Barremian-Aptian calcareous plankton biostratigraphy from the Gorgo Cerbara section (Marche, central Italy) and implications for plankton evolution

Abstract

The Barremian-Aptian boundary interval of the Gorgo Cerbara section (Marche, central Italy) was revisited in order to improve stratigraphic correlations and investigate plankton evolution. A very close sampling at cm scale was carried out in the upper Maiolica and basal Scisti a Fucoidi formations. The latter formation includes the Livello Selli, the landward sedimentary expression of oceanic anoxic subevent OAE la. The study of closely spaced samples revealed that (1) the first diversification among planktonic foraminifera, marked by the appearance of the genus Globigerinelloides, occurred during the Barremian (this event was dated by means of ammonites); thus the first occurrence (FO) of both Globigerinelloides duboisi and Globigerinelloides blowi cannot be used to identify the Aptian; (2) the appearance of Rucinolithus irregularis is the biostratigraphic event which best approximates the Barremian-Aptian boundary; and (3) chron M0, being slightly younger than the FO of R. irregularis, is very close to this boundary. Semiquantitative and quantitative analyses of planktonic foraminifera, calcareous nannofossils and radiolarians show that the three groups fluctuate in abundance and assemblage composition throughout the studied interval. An important change in plankton distribution patterns occurs within the G. blowi foraminiferal Zone and C. litterarius nannofossil Zone, where planktonic foraminifera, radiolarians and calcareous nannofossils start to show large-scale, higher frequency fluctuations in abundance; fluctuations in the three groups are out of phase. These new plankton distribution patterns are interrupted by the Livello Selli, which is preceded and followed by ‘critical intervals’. The distribution and composition of all three planktonic groups lead to the interpretation of the ‘critical intervals’ as representing increased nutrient contents in the surface water and the Livello Selli as a very high fertility event. Among planktonic foraminifers the hedbergellids seem to indicate a more eutrophic habitat than the globigerinelloids. In calcareous nannofossil assemblages, the abrupt crisis affecting the nannoconids prior to deposition of the Livello Selli suggests that these nannofossils are characteristic of more oligotrophic conditions, whereas Zygodiscus erectus is indicative of a more eutrophic environment. Based on our data, the early Aptian Livello Selli has the same oceanographic significance as the latest Cenomanian Livello Bonarelli, i.e., it represents a high fertility event on a global scale.