Calcareous nannofossils of the Barremian – Aptian interval from the southeastern Tethys (Zagros Basin, West Iran) and their paleoceanographic implications: A record of Oceanic Anoxic Event 1a

Abstract

Abstract Calcareous nannofossils and stable isotope data (δ13Ccarb) from a pelagic section of the Zagros Basin (west Iran) were used for an integrated oceanographic analysis of the Barremian – Aptian interval. The study aims at analyzing various oceanographic shifts recorded from this period, including the Oceanic Anoxic Event 1a (OAE 1a), in an expanded carbonate rich mudstone sequence of the eastern Tethys. The extreme thickness (510 m) of the succession allows us to study the environmental changes known from the Barremian and Aptian interval in much more detail than previously done. By looking at closely spaced samples it is possible to better understand the onset, the course and the aftermath of the OAE 1a event. Calcareous nannofossils suggest an early Barremian (nannofossil zone CC5/NC5C) to late Aptian age (nannofossil zone CC7b/NC7B) for the studied sequence. The biostratigraphic zonation is based on the last occurrence of Calcicalathina oblongata, the first occurrences of Rucinolithus irregularis and Eprolithus floralis and the last occurrence of Micrantholithus hoschulzii. A total of 98 species has been encountered in the studied interval with good to moderate, in few cases poor preservation. Based on the calcareous nannofossil assemblages, signals for a warming event in the mid-Barremian are characterized by high relative abundances of warm water specie (e.g., Rhagodiscus asper) along with neritic taxa (Micrantholithus spp., Nannoconus spp.) The OAE 1a was identified via calcareous nannofossil biostratigraphy in combination with the δ13Ccarb record. For the OAE 1a (early Aptian), warm and eutroph conditions were likely, as suggested by high relative abundances of R. asper, Diazomatolithus lehmanii, small Zeugrhabdotus spp. and Lithraphidites carniolensis. The late Aptian is characterized by cool surface water conditions.