Réf. Scapozza & al. 2009 - A

Référence bibliographique complète

SCAPOZZA, C., LAMBIEL, C., REYNARD, E., FALLOT, J.-M., ANTOGNINI, M., SCHOENEICH, P. 2010. Radiocarbon Dating of Fossil Wood Remains Buried by the Piancabella Rock Glacier, Blenio Valley (Ticino, Southern Swiss Alps): Implications for Rock Glacier, Treeline and Climate History. Permafrost and Periglacial Processes, 21, 90-96.

Abstract: Fossil wood stem remains of larch (Larix decidua) found 1m below the surface at the base of the front of the Piancabella rock glacier had a conventional age range of 845±50 14C y BP, corresponding to a calibrated calendar age range of 1040–1280 AD (790±120 cal BP) with a statistical probability of 95.4 per cent. Based on geomorphological, climatological and geophysical observations, [the authors] infer that (1) the treeline in the Medieval Warm Period was about 200m higher than in the middle of the 20th century, which corresponds to a mean summer temperature as much as 1.2°C warmer than in AD 1950, and (2) that ice within this rock glacier is probably several centuries old and so predates recent climatic events such as the Little Ice Age.

Mots-clés
Medieval Warm Period; Permafrost; Radiocarbon dating; Rock glacier; Swiss Alps; Treeline

Organismes / Contact

• Institute of Geography, University of Lausanne, Lausanne, Switzerland - cristian.scapozza@unil.ch
• Natural History Museum of the Canton Ticino, Lugano, Switzerland
• Institute of Alpine Geography, University of Grenoble, 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 Forest (treeline), Permafrost (rock glacier)    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Southern Switzerland Sceru Valley (eastern part of the Blenio Valley, Lepontine Alps, Ticino) Piancabella rock glacier   2480m a.s.l.  

(1) - Modifications des paramètres atmosphériques
Reconstitutions

From radiocarbon dating of fossil wood stem remains of larch found at the front of the Piancabella rock glacier and based on geomorphological, climatological and geophysical observations, the authors infer that the treeline in the Medieval Warm Period was about 200m higher than in the middle of the 20th century, which corresponds to a mean summer temperature as much as 1.2°C warmer than in AD 1950.

Observations
 
Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
[See below]

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

From radiocarbon dating of fossil wood stem remains of larch found at the front of the Piancabella rock glacier and based on geomorphological, climatological and geophysical observations, the authors infer that the treeline in the Medieval Warm Period was about 200m higher than in the middle of the 20th century, which corresponds to a mean summer temperature as much as 1.2°C warmer than in AD 1950:

Radiocarbon dating:
Radiocarbon dating of the fossil wood stem remains of larch (Larix decidua) found at the front of the Piancabella rock glacier gave a mean conventional age of 845±50 14C y BP. Calibration of the radiocarbon dating gave an age range of 1040–1280 cal AD (790±120 cal BP) with a statistical probability of 95.4 per cent.

The age range for the larch sample corresponds to the end of the Medieval Warm Period (MWP) (c. AD 800–900 to AD 1250–1300), a complex climatic period with frequent and intense oscillations of temperature, preceding the Little Ice Age (LIA) cool period (e.g. Davis et al., 2003; Mangini et al., 2005; Büntgen et al., 2006). According to the Great Aletsch glacier fluctuations reconstructed by Holzhauser et al. (2005), the period corresponds to a phase of general glacier recession in the north and in the south of the Alps, with frontal positions comparable to those in the late 20th century, or perhaps even further up-valley (Grove and Switsur, 1994).

Implications for Treeline and Climate History:
If a natural (i.e. non-anthropogenic) origin is assumed for the fossil wood, the latter’s position allows the minimum elevation of treeline to be inferred. [...] The maximum elevation attained by the treeline corresponds to mean summer temperature values of between 5.5 and 7.5°C and to a soil temperature of about 7°C (Körner, 1999; Paulsen, 2000). Treeline positions do not respond instantaneously to climatic change or to anthropogenic perturbations: for example, today’s treeline position (2150–2200m a.s.l.) is a complex result of the environmental dynamics during the past 150 years. Lags of 50–150 years after climate warming are documented in both palaeoecological and modelling studies (e.g. Bugmann and Pfister, 2000). Considering this, we can estimate that the natural potential treeline for the middle of the 20th century was between 2250 and 2300m a.s.l. In this case, the difference in elevation with the end of the MWP treeline inferred from the Piancabella rock glacier samples is about 200 m.With a local lapse rate of 0.62°C/100m (Bouët, 1985), the difference in elevation of the treeline corresponds to a difference in temperature of 1.2°C compared to the mid- 20th century (i.e. ~AD 1950). [...] The same result is obtained if we consider the mean monthly temperatures at the treeline elevation. Currently, the mean monthly temperatures at the potential treeline (about 2300m a.s.l.) during the growing season (between June and September) are between 5.2 and 7.5°C, which is in the range of temperatures reported by Körner (1999) and Paulsen (2000) for the treeline maximum altitude. Considering that the wood samples were reworked, the treeline elevation at the end of the MWP must have been at about 2500ma.s.l.

It is possible that the regional 20th-century treeline elevation has been under-estimated because of important anthropogenic perturbations due to pasturage practices. For this reason, the difference in mean summer temperatures of 1.2°C between the end of the MWP and today must be considered as a maximum value. This is high compared to typical proposed differences between the warmest decades of the MWP and the 20th century of about 0.7–0.8°C (e.g. Büntgen et al., 2006). However, it corresponds well with the highest temperatures of the MWP reconstructed by Mangini et al. (2005) for an high-elevation cave system (2350– 2500m a.s.l.) in the central Alps of Austria from a δ18O stalagmite record: temperature maxima during the end of the MWP that were on average about 1.7°C higher than the minima in the LIA and similar to present-day values, and with an absolute maxima for the MWP higher than the present-day temperature of 1.8±0.3°C.

Implications for Rock Glacier History:
The radiocarbon date and the inferred palaeotemperatures suggest that the Piancabella rock glacier probably became inactive towards the end of the MWP. [...] If the elevation of the treeline during the MWP corresponds to that of the lower limit of discontinuous permafrost, the wood remnants may indicate the beginning of permafrost degradation and consequently, the termination of rock glacier movement. [...] In a periglacial context, if climatic inactivation stops the aggradation of ice within a rock glacier, ice within the Piancabella rock glacier is probably several centuries old and therefore predates recent climatic events such as the LIA, as also indicated, for example, for the Murtèl- Corvatsch rock glacier in the eastern Swiss Alps (Haeberli et al., 1999).

It has been calculated by dating that rock glaciers within the Italian Alps resulted from cold episodes from 1000 to 5000 years BP (Dramis et al., 2003). Since these ages also indicate the timing of permafrost aggradation, an important phase of rock glacier development must have taken place between the end of the Younger Atlantic and the end of the LIA. However, the absence of ages older than 5000 BP in the Alps does not preclude the possibility of Early Holocene phases of permafrost aggradation (Dramis et al., 2003).

The Piancabella rock glacier, because of the phase of warmer air temperatures during the end of the MWP which likely corresponded to elevation of the lower limit of discontinuous permafrost, would have become inactive before the beginning of the LIA. It is difficult to determine if this behaviour is typical for rock glaciers situated close to the present regional lower limit of discontinuous permafrost (as are the Piancabella and the Val Maone rock glaciers). However, it is clear that, during the last millennium, the Piancabella rock glacier was the site of significant changes in dynamics, rheological properties and thermal conditions.

Observations
 
Modélisations
 
Hypothèses
 

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

Treeline positions do not respond instantaneously to climatic change or to anthropogenic perturbations: for example, today’s treeline position (2150–2200m a.s.l.) is a complex result of the environmental dynamics during the past 150 years. Lags of 50–150 years after climate warming are documented in both palaeoecological and modelling studies (e.g. Bugmann and Pfister, 2000). Considering this, we can estimate that the natural potential treeline for the middle of the 20th century was between 2250 and 2300m a.s.l.

This paper presents the results of radiocarbon dating of fossil wood stem remains of larch (Larix decidua) found at the front of the Piancabella rock glacier in September 2005 within the framework of geomorphological and geophysical investigations of the Late-glacial and Holocene glacier/ permafrost evolution in the southern Swiss Alps (see Scapozza and Reynard, 2007; Scapozza, 2008).

In total, eight fragments of wood, of brown/ grey colour, slightly jagged and in a good state of preservation, were found. The wood fragments do not result from a stump rooted in situ. Instead, the authors believe that the larch grew some metres above the location.

Wood analysis, species determination and necessary preparation and pre-treatment of the sample material for radiocarbon dating by AMS (accelerator mass spectrometry) were carried out. Calibration of the radiocarbon dating was performed with the software OxCal 3.10 (Bronk Ramsey, 2001) using the radiocarbon calibration curve IntCal04 (Reimer et al., 2004).

If a natural (i.e. non-anthropogenic) origin is assumed for the fossil wood, the latter’s position allows the minimum elevation of treeline to be inferred. The treeline is the upper limit of erect arborescent growth and it is sometimes difficult to differentiate it from the krummholz line, which is the upper limit of stunted scrub-like trees (Price, 1981). The authors consider it to be a minimum elevation because the samples were reworked.


(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

Two main conclusions can be drawn from observations, measurements and radiocarbon dating at the study site.

1. The 14C age obtained from the Piancabella rock glacier and observations show that the rock glacier probably became inactive during the end of the MWP. From a palaeoecological point of view, it seems likely that the treeline limit in the MWP was situated within the lower belt of discontinuous permafrost or close to its lower limit. Treeline was about 200m higher than in the middle of the 20th century, which corresponds to a mean summer temperature as much as 1.2°C higher than in AD 1950.

2. Concerning the dynamics of the rock glacier, the 14C date obtained can be interpreted as the minimum age for inactivity of the Piancabella rock glacier, which suggests that ice within this rock glacier is probably several centuries old and hence predates recent climatic events such as the LIA.

Références citées :

Bouët M. 1985. Climat et Météorologie de la Suisse Romande, 2nd edition. Payot: Lausanne.

Bronk Ramsey C. 2001. Development of the radiocarbon program OxCal. Radiocarbon 43: 355–363.

Bugmann H, Pfister C. 2000. Impacts of interannual climate variability on past and future forest composition. Regional Environmental Change 1: 112–125. DOI: 10.1007/ s101130000015.

Büntgen U, Frank DC, Nievergelt D, Esper J. 2006. Summer temperature variations in the European Alps, A.D. 755-2004. Journal of Climate 19: 5606–5623. DOI: 10.1175/jcli3917.1.

Davis BA, Brewer S, Stevenson AC, Guiot J. 2003. The temperature of Europe during the Holocene reconstructed from pollen data. Quaternary Science Reviews 22: 1701– 1716. DOI: 10.1016/s0277-3791(03)00173-2.

Dramis F, Giraudi C, Guglielmin M. 2003. Rock glacier distribution and paleoclimate in Italy. In Proceedings of the 8th International Conference on Permafrost, Zurich, Switzerland, 21–25 July 2003, Vol. 1, Phillips M, Springman SM, Arenson L (eds). Balkema: Lisse; 199–204.

Grove JM, Switsur R. 1994. Glacial geological evidence for the Medieval Warm Period. Climatic Change 26: 143–169.

Haeberli W, Kääb A, Wagner S, Vonder Mu¨hll D, Geissler P, Haas JN, Glatzel-Mattheier H, Wagenbach D. 1999. Pollen analysis and 14C age of moss remains in a permafrost core recovered from the active rock glacier Murtèl-Corvatsch, Swiss Alps: geomorphological and glaciological implications. Journal of Glaciology 43: 1–8.

Holzhauser H, Magny M, Zumbühl HJ. 2005. Glacier and lake-level variations in west-central Europe over the last 3500 years. The Holocene 15: 789–801. DOI: 10.1191/ 0959683605hl853ra.

Körner C. 1999. Alpine Plant Life. Springer: Berlin.

Mangini A, Spotl C, Verdes P. 2005. Reconstruction of temperature in the Central Alps during the past 2000 yr from a δ18O stalagmite record. Earth and Planetary Science Letters 235: 741–751. DOI: 10.1016/j.epsl.2005.05.010.

Paulsen J. 2000. Tree growth near treeline: abrupt of gradual reduction with altitude? Arctic, Antarctic, and Alpine Research 32: 14–20.

Price LW. 1981. Mountains and Man. A study of process and environment. University of California Press: Berkeley/Los Angeles.

Reimer PJ, Baillie MG, Bard E, Bayliss A, Beck JW, Bertrand C, Blackwell PG, Buck CE, Burr G, Cutler KB, Damon PE, Edwards RL, Fairbanks RG, Friedrich M, Guilderson TP, Hughen KA, Kromer B, McCormac FG, Manning S, Bronk Ramsey C, Reimer RW, Remmele S, Southon JR, Stuiver M, Talamo S, Taylor FW, van der Plicht J, Weyhenmeyer CE. 2004. IntCal04 Terrestrial Radiocarbon Age Calibration, 0- 26 cal kyr BP. Radiocarbon 46: 1029–1058.

Scapozza C. 2008. Contribution à l’étude géomorphologique et géophysique des environnements périglaciaires des Alpes Tessinoises orientales. Institute of Geography, University of Lausanne. http://doc.rero.ch/record/8799?ln=fr [5 October 2009].

Scapozza C, Reynard E. 2007. Rock glaciers e limite inferiore del permafrost discontinuo tra la Cima di Gana Bianca e la Cima di Piancabella (Val Blenio, TI). Geologia Insubrica 10(2): 29–40.