Hormes & al. 2001 The Alps with little ice...

Référence bibliographique complète

HORMES A., MULLER B.U., SCHLUCHTER C. The Alps with little ice: evidence for eight Holocene phases of reduced glacier extent in the Central Swiss Alps. The Holocene, 2001, Vol. 11, p. 255-265.

Abstract: Glacially deformed pieces of wood, organic lake sediments and clasts of reworked peat have been collected in front of Alpine glaciers since ad 1990. The palaeoglaciological interpretation of these organic materials is related to earlier phases of glacier recession surpassing that of today's shrunken glaciers and to tree growth and peat accumulation in the valleys now occupied by the glaciers. Glacial transport of the material is indicated by wood anatomy, incorporated silt, sand and gravel particles, missing bark and deformed treerings. A total of 65 samples have been radiocarbon dated so far, and clusters of dates provide evidence of eight phases of glacier recession: 9910-9550, 9010-7980, 7250-6500, 6170-5950, 5290-3870, 3640-3360, 2740-2620 and 1530-1170 calibrated years BP. Allowing for the timelag between climatic fluctuations, glacier response and vegetation colonization, these recession phases may lag behind climatic changes by 100-200 years.

Glacier variations, glacier retreat phases, minimal glacier extension, radiocarbon dating, climatic change, Holocene, Swiss Alps.

Organismes / Contact

Quartärgeologie, Geologisches Institut, Baltzerstrasse 1, CH-3012, Bern, Switzerland.
hormes@anpa.it

(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

Massif / Secteur

Site(s) d'étude

Période(s) d'observation

(1) - Modifications des paramètres atmosphériques

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

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

The investigated glaciers were smaller-than-today during the phases which are represented by the radiocarbon dates, the subfossil samples having been melted out of glaciers following subglacial and englacial transport. The radiocarbon dates do not indicate a random distribution over the Holocene but form eight clusters indicating phases of glaciers contraction and climatic amelioration with development of vegetation at higher elevations and glaciers smaller than present. These cluster recession phases are: 9910–9550, 9010–7980, 7250–6500, 6170–5950, 5290–3870, 3640–3360, 2740–2620 and 1530–1170 cal. yr BP.

The two oldest samples (9910–9550 cal. yr BP) were found at Riedgletscher in the Valais. Five clusters are represented in the western Alps (Valais): 9910–9550, 8980–8060, 6870–6500, 6170–5950 and 1530–1420 cal. yr BP. At Unteraargletscher six of the clusters are represented: 9010–7980, 7150–6570, 5290–3870, 3640–3360, 2740–2620, 1260–1170 cal. Yr BP. One data cluster is indicated in the Bernina Massif between 7250 and 6950 cal. yr BP. The youngest cluster has been found at two different sites in the Valais and Berner Oberland between 1530 and 1170 cal. yr BP. The cluster between 6170 and 5950 cal. Yr BP is only represented at Riedgletscher. One sample has a modern age: a wood sample from Glacier du Trient.

Considering the start of tree growth after glacier recession of approximately 100 years, a moderate climate may have existed prior to data points and periods of smaller-than-today glaciers should therefore be prolonged by some 100–200 years.

The phases of glacier recession are consistent with other palaeoclimatic data sets. Some previously published data should be reconsidered in the light of the new data set: the Misox cold period between 8100 and 7400 cal. yr BP; the glacier advances Rotmoos and Piora I and II around 6100–5700 and 5500–5000 cal. yr BP; and also the Löbben cold period recorded in the Austrian and Swiss Alps. There is also some agreement between the reconstructed phases of glacier recessions obtained here and glacier behaviour beyond the Alps, and especially in Scandinavia.

Sensibilité du milieu à des paramètres climatiques

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

Organic material, including pieces of peat and wood of trees, occurs in basal shear planes of the glacier and in the proglacial outwash. Several indications are useful for identifying wood that has undergone glacial transport, including: (1) alteration of wood anatomy with compressed cell walls, in some cases, destroyed or wavelike; (2) gravel particles pressed into the wood clast; (3) missing bark; and (4) deformed tree-rings. The authors conclude that the clasts of fossil wood are primarily evidence for englacial and subglacial transport and that trees were not transported by avalanches or humans to the places of sampling.

Subfossil wood and peat samples from six glacier forelands in the Central Swiss Alps have been collected, and 65 samples have been radiocarbon dated (11 samples were multi-dated). The glaciers considered here are: Unteraar (Central Swiss Alps), Mont Miné (Valais), Tschierva and Forno (Bernina Massif), Ried (Mischabel group), and Trient (Mont Blanc Massif). Given the locations of the samples, the authors are considering organic sedimentation and tree growth in the basins now occupied by the glaciers. In order to constrain the time of advance, only the outer 5–10 tree-rings without bark were dated.

All conventional radiocarbon measurements reported here were carried out using low activity copper proportional counters filled with CH4 (Fairhall et al., 1961). Screening techniques and the method of proportional gas counting are described in previous work (Loosli et al., 1980; Geyh and Schleicher, 1990). Radiocarbon dates are reported in radiocarbon years Before Present (yr BP, where the present is defined as ad 1950). All calibrations of radiocarbon dates (expressed as cal. yr BP) were made using Calib 4.1 by Stuiver and Reimer (1999) based on the calibration data set of Stuiver et al. (1998).

(3) - Effets du changement climatique sur l'aléa

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

(1) - Modifications des paramètres atmosphériques
Reconstitutions
Observations
Modélisations
Hypothèses

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions	The investigated glaciers were smaller-than-today during the phases which are represented by the radiocarbon dates, the subfossil samples having been melted out of glaciers following subglacial and englacial transport. The radiocarbon dates do not indicate a random distribution over the Holocene but form eight clusters indicating phases of glaciers contraction and climatic amelioration with development of vegetation at higher elevations and glaciers smaller than present. These cluster recession phases are: 9910–9550, 9010–7980, 7250–6500, 6170–5950, 5290–3870, 3640–3360, 2740–2620 and 1530–1170 cal. yr BP. The two oldest samples (9910–9550 cal. yr BP) were found at Riedgletscher in the Valais. Five clusters are represented in the western Alps (Valais): 9910–9550, 8980–8060, 6870–6500, 6170–5950 and 1530–1420 cal. yr BP. At Unteraargletscher six of the clusters are represented: 9010–7980, 7150–6570, 5290–3870, 3640–3360, 2740–2620, 1260–1170 cal. Yr BP. One data cluster is indicated in the Bernina Massif between 7250 and 6950 cal. yr BP. The youngest cluster has been found at two different sites in the Valais and Berner Oberland between 1530 and 1170 cal. yr BP. The cluster between 6170 and 5950 cal. Yr BP is only represented at Riedgletscher. One sample has a modern age: a wood sample from Glacier du Trient. Considering the start of tree growth after glacier recession of approximately 100 years, a moderate climate may have existed prior to data points and periods of smaller-than-today glaciers should therefore be prolonged by some 100–200 years. The phases of glacier recession are consistent with other palaeoclimatic data sets. Some previously published data should be reconsidered in the light of the new data set: the Misox cold period between 8100 and 7400 cal. yr BP; the glacier advances Rotmoos and Piora I and II around 6100–5700 and 5500–5000 cal. yr BP; and also the Löbben cold period recorded in the Austrian and Swiss Alps. There is also some agreement between the reconstructed phases of glacier recessions obtained here and glacier behaviour beyond the Alps, and especially in Scandinavia.
Observations
Modélisations
Hypothèses

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

Pays / Zone	Massif / Secteur	Site(s) d'étude	Exposition	Altitude	Période(s) d'observation
Switzerland	Alps	6 glaciers			Holocene