Cretaceous oceanic anoxic events and radially elongated chambered planktonic foraminifera: paleoecological and paleoceanographic implications.

Chamber elongation is a recurring morphological character in Cretaceous and Cenozoic planktonic foraminiferal evolution suggesting that the clavate body-plan takes advantage through particular environmental conditions. Cretaceous planktonic foraminifera bearing radially elongated chambers become repeatedly a consistent component of assemblages in correspondence with the deposition of remarkable organic layers that record the effects of oceanic anoxic events (OAEs). Due to this apparent correspondence, chamber elongation has been recently interpreted as an adaptation to low oxygen levels in the upper water column. The paleoceanographic significance of planktonic foraminifera with radially elongated chambers remains however unclear. We investigated in great detail the distribution and abundance of elongated chambered planktonic foraminifera in an interval of about 40 myr across the Cretaceous OAEs in different well-dated sections. The increased abundance of planktonic foraminifera with elongated chambers can be a markedly evident feature across the Faraoni, the Selli and the Bonarelli OAEs. However each event shows its own peculiarities. Remarkably, the first radiation of the Cretaceous elongated chambered morphotypes just predates the latest Hauterivian Faraoni Event. The Selli Event records the greatest diversification (number of species) of the group bearing elongated chambers. However, this was a period of great diversification in the planktonic foraminifera. This evidence may suggest that radiation of these forms was driven by more than upper water column anoxia per se (e.g., increased upper water column stratification, greater productivity, and/or greater seasonality). The Albian OAE1b, OAE1c, and OAE1d display the lowest abundance and diversity of this group. Conversely, according to the available data, the first appearances of the species Muricohedbergella simplex and of the genus Schackoina occur close to the OAE1b and OAE1d, respectively. In the Italian record, the relative abundance of radially elongated chambered forms appears to be larger before the onset of the Bonarelli Event with respect to the Selli Event. The pattern involving this group across the Bonarelli Event appears more regular, thus indicating that the onset and the end of the OAE2 depend on very powerful global factors and are less influenced by local aspects. We speculate that the relative abundance of planktonic foraminifera with radially elongated chambers may be proportional to the strength of the environmental perturbation related to the OAEs, thus reinforcing the idea that the ecological perturbation related to the OAE2 was more severe for the former. Several lines of evidence suggest that water oxygenation could not have been the single controlling factor governing the development of elongate chambers. The role of availability of food and the nature of food has been so far probably underestimated. It is most likely that an interplay of several influential physical–chemical and ecological factors (i.e., dissolved oxygen concentration, temperature, salinity, nutrients, type of food, trace elements) was responsible for such a morphological adaptation. We document linkages between the distribution, abundance, morphometric changes and evolution of the Cretaceous radially elongated chambered planktonic foraminifera and the major igneous, geological, geochemical, climate, nutrification and bioticevents and biocalcification trends.