Environmental and depositional controls on laminated freshwater carbonates: An example from the Roman aqueduct of Patara, Turkey

Carbonate deposits in aqueducts are a new high-resolution data source for environmental changes during the time of the Roman Empire, notably in the fields of palaeoclimate and spring hydrology. In order to distinguish environmental effects from those related to depositional setting, laminated carbonate deposits were compared along the entire length of an ancient aqueduct channel at Patara, Turkey. The carbonate deposits, up to 80 mm in thickness, are composed of lamina couplets up to 1 mm thick of alternating porous microspar and dense, columnar sparite. The former formed in the dry, warm season and the latter in the wet, cool season. The presence of biofilms seems to play a role in the development of the porous laminae. The relative importance of both lamina types depends also on the location within the aqueduct: dense, coarse-crystalline laminae dominate in steep sections of the aqueduct, while porous, fine-crystalline laminae are mostly found on gentle slopes. This is attributed to the flow velocity of the water, since fast flow on steep slopes hampers the development of biofilms. Carbonate microstructures can vary considerably down the line of a specific aqueduct; hence multiple sampling is needed along the channel in order to collect specimens suitable for detailed research. δ18O profiles show oscillations with increasing amplitude downstream reflecting rising water temperatures in the channel, while evaporation plays a minor, damping role on these oscillations. The periodicity of the δ18O values can be used to recognise annual layering in order to establish the number of years an aqueduct was operating. δ13C values are higher in open masonry channel sections than in ceramic pipes, probably due to stronger degassing of CO2 in the open channel. Carbonate deposits in Roman aqueducts hold high promises as seasonally resolved archives of hydrology and climate and, possibly, of earthquakes.