Rock-Magnetic Proxies of Dredging and Flood Events in Portus, the Ancient Harbor of Imperial Rome

French Title: Signatures magnétiques d'évènements de dragage et de crues à Portus, l'ancient port de Rome

Agathe Lisé-Pronovost1, Guillaume St-Onge2, Andy IR Herries1, David Heslop3, Ferréol Salomon4 and Jean Philippe Goiran5, (1)La Trobe University, The Australian Archaeomagnetism Laboratory, Melbourne, Australia, (2)University of Quebec at Rimouski UQAR, Canada Research Chair in Marine Geology, Rimouski, QC, Canada, (3)Australian National University, Research School of Earth Sciences, Canberra, ACT, Australia, (4)University of Southampton, Department of Archaeology, Southampton, United Kingdom, (5)Centre national de la recherche scientifique (CNRS), Maison de l’Orient et de la Méditerranée, UMR 5133, Lyon, France

Contact First Author: Agathe Lisé-Pronovost; a.lise-pronovost@latrobe.edu.au

Previously Published Material: Part of it was presented at the Casthe meeting 2014.

Abstract ID#: 34012

 

English Abstract:
Exceptionally high average accumulation rates (up to 1 cm/a) were recently reported in the ancient harbor of Rome, Portus, which was constructed on the Tiber delta (Tyrrhenian Sea) during the first century AD and filled with fine sediments during the first millennium AD. With such rapid harbor infill, Romans had to dredge sediments to allow passage. Dredging complicates the work of geoarchaeologists by introducing chronostratigraphic hiatuses and reworked sediments that are not readily identifiable in extruded sediment cores. Here we present high-resolution physical, rock-magnetic and paleomagnetic data from an undisturbed piston core in order to better constrain the chronostratigraphy of Portus.

Core CPS-1 contains ca. 4 m of harbor sediment and was recovered in the canale Traverso, an artificial paleochannel that linked the Tiber River to the harbor. High-resolution analyses were performed, including CT scanning, high-resolution digital imaging, density measurement by gamma ray attenuation, spectrophotometry, magnetic susceptibility, hysteresis properties, natural, anhysteretic and isothermal remanent magnetizations. The CT scans, digital images and data reveal a dredging scar within laminated harbor mud and overlaid by 0.8 m of reworked sediment. A hyperpycnal flow deposit (0.6 m) was also identified using magnetic susceptibility, paleomagnetic inclination and ferrimagnetic grain size and coercivity indicators. The rock-magnetic signature of these dredging and flood events contrast with the background harbor sediment, which is dominated by low coercivity magnetic minerals, stable natural remanent magnetizations and inclinations varying around the expected geocentric axial dipole, suggesting a genuine geomagnetic signal. The precise identification of an event-corrected harbor sequence combined with high-precision radiocarbon dating and possible paleomagnetic dating are useful to better constrain the chronostratigraphy at Portus and highlight the potential of rock-magnetism and paleomagnetism for geoarchaeology.

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