Ancient and modern salt marshes in the Lagoon of Venice

The Lagoon of Venice was formed during the Late Holocene transgression and anthropogenic remains are common within its sedimentary sequences. This study is based on the foraminiferal association of 58 samples from 7 cores obtained from salt marshes north of Venice. Below horizons of lagoonal origin, these samples reveal the presence of faunas with Trochammina inflata (Montagu) and Helenina anderseni (Warren) which indicate conditions that are characteristic of present-day salt marshes. Foraminiferal faunas can identify the different lagoonal environments and indicate the relative position of the various horizons with respect to sea level. Faunas obtained from the lower section of the cores are analogous to present-day assemblages and they indicate, below a transgressive phase, the existence of buried ancient salt marshes occupied by human settlements. The absolute age of these horizons indicates that the establishment of human settlements is much older than previously thought and that the average rate of Holocene relative sea level rise for the last 5000 years is of the order of 1.2 mm/yr. The formation of these salt marshes, and their subsequent submergence, occurred with rates of accretion and subsidence markedly variable and with occasional erosional phases. This study also shows that, within the present lagoonal environment, together with erosional processes there are areas of accumulation with the formation of new salt marshes.