The sea cliffs at Zumaia provide one of the most complete and expanded sections of the Paleocene in open-marine facies from Western Europe and the Mediterranean region. Different studies carried out during the last two decades have allowed the identification at Zumaia of a complete sequence of calcareous plankton and benthic biotic events across the whole Paleocene succession, as well as reliable records of both paleomagnetic reversals and cyclestratigraphy. Based on these results and the completeness, good preservation and accessibility of the outcrop, the Zumaia section has been recently proposed by the Paleocene Working Group as the formal candidate to place the GSSPs for both the Danian-Selandian and Selandian-Thanetian boundaries.
In this contribution we present the results from the detailed analysis of planktonic foraminifers and calcareous nannofossils across the Danian/Selandian boundary at Zumaia. The study involved the biostratigraphic analysis of up to 60 samples from a 20m thick succession, which was at most sampled bed by bed. In addition, we have also carried out the bio-stratigraphic analysis of a similar number of samples (provided by B. Schmitz) from the Qreiya section, in Egypt, and have integrated the results from both sections in a chronostratigraphic framework (Bernaola et al., in press) that also takes into account the data from a recent analysis of the Selandian stratotype sections in Denmark (Clemmensen and Thomsen, 2005).
As a result, up to 14 different biotic events have been identified in the Danian/Selandian boundary interval. Of particular interest among them are those four associated with the marked lithological change that has been selected for the placement of the Danian/Selandian GSSP: 1) a diversification pulse of the genus Fasciculithus; 2) a sharp decrease in the abundance of Braarudosphaera; 3) a decrease in Morozovella(mainly in biconvex morozovellids), and 4) the onset of a marked change in the sense of coiling of Morozovella occlusa. These four events in calcareous plankton are coincident with a striking event in benthonic foraminifers marked by an increase in trochamminids and Spiroplectammina(Arenillas et al., 2008).
All the biotic events identified across the Danian/Selandian attest to marked environmental and evolutionary changes across this boundary interval and will allow a precise definition of the boundary using calcareous plankton in other open-marine sections worldwide.
This research is a contribution to Projects CGL2005-01721/BTE and CGL2005-027701/BTE (Ministerio de Educación y Ciencia, Spanish Government). J.I.B. thanks support from a "Ramón y Cajal" research position funded by the Ministry of Education and Science of Spain and the University of the Basque Country.