Late Barremian-early Aptian climate of the northern middle latitudes: Stable isotope evidence from bivalve and cephalopod molluscs of the Russian Platform

Palaeotemperatures during the late Barremian-early Aptian (Early Cretaceous) on the Russian Platform have been determined on the basis of oxygen isotope analysis of aragonitic bivalve molluscan and ammonoid shells and belemnite rostra with well-preserved microstructure from the Ulyanovsk area. Those obtained from the planispiral and heteromorph ammonoid shells from the lower Aptian Volgensis Schilovkensis, Deshayesi-Tuberculatum, and Deshayesi-Renauxianum zones range from 26.7 to 33.2 degrees C, from 292 to 33.1 degrees C, and from 27.0 to 29.5 degrees C, respectively. A heteromorph Helicancylus? cf. philadelphius shell from the uppermost lower Aptian Bowerbanki Zone was secreted in highest temperature conditions (32.8-35.2 degrees C). In contrast, upper Barremian molluscs (bivalve Cyprina sp. and belemnite Oxyteuthis sp.) of the Ulyanovsk area show significantly lower palaeotemperatures: 16.9-18.5 degrees C and 7.9-17.8 degrees C, respectively, which is in accordance with known palaeogeographic and palaeobotanical evidences, showing that a distinct climatic optimum seems to have occurred during the late early Aptian, when warm Tethyan water penetrated into the basin. Marked changes in calculated growth temperatures for investigated molluscs from the Russian Platform were most likely connected with both the general warming trend during the late Barremian early Aptian and local palaeonvironmental conditions. New data from the Bowerbanki Zone of the Russian Platform provide evidence on existence of the positive carbon isotope anomaly (2.4-6 parts per thousand,) at the end of the lower Aptian. There were apparently the three positive C-isotope anomalies during the late Barremian early Aptian. The onset of mid early Aptian Oceanic Anoxic Event (OAE) 1 a seems to coincide with both the beginning of significant warm conditions (followed by short-term cooling) and the abrupt decline in heavy carbon isotope concentrations in marine carbonates, which partly were the likely consequences of the intensive release of CO2 (biased by volcanic activity) and/or dissociation of methane gas hydrate. (C) 2013 Published by Elsevier Ltd.