Holocene sea-level change in the Cagliari coastal plain (South Sardinia, Italy)

In this research, we studied the marine flood risk of the Cagliari plain in relation to the depressed coastal area, which includes a zone of the extremely high population density (almost half of the population of Sardinia) and major industrial infrastructures. The critical inundation altitudes in the Cagliari plain occur well above the average sea level, as demonstrated by the fact that, during meteorological -marine events with SW and SE winds, the normal and extreme meteorological tides reach heights of approximately +1.0 m in the Santa Gilla coastline and “high water” of + 1.5 m on the Santa Gilla lagoon, located in the western sector of the plain. In order to analyze the way the lagoon fills in and to recognise any active subsidence processes the research was concentrated across the entrance of the lagoon using the stratigraphic-sedimentological facies of the information contained in three continuous boreholes. The mouth of the lagoon is a covered palaeo-riverbed that formed during the last glacial stage at the time of the LGM low stand (MIS 2). The geometries of the different stratigraphic facies and their relative depositional paleo-environments related to the Holocene sea level rise have been reconstructed. AMS Radiocarbon dating allows the start of the lagoon’s filling processes to be placed immediately before 9,6 ky BP, with the deposition of paralic sandy muds upon the gravels of the lower fluvial terrace. The filling processes were followed by a marine inundation, marked by muddy onshore beach sands with Posidonia oceanica. This took place 7,4 ka BP and is identified by alternations of fine and coarse sediments that prove the closing and opening of the ancient lagoon mouth. Fine gravels and coarse-medium sands indicate a period of significant opening of the lagoon during the Upper Atlantic-Sub Boreal stage (4,0 ka BP), followed by a sedimentation of organic rich deposits from the Sub-Atlantic to Present. The study of the thanatocoenosis, mainly composed of Gastropods and Bivalves, precisely highlights the paleo-geographical evolution of the plain and the passages between paralic and marine-littoral and lagoon environments. The living environment of the lagoonal dated organisms, with an error bar of ±1.5 m, (I don’t understand this) allows a precise reconstruction of the sea level rise with a minimum margin of error. The comparison of the predicted sea level curves calculated for the Cagliari Plain with the observed data, indicates good agreement as is consistent with an absence of vertical land movements other than those caused by glacio-hydro isostasy. The Upper Pleistocene-Holocene relative tectonic stability of the plain is also demonstrated by geo-archaeological data; and by the evolution of the fault slope deposit system of the western border of the plain. However, subsidence may have occurred after ~5 ka BP, at a rate of 0.8±0.02 mm yr-1, and this is interpreted as a consequence of compression of the holocene deposits (Posidonia oceanica peats).