Response Of A Tidal Environment To Changes In The Forcing: Insights From The Southern Venice Lagoon (Italy)

Abstract:

The Venice Lagoon represents an outstanding example of man-landscape co-existence, where salt marshes are currently exposed to possibly irreversible transformations due to the effects of climate changes and human activity. The increasing rate of relative sea level rise and the decreasing sediment supply are the dominant forcings controlling salt-marshes survival, as in other cases worldwide. Analysing signatures of landscape changes in the stratigraphic record is crucial to refine our knowledge of tidal landform dynamics and it is a first step to develop predictive morphodynamic models. The southern Venice lagoon is suited to analyze the response of tidal landscape to changes in environmental forcing. The upper part of the Holocene sedimentary succession accumulated as the effect of a “human-induced transgression”, which caused a considerable salt-marshes contraction since the 16^th century. In the southern Venice lagoon, re-directions of the Brenta River during 16^th and the 19^th century caused significant changes in terms of freshwater and sediment supply. To analyze the paleomorphodynamic response to these main changes, we collected 25 cores (1.0 to 1.5 m deep) along a NE-SW linear transect about 5 km long cutting through salt-marsh, tidal-flat and subtidal-platform deposits. Through sedimentological analyses we defined the spatial arrangement of swamp, salt-marsh, wave-worked and tidal-flat mud deposits along this transect. Magnetic susceptibility and organic/inorganic content were measured in the study succession, which was dated through ¹⁴C and ¹³⁷Cs geochronological analyses. Our results suggest that salt-marsh sedimentation occurred above deltaic deposits since the 14^th century. Salt-marsh aggradation stemmed out from both mud settling and organic accumulation, although magnetic susceptibility revealed some intervals during which the inorganic deposition dominates over organic accumulation. Salt-marsh aggradation occurred in parallel with the decrease in salt-marsh extent and the tidal-flat expansion.