Tracking early Eocene Hyperthermals through calcareous nannofossil assemblages at the Site IODP U1443 (Ninetyeast Ridge, Indian Ocean)

The early Paleogene was a period of global greenhouse warmth characterized by a series of events called “hyperthermals”. These were pronounced episodes of warming associated with negative carbon isotope excursions, possibly caused by rapid injections of greenhouse gases into the ocean-atmosphere systems (Zachos et al., 2008). The most prominent among them, the Paleocene-Eocene Thermal Maximum (PETM), was marked by a global temperature increase of about 5-8° C, that affected the terrestrial and marine organisms and their geographic ranges, their evolution and extinction (McIerney & Wing, 2011). To better understand these past climate changes, we investigated the calcareous nannofossil assemblages from Site IODP U1443, in the northeastern Indian Ocean. 4.5 m spanning the late Paleocene to the early Eocene (∼56–53 Ma) were analyzed at an average resolution of about 10 cm. The calcareous nannofossil assemblages identified throughout the studied interval comprise approximately sixty species, indicating a diverse and well-preserved flora. Among these Coccolithus, Discoaster, Sphenolithus and Toweius represent the most abundant taxa across most sample. The obtained results allowed us to refine the biostratigraphy and improve the chronological framework of Site U1443 with respect to the original shipboard data and more recent literature (e.g., Clemens et al., 2016). Changes in nannofossil assemblages during the hyperthermals were also investigated to assess their response to warming, variations in trophic conditions, and associated paleoceanographic changes. Preliminary results indicate that the PETM was characterized by more pronounced changes in calcareous nannofossil assemblages compared to the other hyperthermal events. Variations in the relative abundance of several taxa are observed over the studied interval, with the genus Discoaster showing a marked increase in abundance during the warming phases. Our data also support the absence of Calcareous Nannofossil Excursion Taxa (CNET) and most taxa belonging to the Rhomboaster–Tribrachiatus plexus, with the exception of the last representative, Tribrachiatus orthostylus, whose first occurrence seems consistent with previously published records from other sites and with its established first appearance close to ETM2 (Agnini et al., 2014).