Reoccupation of late Quaternary relative sea level indicators in a tectonically quasi-stable coastal area in Southern Italy (Cilento headland): Insights into the Last Interglacial stillstands

A geomorphological-stratigraphical study, integrated with a modelling approach, has been employed to constrain the age of relative sea-level indicators (RSLi) along the quasi-stable Mt. Bulgheria coast in the southern Apennines, located on the eastern margin of the Tyrrhenian Sea. Focusing on the geological evidence of late Quaternary sea-level fluctuations within the 0 to 12 m above sea level (a.s.l.) elevation range, we describe RSLi along ca. 8 km of coastline and constrain their elevation, including those of previously undated RSLi in the same area. The reassessment of field data was needed to address the phenomenon of the reoccupation of older RSLi by younger ones. The approach employs the synchronous correlation method, integrating highstand peak elevations from global sea-level curves and a calibrated uplift rate value, derived from the correlation of RSLi-2 (currently at 8 ± 1 m a.s.l.) with Marine Isotope Stage (MIS) 9c, as suggested by recent findings. Overall, four RSLi are identified in the investigated coastal zone: RSLi-1 (11 ± 1 m a.s.l.), correlated with MIS 11; RSLi-2 (8 ± 1 m a.s.l.), correlated with MIS 9c and reoccupied during the first MIS 5e peak; RSLi-3 (4 ± 0.5 m a.s.l.) formed during an earlier, undefined stage and reoccupied during MIS 5e; RSLi-4 (2.5 ± 0.5 m a.s.l.) also assigned to MIS 5e. Notably, two cases of reoccupation of older RSLi by younger ones are here reported. By testing multiple global sea-level curves, we derived a range of possible uplift rates for the chronologically constrained RSLi-2. Consequently, assuming a constant uplift rate throughout the final part of the Middle Pleistocene, we sought the optimal match between all observed RSLi and predicted elevation of palaeo sea levels during past highstands. The preferred best fit was obtained using a composite eustatic curve from Waelbroeck et al. (2002) and Kopp et al. (2013, for within the MIS5e), and a constant uplift rate of 0.009 mm/yr. Our modelling of MIS 5e (Last Interglacial) identifies three sea-level stillstands and a rapid drop in sea level following the first and higheststillstand. Our results emphasize the importance of the reoccupation problem in sea-level reconstruction studies, especially for RSLi located in regions that are tectonically stable or with relatively low rates of crustal vertical movements. In such contexts, the sea level during younger interglacials may reach or exceed the elevations of older interglacials, reusing or modifying existing RSLi and complicating the identification of which sea-level stand created a specific shoreline feature. The calibrated model supports scenarios where MIS 5e, MIS 9c and MIS 11 had higher peaks than the Holocene (so far). Finally, this work challenges the assumption that RSL indicators ranging between 5 and 8 m a.s.l. in stable regions were exclusively formed during MIS 5e, emphasising the need for precise age constraints in these interpretations.