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

Abstract

Ostia, 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.

Automated facies identification by Direct Push‐based sensing methods (CPT, HPT) and multivariate linear discriminant analysis to decipher geomorphological changes and storm surge impact on a medieval coastal landscape

“…Compared to vibracoring, CPT and HPT sensing is considerably more time-efficient, and results remain unaffected by artificial compaction or loss of sediment (for further reading on methodological details, see Obrocki et al, 2020). Yet, as the data needs to be calibrated, a combined approach of CPT and HPT sensing and vibracore key sites significantly improves palaeogeographic reconstructions of a site (Hadler et al, 2020).…”

“…The larger the initial dataset, the greater is the potential for a large-scale application and a significant improvement of classification results. Still, DPbased sensing in-between the generally few vibracoring sites of a study area then allows a by far better and more detailed palaeoenvironmental reconstruction than the geoscientific standard approach (Hausmann et al, 2018;Hadler et al, 2020), especially in combination with geophysical prospection.…”

“…During the last years, Direct Push (DP)-based in situ sensing methods like electrical conductivity (EC) logging, Cone Penetration Testing (CPT), the Hydraulic Profiling Tool (HPT) or Colour Logging Tool (CLT) are increasingly applied in geoscientific and geoarchaeological research and successfully used for palaeogeographical reconstructions (Koster, 2016;Hausmann et al, 2018;Fischer et al, 2020;Hadler et al, 2020;Rabiger-Völlmer et al, 2020;Verhegge & Delvoie, 2021, Verhegge et al, 2021. Combined with geophysical prospection data, the site-specific data from DPbased in situ sensing methods can even be spatially extrapolated (Bates et al, 2007;Fischer et al, 2016;Obrocki et al, 2020).…”

Tsunami deposits: Present knowledge and future challenges

“…A new methodological approach combining electrical resistivity tomography, direct pushelectrical conductivity sensing and vibracores was used in the Tiber river (Italy) (Hadler et al, 2020). Within the dominant fluvial deposits, sediments most likely related to tsunami inundations were described for the first time and linked to events that occurred some time before the 2 nd Century BC, in the early 1 st Century AD and in the 17 th Century AD or later.…”

Palaeotsunami deposits at the Tiber River mouth (Ostia Antica, Italy): Do they really exist?

“…In parallel, geoarchaeological research was conducted in the palaeomeander of Ostia that was flowing in the city of Ostia during the Roman period. The first team conducted studies in the area of the Castello Giulio II and in the area of the Castrum of Ostia (Salomon 2013;Pepe, et al, 2016;Salomon et al 2017Salomon et al , 2018, although the second team studied two cross-sections of the palaeomeander of Ostia between these two locations (Wunderlich et al, 2018;Hadler et al, 2020).…”