An isotopic appraisal of the Late Jurassic greenhouse phase in the Russian Platform

Oxygen- and carbon-isotope ratios have been determined from Late Jurassic (Callovian–Volgian) belemnites from three locations on the Russian Platform (Gorodischi, Khanskaya Gora and Marievka). All samples were examined by means of trace element geochemistry and petrography in order screen for diagenetic alteration. Oxygen and carbon isotopes from well-preserved belemnites range from − 2.24 to − 0.09‰ and − 0.57 to 1.77‰ respectively. Oxygen isotopes, if interpreted in terms of temperature, reveal a rise of temperatures during the Oxfordian–Early Kimmeridgian and indicate a prolonged episode of warmth during the Kimmeridgian–Volgian. The isotope data only equivocally reflect a number of significant changes in Boreal–Tethyan ammonite assemblages. A positive carbon isotope excursion is observed within the Volgian, but not seen within composite carbon-isotope stratigraphies of the western Tethys. Hence the Jurassic may have been characterised by regional δ13C excursions related to non-simultaneous organic matter deposition resulting from localised ponding, semi restricted ocean circulation and a lack of tidal mixing.