Discovery of Roman and early medieval palisades in Grado (Italy) constraining relative sea level and paleoenvironment in the northern Adriatic Sea

The Roman settlements on the northern Adriatic provide evidence for the temporal fluctuations of relative sea level (RSL). The discovery of three buried palisades during an archaeological excavation in Grado (Italy), one of the most important Roman harbors, allowed us to reconstruct the position of the sea level on the northeastern Adriatic coast. Two of these wooden barriers provided the islanders with better access to the sea and more effective protection against coastal erosion. The oldest palisade (mid/end I–early II century AD) was intended to support the part of the artificial deposit closest to the shore and enabled the inhabitants access to it. Based on its position and a dated wood sample, which correlates with the chronology of the pottery, the RSL in the early imperial Roman age was -1.20 ± 0.30 m. The most recent palisade (VI century AD) shows a significant sea level rise of + 0.40 m RSL; it was probably built to compact part of the inundated soil and provided reinforcement in response to the sea level rise. The higher, but equally old palisade from the VI century AD, was probably built to consolidate the base of the Castrum of Grado. Based on these results, we add new data to reconstruct the RSL history of the northeastern Adriatic coast and delineate its environmental configuration, in which a lagoonal landscape developed shortly thereafter.