Stable Isotope Analysis of Gryphaea arcuata Reveals the Prevalence of Humid Tropical Conditions During the Early Sinemurian of Normandy (Fresville), Northwestern France

Marine deposits in Western Europe provide insight into the interplay between the warm Tethys and cooler Boreal domains, offering a climatic context for the radiation of Early Jurassic species. Reconstructions of temperature for the Hettangian and Sinemurian periods are scarce, with inferred marine temperatures of 15–20°C based on δ18O values, which are lower than those of subsequent Jurassic stages. This emphasises the necessity for supplementary data in order to enhance our comprehension of the climatic dynamics that characterised the Early Jurassic period. This study analyses 75 invertebrate samples, including 53 specimens of Gryphaea arcuata, from Early Sinemurian marine sediments in the Fresville quarry, Normandy, France. The present study employs a multi-proxy approach, utilising δ13C and δ18O values in conjunction with Sr and Mg contents, to assess the processes of fossil diagenesis, marine productivity and seawater temperatures. Significant post-depositional alteration was observed in the geochemical compositions of 22 bivalve shells assigned to the genera Pseudolimea, Plagiostoma and Chlamys, which were originally composed of aragonite. However, the low-Mg calcite shells of Gryphaea arcuata, which are renowned for their diagenetic resistance, retained the majority of their isotopic integrity. The results of the statistical analyses indicate that there was minimal late pervasive diagenesis involving meteoric waters at Fresville. This is in accordance with the typical decrease in δ13C, δ18O values and Sr and Mg contents that such processes would otherwise cause. Published isotopic data from Sinemurian marine fossils (plesiosaur and shark teeth) were used to estimate seawater δ18O (~ -1‰ VSMOW or -1.27‰ VPDB) and surface temperatures (~24°C). The calculated benthic temperatures of Gryphaea (17°C) correspond to habitats at depths of about 50 m. These findings suggest a positive hydrological balance, euhaline conditions, and elevated marine productivity in a humid tropical climate context.