Geoarchaeological evidence of a buried navigable Roman canal in the Rhône delta (France): The Marius canal hypothesis

• Identification of a canal located in the Rhône Delta between Arles and Fos-Sur-Mer. • Chrono-stratigraphical and palaeoenvironmental analyses of the ancient canal. • The canal was navigable during the Roman period. • A palaeochannel from the 1st millennium BCE existed before this Roman canal. The Marius Canal is considered the first significant Roman hydraulic infrastructure in Gaul. This navigable canal, constructed at the end of the 2nd century BCE, was located in the Rhône Delta in southern France, connecting the Rhône River to the Mediterranean Sea. In the period following the construction of the canal, the large port known as Fossae Marianae was built on the coast. Despite numerous references in ancient sources, the precise location of the canal has remained unknown for the last two millennia. However, recent geophysical surveys in the eastern Rhône Delta have revealed a linear anomaly, alongside the discovery of Roman artefacts, which may indicate the presence of a Roman canal. The objective of this study is to examine morphological, sedimentary and chronological attributes of this structure, postulated to be the Canal of Marius. Sedimentary cores extracted from the supposed canal and the banks are studied on a high-resolution scale using a detailed multi-proxy methodology (grain-size, carbonate content, organic matter, magnetic susceptibility) combined with twenty-one 14 C dates. The morphological analyses and palaeoenvironmental data are consistent with the hypothesis of a navigable canal operable during the Roman period, built in a complex area where an ancient lagoon was partly eroded by a palaeochannel of the Rhône dated to the 1st millennium BCE. However, further archaeological research is needed to definitely confirm that this is the canal known as the Marius Canal.