Calcareous nannofossils across the Eocene-Oligocene transition at Site 756 (Ninetyeast Ridge, Indian Ocean): implications for biostratigraphy and paleoceanographic clues

Abstract

The timing and modalities of calcareous phytoplankton community and evolutionary responses to the Eocene-Oligocene transition (EOT, ~ 34 Ma) are still under-investigated. In order to better constrain the dynamics of these pelagic primary producers during the climate transition, we conducted high resolution assemblage analysis on calcareous nannofossils across a ~19 m-thick interval of nannofossil ooze at Ocean Drilling Program (ODP) Site 756 (Ninetyeast Ridge, Indian Ocean; Peirce et al. 1989) (paleolatitude ~43°S; Zachos et al. 1992). We explored the diversity patterns against a new integrated planktonic foraminifera and calcareous nannofossil biostratigraphy produced for the site, as well as new benthic foraminifera stable isotope (C, O) record, which documents ocean-climate changes, and provides independent chemostratigraphy. The study section spans nannofossil Zones NP20-NP23 (equivalent to CNE20-CNO4) and lasts 5.6 Myr. The results show that the hankeninid extinction falls within the ~4.5 m-thick EOT isotopic interval (0.67 m below the base of the second positive δ 18 O shift -EOIS), which is consistent with previous studies, making Hole 756C one of a few sites globally boasting both the familiar stepped δ 18 O and δ 13 C structure of the EOT and the primary biostratigraphic marker defining the base of the Oligocene. A series of potentially useful new calcareous nannofossil bioevents were identified that could help improve dating and correlation of this crucial interval. In this context, changes in calcareous nannofossil assemblages observed across EOT are interpreted in terms of modifications of paleoecological parameters that typically control the abundance and distribution of different taxa. Variations in sea surface temperature and nutrient availability are considered to be the most likely triggers for the calcareous phytoplankton changes observed across EOT. Specifically, our data suggest that increased nutrients in the mixed layer played a key role in shaping the late Eocene -early Oligocene calcareous nannofossil assemblages.

Calcareous Nannofossils and Paleoclimatic Evolution Across the Eocene‐Oligocene Transition at IODP Site U1509, Tasman Sea, Southwest Pacific Ocean

“…from 250.19 to 183.30 m), the carbonate content has a mean value of 78.3% (±6.8%). Therefore, the carbonate content increased during Interval 2 but not during the EOGM, in contrast to the findings of other sites, specifically in the Indian (ODP Site 756; Viganò, Coxall, et al, 2023) and in the Pacific Ocean (Coxall & Wilson, 2011;Coxall et al, 2005).…”

“…This latter assemblage supports the hypothesis that, once the steady‐state cosmopolitan phase was reached, the new environmental conditions, characterized by cold and nutrient enriched waters, persisted for at least 1.8 Myrs. Further support for an early Oligocene eutrophication at Site U1509 is provided by the presence of small opportunist reticulofenestrids (<4 μm), as observed at ODP Site 756 (Viganò, Coxall, et al., 2023).…”

“…Specifically, the decrease in warm-water oligotrophic taxa (i.e., D. barbadiensis, D. saipanensis and E. formosa) is consistent with both enhanced eutrophic conditions and decreased temperatures, with the assemblage shifts primarily controlled by temperature and nutrients (Figure 6). The shift toward cooler and eutrophic assemblages appears to be a consistent feature of the EOT (Viganò, Coxall, et al, 2023), since published records from both low-middle and high latitudes suggest increased nutrient availability and cooler conditions (Bordiga et al, 2015;Diester-Haass, 1995;Diester-Haass et al, 1998;Diester-Haass & Zahn, 1996Fioroni et al, 2015;Jones et al, 2019;Persico & Villa, 2004;Villa et al, 2008Villa et al, , 2014Villa et al, , 2021. Based on trends and shifts in the diversity indices, PCA scores, and paleoecological groups, we subdivided the study interval into four phases: (a) late Eocene warm-oligotrophic Figure 5.…”

“…The approximate position of the pSTF is inferred from Nelson and Cooke (2001). Site U1509 (this study) and Site 756 (Viganò, Coxall, et al., 2023) (used for comparison) are reported. pACC: proto‐Antarctic Circumpolar Current; pTF: proto‐Tasman Front; pSTF: proto‐Subtropical Front; EAC: East Australian Current; pRG: proto‐Ross Gyre; TG: Tasman Gateway; DP: Drake Passage; pWG: proto‐Weddell Gyre; pIOG: proto‐Indian Ocean Gyre.…”

“…The oxygen isotopes measured in the bulk are also utilized to compare conditions in the mixed layer with those at the bottom, represented by the δ 18 O benthic signature. Ultimately, δ 18 O bulk data provides a valuable tool for correlation and evaluation of the completeness of the studied section based on the globally recognized stepwise pattern of oxygen isotopes, proving essential for site‐to‐site correlations (Viganò, Coxall, et al., 2023).…”