Coastal dynamics under conditions of rapid sea-level rise: Late Pleistocene to Early Holocene evolution of barrier lagoon systems on the northern Adriatic shelf (Italy)

This multidisciplinary case study of two preserved barrier systems combined the analysis of radiocarbon datings, grain-size distributions, high-resolution seismics, and shelf bathymetry with reconstructions of palaeo-environmental conditions (tides, waves, sea-level change) and forward modelling of barrier–lagoon systems, to provide an integrated view of the coastal transgressive evolution of a large sector of the northern Adriatic shelf between 15 and 8 ka BP. Palaeo-environmental reconstructions point to increased tidal amplitude, low-energy wave climate and high rates of sea-level rise (up to 60 mm/a) during the formation of the oldest preserved barrier system (about 90 m water depth; 14.3 cal ka BP). A younger barrier system (42 m water depth; 10.5 cal ka BP) formed under conditions of lower tidal amplitude, higher wave energy and a lower rate of sea-level rise (10 mm/a). Forward modelling suggests that the probability of barrier-island overstepping during transgression is inversely proportional to tidal amplitude, if all other factors are assumed equal. The oldest barrier–lagoon system developed under conditions of large tidal amplitude, which permitted rapid transgression. However, this system apparently failed to keep up with the anomalously high rate of sea-level rise resulting from melt-water pulse 1A. The youngest barrier system appears to have drowned in place due to antecedent topography. As the barrier system transgressed over an ancient Pleistocene alluvial plain, the rapid increase in backbarrier accommodation caused an abrupt disequilibrium between shoreface and backbarrier sedimentation, which led to barrier overstepping. Although BarSim modelling indicates that tidal deposition can reduce the probability of barrier overstepping, there are other driving mechanisms (in our case extremely rapid sea-level rise and antecedent topography), which are more determinative in explaining the transgressive coastal evolution of barrier–lagoon systems in the northern Adriatic Sea. Grain-size analysis of shoreface deposits sampled above the transgressive ravinement surface across the northern Adriatic shelf indicate a distinct relation between the sediment grain size and the rate of sea-level rise during deposition, which implies that progressive sorting must have been highly effective.