Réf. Borgatti & Soldati 2010 - A

Référence bibliographique complète

BORGATTI, L., SOLDATI, M. Landslides as a geomorphological proxy for climate change: A record from the Dolomites (northern Italy). Geomorphology, 2010, Vol. 120, 56–64. doi:10.1016/j.geomorph.2009.09.015

Abstract: This study investigates the relationships between climate changes and hillslope evolution in the Dolomites (eastern Alps, Italy), during the Late Quaternary, with particular attention paid to landslide processes. The basic premise is that modifications in landslide frequency may be interpreted as changes in the hydrological conditions of slopes, which are in turn controlled by climate.
After the statistical analysis of a data set composed of 73 conventional radiocarbon ages, obtained from 24 landslides, four periods of enhanced landsliding have been identified: I. from 10,700 to 8400 cal BP, between Younger Dryas and the Preboreal; II. from 8200 to 6900 cal BP, during the older Atlantic; III. from 5800 to 4500 cal BP, between Atlantic and Subboreal; and IV. from 4000 to 2100 cal BP, between Subboreal and Subatlantic.
These periods have been compared with different Lateglacial and Holocene paleoclimatic records, to check the correspondence between periods of enhanced landslide activity and cold and humid spells recognized at different spatial scales. As the records show, in the study areas, slope instability processes can be considered geomorphological indicators of climatic changes and to a certain extent reliable proxies of environmental evolution.

Mots-clés
Landslide events, Climate changes, Holocene, Dolomites, Italy

Organismes / Contact

• Dipartimento di Ingegneria delle Strutture, dei Trasporti, delle Acque, del Rilevamento, del Territorio - DISTART ALMA MATER STUDIORUM - Università di Bologna
Viale Risorgimento, 2, 40136 Bologna, Italy – lisa.borgatti@unibo.it (L. Borgatti)
• Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia. Largo S. Eufemia, 19, 41121 Modena, Italy


(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, precipitation   Landslides  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Italy Dolomites Cortina d’Ampezzo and Corvara Badia     Holocene

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

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

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

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

 

 


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

Landslide activity records in the Dolomites:
[...] In the area of Corvara the age distribution shows a persistent landslide activity during the entire time span of the Holocene, whereas in Cortina the ages are more scattered. This could be linked to the number of dated samples, i.e., 24 samples in Cortina, 49 in Corvara. Anyway, some clusters are recognisable in both records (around 5 ka cal BP and from 2 to 3 ka cal BP), while others are not analogous, such as the 11,000–9500 temporal cluster in Cortina, which is not so marked in Corvara.

If the distribution of probability is considered for both study sites together, the picture is different and the clustering around certain ages becomes clearer. By analysing the complete data set, four periods of enhanced landsliding can be outlined: I. from 10,700 to 8400 cal BP, between Younger Dryas and the Preboreal; II. from 8200 to 6900 cal BP, during the older Atlantic; III. from 5800 to 4500 cal BP, between Atlantic and Subboreal; and IV. from 4000 to 2100 cal BP, between Subboreal and Subatlantic.

Many initial failures of large landslides occurred between 11,000 and 10,000 cal BP, i.e. during the Lateglacial-Holocene transition (Soldati et al., 2004). The higher number of dated events in this time span may not indicate an increase in landslides frequency, being a consequence of the amplified chance of finding buried plants debris. Otherwise, assuming a climatic significance for this cluster, it could be related with slope release following the definitive permafrost melting and the subsequent increase of water availability at high altitudes.

Period II could be related to the effects of the 8200 ka event (Bond et al., 1997), as in the case of many other records from different proxies that carry the signal of this global climatic event. The enhanced activity in the Upper Holocene is related to more cold and humid periods throughout the Subboreal and Subatlantic. In particular, starting from the Subatlantic, some Authors found that the increase of slope instability phenomena is related to human impact, mainly because of deforestation (Dapples et al., 2002). These studies show that anthropogenic factors must be considered at least since the Bronze Age, possibly as amplificators of climatic factors.

Discussion: climate as a causal factor for landsliding at a broad temporal scale:
[...] Despite the intrinsic difficulties in correlating [the palaeoenvironmental] records [see details in the study], which are mainly due to different spatial scales (local, regional and global), dissimilar time resolutions and several dating constraints, some remarkable indication are apparent. The periods of enhanced slope instability found in the Dolomites display quite a good correlation especially with the indicators of cold and humid climate, suggesting that these phases could have been climatically-driven, and, in particular, that a positive moisture balance could have played a major role in conditioning landslide activity at the hundred to thousand years time scale. It is clear that a positive moisture balance could have occurred under different environmental conditions. On the one hand, an increase in intensity and/or duration of rainfall could be significant at every time scale, together with evapotranspiration variability driven also by temperature regime changes.

The phases of minimum landslide frequency identified from Borgatti et al. (2007) alternating with the enhanced landsliding periods fall within phases of reduced glacier extent, with warmer summers and/or reduced precipitation.

Mass wasting processes are primarily controlled by the geological and structural predisposing factors, which may differ from region to region, but this record shows that the apparent modulations are clearly induced by the centennial–millennial scale climate changes. In addition, in formerly glaciated mountain belts, the long-term effects of the deglaciation and permafrost melting may result in effects opposite to the actual climate tendencies, as in the case of the clustering around the onset of the Holocene. Also the effects of cold spells are to be stressed, as in the case of the 8.2 ka event, that seems to have left a clear signature in the landscape. Finally, during the upper Holocene, the long-term tendency towards an increased slope stability after the last deglaciation could then be counterbalanced by the effects of human activity, starting from 4 ka cal BP.

Observations
 
Modélisations
 
Hypothèses
 

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

Field observation of present-day activity and historical records show that first-time failures of large landslides follow a complex hydrological and mechanical behaviour (Corominas, 2001). In fact, first-time failures are the result of long-term evolutionary processes of the slope rather than the near-immediate response to a specific trigger. On the other hand, the influence of moisture balance is evident in the case of reactivations of dormant landslides, the acceleration of active movements and in the triggering of shallow slope failures.
Previous research has clearly demonstrated the linkages between climate change and landslide activity (Buma and Dehn, 1998; Dehn et al., 2000). In particular, changes in the hydrological balance, resulting from the temporal distribution of temperature and rainfall, and the resulting evapotranspiration, directly influence the hydrological regime of slopes, which in turn governs the type, the rate and the temporal and spatial evolution of mass movements. Consequently, in the analysis of the relationships between landslides and climate, it is necessary to focus on temperature changes as well as on the timing, frequency and magnitude of rainfall.
It can be assumed that the relationship between climate, in particular positive moisture balance and landslide activity exists at every time scale. At a broad temporal scale, the relationship between landslide activity and triggering mechanisms can be established from the temporal clustering of dated landslides, starting from the assumption that a concentration of dated landslides around a specific agemay have a similar cause.
As regards the factors predisposing and triggering landslide events, seismicity, climate and human activities are the only factors able to produce temporal and spatial concentrations of single landslides events. Also fluvial undercutting related to stages of landscape evolution can produce concentration of events, but this requires the same terrains to be involved and the same landscape history. Both rainfall and earthquakes can mobilise debris flows, rock falls, rotational and translational slides, and unless additional information can be retrieved, no distinctive morphological feature can discriminate isolated landslides triggered by rainfall from ones having other causes. The first two triggers cause widespread landsliding, while human activity usually causes local instability phenomena. Therefore, a correct procedure for establishing a scheme of temporal recurrence of landslides carrying a climatic signal, could be given by a critical analysis of the instability events, in order to exclude those landslides ascribable to non-climatic causes. This goal can be achieved through a multidisciplinary approach directed to the appraisal of the paleoenvironmental conditions at the time of the landslides, in order to discriminate between the climatic and nonclimatic factors which conditioned the slope-system in the short-, medium- and long-term period. As a direct consequence, besides the development of a landslide events record, other proxy records have to be considered in order to disentangle the possible interactions between natural systems, in this case the slope-system, climate and humans. [...]

In order to analyse the temporal occurrence of landslide events, the distributions of probability for each dating have been summed, producing plots for the single study sites and for both together. In this procedure, besides the direct datings of landslide events, also the ages obtained from lake sediments have been considered [...].

Starting from the synthesis of the set of data presented in Soldati et al. (2004) and from eleven new datings, the sequence of enhanced slope instability has been further developed, by analysing the statistical distribution of calibrated radiocarbon ages. [...]

As far as the reconstruction of climate evolution during the Holocene is concerned, thanks to pollen databases a detailed temperature reconstruction on a time resolution up to 100 years at the regional scale of the Alps is available [see references in the study]. It can be concluded that Holocene climate variability was driven by changes in precipitation regimes, in term of intensity and duration.
Therefore, considering the significance of rainfall regime and the resultant hydrologic response of the slopes in triggering mass movements, after excluding the direct influence of seismicity and human activity, the periods of enhanced landsliding have been compared to different Lateglacial and Holocene paleoclimatic records, in order to verify the climatic control on the clustering of landslide events. [...]

The records from the Dolomites show the periods of enhanced landsliding documented in this study and the periods of reduced landslide activity as described in Borgatti et al. (2007). [...]


(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

Références citées :

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I., Bonani, G., 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and Glacial climates. Science 278, 1257–1266.

Borgatti, L., Ravazzi, C., Donegana, M., Corsini, A., Marchetti, M., Soldati, M., 2007. A lacustrine record of early Holocene watershed events and vegetation history, Corvara in Badia, Dolomites, Italy. Journal of Quaternary Science 22, 173–189.

Corominas, J., 2001. Landslides and Climate. In: Bromhead, E.N. (Ed.), Keynote lectures, VIII ISL, Cardiff, June 2000, CD-ROM.

Dapples, F., Lotter, A.F., van Leeuwen, J.F.N., van der Knaap, W.O., Dimitriadis, S., Oswald, D., 2002. Paleolimnological evidence for increased landslide activity due to forest clearing and land-use since 3600 cal BP in the western Swiss Alps. Journal of Paleolimnology 27, 239–248.

Soldati, M., Corsini, A., Pasuto, A., 2004. Landslides and climate change in the Italian Dolomites since the Lateglacial. Catena 55 (2), 141–161. [Fiche Biblio]