Marine geology methods unveil history of ancient Roman port

The results of an international research project including researchers from the University of Melbourne, La Trobe University and ANSTO in Australia, and international collaborators in Canada and France, is published this month in Quaternary International. The team innovated by applying marine geology methods for the first time at a port archaeological site in Italy. McKenzie Fellow Agathe Lisé-Pronovost from the School of Earth Sciences at the University of Melbourne led this project with geoarchaeologist Jean-Philippe Goiran from CNRS (France).

The sediments accumulated in the old ports of the Mediterranean Sea can record very high accumulation rates and are rarely equaled in the natural environment. These outstanding sedimentary records have unprecedented temporal resolution and are therefore ideal for reconstructing human and climatic history in detail. This is the case for Portus, the ancient seaport of Rome, which was ingeniously built in the first century AD in the Tiber Delta. Portus was a complex of harbour basins and canals that formed the hub of commerce in the capital of the Roman Empire.

A 16th-century fresco in the Vatican Palace shows an idealized reconstruction of Portus’ grand architectural and engineering features.
A 16th-century fresco of Portus.

A major challenge in the geoarchaeology of ancient ports is to determine the age of drilled sediments. It is difficult to date the harbour sediments since their accumulation is not constant as in marine environments far from the continents and human activity. Indeed, a port can receive deposits of floods, storms and tsunami. In addition, the Romans dragged their infrastructure to maintain enough water depth for ship access. They may also have used gates to close canals in times of war or to avoid silting up during floods. The innovative method chosen for Portus sediment analysis was to obtain data from a multitude of high-temporal-resolution parameters to address this dating issue. Parameters studied include piston coring, computed tomography, magnetic properties, clay mineralogy, grain size and radiocarbon dating (14C).

The results reveal that in addition to the harbour sediment accumulated over time, more than one and a half metres of sediment (approximately 40% of the total sediment) has been remobilised. The remobilised sediment thickness has important implications for the reconstruction of the water depth and the type of vessels capable of accessing the port infrastructure. Two major remobilised deposits are identified. The first major deposit is a dredged deposit from the 2nd century AD, nearly two centuries earlier than previous studies. The second major deposit is a thick 66 cm flood deposit that indicates a strong hydrodynamic event. Although storms, tsunamis and floods could form this type of repository in the natural environment, in Portus, it is rather linked to the reopening of the harbour canal after the dredging operations. While the floods of the Tiber are abundantly reported in historical documents, the sediment analysis does not show a time series of flood deposits, but rather a constant hydrodynamism which indicates an efficient management by the Romans of the river flows in their seaport. These data demonstrate that the Romans used gates for operations on their channels as early as the 2nd century AD.

Sedimentation history of Portus.

Sedimentation history of Portus from the core studied by Lisé-Pronovost et al (2019).

This is the first time that sedimentary magnetism has been applied to the geoarchaeology of an ancient port. The magnetic properties allowed the correlation of sedimentary cores, the identification of remobilised deposits and the reconstitution of river input. Sedimentary magnetism is a versatile tool that can be added to the geoarchaeologist's toolbox.

Gold coin depicting Portus.
Coin from the 1st century AD depicting Portus. Photo: Christophe Jacquand (CC BY-SA 4.0).

This project was funded by a Postdoctoral Fellowship from the Research Fund Quebec - Nature and Technology (FRQNT), La Trobe University Transforming Human Societies RFA and DVCR Research Fellowships to Agathe Lise-Pronovost. Fieldwork was supported by the Geological Society of America (GSA) and radiocarbon dating and particle grain size analysis were supported by a research grant from the Australian Institute of Nuclear Science and Engineering (AINSE ALNGRA15016) to A. Lisé-Pronovost.

To access the article: A. Lisé-Pronovost, F. Salomon, J.-P. Goiran, G. St-Onge, A.I.R. Herries, J.-C. Montero-Serrano, D. Heslop, A.P. Roberts, V. Levchenko, A. Zawadzki, H. Heijnis, 2019. Dredging and canal gate technologies in Portus, the ancient harbour of Rome, reconstructed from event stratigraphy and multi-proxy sediment analysis. Quaternary International 511, 78-93.