New constraints on the evolution of planktic foraminifers and calcareous nannofossils across the Paleocene-Eocene Boundary interval: the Zumaia section revisited.

The Zumaia Section (western Pyrenees) contains one of the most complete and expanded deep-water successions across the Paleocene-Eocene transition so far described. The succession, which is rich in well preserved microfossils, outcrops continuously along sea cliffs and has been the subject of cyclostratigraphic, micropaleontologic, magnetostratigraphic and chemostratigraphic analyses. In a new, high-resolution biostratigraphic and paleoecologic study of planktic foraminifers and calcareous nannofossils of the 67 m thick interval from the upper part of the magnetochron C25r to the lower part of the calcareous nannofossil Zone NP11, including the Initial Eocene Thermal Maximum (IETM), the last and first occurrences (LO, FO) of the most important species, and their relative position within chrons C25r, C25n and C24r have been pinpointed. Some of our results on the distribution of planktic foraminifera are at odds with previous studies, the following ones being particularly noteworthy: (a) the LO of Globanomalina pseudomenardii coincides with the P/E boundary, although specimens within the last 10 meters of the Paleocene succession are distinctly smaller than those found within Biozone P4; (b) the FO of Morozovella subbotinae occurs in the upper part of chron C25n, about 28 m below the IETM; (c) the LO of Igorina laevigata is observed 7 meters above the P/E boundary; (d) Globanomalina luxorensis is very scarce in all of the studied samples (less than 1% of the assemblage), including those above the IETM; (e) the proportion of acarininids does not increase at or near the P/E boundary, but only 6,5 meters above it; and (f) we have not found specimens of Acarinina sibaiyaensis neither Morozovella africana in connection with the P/E boundary. The distribution of the calcareous nannofossil concurs with previous studies in showing a progressive evolution of this group from the FO of Discoaster multi-radiatus, 31m below the P/E boundary, to the FO of Tribrachiatus orthostylus, 20,5 m above it. This evolution is particularly rapid during the IETM when the Rhomboaster-Tribrachiatus lineage and some peculiar discoasters have their FO, and a short lived species association characterized by nannoliths with anomalous structure evolved. In Zumaia, the response of the calcareous plankton to the IETM changes is similar to that observed in the Tethys area but different to that in higher latitudes. Moreover, changes affecting the nannofossil assemblages at the P/E transition begin some 5 meters below the onset of the IETM, where richness, diversity and relative abundance of some dominant species (i.e. Zygrhablithus bijugatus, Semihololithus) show a marked decrease