Late Holocene coastal landscape evolution and extreme wave event history of Vatika Bay, SE Peloponnese (Greece): A multi-proxy approach

Strongyli Lagoon, in the Vatika Bay, is a highly dynamic coastal wetland, located along the forearc of the Hellenic Subduction Zone, one of the most tsunamigenic regions in the Mediterranean. The combined effects of local tectonic activity, isostatic sea-level change, coastal morphodynamics, and multiple extreme wave events have shaped the bay. This study explores the sedimentary archives of the western Vatika Bay to reconstruct the paleoenvironment and identify sedimentary signatures of extreme wave events, contributing to the broader understanding of marine geohazards in Greece.

A multi-proxy analysis was carried out on four sediment cores recovered from the eastern and western margins of Strongyli Lagoon, including granulometry, magnetic susceptibility, inorganic geochemistry, micropaleontology, and radiocarbon dating, allowing a detailed characterization of the depositional facies and high-energy event history.

The stratigraphic record reveals a gradual transition from an alluvial plain dominated by terrigenous input to present-day coastal plain conditions influenced by lagoonal and aeolian sedimentation. Within the sedimentary sequence, three distinct event layers exhibit significantly different properties from the background sediments, presenting several tsunami related features, such as fining upwards and landward-thinning sequences, erosive basal contacts, sharp increases in foraminiferal abundances, and elevated marine geochemical concentrations and ratios (e.g., Ca, Sr, S, Ca/Ti, Ca/Fe, Ca/Al, Sr/Al).

The oldest high-energy event deposit, recorded on the eastern margin of the lagoon, corresponds to the well-documented 365 CE tsunami in the Aegean Sea. On the western margin of the lagoon, an abrupt change in the depositional environment dated to between the 5^th and 10^th centuries could reflect localized co-seismic vertical movements linked to normal faulting that generated a small-scale marine inundation, rather than a major tsunami event. A younger event deposit identified on the eastern margin of the lagoon, dated between the 19^th and 20^th centuries CE, is marked by subtle marine geochemical signals, but exceptionally abundant deep-water foraminiferal assemblages, indicating an offshore sediment source and high-energy marine incursion.

Overall, Strongyli Lagoon preserves a detailed and spatially variable record of the Late Holocene coastal evolution and the marine extreme wave event history of the Vatika Bay. This research highlights the high potential of lagoonal geoarchives for preserving deposits of extreme wave events, providing new insights into the frequency and diversity of tsunamigenic sources affecting the Laconian Gulf, refining our understanding of coastal hazards in tectonically active regions.