Sea level during Marine Isotope Stage 5a in the Messiniakos Gulf (Peloponnese Peninsula, Greece)

Reconstructing past sea levels is essential for understanding the long-term interactions between climate change, ice sheet stability, tectonic activity, and coastal impacts. Here, we present new constraints on the Marine Isotope Stage (MIS) 5a sea-level highstand from the tectonically active Messiniakos region of southwestern Peloponnese Peninsula (Greece), in the eastern Mediterranean. Using 4 ± 2 m above present sea level (mapsl) as a reference eustatic value for MIS 5e, we estimate the sea-level position for MIS 5a by integrating multiple lines of evidence. These include a U-series dated hiatus (~81.0 to ~71.0 ka) in a stalagmite at 18 mapsl, a marine terrace, and a presumed MIS 5a Lithophaga-bored notch found at 16.5 mapsl, with a shell U-series age of 85 ± 8 ka. The hiatus in the stalagmite is further supported by a rise in ⁸⁷Sr/⁸⁶Sr values within the hiatus zone, likely influenced by seawater aerosol input during sea-level highstand. Accounting for regional tectonic uplift, which places the MIS 5e shoreline at 32 mapsl, our findings suggest that the MIS 5a paleo-sea level was −0.7 ± 3.2 m below present sea level. Although this estimate does not include corrections for glacial isostatic adjustment, it provides a valuable baseline for understanding past sea-level highstands in the eastern Mediterranean, where regional reconstructions remain limited despite numerous studies of marine terraces.