A geochemical perspective on Parisian urban history based on U–Th dating, laminae counting and yttrium and REE concentrations of recent carbonates in underground aqueducts

• We test the potential of urban speleothems as a natural archive for the history of the impact of urbanization.
• We compare absolute (U–Th) and relative chronologies for young speleothems.
• We discuss the corrections for detrital 230Th for U–Th dating using stratigraphic constrain.
• We evidence a high rare-earth and yttrium content in infiltration waters in Paris since the mid-twentieth century.
• We discuss the presence of a negative Ce anomaly in the urban speleothems.

Two laminated carbonate deposits from the wall of an underground aqueduct, built between the 13th and 15th century, in Paris, France, were studied to provide historical information about nearby human occupation and urban development. To obtain a robust chronology for these small (29 and 45 mm thick) and very young deposits (< 800 a based on the probable date of aqueduct construction), we adopted two methods: laminae counting and U–Th analysis. For laminae, we tested the hypothesis that their deposition is bi-annual. Concerning U–Th dating, the influence of inherited 230Th on calculated ages is discussed and detrital 230Th/232Th values for each speleothem was determined using stratigraphic constraints and found to be significantly different despite their close proximity. In these speleothems, that precipitated over approximately 300 a according to U–Th dating, the number of laminae is similar, and two laminae were deposited each year. The lamina-counting chronology was used to interpret the rare earth elements and yttrium content (REE + Y) variations measured by ICP-QMS as historical variations in water quality in relation with the anthropogenic use of these elements. There is a marked increase of the REE + Y concentrations since the second half of the twentieth century. A marked negative Ce anomaly in the NASC-normalized REE trend is evidenced, and is more pronounced for post 1850s. This anomaly could be due to prior precipitation of Ce4+ before the water reached the aqueduct, related to a high organic matter content of the water in oxidizing conditions. The increase in the Ce anomaly could indicate a second source of water, from a wastewater collection system or a modern water conveyance system. We thus demonstrate the interest of using these annual carbonate deposits in urban areas as a proxy for the history of urbanization or human activities.