Based upon ammonite biostratigraphy from Crimea, and in conjunction with a composite Tethyan marine δ13Ccarb curve, several conclusions can be drawn: (1) the δ13Cplant 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. δ13Cplant values fluctuate around –23‰ to –22‰ for the Valanginian–Hauterivian, except during the Upper Valanginian where δ13Cplant values record a positive excursion to ~–18‰. ![]() 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. 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. Our understanding of the ancient ocean-atmosphere system has focused on oceanic proxies. The ammonoid faunas of the two basins are compared here, and a detailed biostratigraphic division of the sequences is discussed and compared with the Mediterranean succession. In contrast, in the Neuquen Basin evaporites and continental clastics of the Huitrin Formation mark the beginning of a long disconnection with the Pacific Ocean, though a short-lived marine incursion is represented by the carbonates of La Tosca Member of the Huitrın Formation. Marine conditions persisted in the Chan˜arcillo Basin till Early Albian times the associated ammonoid faunas include pandemic, Pacific and Antarctic genera. ![]() They include both Andean and near-pandemic forms, the latter providing some good correlation levels with the ‘standard’ Mediterranean sequence. The Berriasian to Lower Barremian sequences were mainly marine and include rich ammonoid faunas, with many taxa in common to both the basins. The Chan˜arcillo and Neuque´n basins of the Central Andes shared a common geological history in the earlier part of the Early Cretaceous but from Barremian times onward their evolution began to diverge, probably due to an increasing activity of an intervening volcanic arc.
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