Holocene relative sea-level changes and coastal dynamics in Southern Latium, Italy: an interdisciplinary investigation 

Abstract

Understanding the historical changes in Relative Sea Level (RSL) and coastal responses in stable regions is crucial for unraveling the intricate relationship between natural dynamics and human adaptation. This interdisciplinary study seeks to explore the Holocene sea-level fluctuations in the stable area of Southern Latium, shedding light on how past societies adapted to coastal changes.The study area, located in the historical Sinus Formianus, between the Fondi and Garigliano coastal plains, played a key role in ancient times. Formia, a strategic monitoring point for the Tyrrhenian Sea, was a thriving commercial hub during Roman occupation. During this period, the coastal stretch from Formia to Gaeta witnessed substantial urbanization, leaving behind well-preserved remnants visible today in submerged or semi-submerged coastal structures along the present shoreline.This study reconstructs the Holocene morpho-evolution and RSL changes in the study area by creating a geodatabase following international guidelines for sea-level markers (SLMs). A comprehensive dataset of 52 SLMs was compiled from direct geoarchaeological measurements, stratigraphic and palaeoecological interpretations of new borehole data, and reinterpreting bibliographic information. Archaeological site selection involved analyzing bibliographic, cartographic, and video materials for ruins' details and dating. Additionally, public institutions provided access to an unpublished stratigraphic dataset from five deep boreholes drilled between Fondi and Formia plains in 2023. Three samples were collected from the stratigraphic columns of the analyzed boreholes in Formia Plain and dated using the radiocarbon dating technique. One sample, derived from a lagoonal deposit, presented an age exceeding the dating technique's accuracy range and older than 54 ka BCE. Despite this, the dating provided valuable information on the initiation of backshore formation. The other two dated samples, derived from a second drilling and collected inside layers of peat deposits, were interpreted as Terrestrial Limiting Points (TLPs) defining an upper limit of -4.20 m MSL for the RSL position at about 7.5 ka BP.Accordingly, based on the collected data, between 8.0 and 7.5 ka BP, the sea level in the study area rose from -23 to -5 m at a rate of 25 mm/yr. Subsequently, the rate slowed to less than 5 mm/yr, stabilizing at its current position. In particular, the results coming from the geoarchaeological surveys suggest that the local sea level during the Roman period (I century BCE) was no higher than - 0.55 ± 0.29 m MSL. Overall, the RSL data included in the geodatabase highlights the tectonic stability of this sector during the last 2.0 ka, testified by the position of the SLMs in accordance with the GIA models and supported by the determination of average vertical ground movements rates of -0.017 ± 0.23 mm/yr.Finally, the interplay between new data from geoarchaeological surveys, bibliographic sources, and LiDAR-based geomorphological analysis allowed the creation of a paleogeographic scenario for the study area in the 1st century CE. This highlights the significant landscape modifications induced by anthropic activities during that period.