CONTRIBUTION DU SIG A LA RECONSTITUTION DES PROCESSUS DE PROGRADATION DE LA MAJERDA : quels enjeux pour le port d’Utique?

International audienceThe Majerda delta formation has been the subject of many studies since the late XIXe century. The reconstruction of the fluvial palaeo-environments of the lower Majerda valley is significant as it is critical to understand the decline of Utica following the siltation of the harbor, which is presently buried under sediments. The progression of the delta across the corridor separating the Southern and the Northern compartments of the ancient Utica bay is the cornerstone of our research on the delta. On the basis of a GIS we propose to identify hydro- and morpho-sedimentary processes of the deltaic progradation dynamics. They lead to innovative ways of considering the initial implantation site of Utica, the conditions of harbor siltation and the Majerda River itself across the Northern compartment. Observations and analysis from the GIS allowed highlighting a double system of alluvial fans over the whole delta. The first component which may be described as “torrential” is present throughout the delta fringe, while the second category of flat alluvial fans occurs at the Majerda defluviation sites, forming coalescing alluvial lobes. The ancient merging of these two morphological systems close to Utica could reflect a process of harbor silting more complex than previously considered, the Majerda River being considered as the unique factor of change so far

Rome’s urban history inferred from Pb-contaminated waters trapped in its ancient harbor basins

Heavy metals from urban runoff preserved in sedimentary deposits record long-term economic and industrial development via the expansion and contraction of a city’s infrastructure. Lead concentrations and isotopic compositions measured in the sediments of the harbor of Ostia—Rome’s first harbor—show that lead pipes used in the water supply networks of Rome and Ostia were the only source of radiogenic Pb, which, in geologically young central Italy, is the hallmark of urban pollution. High-resolution geochemical, isotopic, and 14C analyses of a sedimentary core from Ostia harbor have allowed us to date the commissioning of Rome’s lead pipe water distribution system to around the second century BC, considerably later than Rome’s first aqueduct built in the late fourth century BC. Even more significantly, the isotopic record of Pb pollution proves to be an unparalleled proxy for tracking the urban development of ancient Rome over more than a millennium, providing a semiquantitative record of the water system’s initial expansion, its later neglect, probably during the civil wars of the first century BC, and its peaking in extent during the relative stability of the early high Imperial period. This core record fills the gap in the system’s history before the appearance of more detailed literary and inscriptional evidence from the late first century BC onward. It also preserves evidence of the changes in the dynamics of the Tiber River that accompanied the construction of Rome’s artificial port, Portus, during the first and second centuries AD

The Canale di Comunicazione Traverso in Portus: the Roman sea harbour under river influence (Tiber delta, Italy)

Portus was Rome’s maritime port during the Roman Empire. In AD 42, the harbour location was selected about 3 km north of Ostia, along the Tyrrhenian coast, on the margin of the Tiber River. Portus and its maritime façade are well known, however the fluvial aspects of Portus are poorly documented. How did Roman engineers preserve a continuous waterway from the basins of Portus to the Tiber River without accelerating siltation inside the harbour? Were their choices efficient? The present Canale di Comunicazione Traverso is the only canal attested to link the Tiber River and the harbour basins. The objective of this work is to analyse the Canale Traverso sediments infill in order to establish the role of this canal in relation to the infill deposits of the harbour basins and to define the functions of the canal. This study is based primarily on a reinterpretation of the available archaeological data, as well as mainly on sedimentological analysis and the interpretation of the Passega diagram. A Passega diagram is presented for understanding the deposit processes for the harbour environments at the entrance to the Trajanic basin (TR-XIV). This diagram is compared to the Passega diagram of the Canale Traverso (CT-1) which is characterised by a stronger influence of the Tiber River. This study concludes that the Canale Traverso was a canal that was well protected from the influence of the Tiber River, with the exception of occasional flooding. Also, this paper presents the patterns of sedimentation at Portus and allows us to define the use of the canal and the maintenance procedure

The record of human impact in the sedimentary record at Portus, the harbor of ancient Rome

International audienceThe present study focuses on the analysis of palaeo-pollutions and the sedimentary environments in which they were trapped in the Roman Portus harbor. Portus received heavy-metals pollution both from local foundries, fulling, and tanning and from distal upstream development in Rome. Rome wastewaters, which accounted for up to 3 percent of the total Tiber discharge, were forwarded to Portus through a network of canals (Canale Romano and Canale Trasverso) connecting the river to the sea. In this manner, harbor basins accumulated both allochthonous and autochthonous heavy metals. We determined major and trace element concentrations as well as Pb isotope compositions in a high-resolution set of samples from sediment cores recovered in the Portus area. Principal component analysis of elements that are less prone to the influence of human activities, such as Ca, Mg, Mn, Zr, K, Al, Ti, Na, Sr, and Mn, was used in conjunction with metallic elements to break down the sedimentary load into local and regional components. The record of Pb concentrations and isotopic compositions reveals an overall general trend on which other signatures are superimposed. The geochemical background of the Tiber catchment (24.7-26.2 ppm Pb and 206 Pb/ 207 Pb ~ 1.198) represents geologically young (model age Tm <50 Ma) Pb derived from natural runoff over young sediments and volcanics in the Latium. From the 1 st century AD to the end of the roman period, the harbor regime evolved from a dominant fluvial (enriched in Al, Ti, Mg, K, and Zr) to a more marine influence (high Ca/Mg, Na/Al, Sr, and CaCO3) in the upper part of the harbor unit. "Imperial" Pb (90.5-35.4 ppm Pb and 20

A lead isotope perspective on urban development in ancient Naples

The influence of a sophisticated water distribution system on urban development in Roman times is tested against the impact of Vesuvius volcanic activity, in particular the great eruption of AD 79, on all of the ancient cities of the Bay of Naples (Neapolis). Written accounts on urbanization outside of Rome are scarce and the archaeological record sketchy, especially during the tumultuous fifth and sixth centuries AD when Neapolis became the dominant city in the region. Here we show that isotopic ratios of lead measured on a well-dated sedimentary sequence from Neapolis’ harbor covering the first six centuries CE have recorded how the AD 79 eruption was followed by a complete overhaul of Neapolis’ water supply network. The Pb isotopic signatures of the sediments further reveal that the previously steady growth of Neapolis’ water distribution system ceased during the collapse of the fifth century AD, although vital repairs to this critical infrastructure were still carried out in the aftermath of invasions and volcanic eruptions

ETUDE DES SÉDIMENTS COTIERS DU CAP CORSE: RECONSTRUCTION PALÉOENVIRONNEMENTALE ET SUIVI DE LA CONTAMINATION EN ÉLÉMENTS TRACES MÉTALLIQUES AU COURS DE LA PÉRIODE HISTORIQUE

Les sédiments sont d’excellentes archives de la contamination de notre environnement. Depuis l’Antiquité, les activités humaines génèrent des aérosols anthropiques enrichis en métaux lourds (e.g. Pb, Zn, Cu, Cd, Hg) ou associés (As, Sb). Ces polluants métalliques se mélangent aux aérosols naturels (altération des roches) et leurs retombées sont incorporées dans les sols et les sédiments où ils sont préservés (Boyd, 2004). Sur des sédiments datés, une approche géochimique permet de quantifier l’apport anthropique en éléments métalliques par rapport aux sources naturelles et de reconstituer l’historique de la pollution. Les côtes méditerranéennes sont caractérisées par d’intenses échanges commerciaux, et ce depuis l’Antiquité. Nous proposons une reconstruction paléoenvironnementale ainsi qu’un suivi de la contamination anthropique au cours de la période historique pour différents sites du Cap Corse (France ; Figure 1). Cette région de Méditerranée se caractérise par une riche activité économique au cours de la période historique. Depuis le VIème siècle, la Corse a été colonisée successivement par les Grecs, les Carthaginois, les Etrusques puis par les Romains. En milieu continental, des témoins archéologiques (sépulture, oppidum, chapelle) attestent d’une occupation de cette zone sur une période assez longue. Cependant de nombreuses interrogations subsistent quant à l’importance des établissements, leurs périodes d’occupation, leurs activités économiques et leurs rapports commerciaux avec les autres cités du Cap Corse et des côtes méditerranéennes (de La Brière, 2010). Des carottes de 1 m à 1,5 m ont été prélevées à la tarière dans différents sites du Cap Corse. Les concentrations en éléments majeurs et en éléments traces métalliques ont été mesurées par spectrométrie de masse (ICP-MS), activation neutronique (INAA) et par XRF core scanner. Les datations 14C réalisées sur des macrorestes de matières végétales et charbons montrent que ces carottes couvrent toute la période historique, à l’exception de celle prélevée à Méria qui ne couvre qu’environ 300 ans. Cette carotte a également la particularité de contenir d’importantes concentrations en métaux lourds tels que le Sb (2000 ppm) et l’As (300 ppm). Cette contamination est d’origine locale et liée à la présence d’une ancienne mine d’antimoine à 2 km en amont de Méria. Afin d’obtenir un enregistrement sur les 2000 dernières années, un carottage plus long sera réalisé lors d’une nouvelle campagne de terrain. D’autres carottages seront également réalisés entre la mine et la côte actuelle afin d’observer la variation spatiale dans le but de quantifier l’impact environnemental des activités humaines locales et régionales

Humans as the third evolutionary stage of biosphere engineering of rivers

We examine three fundamental changes in river systems induced by innovations of the biosphere, these being: (1) the evolution of oxygenic photosynthesis; (2) the development of vascular plants with root systems; and (3) the evolution of humans. The first two innovations provide context for the degree of human-induced river change. Early river systems of the Precambrian Archean Eon developed in an atmosphere with no free oxygen, and fluvial sediments accumulated ‘reduced detrital’ minerals. By 2.4 Ga the evolution of oxygenic photosynthesis produced an oxygenated atmosphere and ‘reduced detrital’ minerals mostly disappeared from rivers, affording a distinct mineralogical difference from subsequent fluvial deposits. Rivers of the Precambrian and early Phanerozoic were dominantly braided, but from 0.416 Ga, the evolution of vascular plants with roots bound floodplain sediments and fostered fine-grained meandering rivers. Early meandering river deposits show extensive animal activity including fish and arthropod tracks and burrows. Homo sapiens, appearing about 150 ka BP, has, in recent millennia, profoundly modified river systems, altering their mineralogical, morphological and sedimentary state. Changes in sediment fluxes caused by human ‘reverse engineering’ of the terrestrial biosphere include deforestation, irrigation and agriculture. Sediment retention has been encouraged by the construction of dams. Modern river systems are associated with extensive human trace fossils that show a developing complexity from ancient civilizations through to megacities. Changes induced by humans rank in scale with those caused by earlier biosphere innovations at 2.4 and 0.416 Ga, but would geologically soon revert to a “pre-human” state were humans to become extinct

Portus

Pour l’année 2011, nous avons centré nos recherches sur les relations entre des carottages dans les bassins et sur les structures portuaires dans le secteur des Grandi Magazzini di Traiano, de la Darsena et du chenal d’accès (fig. 1). Fig. 1 - Portus, secteur des recherches. Nous avons obtenu une coupe transversale Sud-Nord dont tous les éléments sont calés par rapport au niveau marin biologique antique et actuel (fig. 2). Deux échelles altimétriques de part et d’autre de la figure permetten..

Definition of a new approach in ancient harbor geoarchaeology: geochemistry and ostracod analyses at Portus (Tiber delta, central Italy)

International audiencePortus constituted the harbour complex of imperial Rome from the 1st century AD onward. Located in a deltaic environment, the harbour was subject to influences of the Tyrrhenian marine environment and of the fluvial waters of the Tiber River. Such complex sedimentary processes registered also rapid environ-mental changes. Our research is based on the study of a sediment core (TR14) drilled in the access channel of the hexagonal basin of Trajan (Fig. 1). The goal of our study is the reconstruction the palaeo-environmental dynamics of the water column of the harbour according to a geochemical and an ostra-codological approach. The analysis of the ostracod assemblage has evidenced several palaeo-environmental phases. These phases could be identified through the autoeco-logical analysis of the 25 recognised taxa and their classification in four eco-logical groups (MAZZINI et al., 2011). The results obtained present a succession of four environments, defined by variations in salinity and oxygenation. From the bottom to a depth of 643 cm, the environment was a marine to brackish lagoon influenced by strong marine inputs as evidenced by the vic-ariance of Cyprideis torosa and Pontocythere turbida. P. turbida is a typical phy-tal ostracod often found together with Posidonia oceanica remains. From 600 to 454 cm the coastal marine environment becomes more stable. This could be linked to the establishment of the harbour activities and maintenance. The sta-bility of the environment is still evident in the subsequent group, from 435 to 305 cm, characterised by the dominance of C. torosa and the lack of any fresh-water influence. The increase in salinity could be related to evaporative effects on a closed body of water. From 300 cm the freshwater input becomes clear and constant although the influence of the close sea/brackish body of water is still noticeable. In the last metres a typical freshwater assemblage is represent-ed, dominated by truly freshwater taxa (Candona sp. and Pseudocandona marchica), evidence for the occurrence of a freshwater marsh. The geochemical results were analysed by Principal Components Analy-sis (PCA). The samples were assembled in several groups based on a Hierar-chical Ascendant Classification (cluster analysis). Through PCA analysis, the geochemical and ostracological results of the "functional" harbour unit (800-300 cm) were combined. The different elements recorded in the geochemical analysis are distributed according to the main ostracod assemblages (from fresh water to brackish water and sea water). These elements have been grouped according to the model proposed by SAGEMAN & LYONS (2003). An evolution of these deposits in three stages is proposed. While the fluvial envi-ronments were quite well oxygenated, the harbour environment was more characterised by an increasing anoxia of the water column as it was observed by ELMALEH et al. (2012) in the harbor of Tyre. In a second phase, the varia-tions in salinity distinguished the nature of the fluvial environments and the degree of openness to the sea of the harbour environment. The final phase is specific to a body of water completely isolated from fluviatile and marin