Rockglaciers genesis and growth in a degrading mountaincryosphere (Southern French Alps) [Monique Fort]

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Rockglaciers genesis and growth in a degrading mountaincryosphere (Southern French Alps). Presented by Monique Fort at the "Perth II: Global Change and the World's Mountains" conference in Perth, Scotland in September 2010.

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Rockglaciers genesis and growth in a degrading mountaincryosphere (Southern French Alps) [Monique Fort]

  1. 1. Rockglaciers genesis and growth in a degrading mountain cryosphere (Southern French Alps) E. Cossart1, M. Fort1, D.L. Bourlès2, R. Braucher2, J. Carcaillet3 (1) PRODIG, UMR 8586 – CNRS, Universités Paris 1 & Paris-Diderot (Paris 7), 2 rue Valette, F-75005 France (2) CEREGE, UMR 6635 – CNRS, Université Aix-Marseille 3, Europôle Méditerranéen de lArbois, BP 80, 13545 Aix en Provence Cedex 04, France (3) LGCA, Laboratoire de Géodynamique des Chaînes Alpines, Grenoble.Global Change and the World’s Mountains Perth, Scotland, September 26-30, 2010
  2. 2. Rockglaciers genesis and growth in a degrading mountain cryosphere (Southern French Alps) E. Cossart1, M. Fort1, D.L. Bourlès2, R. Braucher2, J. Carcaillet3 (1) PRODIG, UMR 8586 – CNRS, Universités Paris 1 & Paris-Diderot (Paris 7), 2 rue Valette, F-75005 France (2) CEREGE, UMR 6635 – CNRS, Université Aix-Marseille 3, Europôle Méditerranéen de lArbois, BP 80, 13545 Aix en Provence Cedex 04, France (3) LGCA, Laboratoire de Géodynamique des Chaînes Alpines, Grenoble.OUTLINE• Issue: deglaciation and rockglacierdevelopment vs climate change• Studied area• Methods• Results: chronology of deglaciation vsrock glaciers development• Discussion: control factors and significanceat the regional scaleGlobal Change and the World’s Mountains Perth, Scotland, September 26-30, 2010
  3. 3. Introduction - Glacial and periglacial features widespread in European Alps  potential record of climatic and environmental change But…  small number of valuable ages of both glacial and periglacial landforms  complex significance of some landforms (i.e. rock-glaciers)  What alpine landscape evolution since the Last Glacial Maximum?  Patterns of rock-glacier genesis? - Study area : Briançonnais area, Southern French Alps  very significant deglaciation pattern of the French Alps since the LGM  many rock-glaciers identified, pristine or fresh (Evin 1987, Francou 1988) But debates on…  The Late-Glacial glaciation pattern: valley or cirque glaciation?  Age of rock-glaciers? mostly assumed to be Late-Glacial features  Application of numerical (CRE ages) and relative dating methods (weathering rind thickness)
  4. 4. Introduction - Glacial and periglacial features widespread in European Alps  potential record of climatic and environmental change But…  small number of valuable ages of both glacial and periglacial landforms  complex significance of some landforms (i.e. rock-glaciers)  What alpine landscape evolution since the Last Glacial Maximum?  Patterns of rock-glacier genesis? - Southern French Alps:  Very significant deglaciation pattern since the LGM  Many rock-glaciers identified, pristine or fresh (Evin 1987, Francou 1988) But debates on…  The glaciation pattern during the Late-Glacial: valley or cirque glaciation ?  Age of rock-glaciers? mostly assumed to be Late-Glacial features  Application of numerical (CRE ages) and relative dating methods (weathering rind thickness)
  5. 5. Study area: Briançonnais - Former Durance glacier during the Last Glacial Maximum (LGM):  one of the major valley glacier in the French Alps (more than 100 km long)  trunk glacier receiving most of the glaciers of the Southern French Alps  Ice thickness reaching at least 1000 metres Mt Blanc in the upper part of the watershed (Briançon) After Campy & Buoncristiani; in Ehlers and Gibbard (2004)
  6. 6. Northern part: Clarée valley Clarée glacier during the LGM  Right-bank tributary of former Durance glacier  Ice at least 850 metres thick at the confluence north of Briançon Durance glacier
  7. 7. Current permafrost andglacier extent in theClarée valley- Lower Limit of Permafrost(LLP):  Possible permafrost: 2510m  Probable permafrost: 2700m(in Cossart et al., 2008)- Glacier extent?  no more glacier  Regional Equilibrium LineAltitude (Vallouise): 3200m E 3150 m W Large extent of « periglacialbelt »
  8. 8. Methods (1): Field mapping Identification of former glacier remnants  relative chronology of deglaciation  Morainic ridges: position of former glacial fronts  Roches-moutonnées: reconstruction of minimal thickness of valley glacier Identification of erratics, cf. lithological Quartzite Rock-bar contrasts Erratics (dolomite) Moraine made of dolomites
  9. 9. Methods (1): Field mapping Identification of former glacier remnants  relative chronology of deglaciation  Morainic ridges: position of former glacial fronts  Roches-moutonnées: reconstruction of minimal thickness of valley glacier Identification of erratics, cf. lithological Quartzite Rock-bar contrasts Inventory and description of rock- glaciers (RG)  Geometric extent  Subdued vs. fresh landforms Moraine made of dolomites Assumption: Rock Glacier development after glacier decay
  10. 10. Methods (2): Relative chronology Position of morainic landforms  External ones  Older  Internal ones  Younger Characterization of the freshness/ activity of rock-glaciers  Vegetation extent  Steepness of the front Measurements of weathering rinds (on sandstone boulders)  Yellow to red oxydation cortex (5YR to 10R)  Samples on top of ridges (avoid late snow influence; maximize stability of boulders)  15 to 20 samples for each site  5 to 10 thickness measurements for each sample (accuracy 0.5 mm)
  11. 11. Methods (3): Cosmogenic Ray Exposure (CRE) dating, 10BeSampling strategy  Sampling on stable, roches-moutonnées surfacesAssessment of:- retreat of glacier front  Sampling at various sites from down- toupvalley- ice-thickness lowering  Sampling along cross-sections of valley slopesCRE clock is set when therock-bar is free of eitherice or till-coverSampling:• On the edge (toss side) ofrock-bars• Next to the steep lee sideof rock-bars• On surfaces affected bystriae (avoid rejuvenatedsurfaces)
  12. 12. Results (1): 3 post-LGM glacial stages Stage 1: Most external moraines identified ≈150 m below the trimline (yet post- LGM) = lateral moraines = valley glaciation Stage 2: frontal moraines, located at the outlet of cirques  Incipient cirque glaciation Stage 3: small frontal moraines at the foot of cirque faces  last stage before complete deglaciation
  13. 13. Younger 3 generations of rock-glaciers Class 3  15 small samples: 103 to 103 m3  Front elevation: above 2600 m.a.s.l  no vegetation, water at 0.2-1°C  Active landforms (creeping screes) Class 2  14 samples  Volume: 103-104 m3  Front altitude: 2500m  Symptoms of degradation, yet water seepages at 0.2-1.5°C Class 1  2 samples: 104 m3  Front altitude: 2380 m  Completely vegetatedOlder  Relict landforms
  14. 14. Relative scenario Most recent features Younger features Oldest features Thinner weathering rinds Weathering rinds reach their maximal value
  15. 15. CRE dating: 1 Late Glacial stage, 2 Holocene stages Disappearance Cosmic ray exposure : of valley glacier  2 main stages identified LGM  LGM between 25 and 30 ka BP (cf. trimline) Cirque Glaciation  Preservation of a small glacier in Vallouise tongue during the Late Glacial (between 9 - 11 ka BP)  Cirque glaciation after 8 ka BP Vallouise Valley :  same pattern as in Clarée valley  Cirque glaciation at the beginning of the Holocene
  16. 16. Probable scenario LIA features Age younger than 5.0 ka CRE Age = 11.0 ka Late Glacial features  Second half of the Holocene CRE Age = 7.9 ka
  17. 17. Synthesis and interpretationGlacier variation: A re-assessment of the Late-Glacialperiod:  A low ELA (~ 2200 m.a.s.l)  Persistance of a valley glacier YD 2 stages of cirque glaciation during theHolocene:  Stage 2 = probably Subboreal  Stage 3 = LIARock-glacier chronology: Class 1: only a few Late-Glacialfeatures Class 2: main stage of rock-glacierdevelopment during the second half ofthe Holocene A general, altitudinal shift of RG:indicator of a rise of the Lower Limit ofPermafrost ?
  18. 18. Climatic vs. geomorphicsignificance of Rock-Glaciers Class 1 rock-glacier Deglaciated plateauClass 1 rock-glaciers: only 2 samples Rock-glacier development possibleon an early deglaciated plateau (ice-free during the Late Glacial) Disconnectivity between thedeglaciated plateau and the Late-Glacial tongue Limited debris source no evacuation of RG debrisClass 3 rock-glaciers: 15 small active samples permafrost conditions Rock-glacier development hamperedby limited debris sources and supply The altitude of RG front reflects -a Small creeping featuresminima- the extent of permafrost belt
  19. 19. Conclusions Climatic vs. Geomorphic significance of rock-glaciers - Three generations of rock-glaciers but: - Late Glacial: development of rock-glaciers limited by lack of deglaciated areas - Present: growth of rock-glacier limited by lack of debris supply - Most rock-glaciers probably developped during the second half of the Holocene (Subboreal?)  Location and age of RG depend not only on climatic but also on glaciological and geomorphic conditions CRE results provide new constrains on glacial retreat in SouthwesternFrench Alps: - Useful method for glacial erosional landforms dating - Our data not in agreement with the hypothesis of an early retreat of glaciers within the whole Southern French Alps - valley Clarée glacier front at altitude lower than 2000 m.a.s.l. during the Late- Glacial - major stage of recession: only at the beginning of the Holocene (as in the Western valleys)
  20. 20. Conclusions Climatic vs. Geomorphic significance of rock-glaciers - Three generations of rock-glaciers but: - Late Glacial: development of rock-glaciers limited by lack of deglaciated areas - Present: growth of rock-glacier limited by lack of debris supply - Most rock-glaciers probably developped during the second half of the Holocene (Subboreal?), i.e. earlier than previously thought  Location and age of RG depend not only on climatic but also on glaciological and geomorphic conditions CRE data provided new constrains on glacial retreat in SouthwesternFrench Alps: - Useful method for glacial, erosional landforms dating - Our results: not in agreement with the former hypothesis of an early retreat of glaciers within the whole Southern French Alps - valley Clarée glacier front: still at elevation lower than 2000 m.a.s.l. during Late- Glacial - major stage of recession: only at the onset of Holocene (as in the Western Alpine valleys)
  21. 21. Thank you for your attentionFirst author Etienne COSSART For further details, see COSSART, FORT & al., Catena 80 (2010) 204-21

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