Miocene (Burdigalian) seawater and air temperatures estimated from the geochemistry of fossil remains from the Aquitaine Basin, France
Résumé
Construction work of the Highway A65 led the top of the Burdigalian ‘Molasses de l'Armagnac’ Formation to outcrop in two close localities near the town of Bazas and Marimbault, situated on the western edge of the Aquitaine Basin, France. From this formation, a rich fossil assemblage has been recovered and includes both marine and terrestrial fossil remains which offered the opportunity to reconstruct both seawater and terrestrial air temperatures at a regional scale during the Burdigalian. For this purpose, two sets of fossil samples representative of the whole assemblage have been selected for each locality. They include teeth and bones of sharks, fish, rays, reptiles and mammals, as well as tests of sea urchins and shells of bivalves. First, we performed rare earth element (REE) analyses of 53 apatite samples from both localities in order to characterize the diagenetic history of the assemblage. We then analysed the oxygen isotope composition of 49 biogenic apatite phosphate samples and 6 biogenic carbonate samples representing both marine and terrestrial organisms. Using published isotopic fractionation equations, both seawater and terrestrial air temperatures were reconstructed. Calculated seawater temperatures using phosphate and carbonate isotopic thermometers are + 23 ± 4 °C and + 25 ± 1 °C respectively for the locality of Mendouillet (Bazas) and + 24 ± 4 °C and + 26 ± 1 °C respectively for the locality of Monbalon-Miron (Marimbault). Calculated terrestrial air temperatures are + 18 ± 2 °C for the locality of Mendouillet and + 17 ± 2 °C for the locality of Monbalon-Miron. These regional temperatures are in good agreement with seawater and terrestrial temperatures calculated for other European localities of contemporaneous periods. These results also fit the global climatic context of the Mid-Miocene Climatic Optimum starting at the end of the Burdigalian. Even though the global climatic conditions were warmer than today during the end of the Burdigalian, calculated continental and seawater thermal gradient at mid-latitude appear to have been comparable to present-day ones.