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

Abstract

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. d 13 Cplant values fluctuate around ! 23x to ! 22x for the Valanginian‐Hauterivian, except during the Upper Valanginian where d 13 Cplant values record a positive excursion to ~! 18x. Based upon ammonite biostratigraphy from Crimea, and in conjunction with a composite Tethyan marine d 13 Ccarb curve, several conclusions can be drawn: (1) the d 13 Cplant 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 d 13 C 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. D 2005 Elsevier B.V. All rights reserved.