Calcareous nannofossil assemblages and their response to the Paleocene-Eocene Thermal Maximum event at different latitudes: ODP Site 690 and Tethyan sections

A major change in calcareous nannofossil assemblages has been reported at the Paleocene-Eocene Thermal Maximum (PETM) on a global scale. To document the response of the nannoplankton communities below, within, and above the PETM, we studied in detail six successions, representing a wide range of environments and latitudes. Calcareous nannofossil response was different in discrete paleogeographic areas. Several classical Tethyan sections (Alamedilla, Caravaca, Zumaia [Spain], Contessa [Central Italy], and Wadi Nukhl [Egypt]), plus the high-latitude Ocean Drilling Program reference Site 690 (Weddell Sea) were re-investigated using high resolution calcareous nannofossil quantitative analyses. Five assemblage zones were identified: two before the onset of the Carbon Isotope Excursion (CIE) and three after it. Before the PETM, several changes were observed in both high and low latitudes that are characterized by well-defined increases of r-selected taxa (Biscutum and Prinsius). These changes probably were in response to an upwelling pulse that increased nutrients in surface waters. These events, which predate the geochemical and oceanic changes at the PETM, indicate that there were global events occurring before the actual CIE onset. At Site 690, the principal calcareous nannofossil change coincides with the onset of the CIE and is characterized by the rapid replacement of cold-water taxa by warm-water taxa. This change resulted from a sudden expansion of warm-water low-latitude assemblages into higher latitudes, probably due to an abrupt increase of surface-water temperatures. An increase in species richness here is due to the migration of several genera (i.e., Discoaster and Fasciculithus) south from warmer areas and to decreased dissolution. Moreover, an increase in abundance of Thoracosphaera spp. (calcareous dinoflagellate) below and within the CIE also indicates a stressed surface-water environment. In the Tethyan sections, the response of the calcareous nannofossil assemblages to the PETM is more complex. As at the Southern Ocean Site 690, calcareous nannofossil fluctuations begin below the onset of the CIE and increase in frequency and amplitude at the benthic foraminifera extinction (BFE). At this level, calcareous nannofossil diversity and abundance abruptly decrease, and the Rhomboaster spp.-Discoaster araneus (R-D) association appears. The occurrence of the R-D association together with Thoracosphaera suggests that during the PETM there was a change to stressed ocean-surface conditions. Calcareous nannofossil recovery occurred later in the Tethys than at the southern high latitudes, where it occurred before the CIE recovery. Furthermore, the nannofloral assemblages after the delta C-13 recovery still indicate stressed conditions, suggesting that the plankton communities did not completely recover until later.