Biozonation and biochronology of Paleogene calcareous nannofossils from low and middle latitudes

Calcareous nannofossils have represented a powerful biostratigraphic tool since the 50 and 60, when several milestone papers began to highlight their potential use in dating. Cenozoic sediments and rocks. Here, we present a new calcareous nannofossil biozonation for the Paleogene Period, which is based on biostratigraphic data collected during the past 30 years. Semi-quantitative counting methods applied on DSDP/ODP Sites and marine on-land sections have been used to demonstrate the details of the abundance patterns of each biostratigraphically useful calcareous nannofossil taxon. This new biozonation still partly relies on older biozonations and thus represents an integration between those classical biohorizons that proved reliable and new biohorizons proposed to substitute bioevents considered problematic. Thirty eight new Paleogene biozones are proposed using a new code system: 11 Paleocene biozones (CNP1\u2013CNP11), 21 Eocene biozones (CNE1\u2013CNE21) and 6 Oligocene biozones (CNO1\u2013CNO6). The new scheme uses a limited number of biohorizons, one for each biozone boundary, which guarantee more stability although with a coarser resolution. A series of additional biohorizons are included, however in almost every biozone. This new Paleogene biozonation has an average duration of 1.1 Myr per biozone, varying from 0.9 in the Paleocene, to 1.0 Myr in the Eocene, and to 1.8 Myr in the Oligocene. Age estimates provided for calcareous nannofossil biohorizons are calculated using both magnetostratigraphic and astronomically tuned cyclostratigraphic data

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\u2013C18n transition (ca. 40 Ma) and lasting ca. 500\u2013600 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\u2013large 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

Is the Middle Eocene Climatic Optimum (MECO) recorded in the central-western Tethys?

We have established the carbonate content (%) and the stable oxygen and carbon isotope composition of the bulk carbonates in 535 samples from the Alano di Piave section, located in the Venetian southern Alps, NE Italy. The biomagnetostratigraphic data allow us to establish a sound chronology that indicates that the Alano section extends from the upper part of the Chron C18r (ca. 39.3 Ma) to the base of Chron C16r (ca. 36.5 Ma). Our records show a marked long-term and high-frequency variability. In particular, isotope reveal an interval of striking variability between 13.60 and 25.30 m levels, in which is comprehended a sapropel-like interval. Major and distinct shifts in both \u3b413C and \u3b418O records are present within this interval. Specifically, a negative shift in \u3b418O values (ca. 1.2-1.3 \ub0) mirrors an articulated change in \u3b413C values. A rapid negative shift of the \u3b413C values of ca. 0.7-0.8 \ub0 is followed by a pronounced positive shift in correspondence with the onset of the sapropel-like interval. This positive \u3b4 13C excursion precedes a short-lived \u3b413C negative excursion that leads a second rapid \u3b413C increase. This prominent perturbation in the global carbon cycle terminates abruptly when the carbonate content, \u3b413C and \u3b418O values simultaneously return to more stable conditions, albeit the \u3b413C curve shows a post-event interval of instability around the "stable" condition beyond the termination of the event. This isotope excursion interval, here referred to as Chron C18r/C18n event, is well constrained in time occuring from the upper Chron C18r (ca. 40.3 Ma) to the upper Chron C18n.1r (ca. 39,6 Ma). Interestingly, this event correlates exactly with the positive ca. 0.6 \ub0 \u3b413C positive excursion recorded by Jovane et al. (2007) in the Contessa Highway section. In agreement with Jovane et al. (2007), we correlate this positive \u3b413C excursion with the MECO event of Bohaty & Zachos (2003) in the Southern Ocean. These results suggest that the interval of time between 40.35 and 39.6 Ma is characterized by a strong climatic instability, with a warming that significantly interrupts the overall cooling trend of the Middle Eocene. The pronounced changes of the \u3b413C values, in bulk carbonates and the coeval oscillations in global CCD (Tripati et al, 2005) strongly suggest that the carbon cycle played an important role in driving the climatic evolution in the doubthouse world of the terminal part of the Middle Eocene