Abstract
Carbonate deposits in aqueducts are potential valuable archives of the paleo-hydrological record for the Roman period. Here we present a new high-resolution record from carbonate deposits in the aqueduct of Traconnade built for Aix-en-Provence, the Roman city of Aquae Sextiae. As its name indicates, Aquae Sextiae was built around an abundant source of water but its development, like other ancient cities, required good management of this resource. Through a combination of micromilling and in situ analysis (LA–MC–ICPMS) on thin sections, we measured geochemical variations (stable isotopes and trace elements) and performed sampling for absolute UTh dating. Geochronological results based on classical correction and isochron along with archaeological observations confirm that the carbonate concretion was deposited in Roman times on the walls of the Traconnade aqueduct, built no later than 140 CE. Carbonate δ18O records document the presence of clear geochemical cycles that were mainly controlled by seasonal variations in temperature and δ18O from water circulating in the aqueduct. Minima and maxima in δ18O values were attributed to summer and winter conditions, respectively in agreement with the modern spring’s stable isotope values. The other tracers (Mg/Ca, Sr/Ca and δ13C) also show marked cyclicities with maximum values occurring in summer. The enrichment in trace elements and δ13C were linked to processes occurring during dry periods (1) prior precipitation of calcite in the epikarst, and/or (2) a greater degree of CO2 degassing. The geochemical variations in Ba/Ca, U/Ca and Rb/Ca behave differently and their co-variations may reflect the water-rock interactions in the karst system. We highlight that these tracers can be used as a proxy for intense precipitation events, occurring frequently during autumn. The Traconnade aqueduct carbonate provides sufficient temporal resolution to explore environmental variations on a seasonal scale. These results suggest the occurrence of a double rainy season (spring and autumn) in the Aix-en-Provence region, similar to present-day climatic conditions. Our study confirms the high potential of aqueduct concretions for paleohydrological studies.
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