New insights into the Barremian–lower Aptian calcareous nannofossils of the Mediterranean Tethys: Chronostratigraphic and paleobiogeographic implications

Abstract

A detailed study of calcareous nannofossil assemblages from twelve uppermost lower Barremian–lower Aptian sections in the Subbetic Domain of the Betic Cordillera was performed. Seven new nannofossil species (Crucibiscutum bastetanum, Crucibiscutum gracile, Chiastozygus lamprostauros, Cyclagelosphaera platyaspis, Lithraphidites aichmoides, Lithraphidites pugio, and Rhagodiscus sicutclipeus) are described, one species is emended (Lithraphidites magnus) and the taxonomic concept of the marker species Hayesites irregularis is discussed and clarified. The detailed stratigraphic ranges of the new species, together with those of other relevant taxa, are determined and correlated to standard ammonite biostratigraphy. Age estimates of biostratigraphically relevant calcareous nannofossil biohorizons are calculated using astrochronologically tuned cyclostratigraphic data. Five new calcareous nannofossil subzones are proposed which enhance upper Barremian biostratigraphic resolution at a regional scale and are directly correlated with respect to the standard Tethyan ammonite zonation. Two of the new species described here are used as biostratigraphic markers for the newly proposed subzones. The duration of each subzone is provided through astrochronological calibration. This study allows the refinement of the calcareous nannofossil zonation for the Mediterranean–Atlantic province of the Tethyan Realm. The implications of these new results are discussed regarding the extant definition and use of the Barremian/Aptian boundary. The morpho-evolutionary trends of selected nannofossil groups are reviewed in relation to the latest Barremian–Aptian paleogeographic changes, showing these were a prominent factor controlling calcareous nannoplankton evolution and biogeographical distribution in the west European-Atlantic region.