Planktic foraminiferal turnover across the Aptian/Albian boundary and new criteria to define the GSSP for the base of the Albian Stage

Apart from the terminal Cretaceous extinction, the planktic foraminiferal turnover across the Aptian/Albian boundary interval is the most dramatic event in the Cretaceous evolutionary history of planktic foraminifera, with a change from large-sized and heavily ornamented species in the latest Aptian to small-sized, globigeriniform specimens in the earliest Albian. The detailed documentation of planktic foraminiferal species content and abundance across a continuous Aptian–Albian sedimentary sequence exposed at the Col de Pré-Guittard section in the Vocontian Basin (southeast France) permits a reliable description of the magnitude and rate of planktic foraminiferal turnover at the Niveau Kilian organic rich bed. The latest Aptian assemblage is dominated by few long-ranging Hedbergella and large-sized Paraticinella that completely disappear near the base of the Niveau Kilian organic-rich level. Planktic foraminiferal assemblages from across the Niveau Kilian to the top of the studied section are composed of minute, but very distinctive smooth-surfaced species of Microhedbergella miniglobularis and Mi. renilaevis. The appearance of Mi. renilaevis in the middle part of the Niveau Kilian represents a major step in the evolution and diversification of the Albian planktic fauna. The lowest occurrence of this taxon is recorded immediately above the extinction level of the Aptian hedbergellids and corresponds with a dramatic decrease in the number of planktic and benthic foraminifera specimens and with a 1‰ negative δ13C excursion in bulk carbonate. The same sequence of events was reported from several deep-sea sites in the north and south Atlantic and Indian Oceans. Understanding the causes of the major planktic foraminiferal extinction event will require the study of additional biostratigraphically complete sections yielding well-preserved calcareous plankton assemblages. Nevertheless, the extinction of Paraticinella eubejaouaensis, the planktic foraminiferal turnover, the appearance of Microhedbergella renilaevis and the negative carbon-isotope shift in the Pré-Guittard section are globally identifiable events that could be used as primary and secondary criteria for defining the Global Boundary Stratotype Section and Point for the base of the Albian Stage.