Carbonate deposits from the ancient aqueduct of Béziers, France — A high-resolution palaeoenvironmental archive for the Roman Empire

• Calcite deposits from Roman aqueducts inform on the ancient palaeoenvironment.
• Roman age temperature and precipitation patterns are recorded in the deposits.
• Aqueduct calcite deposits can provide palaeoclimate records with a daily resolution.
• Dense carbonate deposits form in fast flowing water due to the absence of biofilms.
• Incrustations from Roman aqueducts do not always show annual cyclicity.

Carbonate deposits from a Roman aqueduct in Béziers, southern France, record environmental conditions during the late first century C.E. These deposits formed in a steep section of the aqueduct with a high flow velocity, which caused rapid deposition of up to 11 mm of calcite per year over a period of 22–24 years. The microstructure, trace element and stable isotope composition show that regular deposition was interrupted by high-discharge events, probably in response to heavy rainfall during autumn and winter, transporting colloidally- and particle-bound elements and depositing calcite with elevated δ18O values. Individual autumn high-discharge events coincide with abrupt decreases in δ13C from − 8 to − 12‰ giving rise to a saw-tooth profile. In some years, several high flow events persisted throughout the winter, suppressing this profile. Event horizons of micrite capping sparite growth surfaces, enriched in Mg, may represent anomalously low water levels or periods when the channel fell dry. In comparison to carbonate deposits from Roman aqueducts in the Eastern Mediterranean, visible layering is less regular and pronounced in Béziers, reflecting a more complex precipitation pattern.