Biostratigraphic and isotopic record of the Cenomanian-Turonian deposits in the Ohaba-Ponor section (SW Hateg, Romania)

The middle Cenomanian-lower Turonian deposits of Ohaba-Ponor section (Southern Carpathians) were studied from biostratigraphic and isotopic points of view. Both the qualitative and semiquantitative nannofloral analyses, as well as the stable isotope (delta C-13 and delta O-18) data support significant palaeoenvironmental changes in the investigated interval. Two delta C-13 positive excursions were recognized: (1) an excursion up to 1.8 parts per thousand (PDB) within the middle/late Cenomanian boundary; (2) an excursion up to 2.2 parts per thousand (PDB) in the Cenomanian/Turonian boundary interval. The oldest delta C-13 positive excursion recorded (placed within the Acanthoceras jukes-brownei/Eucalycoceras pentagonum Ammonite Zone boundary interval, and in the NC11 Calcareous Nannofossil Zone respectively) could be assigned to the middle Cenomanian Event II (MCEII). During the above-mentioned event, significant increase in abundance of Watznaueria barnesae, followed by successive blooms of Biscutum constans and Eprolithus floralis, were observed. The youngest delta C-13 positive excursion was identified in the Cenomanian/Turonian boundary interval (in the NC12 and lower part of the NC13 Calcareous Nannofossil Zones). Even the amplitude of this delta C-13 positive excursion is lower in the Ohaba-Ponor section, as generally reported, this may represent the regional record of the OAE2. The successive peaks of the nannofossils Biscutum constans, Zeugrhabdotus erectus and Eprolithus floralis indicate episodes of cooler surface water and high fertility, which preceded and lasted the Cenomanian/Turonian boundary event. Additionally, fluctuations of delta O-18 values between -2 and -6 parts per thousand suggest also cooler conditions within the Cenomanian/Turonian boundary interval. (c) 2008 Elsevier Ltd. All rights reserved.