Absolute age and temperature of belemnite rostra: Constraints on the Early Cretaceous cooling event

Assessing the climatic perturbations during the Cretaceous when atmospheric CO2 levels were high is critical for understanding the nature of environmental responses to greenhouse forcing in the past and future. Paleoclimate reconstructions utilizing various geochemical proxies have revealed a disturbed interval during the Late Jurassic-Cretaceous period. This interval comprised multiple instances of cooling and warming, including notable events such as the Kimmeridgian Warm Interval, Faraoni Thermal Excursion, and Tithonian-early Barremian Cool Interval. Among these perturbations, the late Valanginian stands out as the coolest identified period. Reconstructions of the duration and magnitude of these warming and cooling events mostly rely on biostratigraphy and the delta O-18 values of carbonate minerals, but they remain controversial. In this study, we applied in situ UPb geochronology, Sr-87/Sr-86 ratios, and clumped isotope thermometry to belemnite calcite from the Mahajanga Basin, Madagascar. We used these data to reconstruct seawater temperatures that are linked to absolute ages. The UPb dates represent the timing of when diagenetic cements replaced the original organic matter in the apical line of the belemnite rostrum and filled the space after the death of the organisms. The lower triangle(47)-derived temperatures obtained in the apical zone compared to other well-preserved regions of the rostrum could be explained by the possibility that diagenetic calcite formed on the seafloor and reflects bottom water temperature. Our results identified an abrupt Early Cretaceous cooling event of up to 6 degrees C in the Southern Hemisphere that followed the main phase of carbon isotope excursion, which is explained as a combination of global cooling and the change in belemnite habitat depth. These results demonstrate the necessity for better constraints on the absolute timing and magnitude of climatic fluctuation events and their potential consequences in order to achieve a more complete understanding of the relationships among volcanism, carbon cycle, atmospheric pCO(2), ocean temperature, and continental ice volume.