The site of Caours exhibits a calcareous tufa deposit of >10 000 m² and up to 3.5 m thick, overlying lower terrace fluvial deposits of a Somme River (Northern France) tributary. The whole calcareous tufa sequence is attributed to the Eemian Interglacial (MIS 5e) according to its location within the Somme River terraces system, palaeontological data, and U/Th TIMS dating (average of 124 ± 4 ky BP). Since 2005, archaeological excavations of a Middle Palaeolithic site at the base of the tufa formation have provided long stratigraphic profiles (up to 20m long), supporting multidisciplinary studies. Palaeoenvironmental data from a petrographic study and molluscs were compared to geochemical proxies recording temperature (?¹⁸O) and humidity (?¹³C and trace elements Mg and Sr).
In thin section the main Cyanobacteria taxa precipiting tufa were identified as the fossil genus Broutinella and Ponsinella. Broutinella builds massive “cauliflower” tufas. This subaqueous facies is associated with aquatic molluscs and ostracods. Ponsinella produces the “mille-feuilles” facies with thin laminations testifying of diffuse flow periods. This facies is associated to malacofauna dominated by land snails. The cauliflower facies is dominant in the lower part of the sequence, where more pluvial conditions are suggested by ?¹³C data. This wettest part is also the warmest according to ?¹⁸O, and corresponds to the expansion of woodland molluscs. The maxima of temperature and humidity (from ?¹⁸O and ?¹³C) are correlated with the Climatic Optimum (highest number of forest species diversity and mollusc shells). In the upper part, the mille-feuilles facies becomes better developed as spring flow waned, based on the ?¹³C data. Dry conditions are also indicated by mollusc fauna which record a landscape of mainly open areas and some forest remnants. Clear environmental modifications caused by climatic changes are thus reconstructed from the tufa at Caours especially around the Interglacial Optimum.