The stable O- and C-isotope record of fossils from the upper Barremian-lower Albian of the Caucasus - palaeoenvironmental implications

Late Barremian to early Albian palaeotemperatures have been determined through oxygen isotope analyses of well-preserved brachiopod, bivalve, gastropod and ammonite shells, and belemnite rostra from the Caucasus. Results from calcitic invertebrate skeletal elements from the upper Barremian Sartousiana, Securiformis and Waagenoides zones indicate palaeotemperatures of 19.2, 12.0-17.1 and 11.3-14.3 °C, respectively. In contrast, early Aptian fossils of the Weissi and Deshayesi zones reveal significantly higher palaeotemperatures (20.5-22.3 and 17.1 °C, respectively), than those obtained from brachiopod shells of the Securiformis Zone. Isotopic palaeotemperature data are not available for the Furcata Zone in the Caucasus. The warmest conditions in our study interval developed during the late Aptian Melchioris-Abichi Chrone according to data on bivalves, gastropods and the ammonite Parahoplites (20.8-27.1 °C) and the Subnodosocostatum Chrone (?25.3 °C), on the basis of a shell of the oyster Amphidonte. However, the potential influence of minor freshwater incursions into the upper epipelagic zone in which Amphidonte lived, as well as the predominance of ammonites that preferred cool conditions in the Subnodosocostatum Zone, leads to an alternative interpretation in which cooling initiated at the beginning of the late Aptian. There remains considerable uncertainty in the interpretation of δ18O values in fossils from the Subnodosocostatum Zone but it is possible that one or two cooling events interrupted the warmest conditions during the late Aptian. The highest δ13C values derived from fossils found in the Subnodosocostatun Zone (up to 8.3 and 3.9‰ for shallow-water bivalves and deeper-dwelling belemnite taxa, respectively) have been linked to an episode of late Aptian volcanic activity. On the basis of our stable oxygen isotope record, two ethological groups can be recognised among Aptian ammonites from the Caucasus: cool and thermophilic dwellers. All Early Cretaceous belemnites from this area seem to have had a preference for cool-water conditions.