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

Abstract

Abstract 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 aggradation was 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.