Holocene History of the Lower Seine Estuary and the Commerce Valley Tributary (Normandy, France): A Palaeoenvironmental Framework for the Roman Landscape of <i>Juliobona</i> (Lillebonne) and Implication for Port Conditions

ABSTRACT The sediment infill deposits of the Lower Commerce Valley in the Seine estuary, located near the Roman city of Juliobona (now Lillebonne, Normandy), provide valuable data on environmental changes during the Holocene, including sea‐level changes, sediment supply and anthropogenic influences. A 10‐m deep sediment core was collected from the valley deposits, and a multidisciplinary geoarchaeological approach was applied, combining geophysical surveys, dating, sedimentological and diatom analyses. The results reveal a succession of marine to fluvial sediment sequences. At the base, a marine to estuarine environment was identified and then subjected to increasing fluvial contributions. A tipping point is recorded between the fourth and second centuries bce (ca. 2400–2100 cal. BP) with the establishment of a brackish environment, indicating the progressive disconnection of the valley from marine influences. From the second century bce , the Lower Commerce Valley appears to have been fluvial‐dominated, with no marine influence. This implies that the environment to the south of the Roman city was under freshwater influences. These new geoarchaeological data show no evidence of clogging of the estuarine valley during the Roman period, challenging previous hypotheses linking the decline of the city to the silting up of the valley.