Palaeopollutions and human impacts on the Tiber delta – POLTEVERE

Our approach of ancient environments is based on the analysis of cores which are performed using a mechanical rotary sampler, which provides deep and stratigraphically representative samples and solve the problem of groundwater, that can make archaeological excavation difficult or impossible beyond 2 m.

A range of analyses will be performed on these cores, in order to:

- Define the chronological frame through two dating methods: (1) AMS (Accelerator Mass Spectrometry) radiocarbon dating and (2) OSL (Optically Stimulated Luminescence) dating, which has the particularity to be directly applicable on the sediment.

- Reconstruct the palaeo-landscapes, through the study of two types of indicators:
(1) sedimentological indicators. Particle size analysis is used to evaluate the intensity of the energy needed to transport the sediment particles, and thus to clarify the processes involved in the successive phases of deposits that characterize different sedimentary environments.
(2) Biological indicators (pollen, macro- and micro-fauna, plant macro-remains,...). Some organisms may be the markers of specific ecological environments and are thus characteristic of a typical environment. Their analysis allows an accurate reconstruction of the landscape’s evolution and the highlighting of the human-induced disturbances.

Methods/Approach (end)
- Assess the nature and intensity of palaeo-pollution
Characterization of cocktails of pollutants is carried by the analysis of forty major and trace elements, including a wide range of heavy metals (Pb, Zn, Cu, Ni, As, etc.). Lead was chosen because of its widespread use in Roman society. The simple measurement of Pb concentration, however, is insufficient in deltaic environments, where sediment mineralogical variations fluctuate rapidly. These variations influence the concentrations of Pb in sediments, depending on the intensity of detritism, so different isotopes of Pb were measured to distinguish the proximal from the distal Pb.

Results
Lead pollution in Roman society, the city of Portus:
Pb concentrations normalized with Al (Pb/Al) clearly show a change in pollution levels over the past two millennia. After a peak of Pb/Al during the period of Roman Empire, characteristic of the height of the Roman Empire, begins a slow decline since the late Roman Empire. The three independent geological parameters µ-T-?, calculated from the Pb isotopic data (Albarède et al., 2012), suggest changes in sources of lead ores supply, fluctuating between the exploitation of Hercynian and/or Alpine mining districts.
Through this geochemical study, it was also a matter of developing a new methodological approach to the study of harbour deposits in geoarchaeology. The example of the Rome’s harbour has allowed us to reconstruct the palaeoenvironmental dynamics of the water column geochemically defined by changes in salinity and oxygen.

Chrono-stratigraphy of an anthropogenic sediment trap, the river harbour of Ostia Antica:
In the northwest of the ancient city of Ostia, the analysis of two cores helped to highlight a chrono-stratigraphic break which can be interpreted as the digging of a harbour basin. This one, dated between the 4th and the 2nd century BC, was contemporary of the ancient city of Ostia, and was 6m depth, which allowed its access to heavy tonnage vessels. Following the study of its filling, the stratigraphic sequence has been divided in three main units, corresponding to three environment types, namely pre-harbour, harbour and port-harbour environment. The study of the ostracods contained in the harbour sediments helped to highlight alternating phases of marine and fluvial influence, which suggests a mobility of the coastline at the mouth of the Tiber. These elements, combined with geoarchaeological observations, permitted the palaeoenvironmental reconstruction of several phases in this area that we can now interpret as a river mouth harbour basin. These changes in the ecosystem seem more anthropogenic than natural.

1. Communiqué de presse CNRS du 07 décembre 2012, « The first harbour of ancient Rome rediscovered ».

2. Article dans World Archaeology janvier 2013, « Harbouring Secrets ».

3. Article dans Archéologia janvier 2013.

3. Article dans Le Monde Science et Techno du 23 février 2013, « Les sédiments racontent la disparition du port antique d’Ostie ».

4. « Dériver les eaux. Le canal à travers les âges : une réponse technologique aux variabilités socio-économiques » Journée d’étude pluridisciplinaire, 23 et 24 mai 2012. Maison de l’Orient et de Méditerranée. Lyon.

9. archeorient.hypotheses.org/327

6. GOIRAN J.-Ph., SALOMON F., BUKOVIEKY E., BOETTO G., 2011, « Altitudes de structures archéologiques à Portus par rapport au niveau marin antique (quartier Magazzini e Darsena)«, Chronique MEFRA, 123-1, p. 241-249.

2. GOIRAN J.-Ph., SALOMON F., PLEUGER E., VITTORI C., MAZZINI I., BOETTO G., ARNAUD P., PELLEGRINO A., 2012, «Résultats préliminaires de la première campagne de carottages dans le port antique d'Ostie«, Chroniques des Mélanges de l'Ecole Française de Rome, vol. n°123-2.

8. GOIRAN J.-Ph., SALOMON F., TRONCHERE H., CARBONEL P., DJERBI H., OGNARD
C., (2011), «Caractéristiques sédimentaires du bassin portuaire de Claude: nouvelles données pour la localisation des ouvertures«, chapter 2, Portus and its Hinterland, Archaeological Monographs of the British School at Rome, Edited by Simon Keay and Lidia Paroli, p. 31-45.

5. SALOMON F., DELILE H., GOIRAN J.-Ph., BRAVARD J.-P., KEAY S., 2012, «The Canale di Comunicazione Traverso in Portus: the Roman sea harbour under river influence (Tiber delta, Italy«), Géomorphologie : relief, processus, environnement, n° 1, p. 75-90.

7. GOIRAN J.-Ph., SALOMON F., TRONCHERE H., DJERBI H., CARBONEL P., OGNARD C., OBERLIN Chr., 2011, «Géoarchéologie des ports de Claude et de Trajan, Portus, delta du Tibre«, MEFRA, 123-1, p. 157-236.

Between the 1st c. B.C. and 2nd c. A.D., the urban pressure is very strong in the lower Tiber valley. Rome is the first Western city to reach a million inhabitants. No other European city will hit that figure before the nineteenth century. The fluvial harbour of Ostia at the mouth of the Tiber is not enough to feed this population. It cannot accommodate large commercial vessels with large draft. Between the mid-1st c. B.C. and the early 2nd c. A.D., the emperors Claudius and Trajan order the digging of two basins for a huge seaport 4km north of Ostia. Many activities related to the harbour grow around the basins. But, in spite of that, Ostia doesn’t decline. It even sees a renewal of itself. Most of the remains of the ancient city of Ostia that can be visited today date from the 2nd c. A.D..
This strong urban pressure undoubtedly generated a whole assemblage of pollutants, some of which have been preserved in sedimentary archives until nowadays. Among the paleo-pollutions, there are paleo-chemical pollutions (heavy metals) and paleo-biological pollutants (paleo-parasites, intestinal helminths and protozoa). Their analysis will allow us to know (1) the specificity of the wastes of ancient cities and (2) the impacts of these cities on their environments.
The analysis of these paleo-pollutions presupposes the finding of sedimentary traps in which they were kept. It is possible to distinguish two types: (1) artificial sedimentary traps, (2) natural sedimentary traps.
The artificial sedimentary traps are often located within the city or in its immediate vicinity. A major sewer system was designed early in the history of Rome with the Cloaca Maxima (7th – 6th c. B.C.). Preserved sediments will be collected in the sewers of Rome, Ostia and Portus. The analysis in paleo-pollutions will also be conducted on the sediments trapped in the harbours basins of Portus and Ostia.
Between the 1st c. B.C. and 2nd c. A.D., the important growth of Rome is combined with a hydro-climate crisis already identified in numerous study sites in Western Europe. Through paleo-climate studies and geoarchaeology we intend to clarify the manifestations of this climate crisis in the watershed of the Tiber and its impact on the fluvial dynamics downstream of Rome. Everything suggests that the lateral mobility of the Tiber was important at that time. New sedimentary forms were likely produced by accretion and elevation of meander convexities. They may have trapped sediments and pollutions carried by the water. The Tiber is thus considered as the vector of the pollutions discharged by the city of Rome. The Tiber delta is a mosaic of trapping: fluvial mobility of the Tiber that we just exposed; recording of the floods in the lagoons; deltaic progradation. The channels connecting the Tiber to Portus are now filled and constitute in the same way an (artificial) trap for the Tiber sediments.