Reconstructing the palaeoenvironment of the Middle Russian Sea during the Middle–Late Jurassic transition using stable isotope ratios of cephalopod shells and variations in faunal assemblages
Abstract Oxygen and carbon isotope data of well-preserved belemnite rostra and ammonite shells are presented from the Callovian–Oxfordian boundary (uppermost Lamberti to lowermost Cordatum zones) of the Dubki section near Saratov in the Russian Platform. Palaeotemperatures calculated for nektobenthic belemnites (averages of 5 °C and 8 °C for cylindroteuthids and belemnopseids, respectively) show the presence of cold bottom waters in the central part of the Middle Russian Sea during the studied interval. Palaeotemperatures calculated for ammonites, which are assumed to have lived in near-surface waters, are considerably higher (average 13 °C). The presented data show a vertical thermal gradient in the Middle Russian Sea. The belemnite oxygen isotope record and the relative abundances of ammonite families in the Dubki section do not correlate with each other probably as a result of different depth habitats of ammonites and belemnites. A review of literature isotope data shows the climatic zonation in European seas at the Middle–Late Jurassic transition. Despite the flux of cold polar waters to the Middle Russian Sea and the area of Scotland there is no evidence for glaciation at the Middle–Late Jurassic transition. Changes in water circulation during a sea-level highstand were likely a source of spreads of cold bottom waters and cardioceratid ammonite fauna in this time period. The belemnite isotope record of the Callovian–Oxfordian boundary in the Russian Platform is characterized by significant scatter of δ13C values. No temporal carbon isotope trend is observed. The δ13C values of Russian belemnite rostra average 2.6‰ VPDB being 1 to 2‰ higher than the values of coeval Lower Oxfordian belemnites from the area of the Submediterranean ammonite province. Higher (than Submediterranean) δ13C values of Russian belemnite rostra are likely related to high biologic productivity and/or high organic matter burial in semi-isolated Boreal–Subboreal marine basins.