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.
Chron C17n (magnetostratigraphy).Cyclostratigraphic analysis of the Bartonian-Priabonian transition of the Alano section as well as radioisotopic data of the Tiziano tuff layer provide an absolute age (37.710 -37.762Ma, respectively) of this bed and, consequently, of the base of the Priabonian Stage.
A global carbon-isotope curve for the Late Triassic has the potential for global correlations and new insights on the complex and extreme environmental changes that took place in this time interval. We reconstruct the global δ13Corg profile for the late Norian, improving on sparse published data from North American successions that depict a "chaotic carbon-isotope interval" with rapid oscillations. In this context, we studied three sections outcropping in the Lagonegro Basin (southern Italy), originally located in the western Tethys. The carbon-isotope profiles show four negative excursions correlatable within the Lagonegro Basin. In particular, a negative shift close to the Norian/Rhaetian boundary (NRB) appears to correlate with that observed in the North American δ13Corg record, documenting the widespread occurrence of this carbon cycle perturbation. The 87Sr/86Sr and 187Os/188Os profiles suggest that this negative shift was possibly caused by emplacement of a large igneous province (LIP). The release of greenhouse gases (CO2) to the atmosphere-ocean system is supported by the 12C enrichment observed, as well as by the increase of atmospheric pCO2 inferred by different models for the Norian/Rhaetian interval. The trigger of this strongly perturbed interval could thus be enhanced magmatic activity that could be ascribed to the Angayucham province (Alaska, North America), a large oceanic plateau active ca. 214 ± 7 Ma, which has an estimated volume comparable to the Wrangellia and the Central Atlantic Magmatic Province (CAMP) LIPs. In fact, these three Late Triassic igneous provinces may have caused extreme environmental and climate changes during the Late Triassic.
Abstract. The Early Eocene Climatic Optimum (EECO) records the highest prolonged global temperatures over the past 70 Ma. Understanding the causes and timing of Eocene climate change remains a major challenge in Cenozoic paleoceanography, which includes the biotic response to climate variability and the changes among planktic foraminiferal assemblages across the EECO. The symbiont bearing and shallow dwelling genera Morozovella and Acarinina were important calcifiers in the tropical-subtropical early Paleogene oceans but almost completely disappeared at about 38 Ma, near the Bartonian/Priabonian boundary. We show here that morozovellids record a first critical step across the EECO through a major permanent decline in relative abundance from the Tethyan Possagno section and ODP Site 1051 in the western subtropical North Atlantic. Possible causes may include increased eutrophication, weak water column stratification, changes in ocean chemistry, loss of symbiosis and possible complex interaction with other microfossil groups. Relative abundances of planktic foraminiferal taxa at Possagno parallel negative shifts in both δ13C and δ18O of bulk sediment from Chron C24r to basal Chron C20r. The post-EECO stable isotopic excursions towards lighter values are of modest intensity. Significant though ephemeral modifications in the planktic foraminiferal communities occur during these minor isotopic excursions. These modifications are marked by pronounced increases in relative abundance of acarininids, in a manner similar to their behaviour during pre-EECO hyperthermals in the Tethyan settings, which suggest a pronounced biotic sensitivity to climate change of planktic foraminifera even during the post-EECO interval.
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