U/Th Dating of Secondary Carbonate Deposits in Underground Galleries of Fourvière Hill (Lyon, France) Reveals a Water Supply System in Operation From the Roman Period to the Middle Ages

The recent discovery of a water draining structure known as the Antiquaille galleries in Fourvière hill, in the center of Lyon, France (ancient Lugdunum), sheds light on the water drainage system built by the former inhabitants of this area for their water needs. We studied secondary carbonate deposits found in two galleries of this system: a massive flowstone and thin white layers of CaCO 3 found interbedded in a collapsed gallery. The study of these natural archives provides absolute ages (uranium–thorium and 14 C dating) for the period in which water flowed within the galleries. These data unambiguously date the system's installation and period of operation over several centuries. We hereby confirm that these galleries are indeed Roman. Dating showed that deposition of the concretions began in the year 150 ± 80 AD , which is coherent with the dating of a wooden floor of the structure (calibrated 14 C age between the years 127 and 233 AD ). We also provide new information showing that one gallery was in use (water in the gallery) for almost seven centuries, and on the chronology of the filling of another, which took place over almost three centuries. Lastly, a large charcoal found trapped at the top of the flowstone attested that the system was visited, at least once (calibrated age: between the years 673 and 877 ad ), at the end of the deposition period of the flowstone (end of the period with water). This is an interesting historical fact because little is known about these water supply systems, which were forgotten for centuries because Fourvière hill was abandoned by its inhabitants after the Roman period and only urbanized again since the 16th–17th centuries (first slightly and then more intensively from the 19th century).