Ferréol Salomon
email: ferreol.salomon@univ-lyon2.fr

Hugo Delile
email: hdelile@gmail.com

Jean-Philippe Goiran
email: jean-philippe.goiran@mom.fr

Jean-Paul Bravard
email: jean-paul.bravard@univ-lyon2.fr

Simon Keay

Lidia Paroli

Etude du Canale di Comunicazione Traverso à Portus : le port maritime antique de Rome sous influence fluviale (delta

Keywords: relief, processus, environnement, 2012, n°1, p. 75-90 geoarchaeology, Roman harbour, Roman canals, late Holocene, Tiber delta, Rome, Italy géoarchéologie, port romain, canaux romains, Holocène récent, delta du Tibre, Rome, Italie

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

Ferréol Salomon, Hugo Delile, Jean-Philippe Goiran, Jean-Paul Bravard,

Simon Keay

To cite this version: 2). Diverses analyses ont été ées sur les carottes (fig. 5 et fig. 6) : granulométrie, taux de matière organique, susceptibilité magnétique (fig. 4), densité apparente, interprétation du diagramme CM de Passega (1957 ;fig. 7). Ce travail concerne en particulier les données granulométriques et les questions de dynamique sédimentaire en milieu fluvio-marin portuaire. Dans ce secteur de Portus, trois publications récentes [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF][START_REF] Sadori | The plant landscape of the imperial harbour of Rome[END_REF][START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF] développent l'aspect plus proprement paléoenvironemental avec des analyses macro-et microfaunistiques et la détermination des pollens présents. Une synthèse de ces résultats, plus spécifiquement orientée sur l'ostracofaune (fig. 3), confirme une très nette différence entre le bassin du port de Claude, où dominent les eaux salées, et le chenal d'accès au bassin de Trajan marqué par la rencontre des eaux salées et douces. Mais avant tout, pourquoi tant d'études récentes ont-elles concerné ce petit canal ? Le Canale Traverso est le seul lien entre le complexe portuaire de Portus et le système fluvial du Tibre. L'enjeu pour les ingénieurs romains était donc de 1) réaliser une voie navigable continue entre les bassins portuaires et le Tibre tout en 2) évitant l'envasement accéléré de ces bassins par les apports fluviaux [START_REF] Reddé | Mare Nostrum : les infrastructures, le dispositif et l'histoire de la marine militaire sous l'Empire Romain[END_REF]. Le Canale Traverso aurait été pensé pour satisfaire ces deux objectifs [START_REF] Gall | Le Tibre, fleuve de Rome dans l'antiquité[END_REF]. Le présent travail se situe ainsi à l'interface des études de géoarchéologie fluviale et portuaire.

Le Canale Traverso a probablement été creusé dès le I er s. apr. J.-C. [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF]. Le plan dressé par les archéologues [START_REF] Lugli | Il porto di Roma imperiale e l'agro portuense[END_REF][START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF] révèle son implantation assez particulière (fig. 2) puisqu'il est en connexion secondaire avec le Tibre : le Canale Traverso (sud-nord) s'embranche en effet sur le canal de Fiumicino (est-ouest) à 1500 m du Tibre actuel. Ce tronçon amont du Fiumicino correspond à la probable Fossa Traiana antique qui se jetait dans la mer au sud de Portus (fig. 1). Cette position particulière s'accompagne aussi d'une réduction graduelle de la largeur des canaux successifs (Fossa Traiana, Canale Traverso). Le Tibre, aujourd'hui large de 100-120 m dans le delta était d'une largeur comparable dans l' Antiquité (Le Gall, 1953 ;[START_REF] Bertacchi | Elementi per una revisione della topografia ostiense[END_REF]. Grâce à la découverte des berges antiques de la Fossa Traiana sur plusieurs tronçons mis au jour au XX e s., la largeur de ce canal antique est estimée à environ 50 m [START_REF] Testaguzza | Portus: illustrazione dei Porti di Claudio e Traiano[END_REF]. Enfin, cette largeur se réduit encore pour atteindre 25 m dans le Canale Traverso. Chaque embranchement de canal s'accompagne ainsi d'une diminution de moitié de la largeur, réduisant ainsi l'écoulement des eaux fluviales vers les bassins portuaires. Ces données géométriques sont très intéressantes puisqu'elles témoignent sans équivoque de la conscience du problème de l'articulation entre le système portuaire et le système fluvial par les architectes et ingénieurs romains responsables de la construction de Portus.

Mais cette configuration fut-elle efficace ?

La carotte , réalisée dans le Canale Traverso, indique une profondeur de 5,5 m sous le niveau marin antique (calage en référence à J.-P. [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF]. La lame d'eau antique du chenal d'accès au bassin de Trajan se fait plus profonde en direction du bassin de Claude (8 m dans la carotte TR-XI ; [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF]. Dès l'origine, le Canale Traverso semble ainsi ne pas avoir été conçu pour recevoir les plus gros navires antiques [START_REF] Pomey | Le tonnage maximum des navires de commerce romains[END_REF][START_REF] Boetto | Le port vu de la mer: l'apport de l'archéologie navale à l'étude des ports antiques[END_REF]fig. 8). Dans un premier temps, le canal se remblaie très rapidement (2,6 cm/a ; fig. 8). L'en-registrement sédimentaire entre -5,5 et -2,3 m sous le niveau marin antique est composé principalement de vases déposées par suspension uniforme mélangée à des sables déposés par suspension graduée (unités B et C, fig. 5 et fig. 7). Le Canale Traverso en fonctionnement offre ainsi un environnement de dépôt calme soumis aux influences marines et fluviales, proche des conditions existant à l'entrée du bassin de Trajan (fig. 6 et fig. 7). L'étude comparée des diagrammes CM des carottes fig. 7) montre une taille moyenne du D 99 des processus du type 4 (fig. 7), plus élevée dans le Canale Traverso. Ce gradient granulométrique atteste ainsi l'apport de sédiments roulés dans le Canale Traverso en provenance non pas du port de Claude (sableux), mais du fleuve. La texture restant très fine, ces apports grossiers sont très probablement le résultat d'événements hydrologiques ponctuels comme les crues. Dans le dépôt supérieur du Canale Traverso (de -2,3 m au 0 du niveau marin antique ; fig. 5 et fig. 8), la présence plus nette de sédiments fluviaux est marquée par une fraction sableuse plus importante. Les conditions de dépôt se produisent plutôt par un mélange de roulement et de suspension graduée et uniforme. Bien que plus grossier, ce remblaiement est en revanche beaucoup plus lent que dans l'unité sous-jacente : 0,45 cm/a (fig. 8). La datation radiocarbone réalisée à environ 1 m sous le niveau marin antique donne la date la plus récente obtenue dans ce secteur de Portus et correspond à la fin d'utilisation du Canale Traverso : 1415±15 BP, soit 600-660 après J.-C. Ce taux de remblaiement très lent pourrait être expliqué par un (ou plusieurs ?) curage (s) réalisé (s) vers -2,3 m sous le niveau marin antique. Avec une telle lame d'eau, les petits navires de

Introduction

Portus was the seaport of Rome during the Roman Imperial period. The construction of the harbour began in AD 42 under the reign of the Emperor Claudius. The harbour was located about 3 km north of Ostia, along the Tyrrhenian Coast, but on the margin of the Tiber. A few hundred metres east of Portus, the natural Tiber channel curved to the south near Ostia, where it flowed into the sea (fig. 1). At the beginning of the 2nd c. AD, Emperor Trajan complemented the Claudian harbour with a second basin (Juvenal, Saturae, XII, 75-78). To date, many sedimentological and palaeoenvironmental studies have been conducted at Portus, its basins and its maritime entrances [START_REF] Arnoldus-Huyzendveld | Portus, an archaeological survey of the port of imperial Rome[END_REF][START_REF] Giraudi | Carotaggi e studi geologici a Portus: il delta del Tevere dai tempi di Ostia Tiberina alla costruzione dei porti di Claudio e Traiano[END_REF][START_REF] Goiran | Recent geo-archeological findings of Portus, the ancient Harbour of Rome[END_REF][START_REF] Goiran | Portus, la question de la localisation des ouvertures du port de Claude : approche géomorphologique[END_REF][START_REF] Bellotti | Geoarchaeological investigations in the area of the imperial harbours of Rome[END_REF][START_REF] Giraudi | -Late Holocene evolution of Tiber river delta and geoarchaeology of Claudius and Trajan Harbor, Rome[END_REF][START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF], but much less is known about the fluvial features at Portus [START_REF] Salomon | Delta du Tibre -Campagne de carottage 2009 -Géoarchéologie des canaux de Portus : l'exemple du Canale Romano[END_REF]. Inscriptions made during the reigns of Claudius (CIL XIV 85 = ILS 207) and Trajan (CIL XIV 88 = CIL VI 964 = ILS 5797a) were found at Portus. They record the digging of canals linking the Tiber River to the sea. Today, a unique canal, the Fiumicino, is visible and still in use on the southern outskirts of Portus which was proba-bly the Roman Fossa Traiana (Fea, 1924 a and b). This canal is shown on maps of Portus drawn during the Renaissance: for example in a fresco, painted by Ignazio Danti (1582; Galleria delle carte Geografiche of the Vatican Museum). The Canale di Comunicazione Traverso was a NNW-SSE channel almost perpendicular to the Fiumicino, which linked this canal to the southern access of the Trajanic basin (fig. 1). It was the only waterway that existed between the sea basins and the fluvial system [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF]. M. [START_REF] Reddé | Mare Nostrum : les infrastructures, le dispositif et l'histoire de la marine militaire sous l'Empire Romain[END_REF] thinks that the rapid silting up of Portus' basins was probably due to fluvial sedimentary input. How did Roman engineers preserve a continuous waterway from Portus' basins to the Tiber River without accelerating the rate of siltation in the harbour? Or on the other hand, was this canal able to carry sediment away from the basins thanks to the increase in outflow? Sediment cores, allowed the analysis of the characteristics of the Canale Traverso in order to (i) establish its role in the sedimentation of Portus' basins and (ii) to define the function and the operation of this canal.

Setting

Geology and geography

The Tiber River, 405 km long from the source to the Tyrrhenian Sea, originates in the Apennine Mountains (Mount Fumaiolo, 1268 m) and drains a 17375 km² basin. The mean annual discharge at the Ripetta gauging station (Rome, 43 km upstream the mouth) is 225 m 3 /s. The maximum historic discharge exceeds 3000 m 3 /s and the mini-mum discharge 60 m 3 /s. The mean suspended load was 7.2x10 6 t/a at the Ripetta station (1934)(1935)(1936)(1937)(1938)(1939)(1940)(1941)(1942)(1943)(1944)(1945)(1946)(1947)(1948)(1949)(1950), before the impacts of large dams on sediment fluxes occurred [START_REF] Iadanza | Sediment transport time series in the Tiber River[END_REF].

The Tiber delta has been studied since the 19th c. [START_REF] Moro | Lo stagno di Ostia[END_REF][START_REF] Ponzi | Il delta del Tevere: storia naturale del Tevere[END_REF][START_REF] Amenduni | Sulle opere di bonificazione della plaga litoranea dell'Agro Romano che comprende le paludi e gli stagni di Ostia, Porto, Maccarese e delle terre vallive di Stracciacappa, Baccano, Pantano e Lago dei Tartari[END_REF][START_REF] Bocci | Il delta tiberino[END_REF], but knowledge on it has improved in the second half of the 20th c. [START_REF] Dragone | -Note illustrative del Foglio 149 Cerveteri della Carta Geologica d'Italia[END_REF]. Since the 1980's, deep-coring studies have been completed in order to investigate Holocene stratigraphy [START_REF] Belluomini | Evoluzione recente del delta del Tevere[END_REF][START_REF] Belfiore | -Il delta del Tevere: le facies sedimentarie della conoide sommersa. Un'analisi statistica dei caratteri tessiturali, microfaunistici e mineralogici[END_REF][START_REF] Bellotti | -La piana deltizia del Fiume Tevere: analisi di facies ed ipotesi evolutiva dell'ultimo "low stand" glaciale all'attuale[END_REF][START_REF] Bellotti | Sequence stratigraphy and depositional setting of the Tiber delta: integration of high-resolution seismics, well logs, and archeological data[END_REF][START_REF] Bellotti | Physical stratigraphy and sedimentology of the Late Pleistocene-Holocene Tiber Delta depositional sequence[END_REF][START_REF]Depositional setting and high-frequency sequence stratigraphy of the Middle-Upper Pleistocene to Holocene deposits of the Roman Basin[END_REF][START_REF] Amorosi | Late Quaternary depositional architecture of Po and Tevere river deltas (Italy) and worldwide comparison with coeval deltaic successions[END_REF][START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14 C dating[END_REF]. The Tiber delta results from two main influences, a combination of sea and fluvial processes. The rapid sea-level rise from 18000 BP to 7000 BP induced the submersion of the Pleistocene delta. The pace of sea-level rise reduced from 7000 BP to 5000 BP. Consequently, the fluvial sediment input became dominant and the Tiber delta adopted the lobate form, a characteristic of a progradational system [START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14 C dating[END_REF]. The major migrations of the Tiber channel have been divided into three major phases: (i) During the Late Pleistocene, the palaeo-Tiber flowed into the northern part of the delta [START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14 C dating[END_REF]; (ii) The second stream way switched to the middle part of the present delta ca. 7000 BP [START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14 C dating[END_REF]; (iii) During the 9th-8th c. BC, the Tiber mouth moved to the south near Ostia Antica [START_REF] Giraudi | -Late Holocene evolution of Tiber river delta and geoarchaeology of Claudius and Trajan Harbor, Rome[END_REF][START_REF] Bellotti | The Tiber River Delta Plain (Central Italy): Coastal evolution and implications for the Ancient Ostia roman settlement[END_REF].

Over the past twenty years, a lot of research has been conducted on the late Holocene period aiming to locate the successive Tiber palaeochannels [START_REF] Segre | Considerazioni sul Tevere e sull'Aniene nel Quaternario. In Il Tevere e le altre vie d'acqua del Lazio antico[END_REF][START_REF] Arnoldus-Huyzendveld | Alcune considerazioni sullo sviluppo storico dell'ansa dell Tevere presso Ostia e sul porto-canale[END_REF]Arnoldus-Huyzendveld and The Canale di Comunicazione Traverso in Portus [START_REF] Arnoldus-Huyzendveld | Traces of historical landscapes preserved in the coastal area of Rome[END_REF], lagoons [START_REF] Giraudi | Evoluzione ambientale tardo-olocenica nell'area comprente il sto eneolitico di Maccarese (Fiumicino)[END_REF][START_REF] Rita | Holocene environmental instability in the wetland north of the Tiber delta (Rome, Italy): sea-lake-man interactions[END_REF][START_REF] Bellotti | The Tiber River Delta Plain (Central Italy): Coastal evolution and implications for the Ancient Ostia roman settlement[END_REF] and the coastline variations [START_REF] Giraudi | Evoluzione tardo-olocenica del delta del Tevere[END_REF][START_REF] Rendell | Late Holocene Mediterranean coastal change along the Tiber delta and Roman occupation of the Laurentine shore, central Italy[END_REF][START_REF] Bicket | A multiscale geoarchaeological approach from the Laurentine shore (Castelporziano, Lazio, Italy)[END_REF] more precisely. In central Italy, more intense fluvial activity seems to have occurred during Roman Imperial times and reduced during the Late Antiquity and the early Middle Ages [START_REF] Benvenuti | Late-Holocene catastrophic floods in the terminal Arno River (Pisa, Central Italy) from the story of a Roman riverine harbour[END_REF][START_REF] Bicket | A multiscale geoarchaeological approach from the Laurentine shore (Castelporziano, Lazio, Italy)[END_REF]. Some of those past hydrological events have been monitored in Rome and have been mentioned by ancient authors (Le [START_REF] Gall | Le Tibre, fleuve de Rome dans l'antiquité[END_REF]Bersani and Bencivenga, 2001).

The archaeological area of Portus has been the subject of many geoarchaeological and palaeoenvironmental studies. Studies focused on the environments that existed prior to the building of the harbour and on ancient harbour basins [START_REF] Goiran | Géoarchéologie des ports antiques de Méditerranée : problématiques et études de cas[END_REF][START_REF] Marriner | Coastal and ancient harbor geoarchaeology[END_REF]. Preharbour aspects have been approached mainly through the study of the migration of the Tiber River in this area of the delta [START_REF] Segre | Considerazioni sul Tevere e sull'Aniene nel Quaternario. In Il Tevere e le altre vie d'acqua del Lazio antico[END_REF][START_REF] Giraudi | Carotaggi e studi geologici a Portus: il delta del Tevere dai tempi di Ostia Tiberina alla costruzione dei porti di Claudio e Traiano[END_REF][START_REF] Goiran | Recent geo-archeological findings of Portus, the ancient Harbour of Rome[END_REF][START_REF] Giraudi | -Late Holocene evolution of Tiber river delta and geoarchaeology of Claudius and Trajan Harbor, Rome[END_REF][START_REF] Bellotti | The Tiber River Delta Plain (Central Italy): Coastal evolution and implications for the Ancient Ostia roman settlement[END_REF] and through the position of the shoreline during historical times [START_REF] Arnoldus-Huyzendveld | Portus, an archaeological survey of the port of imperial Rome[END_REF]. The harbour itself was studied through the reconstruction of basins' shape, and through detailed sediment analysis [START_REF] Arnoldus-Huyzendveld | Portus, an archaeological survey of the port of imperial Rome[END_REF][START_REF] Giraudi | Carotaggi e studi geologici a Portus: il delta del Tevere dai tempi di Ostia Tiberina alla costruzione dei porti di Claudio e Traiano[END_REF][START_REF] Goiran | Portus, la question de la localisation des ouvertures du port de Claude : approche géomorphologique[END_REF][START_REF] Bellotti | Geoarchaeological investigations in the area of the imperial harbours of Rome[END_REF][START_REF] Giraudi | -Late Holocene evolution of Tiber river delta and geoarchaeology of Claudius and Trajan Harbor, Rome[END_REF][START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF][START_REF] Sadori | The plant landscape of the imperial harbour of Rome[END_REF]. J.-P. [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF] identified different sediment deposition in the two basins of Portus. The Claudian basin was a poorly protected basin that opened out to the sea; its sediment infill was mostly composed of sands as it was open to the influences of the sea wind and swell. The Trajanic basin was better protected; as a result, its sediment infill was mostly composed of silt and clay [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF]. In with the continuity of these results, the present work focuses on the origin of sediments in the Canale Traverso and Trajan basin.

Archaeology and geoarchaeology

The geoarchaeological work carried out at Portus would not have been possible without previous historical and archaeological research [START_REF] Lugli | Il porto di Roma imperiale e l'agro portuense[END_REF][START_REF] Lanciani | Ricerche topografiche sulla città di Porto. Annali dell'Instituto di Corrispondenza[END_REF][START_REF] Testaguzza | Portus: illustrazione dei Porti di Claudio e Traiano[END_REF][START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF]. The available data concerning the geographical units are the following:

-The Fiumicino. In the Late Antiquity, the mouth of the Fiumicino channel was about 2000 m from the Tiber (Procopius, De Bello Gothico, 1, 26, 7-13). The western part of the deltaic system has been attributed to the rapid progradation of the coast over the last five centuries (Le [START_REF] Gall | Le Tibre, fleuve de Rome dans l'antiquité[END_REF][START_REF] Giraudi | Evoluzione tardo-olocenica del delta del Tevere[END_REF][START_REF] Bersani | Evoluzione storica della linea di costa in prossimità della foce del Tevere[END_REF]fig. 1). This canal would have remained navigable until AD 1118 and perhaps even AD 1461 [START_REF] Coccia | Portus Romae" fra tarda antichità ed altomedioevo. In Storia economica di Roma nell'Altomedioevo alla luce dei recenti scavi archeologici[END_REF][START_REF] Coccia | Il recinto fortificato dell'episcopio di Porto come epilogo di una crisi urbana[END_REF][START_REF] Paroli | Il porto di Roma nella tarda antichità[END_REF][START_REF] Paroli | Portus. An Archaeological Survey of th Port of Imperial Rome[END_REF]. Unfortunately, sedimentary archives have disappeared due to successive operations of dredging. For instance, in AD 1612, the canal was restored to its navigable state when Pope Paul V ordered for it to be dredged [START_REF] Chiumenti | La campagna romana antica, medioevale e moderna. Edizione redatta sulla base degli appunti lasciati da Giuseppe e Francesco Tomassetti VI[END_REF][START_REF] Paroli | Portus. An Archaeological Survey of th Port of Imperial Rome[END_REF]; also the restoration of the canal banks was implemented in the early 20th c. [START_REF] Gatti | Fiumicino. Avanzi di antiche fabbriche scoperte nell'Isola Sacra, presso S. Ippolito[END_REF]. In the first monographs on the archaeology of Portus, the Fiumicino canal holds a prominent position. In several studies published in the early 19th c., C. Fea (1824 a and b) focused on this channel and identified the Fiumicino canal with the Roman Fossa Traiana, an ancient canal of Portus cited by Pliny the Younger. For S. [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF], the Fiumicino canal dates back to the Claudian period and was later incorporated into the new Trajanic complex.

-The area of the Canale di Comunicazione Traverso. The Canale di Comunicazione Traverso is a 300 m long and up to 25 m wide channel. This canal is located in an area that was densely occupied after the construction of the harbour from the mid-1st c. AD to the late Middle Ages [START_REF] Paroli | Il porto di Roma nella tarda antichità[END_REF][START_REF] Paroli | Portus. An Archaeological Survey of th Port of Imperial Rome[END_REF]. The Canale Traverso was probably surrounded by warehouses and storage areas as early as the mid-1st c. AD [START_REF] Lugli | Il porto di Roma imperiale e l'agro portuense[END_REF][START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF]. The recent excavation of the Basilica Portuense provided extensive data on the evolution of the urban sector. Three chronological phases were identified: (i) the port area which was occupied by administrative and commercial activities, along with warehouses, (ii) at the end of the 3rd and early 4th c. AD, the area became residential, then (iii) between the end of the 4th c. AD and the second half of the 5th c. AD, the basilica was built (Paroli, 2005, p. 258). The archaeological age of the Canale Traverso is not currently ascertained by any specific architectural studies. However, based on a study of the visible apparatus [START_REF] Testaguzza | Portus: illustrazione dei Porti di Claudio e Traiano[END_REF], the Darsena-Canale Traverso-Fossa Traiana complex is thought to have been built at the same time. Thus, the discovery of a brick from Neronian times in one of the Darsena docks dates the Canale Traverso back to the 1st c. AD [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF][START_REF] Verduchi | Some thoughts on the infrastructure of the port of Rome[END_REF].

A brief synthesis of palaeoenvironmental analysis

Three recent publications [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF][START_REF] Sadori | The plant landscape of the imperial harbour of Rome[END_REF][START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF] develop palaeoenvironmental reconstructions from macrofauna, microfauna (ostracods), and pollen analysis. Ostracod assemblages are related to water salinity [START_REF] Carbonel | Les ostracodes et leur intérêt dans la définition des écosystèmes estuariens et de plateforme continentale. Essais d'application à des domaines anciens[END_REF]. They are the best indicators to reconstruct marine and fluvial influences on a statistical base. Fig. 3 is a synthesis of J.-P. Goiran et al. (2010, in press), and I. [START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF]. Results concerning the ostracods groups were simplified into three palaeobiotopes: freshwater, brackish and marine waters. Each circle corresponds to a harbour basin unit defined by respective authors. Fig. 3 clearly indicates the contrast between the Claudian basin, mostly influenced by marine environment (cores CL-7, CL-2 and TR-IV), and the Trajanic channel area, characterised by a brackish palaeobiotope with temporary freshwater influence. Surprisingly enough, the sediment of Canale Traverso contains fewer freshwater ostracods (PTS-5) than the Trajanic access channel (TR-XIX, PTS-13) considering the basal layers of the harbour and of the canal during their activity. Could these results allow other freshwater sources to be considered? Indeed, near the TR-XIX core, thermae have been found [START_REF] Lugli | Il porto di Roma imperiale e l'agro portuense[END_REF]fig. 2). The freshwater/brackish signals can then be due to inland water discharge from an aqueduct [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF][START_REF] Tata | Le acque e gli acquedotti nel territorio ostiense e portuense. Ritrovamenti e ricerche recenti[END_REF] and not only from the Tiber River through the Canale Traverso.

Methods

While recent articles have focused on macrofaunal bioindicators (shells), ostracods and pollen [START_REF] Giraudi | -Late Holocene evolution of Tiber river delta and geoarchaeology of Claudius and Trajan Harbor, Rome[END_REF][START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF][START_REF] Sadori | The plant landscape of the imperial harbour of Rome[END_REF][START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF], this paper focuses on granulometric data and possible river influence on the canal. Core locations are presented on the fig. 2 with results of the geomagnetic survey obtained by fluxgate radiometer and published in S. [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF].

Cores presented were collected using a mechanical rotary core barrel during several field excursions from 2004 to 2009. The objective of coring TR-XIV and CT-1 was to accurately characterise the sedimentary sequences of the access channel to the Trajanic basin and the Canale Traverso. Thus, the cores are around 10-m deep and include the complete harbour and canal sedimentation sequence. The cores were analysed in the OMEAA laboratory in Lyon.

Magnetic susceptibility was measured three times every centimetre using a Bartington MS2E1 [START_REF] Dearing | Environmental magnetic susceptibility[END_REF]. The value of susceptibility is recorded in CGS (centimeter, gram, 79 Géomorphologie : relief, processus, environnement, 2012, n°1, p. 75-90

The Canale di Comunicazione Traverso in Portus [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF][START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF]. 1: cores locations with portuary units [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF][START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF]Goiran et al., in press); all cores are represented according to the 3rd-5th c. sea level [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF]. Only the portuary units data are shown; 2: harbour bottom; 3: ostracod groups with 3a: freshwater species, 3b: brackish species, and 3c: marine species.

Fig. 3 -Synthèse des données paléoenvironnementales obtenues pour les bassins de Portus par détermination de l'ostracofaune (Goiran et al., 2010 ; Mazzini et al., 2011). 1 : localisation des carottages avec leurs unités portuaires (Goiran et al., 2009; Mazzini et al., 2011; Goiran et al., sous presse) ; tous les carottages sont représentés en référence au niveau marin des IIIè-Vè apr. J.-C. [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF] ; 2 : fond des bassins portuaires ; 3 : ostracodes ; 3a : individus d'eau douce ; 3b : individus d'eau saumâtre ; 3c : individus ca- ractéristiques du milieu marin.

second system) unit (corresponding to SI = CGS value * 0.4; [START_REF] Dearing | Environmental magnetic susceptibility[END_REF]. This non-destructive method efficiently separates different sedimentary units [START_REF] Dearing | Environmental magnetic susceptibility[END_REF], as attested in fig. 4 with a histogram depicting the CT-1 core. One colour rectangle corresponds to each unit. The x-axis represents the magnetic susceptibility in each 5 CGS. The occurrences of MS values in this range of 5 CGS are shown on the y-axis (fig. 4). Magnetic susceptibility offers a range of values with fairly consistent distributions for low values, but very large distributions for high values. Corresponding to the magnetic susceptibility, densitometric analysis was undertaken using an X-ray-scanner. Based on this high-resolution analysis, we sampled every 3 cm to 10 cm. Then, depending on sedimentary facies, we analysed a sample every 10 cm to 40 cm.

Then, to get an overview of the texture of each of these samples, we sieved 30 g of the total fraction to distinguish between silt-clay (<63 μm), sand (63 μm-2 mm) and coarse fraction (>2 mm). Finally, on the sediment fraction <1.6 mm, accurate granulometrical analysis was undertaken using a Malvern Mastersizer 2000. Trask sorting index and the median grain have been used to describe the grain size distribution [START_REF] Folk | Brazos river bar: a study in the significance of grain size parameters[END_REF][START_REF] Rivière | Méthodes granulométriques, techniques et interprétations[END_REF]. The interpretation of granulometric curves was based on the CM diagram, also called the Passega image [START_REF] Passega | Texture as characteristic of clastic deposition[END_REF][START_REF] Bravard | Les sédiments fins des plaines d'inondation dans la vallée du Haut-Rhône[END_REF][START_REF] Arnaud-Fassetta | Dynamiques fluviales holocènes dans le delta du Rhône[END_REF][START_REF] Bravard | The CM image as a tool for the classification of alluvial units and floodplains along the river continuum[END_REF][START_REF] Arnaud-Fassetta | The site of Aquileia (northeastern Italy): exam-ple of fluvial geoarchaeology in a Mediterranean coastal plain[END_REF]. This diagram uses the median (D 50 ) and the coarsest percentile (D 99 ) to determine depositional and transport processes.

During this study, we used the biological mean sea level as determined on the quay of Portus [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF]. It corresponds to the upper biological limit of barnacles dated at 2115±30 BP, calibrated AD 230-450. This biological, Roman sea level (R.s.l.) stands 0.8 m below the current biological sea level [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF]. The studied cores are related to those current and R.s.l. This article will deal especially with the period from the 1st to the 7th c. AD. During this period, the sea level may have changed a few centimetres above and below the sea level of the 3rd-5th c. AD [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF]. We do not exclude the possibility of limited compaction of the harbour sedimentary sequence, but without consequences for our interpretations.

In order to establish an age-depth model, five radiocarbon dates were obtained on core CT-1. We focused on dating 80 Géomorphologie : relief, processus, environnement, 2012, n°1, p. 75-90 Ferréol Salomon et al.

Samples

Laboratory wood fragments, charcoal and seeds using the AMS dating technique. Calib 6 software was used for calibrating dates (tab. 1) with the continental curve [START_REF] Reimer | IntCal04 Terrestrial Radiocarbon Age Calibrations, 0-26 cal kyr BP[END_REF]. A margin of error of 2 sigma was retained.

Results

The detailed description of cores CT-1 and TR-XIV is presented below:

CT-1: Canale Traverso (fig. 5 and fig. 7)

This sedimentary core focuses on the granulometric description. For more information about microfauna and pollen content of core PTS-5 (fig. 2), we refer the reader to L. [START_REF] Sadori | The plant landscape of the imperial harbour of Rome[END_REF] and I. [START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF].

-Unit A is composed of yellow sterile layered sands (82%). The silt and clay fraction corresponds to 18% of the total dry sample. The sediment is moderately well sorted with a mean Trask index of 2. The median grain size of this unit is the highest of the whole sedimentary sequence (197 μm), but it rises progressively from the bottom of the core around 100 μm (graded suspension at a depth of 1183 cm, type 1) to 354 μm (medium sand) at the top (dominance of graded suspension and rolling, type 2). A layer at the bottom of the sequence is composed mostly of silts and clays (65%) with a median grain size around 13 μm. This sandy-silt layer has a coarse fraction with plant remains and seeds. A piece of charcoal has been dated to 4030±30 BP (2620-2475 BC). The global magnetic susceptibility value (MS) of this unit is around 156 CGS, ranging between 34 and 378 CGS.

-Units B and C are composed of dark grey mud with shells and Posidonia, which is a marker of the marine influence. 95% of unit B is composed of silt and clay, a value reduced to 84% in unit C. The average median grain size for most of sequence B and C ranges between 10 μm and 13 μm. These units are characterised by a poor sorting index (mean of 3.5). The depositional processes point mostly to mixed graded and uniform suspensions (type 3 on the CM diagram; fig. 7). However, in units B-C, sample '741' is a mix of rolled particles and graded/uniform suspension (type 4). The organic content of units B and C is high (10% of the dry weighted sediment). The coarse faction (>2 mm) is composed of shell fragments, clasts (rolled and not rolled), plant remains, charcoal, wood fragments and Posidonia. Seeds and charcoal have been radiocarbon dated 1920±30 BP (AD 25-130; Lyon-6895) and 1940±30 BP (AD 5-125; Lyon-6894) respectively, at the bottom of unit B. In the middle of unit C, a fragment of wood has been dated 1838±30 BP (AD 90-245; Lyon-7081). The mean MS is higher in unit B (38 CGS), than in unit C (9 CGS).

-The sedimentary facies of unit D is a grey sandy-silt with shells and artefacts. Compared to unit C, a larger proportion of the dry weight sediment is composed of sand (31%) and coarse fraction larger than 2 mm (19%). The median grain size is very fine sand (67 μm). The sediment of unit D is very poorly sorted (mean of 5.6). Depositional processes are dominated by a mix of rolling and graded suspension (type 4); a mix of graded and uniform suspension (type 3) is represented in the lower part (sample '433', type 3). We can observe a progressive decrease in the percentage of the organic material from 8.5% at the bottom to 5.6% at the top. Fragments of shells, clasts, wood fragments, vegetation remains, a lot of potshards and some Posidonia are present at the bottom of unit D. A later date has been obtained from a piece of charcoal: 1415±30 BP (AD 600-660; Lyon-6869). The MS value increases significantly to a mean of 512 CGS, ranging between 192 CGS and 842 CGS.

-Units E and F (-68 cm to 180 cm in reference to the current sea level) are yellow sandy silt to laminated silt. The fine fraction increases from 67% (unit E) to 85% (unit F). The sorting index is low but slightly higher than in the lower units (4 to 3.6). The organic matter content decreases (5.14% to 3.2%). The depositional processes are mixed (types 3 and 4), even if some layers belong to the uniform suspension (type 5). The MS values are around 600 CGS.

TR-XIV: Access channel to the Trajanic basin (fig. 6).

-Unit A is yellow sterile layered sand similar to unit A at the bottom of the CT-1 core. Sand is dominant, deposited as graded suspension (type 1), with a median grain size between 90 μm and 120 μm. The organic content is low (1.3%). The mean magnetic susceptibility value is 46 CGS.

-The sedimentary environment changes abruptly since units B and D are composed of mud and sandy-mud with shells and Posidonia with 85% of fine sediments (<63 μm). They are characterised by a negative magnetic susceptibility around -25 CGS. The average median grain size is 13 μm. Compared to unit A, the organic matter content is multiplied by 6 with a maximum of 12%. The environment was relatively quiet with a mix of uniform and graded suspension processes (type 3). Between these two units, a sandy layer (70% of sand in unit C) was deposited by mixed rolling and graded suspension processes (type 4). The magnetic susceptibility increases to 30 CGS, in positive values. Sparse potshards have been collected in unit D.

-Units E and F are quite similar to units B and D, but with a smaller proportion of fine sediments (78%) and a greater proportion of organic matter (up to 22%). Shells and Posidonia are more present. The dominant depositional processes are a mix of uniform and graded suspension. The major difference between the sedimentary units are given by the values of magnetic susceptibility. Unit F has similar negative value, around -5 CGS, contrasting with unit E characterised by positive values with a mean of 7 CGS.

-Units G and H are mostly composed of fine sediment (73%) but, between these two layers, the sandy content is similar to the fine fraction. A high frequency of shells has been observed in this intermediate level. In unit G the sandy content was deposited by mixed rolling and graded suspension processes increased. The magnetic susceptibility is positive with a mean value of 7 CGS. The sand content decreases in unit H which displayed a quieter environment with uni- form and graded suspension mixing processes and with a low magnetic susceptibility (mean of -3 CGS).

-The two upper layers (units I and J) are characterised by layers of silt and clay deposited by alternated uniform suspension and decantation. More than 99% of the total fraction is composed of fine fraction. In unit I, the magnetic susceptibility increases from 10/20 CGS at the bottom to 60 CGS at the top. Finally, those values in unit J are highly variable.

Discussion

The analysis of the two studied cores allows us to determine three sedimentary phases: the pre-harbour environments, the harbour and canal in activity, and the final infill.

Pre-harbour deposits

In the CT-1 and TR-XIV cores, basal units A correspond to pre-harbour deposits characterised by yellow sterile layered sands. In support of this interpretation, a piece of charcoal sampled 10 m below current sea level has been dated 4030±30 BP (2620-2475 BC). The sediment facies is similar to all the Portus' basal units (Goiran et al., 2010, in press). On the Passega diagram, sediments plot parallel to the straight line of perfect sorting C (D 99 ) = M (D 50 ). Depositional processes correspond to graded suspension (type 1) and rolling (type 2; fig. 7).

The Canale Traverso geometry and location as evidence of Roman planning

This channel is relatively narrow compared to other channels of the Portus complex. After re-analysing the results of a geomagnetic survey [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF]fig. 2), it seems pretty clear that the channel widens towards the Fossa Traiana, thus facilitating the movement of ships entering this channel. On the side of the docks, the canal opens into an approximately 40-m wide space, opened to the north. Strong brick walls, over 3-m high (4 m above the Roman sea level) are still visible today. In an upstream-downstream perspective (from the Tiber channel to the Canale Traverso), it can be hypothesised that there is a progressive reduction of the hydraulic cross section and of the flow along the derivation canals (fig. 1). The natural Tiber channel was 100 m to 120 m wide in the 1st c. AD, which is confirmed by L. [START_REF] Bertacchi | Elementi per una revisione della topografia ostiense[END_REF] in the former Tiber channel or Fiume Morto near Ostia. The ancient Fossa Traiana was around 50-m wide [START_REF] Testaguzza | Portus: illustrazione dei Porti di Claudio e Traiano[END_REF]. Finally, the Canale Traverso was 25-m wide (see magnetical survey in S. [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF]. In the successive reaches of the canals (Fossa Traiana, Canale Traverso), widths were divided twice into two. We cannot precisely locate the palaeochannel of the Tiber near ancient Portus In this deltaic plain, the flow of water in the Canale Traverso should be small. This geometric data is interesting because it may demonstrate a clear awareness of the problem arising when planning the position of the port and of the river system by the ancient architects and engineers responsible for the construction of Portus. We are now able to see how through a succession of canals and thanks to their optimal orientation, Romans tried to reduce the carrying capacity of the channel entering into the harbour basins, while ensuring the presence of a waterway between the river and the harbour. However, one question remains unanswered: was this configuration effective against sedimentation?

Harbour and canal sedimentation occurred in a common quiet brackish environment

The katolimenic limit corresponds to the bottom of a harbour sedimentary sequence [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF], between pre-harbour deposit (the yellow sterile layered sand) and the harbour mud, characteristic of a quiet environment protected from the sea's influence. In the TR-XIV core, the first potshards were found in unit D. Thus at this location, the channel access to the Trajanic harbour is 7 (unit B) or 6.5 (unit D) meters deep below the R.s.l. From 7 m to 3.30 m below R.s.l. (unit B to H), the sedimentary sequence of the TR-XIV core is mostly composed of muds with shells and Posidonia fibers. The harbour deposits of the TR-XIV core correspond to the other cores described in this area of Portus and deposited under high marine influence (cores TR-XIX, TR-XX in [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF]. The environmental conditions under which deposition occurred were relatively quiet, with a mix of graded and uniform suspension processes. Some levels display energetic hydrodynamic condition with sediment originating from a mix of graded and rolling processes. These sandy units occur mostly in units C, E and G. The Canale Traverso is only 5.5 m below R.s.l. (CT-1). It is an unusual fluvial canal because it has conditions similar to those of a harbour basin. Indeed, units B and C are composed of mud with shells and Posidonia, corresponding to the first phase of deposition. The depositional processes are primarily represented by mixed uniform and graded suspension, similar to those of core TR-XIV. The presence of Posidonia validates the hypothesis of the marine influence in the canal, succeeded by a brackish 84 Géomorphologie : relief, processus, environnement, 2012, n°1, p. 75-90

Ferréol Salomon et al. environment with a freshwater input [START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF]. Most significantly, the fluvial influence is deduced from the presence of Alnus pollen which were found trapped inside the sediments, and probably transported and deposited there by the Tiber [START_REF] Sadori | The plant landscape of the imperial harbour of Rome[END_REF]. The bottom of this canal sediment has been dated from the 1st c. to the beginning of the 2nd c. (seed-Lyon-6895/charcoal-Lyon-6894). Unit C is dated from the end of the 1st c. AD to the middle of the 3rd c. AD (wood-Lyon-7081). It is impossible to specifically date the construction of the canal between Claudius and Hadrian reigns. Whatever the answer, the basal unit B and probably unit C correspond to the first canal deposits.

Last sedimentation of Portus waterways

The closely related evolution, as described in the first metre of the canal's sediments infill (CT-1) and the access channel to the Trajanic harbour (TR-XIV), are not characteristic of the upper units. Unit D of the Canale Traverso demonstrates an increase in hydrodynamism, shown by a coarser median grain size (10-13 μm to 67 μm). The mixed processes of graded suspension and rolling are dominant. This layer has not been found in the TR-XIV core which displays constant conditions from unit B to unit H, with mixed uniform and graded suspension. In unit D of the CT-1 core, one meter below the former R.s.l., a piece of charcoal has been dated to the 7th c. AD (charcoal-Lyon-6869). Unfortunately, no dating was possible in the access channel above -3 m (R.s.l). However, on the fresco painted by Ignazio Danti in 1582, the southern end of the Canale Traverso, close to the Fiumicino canal, is plugged by sediments, and stagnant water is visible in the access channel which is both disconnected from the sea and the fluvial system. The upper units of the CT-1 core can be interpreted as the deposition of this upstream sediment blockage. In the same way, the upper units I and J of the TR-XIV core correspond to swamp and floodplain deposits, showing the alternation of uniform suspension and decantation processes. These deposits correspond to the upper part of the PTS-5 and 13 cores [START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF]fig. 3), with a high frequency of freshwater ostracods groups.

A canal and a harbour silted up with sediment transported by Tiber River floods?

When operational, the Canale Traverso was a quiet environment similar to the environments present at the entrance of the Trajanic harbour. Another fundamental question is whether mixed rolling and graded suspension results from a marine origin (Claudius basin) or from a fluvial origin. In the CM diagram, the mean coarsest grain-size percentile (D 99 ) present in type 4 indicates the maximum transport capacity of particles. This mean D 99 grain of the CT-1 core is coarser (1379 μm) than the mean D 99 of the TR-XIV core (1102 μm), with rolled particles. Therefore, it is hypothesised that there is an influence of the Tiber River's flow (floods in particular) in the Canale Traverso (fig. 7). In this area, future work will ascertain whether fine sediments originate mainly from the Tiber River through the Canale Traverso. A preliminary answer is that the sedimentation rate in the bottom units of the Canale Traverso is higher than in the access channel to the Trajanic harbour (fig. 8; [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF]. So far, it is risky to attribute the presence of flood deposits in the Canale Traverso to a regional hydroclimatic crisis context. Indeed this canal is a specific sediment trap mainly controlled by anthropic influences. Besides, at a certain period, a canal lock may have been installed between the Fossa Traiana and the Canale Traverso [START_REF] Testaguzza | Portus: illustrazione dei Porti di Claudio e Traiano[END_REF].

Water depth and boat draughts

Fig. 8 synthesises all the published radiocarbon data concerning the access channel to Trajan's basin in relation to the R.s.l. [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF][START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF]. These dates are confronted by the altitudinal variations of the bottom of the harbour, called 'mesolimenic limit' [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF], and also to different Roman boat draughts [START_REF] Boetto | Le port vu de la mer: l'apport de l'archéologie navale à l'étude des ports antiques[END_REF]. To compute the sedimentation rate, we used the mean radiocarbon date. First of all, the Canale Traverso was not deep when it was excavated (5.5 m), while the access channel to the Trajanic basin was 6.8 m (TR-XX), 7 m (TR-XIV) to 8 m (TR-XI) deep [START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF]fig. 8). Thus, a slope from the Canale Traverso to the Claudian basin may be deduced from these data. Thus, when it excavated, the canal was probably designed to accommodate vessels with a shallow draughts (fig. 8). This result is synonymous with the relatively limited width of the canal (25 m; [START_REF] Boetto | Les navires de Fiumicino (Italie) : architecture, matériaux, types et fonctions. Contribution à l'étude du système portuaire de Rome à l'époque impériale[END_REF][START_REF] Boetto | Le port vu de la mer: l'apport de l'archéologie navale à l'étude des ports antiques[END_REF]. Using the radiocarbon dates, the sedimentation rate may be divided into two phases. In the TR-XI core from -8 m to -6 m (R.s.l.), the bottom of the access channel aggraded by approximately 1.1 cm/a. The two first metres of TR-XX (from 6.8 m to 4.3 m below R.s.l.) aggraded faster than TR-XI, at a rate of 1.6 cm/a. Finally, the CT-1 core displays a fast accretion rate (2.6 cm/a) for the bottom 2.3 m. It can be deduced that in the access channel to the Trajanic basin, the closer to the Canale Traverso, the faster the accretion of the bottom of the channel. This observation can also apply to the river sediment supply which originated from the Tiber River. The second aggradational phase is more differentiated. The channel bottoms of TR-XI and TR-XX aggraded at a higher rate from 1.6 cm/a (TR-XX -6 m to -3 m R.s.l.) to 4.2 cm/a (TR-XI -4.3 m to -3 m R.s.l.). An inversion of dates occurs in the TR-XI core and could be interpreted as a phase of dredging [START_REF] Marriner | Geoarchaeological evidence for dredging in Tyre's ancient harbour, Levant[END_REF]. Unfortunately, there is no date for the last 3 m (R.s.l.) inside the access channel to Trajan's basin. After a first phase of rapid siltation in the canal (2.6 cm/a), the second phase displays a slower rate of sedimentation with coarser sediments (0.45 cm/a). Theoretically, the canal would have filled with sediments becoming finer at the top of the sequence. In reality, sediment is coarser than below, with a reduction in the rate of siltation. The canal may then have been dredged at the base of Unit D (the abrupt change in magnetic susceptibility could be reconciled with that location). If the depth of the canal is maintained by dredging at about -2.3 m below the Roman sea level, small boats (70 t), like those found in the northern part of Claudius harbour [START_REF] Boetto | Les navires de Fiumicino (Italie) : architecture, matériaux, types et fonctions. Contribution à l'étude du système portuaire de Rome à l'époque impériale[END_REF], could have navigated along this canal before the 7th c. AD. At the beginning of the 5th c. AD, Philostrogius (Ecclesiastical History, XII, 3 in Le [START_REF] Gall | Le Tibre, fleuve de Rome dans l'antiquité[END_REF] described three basins in Portus, probably the Claudius and Trajanic basins, along with the darsena (Le [START_REF] Gall | Le Tibre, fleuve de Rome dans l'antiquité[END_REF]. Also Philostrogius specified that the Fossa Traiana was the middle of the port. He may have seen the Canale Traverso, originating from the Fossa Traiana, flowing into the access channel between the Claudius and Trajanic basins on one side and the darsena on the other side.

Conclusions

The relationship between ancient harbour basins and the fluvial system of the Tiber delta is a complex and crucial question. This paper takes advantage of the plurality of studies published on Portus in the recent years. In a new perspective, the history, harbour and naval archaeology, fluvial and ancient harbour geoarchaeology, were combined with new palaeoenvironmental studies in order to give a renewed description of Portus basins. Environmental and depositional conditions are quite similar at the entrance of the Trajanic basin and in the Canale Traverso. Granulometrical analyses provided new CM diagrams (Passega) about the conditions of sediment deposition in the harbour, with a wider range of processes in the access channel to Trajanic harbour (TR-XIV). The Canale Traverso is prone to higher energy flows coming from the Tiber (CT-1). This approach allows at better understanding of sediment budgets and functioning in the Portus harbour. Obviously, the basins silted up inexorably, despite the initial project of reducing sediment input from the Tiber. The sediment budget depended on preventive actions (e.g., geometry and configuration of the canals sheltered from the Tiber flow direction; or maybe canal lock in the Canale Traverso according to [START_REF] Testaguzza | Portus: illustrazione dei Porti di Claudio e Traiano[END_REF], on adaptive actions (e.g., construction of internal moles; [START_REF] Lugli | Il porto di Roma imperiale e l'agro portuense[END_REF][START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF] and curative actions (dredging). Future research may provide a more accurate vision, in particular the modelling of the waterflow in Portus and in the canals. Finally, the Canale Traverso is the starting point for future research on the canals directly connected to the Tiber, like the Canale Romano and the Northern Canal [START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF][START_REF] Salomon | Delta du Tibre -Campagne de carottage 2009 -Géoarchéologie des canaux de Portus : l'exemple du Canale Romano[END_REF].

Ferréol Salomon ,

 Salomon Hugo Delile, Jean-Philippe Goiran, Jean-Paul Bravard, Simon Keay. The Canale di Comunicazione Traverso in Portus: the Roman sea harbour under river influence (Tiber delta, Italy): Etude du Canale di Comunicazione Traverso à Portus : le port maritime antique de Rome sous influence fluviale (delta du Tibre, Italie). Géomorphologie : relief, processus, environnement, 2012, 1, pp.75-90. 10.4000/geomorphologie.9754. halshs-01099942 Version française abrégée Le coeur de cette étude est un transect de carottages réalisés entre le Canale di Comunicazione Traverso et le chenal d'accès au bassin de Trajan dans le Portus de Rome (CT-1 et TR-XIV ; fig. 1 et fig.

Fig. 1 -

 1 Fig. 1 -Location map of Portus, the ancient sea harbour of Rome on the margins of the Tiber.

Fig. 1 -

 1 Fig. 1 -Localisation générale de Portus, l'ancien port maritime de Rome en marge du Tibre.

Fig. 2 -

 2 Fig. 2 -Geomagnetic survey results (Keay et al., 2005) and archaeological data between the Claudius and the Trajan harbour: synthesis of successive cores location near the Canale Traverso. 1: studied cores in this paper; 2: cores in J.-P. Goiran et al., 2010; 3: cores in C.[START_REF] Giraudi | -Late Holocene evolution of Tiber river delta and geoarchaeology of Claudius and Trajan Harbor, Rome[END_REF][START_REF] Sadori | The plant landscape of the imperial harbour of Rome[END_REF][START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF] 4: bridges? (Lanciani, 1866;[START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF]; 5: streets ?[START_REF] Keay | Portus: An archaeological survey of the Portus of Imperial Rome. British School at Rome, Archaeological Monographs of the British School at Rome[END_REF].

Fig. 2 -

 2 Fig. 2 -Résultats des prospections géomagnétiques (Keay et al., 2005) et données archéologiques situées entre les bassins portuaires de Claude et de Trajan : synthèse des carottages réalisés à proximité du Canale Traverso. 1 : carottages étudiés dans cet article ; 2 : carottages présentés dans J.-P. Goiran et al., 2010 ; 3 : carottages présentés dans C. Giraudi et al., 2009, L. Sadori et al., 2010, I. Mazzini et al., 2011 ; 4 : ponts ? (Lanciani, 1866 ; Keay et al., 2005) ; 5 : routes ? (Keay et al., 2005).

Fig. 3 -

 3 Fig. 3 -Marine to freshwater palaeoenvironments in the Portus basin obtained by ostracods determination -A synthesis[START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF][START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF]. 1: cores locations with portuary units[START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF][START_REF] Mazzini | Late Holocene palaeoenvironmental evolution of the Roman harbour of Portus, Italy[END_REF] Goiran et al., in press); all cores are represented according to the 3rd-5th c. sea level[START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF]. Only the portuary units data are shown; 2: harbour bottom; 3: ostracod groups with 3a: freshwater species, 3b: brackish species, and 3c: marine species.

Fig. 4 -

 4 Fig. 4 -Magnetic susceptibility results for core CT-1 -Relevance of a high-resolution dataset (each centimetre) to determine sedimentary layers. Bars are grouped by sedimentary units and correspond to the occurrence of MS data (y-axis) included in a range of 5 CGS (x-axis).

Fig. 4 -

 4 Fig. 4 -Résultats de susceptibilité magnétique pour le carottage CT-1. Ce graphique valide la pertinence des données de haute résolution (chaque centimètre) pour définir des unités sédimentaires. Les barres sont identifiées selon les unités sédimentaires et correspondent à l'occurrence des valeurs de SM (en ordonnée) incluses dans un intervalle de 5 CGS (en abscisse).

Fig. 7 -

 7 Fig. 7 -Cross-sections from the Canale Traverso to the access channel to Trajanic harbour, and CM diagram of the CT-1 and TR-XIV cores. 1a: Posidonia; 1b: shells; 1c: pot shards; 1d: canal or harbour basin bottom; 2a: coarse sediments; 2b: sand; 3c: silt and clay; C/M diagram interpretation. 3: pure processes: 3a: rolling; 3b: rolling and graded suspension; 3c: graded suspension; 3d: uniform suspension; 3e: decantation; 4: mixed processes: 4a: mixed rolling/graded suspension + uniform suspension; 4b: mixed graded + uniform suspension.

Fig. 7 -

 7 Fig. 7 -Transect longitudinal du Canale Traverso au chenal d'accès au basin de Trajan.Présentation des diagrammes CM pour les carottages CT-1 et TR-XIV. 1a : Posidonie ; 1b : coquille ; 1c : céramique ; 1d : fond du canal ou du port ; 2a : fraction grossière ; 2b : sables ; 3c : limons et argiles ; Interprétation du diagramme C/M. 3 : processus purs : 3a: roulement ; 3b : roulement et suspension graduée ; 3c : suspension graduée ; 3d : suspension uniforme ; 3e : décantation ; 4 : processus mixtes : 4a : roulement/suspension graduée + suspension uniforme mixés ; 4b : suspension graduée et uniforme mixées.

Fig. 8 -

 8 Fig. 8 -Harbour infill, water depth and ships draught data. Comparisons of these parametres by chronology.

Fig. 8 -

 8 Fig. 8 -Compilations des données disponibles à Portus pour comprendre chronologiquement la relation entre le remblaiement du port, la lame d'eau disponible et le tirant d'eau des bateaux.

Le Canale Traverso, analyse de la carotte CT-1. 1a

	Fig. 5 -Canale Traverso, analysis of the core CT-1. 1a: Posidonia; 1b: shells; 1c: pot shards; 2a: coarse sediments; 2b: sand; 3c: silt and clay; C/M diagram interpretation. 3: pure processes: 3a:	rolling; 3b: rolling and graded suspension; 3c: graded suspension; 3d: uniform suspension; 3e: decantation; 4: mixed processes: 4a: mixed rolling/graded suspension + uniform suspension; 4b: mixed	graded + uniform suspension.	Fig. 5 -

: Posidonie ; 1b : coquille ; 1c : céramique ; 2a : fraction grossière ; 2b : sables ; 3c : limons et argiles ; Interprétation du diagramme C/M. 3 : processus purs : 3a: roulement ; 3b : roulement et suspension graduée ; 3c : suspension graduée ; 3d : suspension uniforme ; 3e : décantation ; 4 : processus mixtes : 4a : roulement/suspension graduée + suspension uniforme mixés ; 4b : suspension graduée et uniforme mixées.

Fig.

6

-The

access channel to the Trajanic harbour, analysis of core TR

	sediments; 2b: sand; 3c: silt and clay; C/M diagram interpreta-	tion. 3: pure processes: 3a: rolling; 3b: rolling and graded suspension; 3c: graded suspension; 3d: uniform suspension; 3e: decantation; 4: mixed processes: 4a: mixed rolling/graded suspension +	uniform suspension; 4b: mixed graded + uniform suspension.	Fig. 6 -

-XIV. 1a: Posidonia; 1b: shells; 1c: pot shards; 2a: coarse

Le chenal d'accès au port de Trajan, analyse de la carotte TR

	2a : fraction grossière ; 2b : sables ; 3c : limons et argiles ; Interpréta-	tion du diagramme C/M. 3 : processus purs : 3a: roulement ; 3b : roulement et suspension graduée ; 3c : suspension graduée ; 3d : suspension uniforme ; 3e : décantation ; 4 : processus mixtes :	4a : roulement/suspension graduée + suspension uniforme mixés ; 4b : suspension graduée et uniforme mixées.

-XIV. 1a : Posidonie ; 1b : coquille ; 1c : céramique ;

t du type de ceux retrouvés au nord de Portus[START_REF] Boetto | Les navires de Fiumicino (Italie) : architecture, matériaux, types et fonctions. Contribution à l'étude du système portuaire de Rome à l'époque impériale[END_REF][START_REF] Boetto | Le port vu de la mer: l'apport de l'archéologie navale à l'étude des ports antiques[END_REF] pouvaient emprunter le Canale di Comunicazione Traverso au plus tard jusqu'au VI e -VII e s. apr. J.-C.

Géomorphologie : relief, processus, environnement, 2012, n°1, p. 75-90Ferréol Salomon et al.

Géomorphologie : relief, processus, environnement, 2012, n°1, p. 75-90

Géomorphologie : relief, processus, environnement, 2012, n°1, p. 75-90The Canale di Comunicazione Traverso in Portus

Acknowledgements

support. In particular, we would like to thank Dr Michel Gras (Head of the Ecole Française) and Dr Yann Rivière Many thanks to Vincent Gaertner for his assistance in


Laure Nuninger

Philip Verhagen

Frédérique Bertoncello

Estimating the "memory of landscape" to model changes in archaeological settlement patterns

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.


S Bouccara

G Sitbon

A Fragola

V Loriette

N Lequeux

T Pons

Enhancing fluorescence in vivo imaging using inorganic nanoprobes

Fluorescence imaging is a versatile tool for biological and preclinical studies with steady improvements in performance thanks to instrumentation and probe developments. The sensitive detection and imaging of deep targets in vivo is especially challenging due to the diffusion and absorption of light by the tissues and to the emission of autofluorescence from intrinsic chromophores. Fluorescent inorganic nanoparticles present interesting optical properties that may significantly differ from organic dyes. In this short review, we present recent developments in the design of these nanoprobes and their use for new in vivo fluorescence modalities which provide enhanced imaging capabilities.

Introduction

Fluorescence in vivo imaging is becoming a commonly used tool to study biological functions and pathologies in living model organisms, and is being increasingly developed for clinical diagnostics and treatment. [START_REF] Frangioni | In vivo near-infrared fluorescence imaging[END_REF] Fluorescence imaging typically offers access to tissues a few mm or cm below the surface. It also offers unique advantages that make it a precious, powerful and very popular technique. Besides its ease of use, low cost, noninvasiveness and absence of ionizing radiation, it possesses distinct performance characteristics. For example, it offers high sensitivity, down to the single molecular tracer, as well as straightforward multiplexing capabilities by selecting probes with different spectral properties. It can also reach unrivaled spatial and temporal resolutions. For these reasons, in vivo fluorescence imaging techniques and applications are being developed at an accelerating pace. Several organic chromophores are being used in small animal models and in clinical applications. [START_REF] Frangioni | In vivo near-infrared fluorescence imaging[END_REF][START_REF] Escobedo | NIR dyes for bioimaging applications[END_REF][START_REF] Hilderbrand | Near-infrared fluorescence: application to in vivo molecular imaging[END_REF][START_REF] Vahrmeijer | Imageguided cancer surgery using near-infrared fluorescence[END_REF] In general however, for these dyes, the maximum imaging depth, resolution and sensitivity are limited by the optical properties of the tissues. [START_REF] Richards-Kortum | Quantitative optical spectroscopy for tissue diagnosis[END_REF] More specifically, the major obstacles limiting imaging performance are the absorption and diffusion of excitation and emission photons, causing a loss of signal and resolution, and the emission of autofluorescence from intrinsic chromophores, giving rise to spurious background signal. Inorganic nanoparticles, on the other hand, present unique optical properties that help alleviate some of these limitations. In this short review, we do not aim to offer a comprehensive review on fluorescent nanoparticles (NPs), but instead we will focus on how their optical properties can be used to enhance the performance of fluorescence in vivo imaging.

Near infrared emitting nanoprobes for in vivo imaging

The near infrared window (700-1300 nm) enables deeper imaging thanks to reduced light absorption and scattering compared to the visible range, where intrinsic chromophores such as hemoglobin absorb, and to longer NIR wavelengths, where water and lipids strongly absorb. In addition, autofluorescence from intrinsic chromophores is strongly reduced when exciting and/or detecting in the NIR range. Two near infrared bands are usually distinguished, NIR I (700-950 nm) and NIR II (1000-1300 nm). Several classes of organic fluorophores emit in the NIR I band; indocyanine green, for example, is used for angiography. [START_REF] Slakter | Indocyanine-green angiography[END_REF] Even though continuing progress is being made in the synthesis of brighter and more stable organic dyes, [START_REF] Escobedo | NIR dyes for bioimaging applications[END_REF] nanoparticles offer interesting alternatives as NIR I-emitting probes. These include semiconductor quantum dots (QDs), metal nanoclusters, rare earth doped nanoparticles (RE-NPs) and nanodiamonds (NDs). After initial demonstrations of in vivo imaging based on CdTe and PbS materials, new generations of low toxicity NIR emitting QDs have emerged. [START_REF] Cassette | Design of new quantum dot materials for deep tissue infrared imaging[END_REF] These are mainly composed of CuIn(S/Se) 2 , [START_REF] Cassette | Synthesis and characterization of near-Infrared Cu-In-Se/ZnS core/shell quantum dots for in vivo imaging[END_REF][START_REF] Li | Highly luminescent CuInS 2 /ZnS core/shell nanocrystals: cadmium-free quantum dots for in vivo imaging[END_REF][START_REF] Pons | Cadmium-free CuInS 2 /ZnS quantum dots for sentinel lymph node imaging with reduced toxicity[END_REF][START_REF] Xie | Formation of high-quality I-III-VI semiconductor nanocrystals by tuning relative reactivity of cationic precursors[END_REF] Cu-doped InP, [START_REF] Xie | Synthesis of Cu-doped InP nanocrystals (d-dots) with ZnSe diffusion barrier as efficient and color-tunable NIR emitters[END_REF] InAs, [START_REF] Aharoni | Synthesis of InAs/CdSe/ZnSe Core/Shell 1 /Shell 2 structures with bright and stable near-infrared fluorescence[END_REF][START_REF] Gao | Ultrasmall near-infrared noncadmium quantum dots for in vivo tumor imaging[END_REF][START_REF] Kim | Engineering InAs x P 1-x /InP/ZnSe III-V alloyed core/shell quantum dots for the near-infrared[END_REF][START_REF] Zimmer | Size series of small indium arsenide-zinc selenide core-shell nanocrystals and their application to in vivo imaging[END_REF] and Ag 2 S [START_REF] Chen | Characterization of tumor-targeting Ag2S quantum dots for cancer imaging and therapy in vivo[END_REF][START_REF] Du | Near-infrared photoluminescent Ag 2 S quantum dots from a single source precursor[END_REF] nanocrystals. These QDs offer a high brightness thanks to their high extinction coefficient and quantum yields (QY), and a higher photostability compared to organic chromophores. Their emission is tunable with their size and composition. Metal nanoclusters are a novel class of fluorescent nanomaterials. While the exact underlying photophysical mechanism is not completely elucidated yet, NIR emitting gold nanoclusters have been synthesized. [START_REF] Aldeek | Growth of highly fluorescent polyethylene glycol-and zwitterion-functionalized gold nanoclusters[END_REF][START_REF] Chen | Folate-modified gold nanoclusters as near-infrared fluorescent probes for tumor imaging and therapy[END_REF][START_REF] Xie | Protein-directed synthesis of highly fluorescent gold nanoclusters[END_REF][START_REF] Yuan | Synthesis of highly fluorescent metal (Ag, Au, Pt, and Cu) nanoclusters by electrostatically induced reversible phase transfer[END_REF] These clusters are not as bright in terms of extinction coefficient and quantum yield as QDs, but their small size and lack of toxicity make them an attracting alternative. Carbon-based materials also present interesting optical properties. In particular, N-V centers in nanodiamonds (NDs) fluoresce in the far red-NIR range. [START_REF] Fu | Characterization and application of single fluorescent nanodiamonds as cellular biomarkers[END_REF][START_REF] Chang | Mass production and dynamic imaging of fluorescent nanodiamonds[END_REF] These NDs are inert, which leads to a superior photostability and low toxicity. Finally, the fluorescence properties of rare earth doped nanoparticles rely on 4f transitions of trivalent lanthanide ions inserted in a suitable host matrix. The optical properties of these RE-NPs are therefore tunable by choosing appropriate emissive and sensitizing dopants. The choice of the host materials is determinant for the size control during the synthesis, as well as for the optical properties and colloidal and chemical stability. [START_REF] Chen | Upconversion nanoparticles: design, nanochemistry, and applications in theranostics[END_REF] Surface chemistry of these different nanoprobes is of course crucial to guarantee their colloidal stability, preserve their optical properties in vivo, reduce non specific adsorption and provide specific targeting and functionality. The development of optimized chemistries is an ongoing research effort, which has already allowed most of these NPs to be used in small animals for different imaging applications. These include vasculature [START_REF] Damasco | Sizetunable and monodisperse Tm3+/Gd3+-doped hexagonal NaYbF4 nanoparticles with engineered efficient near infrared-to-near infrared upconversion for in vivo imaging[END_REF][START_REF] Larson | Water-soluble quantum dots for multiphoton fluorescence imaging in vivo[END_REF] and lymph node [START_REF] Pons | Cadmium-free CuInS 2 /ZnS quantum dots for sentinel lymph node imaging with reduced toxicity[END_REF] imaging, tumor detection [START_REF] Gao | Ultrasmall near-infrared noncadmium quantum dots for in vivo tumor imaging[END_REF], cell tracking [START_REF] Idris | Tracking transplanted cells in live animal using upconversion fluorescent nanoparticles[END_REF][START_REF] Wu | Tracking the engraftment and regenerative capabilities of transplanted lung stem cells using fluorescent nanodiamonds[END_REF] and cell lineage. [START_REF] Dubertret | In vivo imaging of quantum dots encapsulated in phospholipid micelles[END_REF] In addition to providing higher detection sensitivity compared to standard NIR I organic fluorophores, these nanoprobes share several unique features. Their large hydrodynamic size, while restricting diffusion into tissues and preventing renal excretion above 5 nm, [START_REF] Choi | Renal clearance of quantum dots[END_REF] also extends the circulation time in the bloodstream and limit leaching from the blood vessels into surrounding tissues, leading to more specific targeting. In addition, tumor detection using nanoprobes benefits from the enhanced permeation and retention (EPR) effect: [START_REF] Matsumura | A new concept for macromolecular therapeutics in cancer chemotherapy: mechanism of tumoritropic accumulation of proteins and the antitumor agent smancs[END_REF] nanoparticles in the 50-150 nm size range can penetrate loose epithelial barriers and remain much longer in the tumor than smaller organic dyes which get washed away much faster. [START_REF] Tang | Investigating the optimal size of anticancer nanomedicine[END_REF] Similarly, endocytosed NPs tend to remain much longer inside targeted cells compared to smaller organic dyes, facilitating long term cell tracking. [START_REF] Fang | The exocytosis of fluorescent nanodiamond and its use as a long-term cell tracker[END_REF] In contrast to monofunctional organic dyes, the large surface of NPs can be used to assemble different components on a single nanoplatform. This potentially enhances the targeting efficiency, thanks to the multiplicity of targeting moieties, or provides multifunctional NPs carrying different targeting probes, drugs, contrast agents for complementary imaging modalities, and/or environment reporters. Interestingly, these nanoprobes also offer optical properties that can be exploited to develop new improved imaging modalities in terms of accessible depth, resolution and sensitivity. In the following sections, we will more specifically review recent developments of NPs that emit in the NIR II range, undergo efficient nonlinear excitation, present slow fluorescence decays for time-gated detection or can be efficiently coupled to other imaging modalities.

NIR II emitting nanoprobes

In vivo imaging in the NIR II band has long remained difficult due to the absence of fluorophores and sensitive detectors in this region. In principle however, the NIR II band offer several advantages compared to NIR I, including better detection sensitivity, thanks to a much lower autofluorescence background, and better spatial resolution deep in tissues, thanks to reduced light scattering. [START_REF] Lim | Selection of quantum dot wavelengths for biomedical assays and imaging[END_REF] In addition, NIR II probes can usually be excited in the NIR I band, which reduces photodamage and improves the excitation penetration. Water soluble NIR II organic dyes are not available so far, and the QY of their hydrophobic counterparts is limited to a few percents in organic solvents. [START_REF] Tao | Biological imaging using nanoparticles of small organic molecules with fluorescence emission at wavelengths longer than 1000 nm[END_REF] For these reasons, three main classes of NIR II nanoprobes are being actively developed (see Table 1). Single wall carbon nanotubes (SWNT) present a broad series of emission peaks between 1,000 and 1,400 nm. [START_REF] Hong | Through-skull fluorescence imaging of the brain in a new near-infrared window[END_REF][START_REF] Welsher | Deep-tissue anatomical imaging of mice using carbon nanotube fluorophores in the second near-infrared window[END_REF][START_REF] Hong | Multifunctional in vivo vascular imaging using near-infrared II fluorescence[END_REF][START_REF] Welsher | A route to brightly fluorescent carbon nanotubes for near-infrared imaging in mice[END_REF] RE-NPs can be designed to emit at different wavelengths in the NIR II band, from 1,200 nm (Er 3+ ) to 1,525 nm (Pr 3+ ). [START_REF] Naczynski | Rare-earth-doped biological composites as in vivo shortwave infrared reporters[END_REF] Ag 2 S and Ag 2 Se QDs have been synthesized with tunable spectra up to 1,300 nm and good quantum yields. [START_REF] Du | Near-infrared photoluminescent Ag 2 S quantum dots from a single source precursor[END_REF][START_REF] Hong | In vivo fluorescence imaging with Ag 2 S quantum dots in the second near-infrared region[END_REF][START_REF] Zhang | Ag 2 S quantum dot: a bright and biocompatible fluorescent nanoprobe in the second near-infrared window[END_REF][START_REF] Dong | Facile synthesis of highly photoluminescent Ag 2 Se quantum dots as a new fluorescent probe in the second near-infrared window for in vivo imaging[END_REF][START_REF] Zhu | Ag 2 Se quantum dots with tunable emission in the second near-infrared window[END_REF] In addition, the high chemical stability of Ag 2 S reduces significantly the potential toxicity of these QDs, compared to other QD materials. These nanoprobes have been used for blood vessel imaging. The Dai group in particular, demonstrated that carbon nanotubes could provide images of cerebral vasculature through the skull and scalp, while maintaining an impressive sub-10 µm resolution at a 2 mm depth. [START_REF] Hong | Through-skull fluorescence imaging of the brain in a new near-infrared window[END_REF] Another example is in vivo tracking of Ag 2 S QD-labeled cells. The absence of autofluorescence led to a low detection limit (< 1000 cells) and allowed tracking these cells for up to 30 days. [

Nonlinear excitation/Upconversion

Most inorganic nanoparticles present higher nonlinear absorption cross sections compared to organic fluorophores. Nonlinear excitation in the NIR enables deep penetration and reduces autofluorescence, thus leading to improved signal-to-background ratio. W. Webb and co-workers demonstrated that 2-photon excitation (2PE) is particularly suited to CdSe quantum dots (QDs) because they have 2-photon absorption cross sections two to three orders of magnitude higher than organic fluorophores, enabling dynamic observation of QDs through the skin of living mice, in capillaries hundred of micrometers deep. [27•]. However, 2PE requires expensive equipment and is better suited for cellular scale than for whole animal imaging where efficient focusing is hard to achieve deep in the tissues. For the latter application, rare-earth doped nanoparticles present the great advantage of being easily and efficiently excited in the near infrared with a continuous laser source by up conversion. [START_REF] Chen | Upconversion nanoparticles: design, nanochemistry, and applications in theranostics[END_REF] The large blue spectral shift between excitation and emission thus obtained and their high resistance to photobleaching contributes to enhance the detection sensitivity of up-conversion nanoparticles (UCNP). Lim et al. demonstrated first in vivo imaging using 50 to 200 nm lanthanide doped NPs in Caenorhabditis elegans (see figure 2a). [START_REF] Lim | Selection of quantum dot wavelengths for biomedical assays and imaging[END_REF] Since then, lanthanide doped NPs have been efficiently detected when injected deep in a rat and have allowed tracking cells in live animals. [START_REF] Idris | Tracking transplanted cells in live animal using upconversion fluorescent nanoparticles[END_REF] Vinogradov and co-workers reported the use of UCNP doped with both Yb 3+ and Er 3+ to acquire vascular images with high contrast and resolution up to 340 microns deep into a mouse brain (see figure 2B). [START_REF] Esipova | Dendritic upconverting nanoparticles enable in vivo multiphoton microscopy with low-power continuous wave sources[END_REF] To enhance the depth of in vivo imaging, further works focused on the synthesis of biocompatible NIR-to-NIR up conversion nanoparticles. [START_REF] Xiong | Long-term in vivo biodistribution imaging and toxicity of polyacrylic acid-coated upconversion nanophosphors[END_REF] Within lanthanides, Tm 3+ ions are thus especially well suited because they provide a main up conversion emission band around 800nm. Prasad et al. recently reported the development of core/shell NIR-to-NIR UCNP with enhanced up conversion efficiency, which enable whole body mice imaging with high signal to background ratio (see figure 2C). [26•] with permission from the American Chemical Society).

Time gated detection

In order to improve detection sensitivity, the signal of interest can be discriminated from the autofluorescence background spectrally (as mentioned above by selecting NIR excited and/or emitting NPs), but also temporally. This can be easily achieved using inorganic NPs that present slow fluorescence decay (from 10 ns to several µs) compared to both extrinsic and intrinsic organic chromophores (a few ns). [START_REF] Richards-Kortum | Quantitative optical spectroscopy for tissue diagnosis[END_REF] In the last decade, two techniques have emerged to perform time-gated detection using pulsed excitation. Time Correlated Single Photon Counting (TCSPC) provides a high temporal resolution with a pixel scanning acquisition. Fluorescence decay maps are then reconstructed from the raw data. This technique thus provides information on the distribution of fluorescence lifetimes, from which the slow NP decay can be deconvoluted from the fast autofluorescence. However, it can detect at most one fluorescence photon per pulse which limits the acquisition speed. It is therefore not well suited for dynamic sample imaging. In contrast, intensified gated cameras (iCCD) directly provide an integrated image for different controllable delays between the excitation and the detection window. Selecting a detection delay longer than the autofluorescence lifetime ensures its rejection, while efficiently detecting the signal from slower decaying NPs. Time-gated detection is particularly interesting to increase NP detection sensitivity in vivo, and several applications have been demonstrated for tumor targeting or cell tracking as illustrated in Table 2 and Figure 3. Table 2. Applications of time-gated detection for biological imaging. Detection enhancement stands for the gain in sensitivity or signal-to-background ratio between a time-gated and an ungated detection. Another way to extract a signal of interest from the autofluorescence background is to use a specific modulation of the fluorescence intensity. NDs fluorescence, for example, depends on the spin properties of their ground state, and can therefore be modulated by an external magnetic field or a microwave irradiation. Since autofluorescence is insensitive to these modulations, the signal from NDs can be extracted with an excellent signal to noise ratio. [START_REF] Igarashi | Real-time background-free selective imaging of fluorescent nanodiamonds in vivo[END_REF][START_REF] Sarkar | Widefield in vivo background free imaging by selective magnetic modulation of nanodiamond fluorescence[END_REF]

Multimodal imaging

The interest of fluorescence in vivo imaging resides in its high sensitivity and spatial and temporal resolution. However, it is still limited to surface tissues; it is not quantitative and often requires additional information on anatomic environment. These limitations can be circumvented by combining fluorescence with other imaging modalities. For example, magnetic resonance imaging (MRI) and computed X-ray tomography (CT) provide whole body images and can be used for preoperative detection while fluorescence can be used intra-operatively for guided surgery. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are very sensitive methods and can provide quantitative information on metabolic activity but suffer from a poor spatial resolution whereas fluorescence possesses a high spatial resolution but is not a quantitative technique. The combination of these modalities with fluorescence imaging can be achieved using nanoparticle-based multimodal probes.

A first strategy consists in coating the surface of fluorescent nanoparticles with active ligands. This method is quite universal as it only requires changing one part of the ligand. For example, the grafting of gadolinium complexes leads to dual MR-fluorescence imaging probes. [START_REF] Kim | Multifunctional nanostructured materials for multimodal imaging, and simultaneous imaging and therapy[END_REF] This method was also used to produce SPECT/fluorescence and PET/fluorescence imaging by grafting 111 In-and 64 Cu-complexes respectively. [START_REF] Xing | Radiolabeled nanoparticles for multimodality tumor imaging[END_REF] Unfortunately, such probes lack stability due to a non-covalent grafting of such these ligands.

Another strategy consists in incorporating different types of nanoparticles in a carrier nanoparticle. Such composite nanoparticles for fluorescence and MRI were obtained by incorporating QDs and superparamagnetic iron oxide nanoparticles (SPIONs) in silica or polymer matrices. [START_REF] Bigall | Fluorescent, magnetic and plasmonic-hybrid multifunctional colloidal nano objects[END_REF] This method provides very stable probes and preserves the individual properties of each nanoparticle. However their use is limited by their generally larger size (typically a few hundreds of nanometers).

Alternatively, the doping of fluorescent nanoparticles with paramagnetic ions or radioactive nuclide is an effective method to obtain small and stable multimodal probes. The main drawback of this method is the limited amount of doping ions successfully incorporated. In the case of RE-NPs, the incorporation (either as component of the host matrix or as codopant) of different ions can be used to implement new modalities: Gd 3+ for MRI and CT and 18 F for PET [START_REF] Chen | Upconversion nanoparticles: design, nanochemistry, and applications in theranostics[END_REF]. The same strategy has been used with quantum dots, mainly by incorporating Mn 2+ ions to produce bimodal MR/fluorescent probes. For example, Sitbon et al have shown that Zn-Cu-In-Se/Zn 0.9 Mn 0.1 S QDs can be used to detect regional lymph nodes in both MRI and NIR fluorescence imaging (Fig. 4). [START_REF] Sitbon | Multimodal Mn-doped I-III-VI quantum dots for near infrared fluorescence and magnetic resonance imaging: from synthesis to in vivo application[END_REF] More recently, the incorporation of 64 Cu by cation exchange enable their use as PET/fluorescence contrast agents [START_REF] Sun | Self-illuminating 64 Cu-doped CdSe/ZnS nanocrystals for in vivo tumor imaging[END_REF]. Another way to obtain multimodal probes is to grow directly a nanoparticle on top of another one. This method provides very stable probes but some charge transfer between the different materials may cause a diminution of QY. This system has been particularly studied for the synthesis of MR/fluorescent probes, as for example in the FePt/CdSe or Fe 3 O 4 /CdSe systems and more recently in the Si/Fe 3 O 4 system. [START_REF] Bigall | Fluorescent, magnetic and plasmonic-hybrid multifunctional colloidal nano objects[END_REF][START_REF] Sato | Size-tunable silicon/iron oxide hybrid nanoparticles with fluorescence, superparamagnetism, and biocompatibility[END_REF]

Conclusions/Perspectives

Inorganic nanoprobes present unique optical properties that can be harnessed to enhance the sensitivity, resolution and imaging depth of fluorescence in vivo imaging. They are therefore bound to play an increasingly central role in the development of new imaging techniques and applications. Continuing progress in the synthesis of these NPs and in the control and understanding of their optical properties, together with the development of adapted instrumentation, should further improve their imaging performance. To translate these advances into pertinent bio-applications, the development of their surface chemistry is also required, in order to better control their biodistribution and improve their targeting efficiency. This should in turn enable a better understanding and control of their potential toxicity. Ultimately, fast clearance of these NPs from the body will be required to bring these imaging modalities to the clinical field. We note some recent progress in this direction, with examples of fluorescent NPs that are either small enough [START_REF] Choi | Renal clearance of quantum dots[END_REF] or degrade into molecular compounds [START_REF] Gu | In vivo timegated fluorescence imaging with biodegradable luminescent porous silicon nanoparticles[END_REF] to undergo partial renal clearance. Future expected developments should also include smart nanoprobes that are able to report on their biochemical environment, or theranostics probes that use local heating, singlet oxygen production or controlled drug delivery as a therapeutic tool in addition to imaging.

Figure 1 .

 1 Figure 1. In vivo through-skull fluorescence imaging of the brain vasculature using SWNT injected in the tail vein, in the NIR I (left) and NIR II (right) range. (Reproduced from ref. [37••] with permission from Macmillan Publishers Ltd.).

[ 26 ]Figure 2 :

 262 Figure 2: Up-conversion nanoparticles imaged in live organisms: A) Fluorescent and bright field images of C. elegans NaYF 4 : Yb,Er@PEI nanoparticles; B) Maximal intensity projection image of a 200-μm-thick image stack showing vasculature in the mouse brain. Scale bar is 100 μm (Reproduced from [48••] with permission from the National Academy of Sciences); C) Whole-body imaging of a nude mouse injected via tail vein with NIR-to-NIR core-shell nanoparticles. (Reproduced from ref. [26•] with permission from the American Chemical Society).

  Finally, in order to completely overcome autofluorescence, le Masne de Chermont et al. have developed persistent luminescence nanoparticles that can be excited before the in vivo injection and followed for up to one hour without the need of an external illumination source.

Figure 3 :

 3 Figure 3: Continuous Wave (CW) and Time-Gated fluorescence images of tumors after injection of NIR luminescent nanoparticles made of porous silicon coated with 5kDa polyethylene glycol into the right shoulder tumor. (Reproduced from ref[START_REF] Gu | In vivo timegated fluorescence imaging with biodegradable luminescent porous silicon nanoparticles[END_REF] with permission from Macmillan Publishers Ltd).

Figure 4 .

 4 Figure 4. MR/fluorescence colocalization of regional lymph nodes with Zn-Cu-In-Se/Zn 0.9 Mn 0.1 S QDs. T1-weighted MRI images of a mouse before (a) and after (b) injection of QDs and the corresponding NIR fluorescence image (c) (Reproduced from ref [60] with permission from the Royal Society of Chemistry)

Table 1 .

 1 Water soluble near infrared emitting nanoprobes, their respective size, QY and emission wavelength. (For RE-NPs, QY is defined for standard one photon excitation, down-conversion fluorescence).

	46]