Ultra-shallow water geophysics surveys for a Pseudo-3D palaeochannel reconstruction, Lagoon of Venice, Italy

Abstract

The Lagoon of Venice (LV) is a shallow lagoon located in the Mediterranean region, NE Italy. It is connected to the Adriatic Sea by three inlets. The LV is the largest Italian lagoon with an area of 550 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and is characterized by the presence of a complex occurrence of salt marshes, tide flats and navigation channels. The LV began to form about 6000 yrs. BP related to the marine transgression occurred in the end of the Wurm glaciation. Since then, a combination of natural processes and intense human activities determined its morphological evolution. Recent studies demonstrated that the use of sub-bottom profilers can be very useful to obtain a description of ancient phases of the lagoon evolution. In this contribution, we present the results concerning ultra-shallow water (<;1 m depth) acoustic survey in the central lagoon area between the industrialized area of Marghera and the city of Venice. The aim of our work is to reconstruct the sub bottom architecture of a very anthropised environment through a multidisciplinary approach using acoustic and environmental data. The surveys were carried out using a modified ELAC echosounder with a 30 kHz transducer. The positions were defined using a DGPS system. The survey lines were done through parallel lines spaced 50 m in south-north direction. In correspondence with interesting acoustical discontinuities 14 ground cores were extracted with the help of a rotation method with water circulation and the position of the cores was done using a DGPS. The geophysics data were acquired using the software Ecosonda v.3.04.vi (CNR-Rome) and processed with the software MostraDati (CNR-Roma). Acoustic data allowed the reconstruction of three paleochannels and their meandering paths and also a pseudo-3D reconstruction by the interpolations of the mapped reflectors depth. In particular, the central one denominated as PC02 was intersected by the core SG25. The stratigraphic record presents mainly clayey-silty sediments between -1.2 to -5.2 m and sandy sediments from -5.2 m to -6.60 m from the m.s.l. The C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> dating on a mollusk shell taken at -5.2 m between the two facies allows the reconstruction of the paleochannel shape 1600 ± 90 cal yrs. BP. In the southern side of the acoustic profile, were identified inclined reflectors who correspond to the palaeochannel point bar migration for about 20-30 m towards north. Was also observed a variation in the depositional form of this point bar migration separated in two different phases: before a higher energetic regime with sand deposition and channel migration, later a finer filling and apparently no migration. This variation could be related to a change of the hydrodynamics of the area due to a climate change between the IV and VI century AD.