in ice content) from those relating to seasonal changes in ground properties (e.g. differing water contents during the sounding campaigns in the active layer and/or within the icy layer), and/or localised changes in the internal structure related to permafrost deformation. Despite these limitations, the diachronic VES measurements can be interpreted using inversion models to depict hypothetical vertical changes in structures composed of homogeneous horizontal layers. Taking into account the infinity of solutions and the well-established electrical properties of similar ground materials (Fabre and Evin, 1990; Hauck, 2001; Delaloye, 2004), changes at the root can be interpreted as being due to thickening of the active layer from 1–2m to 2–3m with no noticeable modification of the icy layer. In contrast, those at the tongue could represent a thickening of the active layer and a lower ice content, and/or a thinning of the icy layer. These results are less reliable, however, due to strong lateral variations of the internal structure which are not ideal for layered modelling.

Relations between Velocities, Ground Surface Temperatures and Climate:
The temperature at the front of the RGL1 is thought to be close to 0°C based on a MAAT of 0.6°C (1975–2006) extrapolated for the tongue and potential global solar radiation in summer around 200W/m². These topoclimatic conditions also suggest that the seasonal supply of liquid water from rainfall or snowmelt inside the rock glacier may influence its deformation. The possible relationships between climate and rock glacier kinematics were examined at three different time spans: 1986–2006, 2000–06 and 2003–04.

The influence of air temperature on rock glacier deformation was first examined by correlating surface velocity with Monêtier monthly air temperature during the months preceding geodetic measurements. The highest Bravais-Pearson coefficient of correlation was observed for the preceding December when a relation significant at p = 0.02 and R² = 0.69 existed for two blocks. However, this statistical pattern is strongly influenced by extreme events (especially two very cold winters).

There was no clear relationship between weather station precipitation or annual snow accumulation on the Sarennes glacier and rock glacier velocity, but there was a significant correlation between snow depth measured in December at La Toura station (2590m asl, 16km to theW) and velocity (R² = 0.71) prior to 2000 when the record ended. It appears that rock glacier creep was enhanced by a thicker December snow cover, probably because of the importance of early winter snow conditions on internal temperatures and hence rock glacier movements (e.g. Haeberli et al., 2006).

In addition, movements of the rock glacier from 2000–06 can be correlated in detail with precipitation parameters and ground surface temperatures. These interpretations both support and are supported by the longer term relations outlined above for 1986–2000:
• Precipitation in October, November and March 2000–01 exceeded monthly normals (1960–91) by 125–180mm, and snow accumulation on the Sarennes glacier was 630mm w.e. above the 1984–2006 mean during winter 2000–01. The early, thick snow cover and its melt in spring and early summer would have, respectively, limited winter cooling of the ground and provided large amounts of liquid water into the rock glacier body, both of which may have caused the high measured rock glacier velocities.
• In contrast, movement slowed in 2001–02 when insulating snow was sparse: precipitation was less than normal and total winter accumulation of 1040mm w.e. on the Sarennes glacier was 750mm w.e. below the 1984–2006 mean.
• The 2004 peak in velocity may have been caused by two successive winters with a thick, early snow cover and the very warm summer of 2003 in between (see below).
• Slowing again occurred from 2004–06 and is linked to the lack of snow and cold air temperatures of 2005 and 2006 which led to a decrease in ground surface temperatures on the RGL1.

Finally, 2004 was not only characterised by a peak in velocity, but also by unusual behaviour of the RGL1 surface. In other years, vertical changes show good agreement with changes in the inter-block distance: compression at the tongue is accompanied by rising up of the surface while extension at the root leads to surface lowering. However, in 2004, lowering occurred over almost the entire surface (mean of 100 mm), even in the compressive parts of the rock glacier. This may have been a response to: (1) a topographically controlled change in flow direction of the tongue towards the orographic right, as mentioned above (Bodin et al., 2008); or (2) a climatically controlled melt of ice in the body of the rock glacier, especially at the permafrost table (Lambiel and Delaloye, 2004). In the absence of adequate geodetic data to fully assess the direction of flow, only the second hypothesis is discussed further.

Surface mass-balance measurements on the Sarennes glacier show that ice ablation was 1.90m w.e. higher in summer 2003 than the 1954–2002 average (Thibert et al., 2008), corresponding to an increase of about 42–65 W/m² of surface melt flux (Vincent et al., 2007). The potential melt of 0.1m of ice within the RGL1 would have required only a surplus of atmospheric power flux of 2.3 W/m² over a period of five snow-free months with an additional 0.03 W/m² if it is assumed that the ice temperature was -28C rather than 0°C. If this hypothesis is correct, lowering must have taken place after the summer 2003 survey (carried out on 27 August 2003), which had not shown any significant change. This indicates that lags of several weeks or months are involved, implying that the influence of snow cover during 2003–04 might also have added to that of the warm air temperature of summer 2003.

Discussion:
The long-term datasets analysed in this study suggest a synchronous increase in surface velocity and DC resistivity between the mid-1980s and the mid-2000s. Both parameters may relate to an increase in permafrost temperatures, which would be coherent with the increase from around –2.5°C to –1.5°C at the depth of 11.6m in the Murtèl borehole over the same period (Harris et al., 2009). However, the Murtèl rock glacier is estimated to be between 400 and 600m above the regional 0°C isotherm (Hanson and Hoelzle, 2004), whereas the RGL1 is located very close to the same isotherm, raising the crucial question of potential degradation of its frozen core. As demonstrated by Kääb et al. (2007), Ladanyi (2006) and Arenson (2002), at an annual time scale the deformation rate of ice-rich permafrost is probably mainly related to its thermal state, which itself represents a complex transient response to changes in climatic parameters (mainly air temperature) and thickness and history of snow cover (first appearance during early winter, development of a thick mantle, melting duration). Where permafrost is close to (or at) the point of thaw, the role of liquid water may also become important (Hausmann et al., 2006; Ikeda et al., 2008), by either changing the thermal properties of the ground (role of latent heat), by reducing friction between particles, or allowing/increasing basal sliding on a water film.

It appears possible that temperatures in the RGL1 are not in equilibrium with the current climate conditions, and this may also be the case for other rock glaciers in the southern Alps. In the Argentera Massif, Italy (around 44°'N) for example, geophysical investigations and ground thermal monitoring indicate that permafrost at elevations of 2450–2550m asl is probably close to thawing (Ribolini and Fabre, 2006). Furthermore, Krysiecki et al. (2008) recently reported destabilisation of the Bérard rock glacier in the southern French Alps, similar to the cases described by Roer et al. (2008) but which finally ended in collapse of the rock glacier during summer 2006. According to Roer et al. (2008), this likely results from higher internal deformation and changes in shearing and basal sliding that relate to modifications of the rheological properties of warming ice.

Although the RGL1 is probably not in a suitable topographic context for major destabilisation, its location at a relatively lowelevation in a Mediterranean influenced climate likely makes it representative of changes being experienced by other rock glaciers in the southern Alps. Therefore, besides its continued long-term monitoring, future efforts should be focused on: (1) characterisation of its internal structure in terms of ice and water content and thermal state; (2) monitoring of relevant parameters controlling the ground thermal state, including snowpack development and melt, water seepage in the ground and thermal effect of the coarse debris layer; and (3) survey of its deformation, possibly combining high temporal resolution with seasonal geodetic measurements and high spatial resolution with a laser scanning campaign every three to five years.

Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

The high thermal inertia of permafrost and the slow motion of rock glaciers (typically decimetres per year) lead the latter to react to decadal to centennial climatic trends (Haeberli et al., 2006). However, recent observations also suggest that the sensitivity of rock glaciers to climate may increase when ground temperatures approach 0°C (Kääb et al., 2007), and this is partially confirmed by studies of inter-annual fluctuations in the movements of surveyed rock glaciers (Roer et al., 2005; Hausmann et al., 2006; Delaloye et al., 2008; Ikeda et al., 2008).

The study of rock glaciers, and especially of the relations between their thermal and mechanical dynamics and climate, requires annual monitoring over the long term. This includes ground temperature measurements in boreholes, or at shallower depths (Delaloye, 2004; Lambiel, 2006; Bodin, 2007) which may indicate how the thermal state of permafrost is reacting to climatic warming (Harris et al., 2003). The deformation of ice-rich permafrost on mountain slopes, which is thought to be partially controlled by the thermal state of permafrost (Arenson et al., 2002; Ladanyi, 2006; Kääb et al., 2007), can be examined using remote sensing (for a synthesis, see Kääb, 2005) or in situ geodetic methods (e.g. Lambiel and Delaloye, 2004).

In the French Alps, data collected over two decades on and around the Laurichard rock glacier (RGL1) are invaluable for examining this Alpine landform, which is located close to the lower limit of discontinuous permafrost (Bodin, 2007).

Datasets from the four closest meteorological stations (Briançon, Monêtier-les-Bains, Saint-Christophe-en-Oisans, La Grave, located, respectively, at 1324, 1459, 1570 and 1780m asl, and at 20, 18, 18 and 8 km from the study site) for the 1960–91 period, mean winter snow accumulation (October to May) recorded from 1960–2006 on the Sarennes Glacier (located 20-km west) and measurements of snowpack thickness in the Combe de Laurichard between 1979 and 1986 (Francou, 1988) were used to analyse the main characteristics of the regional climate.

A geodetic survey was initiated in 1979 by Francou (1981) to regularly monitor surface velocities of the RGL1 along longitudinal and transverse transects. The survey was carried out every two to three years from 1979 to 1999, and subsequently every year. Annual velocity (downslope, vertical and horizontal), actual vertical change after removing total displacement of the block downslope and variation in inter-block distance can be calculated from these data.

A total of 12 vertical electrical soundings (VES) and two electrical resistivity tomography (ERT) profiles have been undertaken on the RGL1 to investigate its internal structure: two VES in 1986 (Francou and Reynaud, 1992), five VES in 1998 (V. Jomelli and D. Fabre, unpublished work), four VES and two ERT in 2004, and one VES in 2006.

Seven temperature loggers, installed just below the blocky surface to protect them from direct solar radiation, have been recording bi-hourly since October 2003. Ground surface temperatures recorded from 2000–04 in a rock glacier in Valais (140-km north in the Swiss Alps, 2500m asl, see Delaloye et al., 2008) were used to help assess evolution of the thermal regime in the Laurichard area.

The possible relationships between climate and rock glacier kinematics were examined at three different time spans: 1986–2006, 2000–06 and 2003–04.


(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

 


(4) - Remarques générales
 

(5) - Syntèses et préconisations

Conclusions:
The two main datasets of surface velocity measurements and geoelectrical soundings enabled to characterise the internal structure and movement of the Laurichard rock glacier (RGL1) and to examine relationships between its dynamics and climate. First, it appears that surface velocities increased during the 1990s and then decreased after 2004, as has been noted for other rock glaciers in the Alps. Second, between 1986 and 2006, a decrease in resistivity and changes in the resistivity curves suggest that changes in the internal structure of the RGL1 took place. However, a potential thickening of the active layer and reduction in ice content cannot be fully confirmed in the absence of other geophysical and/or mechanical investigations. Both the increase in surface velocity and the decrease in resistivity can, nevertheless, be linked to a hypothesised increase in permafrost temperature.

The influence of snowpack development in early winter on surface velocity was apparent for the 1986–2006 period. In the last six years of the record, interannual variability of the latter tracked ground surface temperature, which itself is related to air temperature and snow conditions. A large amount of meltwater due to a thick snow cover may also explain the peaks in velocity in 2001 and 2004. However, 2003–04 was unusual not only because of the peak in velocity, but also because it was accompanied by a generalised lowering of the rock glacier surface attributed to high surface energy inputs causing the melt of 10 cm of ground ice.

Further geophysical investigations and direct observations of the RGL1 are required, particularly to ascertain if the permafrost is at the point of thaw. Such long-term monitoring is needed to understand the way that mountain permafrost in the southern Alps is responding to atmospheric forcing. It is especially important to examine further the short- to mid-term sensitivity of perennially frozen, but warming, debris deposits, as these may become hazardous if destabilised on steep terrain.

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