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 10mthe 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.