Climatic signature of two mid-late Holocene fluvial incisions formed under sea-level highstand conditions (Pisa coastal plain, NW Tuscany, Italy)

A multi-proxy (stratigraphic, geomorphological, palynological, geophysical) study of mid-late Holocene deltaic-alluvial deposits beneath the Pisa Plain (Tuscany, Italy) reveals short-term enhances of fluvial activity under relative sea-level highstand (HST) conditions (last ~7000 cal yr BP). Early HST delta progradation led to the progressive infill of a broad lagoon area (~5000 cal yr BP), followed by the development of a stable alluvial depositional environment (~4000 cal yr BP). The intense phase of alluvial aggradationwas punctuated by two events of enhanced fluvial incision that cut down to 10 m the underlying lagoonal substrate. The two erosive events,which reflect centennial-scale changes in the aggradation/degradation ratio, are chronologically constrained to the Eneolithic-Bronze age transition (~3800 cal yr BP) and to the Bronze-Iron age transition (2900-2800 cal yr BP), respectively. A detailed pollen profile highlights the correlation between these erosive events and two phases of increased humidity (Abies peaks 1 and 2) recorded in several sites of Europe. This correlation suggests a key-role of climate fluctuations in triggering channel incision. The peculiar high compressibility of the lagoonal substrate can also have acted as a factor able to foster the deepening of the channels. In contrast, the role of relative sea-level changes and human impact on the activation of the two erosive processes appears negligible.