Landslide Recurrence as a Proxy of Climate Change.

Landslides and in particular the temporal concentrations of instability events in different periods of the Holocene have not been by now considered as climate proxies, though they could improve significantly the insight of the environmental context as a whole in the reconstructions of past climate changes. The key to answer the questions concerning how and why climate has varied on different time-scales is infact to improve the documentation and understanding of natural variability for periods extending back beyond the instrumental record. Knowledge of past climate changes has to be gained from well-calibrated proxy data derived from different natural archives. These archives should provide accurate records of climate history, should be dated with annual precision on a calendar year time-scale and correlated through time. Actually, no single archive encompasses such properties and information from different sources are to be merged. In this framework, when the geomorphological evolution of a slope can be described by way of surveys, radiometric dating, cross-sections etc., and climate is considered to be the main cause of instability, concentrations of events become significant in a paleoclimatic perspective, if a sufficient number of landslides have been recorded. The records of landslide activity are not likely to be considered as comprehensive proxy archives, but they can give a significant contribution to the establishment of a paleoclimatic multi-proxy database when the environmental context is analysed with a multidisciplinary approach. Within the research carried out in the Dolomites (Italy), the event statigraphy of past landsliding has been traced thanks to the availability of sections and boreholes in which a large number of organic matter samples has been collected and dated with the radiocarbon method. During the Holocene, notwithstanding the evident influence of the geological-structural factors on slope modelling in the study areas, a possible cause-effect relationship between the phases of active slope movements and climatic and environmental changes taking place from the Lateglacial to date can be inferred. A period of enhanced slope instability has been found at the beginning of the Holocene, in the Preboreal and Boreal some 11,500 to 8500 cal yr BP, that reflects the response of the slope-system to the changes in the environmental forcing processes. The retreat of LGM glaciers made the valley flanks sensitive and susceptible of rapid changes and prone to an accelerated geomorphic activity. The response of the slope-system consisted on large rock slides, affecting the dolomite slopes after the withdrawal of Würm glaciers and on complex movements (rotational slides and flows) involving the underlying pelitic formations. A second concentration of landslide events in this area is reported during the Subboreal period (some 5800 to 2000 cal yr BP), when rotational slides and flows mainly took place. The slope movements ascribed to this second phase may likely be considered as reactivations of more ancients events, linked to a humid phase. In any case, the correlation of landslide activity records with the environmental context deduced from other proxies and confirmed by a multidisciplinary approach can validate the assumption that the process of landsliding is an expression of slope-system sensitivity to climate changes.