Beach-dune systems in a transgressive domain along the Sardinian coasts

Coastal transgressive dunefields represent an unusual, decoupled state of the beach-dune system in current temperate zones. Knowledge of the factors responsible for the transgressive activation (high sediment supply, climatic conditions, vegetation disturbance, high wave and wind energy) is a tool for inferring similar conditions in the past and evaluating the potential of sand drift re-activation in the context of future climatic change. The sole examples of currently stabilized and active transgressive dune fields in Italy are on Sardinia's west coast, due to conditions of strong unimodal wind from the north-west. In the context of a project funded by the Italian Ministry of University and Research (MUR-PRIN 2022), we present a series of preliminary results regarding the land-sea continuum of the coast of Piscinas (Sardinia, Italy), where an active dunefield develops, covering 132 ha. The main aim is to investigate the role of transgressive aeolian episodes as potential environmental indicators as part of a larger understanding of their significance on a local and global scale. The first results, regarding the Piscinas site, are based on a detailed geomorphological description obtained by UAV and GPR surveys, as well as a stratigraphic survey of the Pleistocene aeolianites outcropping along the coasts. On the continental shelf facing the study area, geophysical data have been acquired in the last 15 years, including high-resolution morpho-bathymetric surveys, acoustic backscatter maps, and high- and very-high-resolution seismic profiles. The main parabolic dune, aligned with the prevailing Mistral wind (from the northwest), extends from the coast for about 2 km inland to a maximum altitude of 113 m. Topographic sections and GPR profiles show the presence of a geological control due to the gradually sloping Paleozoic rocky basement over which the dune accommodated while climbing. Buried post-LGM (Late Glacial Maximum) aeolian deposits of partially consolidated sands were also recognized. These sands represent the sediment layer of the established transgressive dunefield and serve as the sediment source of the current active dune lobes, migrating downwind. Extensive outcrops of Middle and Upper Pleistocene aeolianites throughout the coasts attested to extensive aeolian activity in the context of sea level rise variation and tectonic stability in the past. A preliminary examination of the marine landforms on the inner shelf reveals the presence of parabolic bodies at a depth range of 31-40 m. These relict coastal landforms, which may have been subjected to early cementation (as previously explored at neighboring sites), attest to an aeolian pulse affecting the exposed continental shelf during the LGM, as a continuum to connect with the analogous signature on land. The integrated analysis using direct and indirect stratigraphic information collected in the coastal area will allow reconstruction of the depositional palimpsest and order of accommodation of several aeolian transgressive bodies during the transgression, highlighting possible differences or analogies between the current transgressive domain and the climatic phases that drove aeolian activity during the late Quaternary.