INFRARED SPECTROSCOPIC INVESTIGATIONS OF THE NORTHERN MOLE OF PORTUS, THE ANCIENT HARBOUR OF ROME. INSIGHTS FOR STRATIGRAPHY AND PROVENANCE OF RAW MATERIALS FOR CONSTRUCTION

Abstract

Despite numerous studies focusing on Ancient Roman concrete especially petro-chemical characterization and provenance investigations of aggregates and mortar, less is known about the raw materials used for the construction of harbour structures where concrete had not been used. Recent geoarchaeological investigations of the northern mole of the Claudius harbour (Imperial port of Rome, Portus), revealed ten meters of continuous stratigraphic succession of raw materials employed for the foundation of the mole structure where the water column at the time of construction was reaching more than 16 meters of depth. The material succession consists of volcanic tuffs of different quality and hardness, carbonate and quartz rich sand, fragments and boulders of magmatic rock and Roman sherds. Attenuated total reflectance infrared spectroscopy and principal component analysis have been applied on drilled core sediments and local reference outcrops including tuffs and pozzolana in the vicinity of Rome with the aim to: (i) characterize the mineral composition of samples and (ii) provide insights about the lithological provenance of raw materials. The results revealed that the majority of materials have a close spectra resemblance with “Tufo Lionato” and “Pozzolane rosse” except leucitite fragments occurring at the top of the harbour structure. By providing spectra similarities between the raw construction materials and the local outcrops, this study provides insights about the sources of materials and building strategies during the construction of the Imperial harbour of Rome and therefore constitute a starting point for further archaeological, geoarchaeological and restoration projects.