HIGH-RESOLUTION (PIXE) ANALYSES OF CARBONATE DEPOSITS IN A ROMAN AQUEDUCT (FRÉJUS, SE FRANCE): IMPLICATIONS FOR THE STUDY OF PALAEOHYDROLOGICAL VARIABILITY AND WATER RESOURCES MANAGEMENT IN SOUTHERN GAUL DURING THE ROMAN PERIOD
Résumé
Urban water was supplied to the Roman city of Forum Julii (Fréjus, southeastern France) for at least 200 years by a 39.4 km long aqueduct, operating in AD 50. Two perennial springs were successively collected, the Foux and the Siagnole, located at the outlet of Triassic and Jurassic karstic reservoirs, respectively. In this study, we performed high-resolution PIXE (Particle- Induced X-ray Emission) measurements of Ca, Si, Fe and Sr concentrations on selected cross-sections of laminated carbonate sampled along the sidewalls, before and after the connection of the two collection channels. Seasonal variations of water composition, suspended sediment load and discharge are recorded by alternating clear and dark layers, and can be traced by their Sr and Fe contents. On the basis of an annual bimodal high-discharge regime for the two karstic reservoirs, the concentration measurements allow the derivation of a chronological record (117.5 years) of water supply and maintenance activities for a part of the operational period of the aqueduct. The water level in the channel was mainly controlled by the extent of carbonate deposition and by dredging, repair and maintenance operations rather than by the past hydrological regime of the two springs.