Past and recent sedimentation rates in the Lagoon of Venice (Northern Italy)

A chronology for the accumulation of subtidal and intertidal sediments in the Lagoon of Venice has been obtained from past and recent biofacies, with the aim of obtaining the sedimentiaton rates in various palaeo-environments and over various time spans. In the sedimentary sequences,changes in benthic foraminiferal assemblages, i.e. ecological transitions, reflect changes in environmental conditions. Quantitative analyses of the foraminiferal assemblages of 89 samples from 26 cores, each about 1-m long, reveal a number of these ecological transitions. Statistical analysis based on the percentage of each foraminiferal species in the samples identified older lagoonal morphologies. In addition, 57 radiometric age determinations on organogenic materal in sediments yielded the conventional BP ages of major events and the various sedimentation rates occurring in different environments in the course of time. Littoral deposits, identified at a depth of 90 cm, are dated for the first time, giving an age of 3180 +/-50 y BP. Various buried salt marshes gave ages ranging from 820 +/-40 to 1887 +/-40 y BP; in the inner part of the lagoon, the cores reach continental deposits, and indicate that the earliest lagoonal deposits go back to historic or proto-historic times, with ages ranging from 820+/-40 y BP to 2403 +/- 37 y BP. The oldest age (3437 +/-38 y BP) was obtained from shells of lagoonal organisms found at a depth of 90 cm in core 2, entirely within lagoonal deposits. The long-term sedimentation rates, inclusive of subsidence and eustasy, are about 1.1 mm/yr between 2500 and 1500 y BP and about 0.5 mm/yr from 1500 y BP to the present. The highest accumulation rates were recorded in the southern part of the Lagoon and the lowest rates around the city of Venice. In view of the decreased sedimentation rates, the recent resumption of the Sea Level Rise after a phse of quiescence may produce the deepening of the lagoonal basin.