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

l'Arbois, 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

OUTLINE

• Issue: deglaciation and rockglacier

development vs climate change

• Methods

• Results: chronology of deglaciation vs

rock glaciers development

• Studied area

• Discussion: control factors and significance

at the regional scale

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

l'Arbois, 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

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)

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)

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

in the upper part of the watershed (Briançon)

After Campy &

Buoncristiani; in

Ehlers and

Gibbard (2004)

Mt Blanc

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

Current permafrost and

glacier extent in the

Clarée valley

- Lower Limit of Permafrost

(LLP): Possible permafrost: 2510m

Probable permafrost: 2700m (in Cossart et al., 2008)

- Glacier extent?

no more glacier

Regional Equilibrium Line

Altitude (Vallouise): 3200m

Large extent of « periglacial

belt »

3150 m E W

Methods (1): Field mapping

Identification of

erratics, cf. lithological

contrasts

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

Quartzite Rock-bar

Moraine made of

dolomites

Erratics (dolomite)

Methods (1): Field mapping

Identification of

erratics, cf. lithological

contrasts

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

Quartzite Rock-bar

Moraine made of

dolomites

Inventory and

description of rock-

glaciers (RG)

Geometric extent

Subdued vs. fresh

landforms

Assumption: Rock Glacier

development after glacier

decay

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)

Methods (3): Cosmogenic Ray Exposure (CRE) dating, 10Be

Assessment of:

- retreat of glacier front

Sampling at various sites from down- to

upvalley

- ice-thickness lowering

Sampling along cross-sections of valley slopes

CRE clock is set when the

rock-bar is free of either

ice or till-cover

Sampling:

• On the edge (toss side) of

rock-bars

• Next to the steep lee side

of rock-bars

• On surfaces affected by

striae (avoid rejuvenated

surfaces)

Sampling strategy Sampling on stable, roches-moutonnées surfaces

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

Class 2

14 samples

Volume: 103-104 m3

Front altitude: 2500m

Symptoms of

degradation, yet water

seepages at 0.2-1.5°C

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 1

2 samples: 104 m3

Front altitude: 2380 m

Completely vegetated

Relict landforms Older

Younger

Oldest features

Weathering rinds reach their maximal value

Younger features

Thinner weathering rinds

Relative scenario

Most recent

features

CRE dating: 1 Late Glacial stage, 2 Holocene stages

Cosmic ray exposure :

2 main stages identified

LGM between 25 and 30 ka BP

(cf. trimline)

Preservation of a small glacier

tongue during the Late Glacial

(between 9 - 11 ka BP)

Cirque glaciation after 8 ka BP

LGM

Disappearance

of valley glacier

Cirque Glaciation

in Vallouise

Vallouise

Valley :

same

pattern as in

Clarée valley

Cirque

glaciation at

the beginning

of the

Holocene

Late Glacial features Age younger than 5.0 ka

Second half of the Holocene

Probable scenario

CRE Age = 7.9 ka

LIA features

CRE Age = 11.0 ka

Synthesis and interpretation

Glacier variation:

A re-assessment of the Late-Glacial

period:

A low ELA (~ 2200 m.a.s.l)

Persistance of a valley glacier YD

2 stages of cirque glaciation during the

Holocene:

Stage 2 = probably Subboreal

Stage 3 = LIA

Rock-glacier chronology:

Class 1: only a few Late-Glacial

features

Class 2: main stage of rock-glacier

development during the second half of

the Holocene

A general, altitudinal shift of RG:

indicator of a rise of the Lower Limit of

Permafrost ?

Climatic vs. geomorphic

significance of Rock-Glaciers

Deglaciated plateau

Class 1 rock-glacier

Small creeping features

Limited debris source

Class 1 rock-glaciers:

only 2 samples

Rock-glacier development possible

on an early deglaciated plateau (ice-

free during the Late Glacial)

Disconnectivity between the

deglaciated plateau and the Late-

Glacial tongue

no evacuation of RG debris

Class 3 rock-glaciers:

15 small active samples

permafrost conditions

Rock-glacier development hampered

by limited debris sources and supply

The altitude of RG front reflects -a

minima- the extent of permafrost belt

Conclusions

CRE results provide new constrains on glacial retreat in Southwestern

French 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)

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

Conclusions

CRE data provided new constrains on glacial retreat in Southwestern

French 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)

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

Thank you for your attention

First author Etienne COSSART For further details, see COSSART, FORT & al., Catena 80 (2010) 204-219