Abrupt planktic foraminiferal turnover across the Niveau Kilian at Col de Pré-Guittard (Vocontian Basin, southeast France) : new criteria for defining the Aptian/Albian boundary

A detailed and quantitative study of foraminiferal assemblages across the Niveau Kilian in the Col de Pré-Guittard section (Vocontian Basin, southeast France) ably documents the planktic foraminiferal turnover across the Aptian-Albian boundary interval. 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, with a 1‰ negative δ13C excursion in bulk carbonate, and evidence for regional rise in sea-surface temperature. Our careful study of the species composition permits refinement of the previous biozonation by using the appearance datum of Mi. renilaevis as a zonal boundary event. Changes include redefinition of the top of the Microhedbergella miniglobularis Zone and designation of a new Microhedbergella renilaevis Zone. The same sequence of events was reported from several deep-sea sites in the Atlantic and Indian Oceans. Therefore, documentation of the planktic foraminiferal turnover, combined with the carbon-isotope stratigraphy in the Col de Pré-Guittard section, provide new criteria for defining the Global Boundary Stratotype Section and Point for the base of the Albian Stage in a stratigraphically complete succession.