The contribution of geochemistry to ancient harbor geoarcheology: The example of Ostia Antica

Résumé En

Once trapped in ancient harbor basins, sediments form environmental archives that have been widely studied by geoarcheologists in recent decades, especially to help reconstruct fluviomarine landscapes of the last millennia. In some cases, classic environmental markers cannot be used for this purpose either because of their scarcity in the sedimentary deposits, or because analytical costs limit the resolution that can be achieved. In order to remedy these shortcomings, and to complement the more commonly used proxies, elemental and isotopic geochemistry has been added to the geoarcheological toolkit. Here we show how to "read" the evolution of the paleo-environmental dynamics in the water column of Ostia Antica (Rome's first maritime harbor) using the geochemical and isotopic record of a 3000-year-old sediment core drilled in the ancient harbor basin. A comparison of the results obtained from Ostia Antica with those of other ancient Mediterranean harbors reveals the nature of the main environmental processes operating during the formation of sedimentary deposits in harbor basins. From this comparative approach, it appears that the respective weight of each control factor is dependent on the coastal geomorphological context of the sites where the harbors were established. Since the discovery of the harbor of Ostia Antica in 2014, this method has provided the means, for the first time, to identify two distinct harbor basin regimes; an initial marine-dominated regime from the middle of the 4 th c. BC to ~ the 3 rd c. BC, and a later freshwater-dominated regime up to the 2 nd c. BC. More generally, we observe the effects of the dynamics of the deltaic progradation of the Tiber, which very early on was subject to a hydro-climatic component, on the processes of alluviation of the harbor basin. Additionally, and also for the first time in harbor geoarcheology, Pb isotope compositions measured specifically on uncontaminated sediments demonstrate their utility for both identifying the geological sources of the sediments of the Tiber delta and discriminating finer from larger particles. The present study further provides an opportunity to test the validity of two hypotheses recently put forward: (1) that a series of three tsunamis is recorded in the harbor silts, and (2) that an initial lagoon-type harbor was constructed at Ostia Antica, which later evolved into a fluvial harbor. Neither of these hypotheses are supported by the present data.