12 kyr record of flood and environmental changes in Southern Alps (Lake Iseo, Italy)
Résumé
The high-resolution sedimentological and geochemical analysis of a 21m sediment sequence from Lake Iseo (Southern Alps, Italy) led to reconstruct the long term hydroclimate variability by the identification of event layers over the last 12 kyr BP. In a previous study undertaken on another core, these layers were attributed to extreme surface runoff events. However, large mass-wasting deposits inducing important hiatus did not permit to establish a continuous record of such events. Our new sediment sequence does not present such disturbances and thus allows us to establish a high resolution continuous Holocene record. Based on the flood chronicle and XRF geochemical analyses, we evidence a major palaeohydrological transition at approximately 4 kyr BP, as previously described in the western Mediterranean region. The first part of the record presents very low frequency of flood events, while since 4 kyr BP the flood frequency increases. This pattern appears to be in phase with Southern Alpine palaeoflood records. This transition is interpreted as a nonlinear climate response to the orbital-driven gradual decrease in summer insolation at 60°N, with the influence of Mediterranean mesoscale precipitation events, typical of the Mediterranean climate. We propose to compare the flood record, with archaeological and historical data coming from the watershed. The human activity attested in the vicinity of the main tributary from the Roman period seems to be one of the key to understand the flood chronicle. This evidences that even in a large Alpine lake, sedimentation process and hydroclimate record can be impacted by the anthropization of the catchment area.