Preservation of Modern and MIS 5.5 Erosional Landforms and Biological Structures as Sea Level Markers: A Matter of Luck?

The Mediterranean Basin is characterized by a significant variability in tectonic behaviour,ranging from subsidence to uplifting. However, those coastal areas considered to be tectonicallystable show coastal landforms at elevations consistent with eustatic and isostatic sea level changemodels. In particular, geomorphological indicators—such as tidal notches or shore platforms—areoften used to define the tectonic stability of the Mediterranean coasts. We present the results ofswim surveys in nine rocky coastal sectors in the central Mediterranean Sea using the Geoswimapproach. The entire route was covered in 22 days for a total distance of 158.5 km. All surveyedsites are considered to have been tectonically stable since the last interglacial (Marine Isotope Stage5.5 [MIS 5.5]), because related sea level markers fit well with sea level rise models. The analysis ofvisual observations and punctual measurements highlighted that, with respect to the total lengthof surveyed coast, the occurrence of tidal notches, shore platforms, and other indicators accountsfor 85% of the modern coastline, and only 1% of the MIS 5.5 equivalent. Therefore, only 1% of thesurveyed coast showed the presence of fossil markers of paleo sea levels above the datum. Thissignificant difference is mainly attributable to erosion processes that did not allow the preservationof the geomorphic evidence of past sea level stands. In the end, our research method showed that thefeasibility of applying such markers to define long-term tectonic behaviour is much higher in areaswhere pre-modern indicators have not been erased, such as at sites with hard bedrock previouslycovered by post-MIS 5.5 continental deposits, e.g., Sardinia, the Egadi Islands, Ansedonia, Gaeta,and Circeo. In general, the chances of finding such preserved indicators are very low.