River channel evolution and tsunami impacts recorded in local sedimentary archives – the ‘Fiume Morto’ at Ostia Antica (Tiber River, Italy)

Abstract

AbstractOstia, the ancient port of Rome at the mouth of the river Tiber (Italy), flourished until the 2nd Century ad, although massive siltation had already caused the abandonment of its lagoonal harbour in the 1st Century bc. In search of an alternative harbour site, geomorphological and geoarchaeological research was carried out in the ‘Fiume Morto’, an abandoned Tiber meander to the immediate north of the town. To reconstruct its complex development and evaluate the suitability of the river channel as a fluvial harbour, a transect perpendicular to the former river channel's flow axis was systematically explored by a new methodological approach combining electrical resistivity tomography, direct push‐electrical conductivity sensing and vibracores. Together with microfossil, mineralogical and geochemical analyses of sediment samples and 50 radiocarbon dates, in‐depth stratigraphic data support a detailed reconstruction of different Tiber channel generations. Results reveal a meander development closely related to distinct ‘cut and fill’ dynamics. Inactive river channels were repeatedly filled with lagoonal sediments, abruptly followed by phases of intense incision. Yet, the overall Fiume Morto channel structure remained stable and almost in situ over centuries, showing nearly no lateral changes since the southward shift of the Tiber River mouth in the first millennium bc. In the 1st Century bc, channel conditions favoured navigation and transportation of goods with large ships. Open‐water conditions, suitable for anchoring and landing activities, dominate from the 1st Century ad until 1557 ad, when the Fiume Morto meander was finally cut off and silted up within a short time. Within the fluvial deposits, sediments most likely related to tsunami inundations are preserved. These events occurred sometime before the 2nd Century bc, in the early 1st Century ad and in the 17th Century ad or later. Results are consistent with traces of tsunami influence found in Ostia's western lagoonal harbour.