Calcareous Nannofossil Response to Transient and Long-term Climate Evolutions: the PETM and Early Eocene in the Southern Alps (Italy)

The Early Paleogene was a time of profound changes in global climate and in both terrestrial and marine biota. Still, the relationships between climate dynamics and the evolution of pelagic ecosystem are sparsely documented. The Veneto region (Southern Alps) is to be considered a classical area for this time interval. Specifically, previous works have already detected the presence of a limestone succession Late Paleocene-Eocene in age in this area (i.e. Possagno). We present new integrated calcareous nannofossil and magnetostratigraphic data from the Carcoselle Quarry section and calcareous nannofossil and geochemical data from the Forada section. The Carcoselle section (located close to Possagno) spans Zones NP9-NP15 of Martini (1971) and is bracketed between magnetochrons C24r and C20r. Both the Paleocene/Eocene boundary and the EECO (Early Eocene Climatic Optimum; Zachos et al., 2001) have been recognized. The Paleocene-Eocene transition outcrops in many other sections along the Piave River Valley. Specifically, we performed high-resolution analyses in the Forada section, which is characterized by higher sediment accumulation rates compared to Carcoselle. We paid special attention to the short-term transient climatic event of the PETM (Paleocene-Eocene Thermal Maximum) at Forada, which is characterized by a continuous carbonate record and is ideally suited for studying the much-debated response of calcareous nannofossils to the PETM. At Carcoselle section, we focused on the prominent climatic feature of the EECO and, in particular, the response of calcareous nannofossils to this long term climatic trend, which seems to produce important changes in the nannoplankton communities. In summary, we tentatively point out the striking and different modifications that affected the nannoplankton associations during either transient and long-term climate changes. In particular, the short-lived PETM caused provisional adaptations (malformations and/or ecophenotypes) in the coccolithophores communities that were reabsorbed upon return to long-term varying climatic conditions. In contrast, the Early-Middle Eocene long-term climatic trend (EECO) was capable to generate true evolutionary trends with the appearance of the very successful clade of the Noelaerhabdaceae, whose offsprings include the most important bloom-forming coccolithophorids in the present-day oceans. This can be considered the time in which nannoplankton communities set course towards modern structures, thus triggering a reconfiguration of the global ocean life chain.