Evolution of the carbon isotope composition of atmospheric CO2 throughout the Cretaceous

Although atmospheric CO2 has been extensively described as a primary driver of Phanerozoic climate and carbon cycle disturbance, little is known about its carbon isotope composition (δ13CCO2) during pre-Cenozoic times. We reconstruct for the first time the evolution of δ13CCO2 during the whole Cretaceous period based on reference curves of δ13C values of Tethyan marine bulk carbonates (δ13Ccarb) and δ18OPO4 values of fish tooth enamel. We test this method against that based on the oxygen and carbon isotope ratios of high-latitude benthic foraminifera (δ13Cforam) recently implemented for the Cenozoic and closely matching δ13CCO2 measurements from ice cores. Highly consistent δ13CCO2 estimates and trends are obtained from the δ13Ccarb method (− 5.90‰ ± 0.28) and the δ13Cforam method (− 6.18‰ ± 0.64) during the phases of available foraminiferal record encompassing the Albian-Maastrichtian interval. However, the δ13Cforam-based δ13CCO2 signal is strongly biased during the main OAEs, during which heat transport to higher latitudes associated with Haline Euxinic Acidic Thermal Transgression (HEATT) episodes may have affected temperature inference from δ18Oforam values. Our results demonstrate the suitability of using δ13Ccarb records to reconstruct past δ13CCO2 values, which opens the opportunity for pre-Cenozoic, worldwide δ13CCO2 reconstructions even during sharp climate change intervals. The consistency between both methods together with our δ13CCO2 estimates and those previously obtained for the Cenozoic provides about 145 Myr of carbon isotope evolution of atmospheric CO2.