Constraining the past and future evolution of a drowning landscape in Northeastern Italy

Many deltas and lagoonal systems are currently under threat of continued sea level rise and face potential future drowning, or transgression of the sea, particularly in lowland lagoonal areas. Resulting drowning of deltas and the impact of potential mitigation measures is subject of many studies. However, timescales and spatial complexity of transgression-related processes often remain unknown or topic of debate. Transgression-forced vegetation changes, salinization, changed these processes are fundamental  for the functioning of the lagoonal complex, determining the stability of tidal inlets and rate of sediment transport. Reconstruction of coastal transitions in the past can quantify rates of change and coastal morphology, which by comparison with model simulations help to constrain the transgressive processes and effects.

The former Caorle lagoon, located in the Veneto region in NE Italy, is currently mostly reclaimed agricultural land. It is under threat of sea-level rise and the past development of this relatively small area acts as a natural laboratory of coastal change, highly relevant for the adjacent lagoon of Venice. We use the output of years of fieldwork (2012 – 2026), a large set of radiocarbon dates derived and proxy records at selected core sites of pollen and foraminifera to constrain salinity gradients, ecological changes and human influence. Exceptionally large infill rates are found in unstable tidal inlets. We find that deposition rates at sites with stable conditions vary between 1-8 mm/yr and are largest in depressed areas (incised valleys) and during rapid sea level rise. Lastly, we find that landscape transitions from fluvial- to tide dominated can be rapid (10 – 100 years) or gradual (100 – 1000 years) depending on relative height, topographic position and river configuration.