Early Cretaceous climate for the southern Tethyan Ocean: Insights from the geochemical and paleoecological analyses of extinct cephalopods

Reconstructing temperatures of ancient oceans is important for understanding Earth's climate in deep time. Belemnites have been used widely to estimate Mesozoic seawater temperatures, but their paleoecology remains controversial. Clumped isotope (Δ₄₇) thermometry applied to belemnite rostra has the potential to significantly improve paleoecological and paleoclimate reconstructions, because this method is insensitive to the chemical composition of seawater. In this study, we obtained Δ₄₇, δ¹³C, δ¹⁸O, Mg/Ca and Sr/Ca data for three well-preserved belemnites sampled from two mid-latitude basins (∼30-45°S) in the Early Cretaceous (Valanginian) Tethyan Ocean. Our aim was to investigate their paleoecology over the course of ontogeny and compare Δ₄₇-based temperatures to reconstructed temperatures from δ¹⁸O values of the same specimens, TEX₈₆, and other proxies globally. We selected different species from a wide range of ocean settings to evaluate the feasibility of seawater temperature estimates for different depths. Our Δ₄₇ analyses results document a range of growth temperatures between 10 and 26 °C. Two main migration patterns and habitat environments were identified by the Δ₄₇ temperatures and rostra morphology, although an influence of seasonal temperature changes cannot be fully excluded. Hibolithes jaculoides with elongate and slender rostra had a temperature range of 10-18 °C, and were probably nektic and fast swimmers that undertook vertical migration in the water column. In contrast, Acroteuthis acrei and Duvalia sp. with stout and depressed rostra likely occupied shallower and warmer waters in the thermocline at temperatures of 18-26 °C. Our temperature estimates suggest relatively warm climatic conditions and a shallower thermocline zone characterized by a rapid decrease in temperature at depths shallower than 200 m in the mid-latitude Tethyan Ocean of the southern hemisphere during the Early Cretaceous greenhouse interval. When high pCO₂ is considered, our reconstructions are close to reproducing modern tropical climatic conditions.