Isotopic seawater temperatures in the Albian Gault Clay of the Boulonnais (Paris Basin): palaeoenvironmental implications

Oxygen isotopes were measured on several types of fossil hardparts from the Gault Clay Formation including benthic and planktonic foraminifera, belemnite guards, and fish small-teeth. Belemnites δ18O values indicate low temperatures (13.5-19.3 °C) with an increase from the Middle to Late Albian. Foraminifera provide variable δ18O values, some too low to be relevant in terms of temperature (until 42 °C). These low values probably result from a diagenetic alteration of the foraminiferal tests even though SEM observations revealed well-preserved microstructures. However, higher foraminiferal δ18O values recorded in some levels indicate temperatures in the range of previously published estimates for the Albian at comparable palaeolatitudes. In these levels, temperatures inferred from benthic and planktonic foraminiferal δ18O range between 15-17 °C and 27-30 °C respectively, during the Middle-Late Albian interval. This slight increase in temperature is coherent with the long-term warming that characterises the Aptian-Cenomanian interval. The temperature difference between sea-surface and bottom waters fits well with a deposition at a palaeodepth of about 180 m in lower offshore environments, assuming a temperature gradient with depth comparable to the modern one in similar epicontinental tropical environments. Fish small-teeth indicate a temperature range from 22 to 28 °C consistent with previously published data from planktonic foraminifera, with a greater variability recorded during the late than during middle Albian. This correspondence suggests that small-teeth assemblages may be dominated by pelagic fishes, thus recording upper ocean temperatures. Finally, the markedly lower temperatures recorded by the belemnite guards compared to other analysed materials suggest a necto-benthic mode of life of belemnites.