The Upper Valanginian (Early Cretaceous) positive carbon-isotope event recorded in terrestrial plants

Our understanding of the ancient ocean-atmosphere system has focused on oceanic proxies. However, the study of terrestrial proxies is equally necessary to constrain our understanding of ancient climates and linkages between the terrestrial and oceanic carbon reservoirs. We have analyzed carbon-isotope ratios from fossil plant material through the Valanginian and Lower Hauterivian from a shallow-marine, ammonite-constrained succession in the Crimean Peninsula of the southern Ukraine in order to determine if the Upper Valanginian positive carbon-isotope excursion is expressed in the atmosphere. δ¹³C_plant values fluctuate around − 23‰ to − 22‰ for the Valanginian-Hauterivian, except during the Upper Valanginian where δ¹³C_plant values record a positive excursion to ∼− 18‰. Based upon ammonite biostratigraphy from Crimea, and in conjunction with a composite Tethyan marine δ¹³C_carb curve, several conclusions can be drawn: (1) the δ¹³C_plant record indicates that the atmospheric carbon reservoir was affected; (2) the defined ammonite correlations between Europe and Crimea are synchronous; and (3) a change in photosynthetic carbon-isotope fractionation, caused by a decrease in atmospheric pCO₂, occurred during the Upper Valanginian positive δ¹³C excursion. Our new data, combined with other paleoenvironmental and paleoclimatic information, indicate that the Upper Valanginian was a cool period (icehouse) and highlights that the Cretaceous period was interrupted by periods of cooling and was not an equable climate as previously thought.