Morphometric changes of the calcareous nannofossil taxon Discoaster multiradiatus across the Paleocene-Eocene Thermal Maximum: biotic and abiotic factors

Studies on a climate extreme as the Paleocene-Eocene Thermal Maximum (PETM, ~ 55 myrs ago) have shown the effects of these critical conditions on global biogeochemical cycles and ecosystem, including the marine and terrestrial biota. A prominent negative shift (~3 ‰) in marine 13C, the Carbon Isotope Excursion (CIE) (Shackleton and Hall, 1984; Kennett and Stott, 1991, Zachos et al., 2001), reflects the input of a large amount of isotopically depleted carbon in the ocean-atmosphere system. Studies of the few complete deep-sea sections recovered to date (e.g., Kelly et al., 2004, 2005; Zachos et al., 2003, 2005), have shown how global climate, atmospheric CO2 levels, marine carbonate chemistry and continental weathering were dynamically related during the PETM. Together, these changes should have strongly influenced the calcifying organisms living in the surface (photic zone) of the oceans. For this reason, recent investigations have focus on the planktonic community response to the shifts in oceanic environments during the PETM, specifically the response of calcifying microplankton to higher CO2 and lower pH, as well as the possible role of plankton in drawing down CO2. Calcareous nannofossils seem to play an important role in these interrelated mechanisms. For this reason we have performed detailed micropaleontologic analysis (using a SEM) of calcareous nannofossil assemblages in selected samples from selected Paleocene/Eocene deep-sea sediment cores with the purpose of documenting possible influence on assemblage composition and preservation. The sediments studied in detail are from ODP Site 1263 (from Southern East Atlantic, Walvis Ridge) that has been chosen as representative of one of the few complete PETM deep-sea cores. Comparative analyses were performed in few selected samples from sections located at different latitudes in the Atlantic and Pacific oceans (ODP Site 929, paleo-equatorial Atlantic, Ceara Rise; ODP Site 689, high-latitude Southern Atlantic, Maud Rise; ODP Sites 1215 and 1221, Eastern equatorial Pacific; ODP Site 1209, central Pacific, Shatsky Rise). This study documents the different “behavior” of nannofossils through the different phases of the PETM, at the onset of CIE, within the CIE, and during the recovery interval, and reveals the presence of peculiar morphotypes of Fasciculithus and Discoasters as probably related to the anomalous amount of carbon in the ocean-atmosphere system. Although the anomalous geochemical conditions seems to have had some influence on the nannofossil assemblage composition, it results that local productivity and overall post depositional (diagenetic) conditions were the major controlling factors on nannofossils.