ИСТИНА

The late Barremian to early Aptian temperature succession for the Russian platform was obtained for the first time on the basis of oxygen isotopic analysis of well-preserved bivalve molluscan (Corbiculidae? and Neocomiceramus) and ammonoid (Dashayesites, Sinzovia, Proaustraliceras, Volgoceratoides, and Arioceras?) shells, and belemnite Oxyteuthis rostra from the Ulyanovsk area. Palaeotemperatures obtained from the aragonitic planispiral and heteromorph ammonoid and bivave shells from the early Aptian Volgensis-Schilovkensis, Deshayesi-Tuberculatum, and Deshaesi-Renauxianum zones are: 26.7–33.2◦C, 29.2–33.1◦C, and 27.0–29.5◦C, respectively. A heteromorph Helicancylus? shell from the end-early Aptian Bowerbanki Zone was even secreted in somewhat higher temperature conditions (32.8–35.2◦C). High delta13C values (up to 5‰) were encountered in the mentioned heteromorph shell. In contrast, the late Barremian molluscs (bivalve Corbiculidae? and belemnite Oxyteuthis) of the Ulyanovsk area demostrate significantly lower palaeotemperatures: 16.9–18.5◦ and 7.9–17.8◦C, respectively, which is in accordance with the palaeogeographic data from the Russian platform. Data obtained agree with the palaeobotanical results from the Russian platform, Caucasus, Crimea, and Far East, which show that a distinct climatic optimum seems to have occurred during the Aptian. The new isotopic data on the Russian platform and some evidences from other areas suggest that the end-early Aptian climatic optimum, corresponding to the Tropaeum bowerbanki Zone of the Russian platform, seems to be the most prominent one within the limits of the Phanerozoic time.