The genesis of these submerged reef, oases of marine biodiversity and geodiversity, dates back to about 7,000 years ago when remnants of fluvial channel of the last glacial period cemented. The study, coordinated by Ismar-Cnr and performed together with OGS, University of Padova, Ispra and Conicet (Argentina), has been published in Scientific Reports journal
The north-western Adriatic seabed is not only a sandy-silty flat underwater landscape, as it is generally thought. In the Gulf of Venice, offshore Chioggia, at about 20 m water depth, localized bio-concretionned rocky buildups rise up to 3-4 m above the seafloor. These submerged reefs host peculiar aquatic ecosystems and offer shelter, reproduction and nursery ground to fish and invertebrate species. Called ‘tegnùe’ in the Venetian dialect because they catch fishermen's nets, they are bio-constructions composed of encrusting organisms and show a significant similarity with the coralligenous habitat. A study coordinated by the Institute of Marine Science of the National Research Council (Ismar-Cnr) has investigated the origins of these surprising marine habitats. The research has involved the University of Padova, the National Institute of Oceanography and Experimental Geophysics (OGS), the Italian National Institute for Environmental Protection and Research (Ispra), the Consejo Nacional de Investigaciones Científicas y Técnicas (Conicet, Argentina) and has been published on Scientific Reports journal.
"These coralligenous buildups are widespread in the north-western Adriatic Sea and represent a ‘rocky geological anomaly’ with respect to the sandy-silty seafloor on which they stand" explains Luigi Tosi as first author of the paper and Ismar-Cnr research scientist. "Because the genetic models obtained by previous investigations were not satisfactory, we have involved multidisciplinary skills".
The research team composed of geologists, oceanographers, geophysics and biologists, has performed geophysical surveys for the sea floor and subsoil mapping. "Based on the new maps, Ismar-Cnr researchers have performed more than 200 dives, often with very low visibility, for geomorphological and geological observations as well as for rock and sediment samplings. Moreover, we have installed particular wells at the sea bottom for the analysis of the submarine groundwater" continues Andrea Bergamasco, oceanographer at Ismar-Cnr and responsible of the scientific underwater SCUBA diving.
"In the laboratory we have performed isotopic, paleo-environmental and electron microscopy analyses examining all the acquired data. It has inferred that ‘tegnùe’ developed along winding morphological structures with meandering-like horizontal paths attributed to ancient river channels flowing over the paleo-plain during the last glacial period, about 20,000 years ago" says Sandra Donnici, geologist at Ismar-Cnr.
In particular, a rock sample has been crucial. "A slab of cemented sand with embedded mollusk shells has allowed to detect the age and the characteristics of the paleo-environment at the time of its cementation" concludes Luigi Tosi. "Like a 'Rosetta Stele' it has allowed to reach a new vision on the origins of coralligenous buildups of the Gulf of Venice. Radiocarbon analyses have dated at about 9,000 years ago the arrival of the sea in this part of the Pleistocene paleo-plain and at 7,000 years ago the cementation process of the sandy channels on which the earliest bioconstructors developed".
The occurrence of coralligenous buildups developed above older fluvial meandering systems is an unusual feature never observed before in the Mediterranean Sea.
This work has been developed in the framework of the Project 'Tegnùe' coordinated by Comune di Chioggia and funded by Regione del Veneto (coordinators: Sandra Donnici and Paolo Montagna), and the Flagship Project Ritmare funded by Miur (coordinator: Luigi Tosi).