Changes in calcareous nannofossil assemblages during the Middle Eocene Climatic Optimum: Clues from the central-western Tethys (Alano section, NE Italy)

We present a study focused on changes in calcareous nannofossil assemblages of the Alano section during the Middle Eocene Climatic Optimum (MECO). This warming event is characterized by a prominent perturbation both in oxygen and carbon stable isotopes around the Chron C18r–C18n transition (ca. 40 Ma) and lasting ca. 500–600 kyr. Semi-quantitative analyses on calcareous nannofossil assemblages have been carried out. Our results show that the MECO interval coincides with a significant shift in the relative abundance of calcareous nannofossil taxa, suggesting a relationship between biotic changes and stable isotope shifts. Paleoecological studies at species level and/or based on morphometric criteria (i.e., small placoliths) sometimes show the opposite behavior between changes observed at the genus level and those observed at lower taxonomic levels. For instance, a taxon thought to be better adapted to oligotrophic/warm waters, e.g. Sphenolithus, shows a prominent decrease if analyzed at genus level, but an increase was instead recorded for S. spiniger. Moreover, taxa preferentially thriving in eutrophic/cold waters, as for instance small reticulofenestrids, increase remarkably in abundance during this warming phase, while medium–large placoliths do not show any significant trend. An increase in reworked, mainly Cretaceous, specimens is also observed during the MECO. These lines of evidence are consistent with a transient enrichment in dissolved nutrients in warmer sea surface waters suggesting that an enhanced nutrient availability could have driven the make-up of the calcareous nannofossil assemblages. The increase in reworking may indicate an increase in terrigenous input, due to increased chemical weathering likely produced by an enhanced hydrological cycle.