Biostratigraphy and isotope stratigraphy and the ‘catastrophic’ extinction of calcareous nannoplankton at the Cretaceous/Tertiary boundary

Abstract

The biostratigraphy of Cretaceous/Tertiary boundary sections from eight localities is summarized and compared in order to define a continuous or complete section. The El Kef, Tunisia, section is apparently the thickest C/T boundary section as yet discribed and contains all the biostratigraphical criteria required to define a complete section; that is, the uppermost Cretaceous Micula prinsiiZone, the “boundary clay” within the lowermost Tertiary Globigerina fringaZine, followed by the Globigerina eugubinaand pseudobulloidesZones. Using these zonations, a correlation of the stable-isotope stratigraphy from the various sections was made. The latest Cretaceous oceans and the earliest Tertiary oceans contained significantly different isotopic signals, which were incorporated into the tests of the calcareous nannofossils. The carbon-isotope signals are apparently global, synchronous, and primarily determined by changes in oceanic fertility, whereas the oxygen-isotope signals are globally induced but modified regionally according to paleogeographic positon and paleocirculation patterns. Further, this combination of biostratigraphy and isotope stratigraphy indicates that Cretaceous nannofossils in the lowest Tertiary sediments, previously thought to be reworked, actually survived the C/T boundary events and continued to reproduce in the earliest Tertiary oceans. These relic species became extinct some tens of thousands of years after the actual C/T boundary, probably as a consequence of the environmental stress following the C/T boundary events rather than as a result of a “catastrophic” extinction coinciding with the C/T boundary.