Calcareous nannofossils across the Eocene-Oligocene transition: Preservation signals and biostratigraphic remarks from ODP Site 1209 (NW Pacific, Shatsky Rise) and IODP Hole U1411B (NW Atlantic Ocean, Newfoundland Ridge)

Allyson Viganò*, Thomas Westerhold, Paul R. Bown, Tom Dunkley Jones, Claudia Agnini

*Corresponding author for this work

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Abstract

This work provides a detailed biostratigraphic correlation through the Eocene-Oligocene Transition (EOT), based on an integrated stratigraphic approach and the study of calcareous nannofossils, between two disparate sites, one in the NW Atlantic (IODP Hole U1411B) and one in the NW Pacific (ODP Site 1209). The precise site-to-site correlation provided by these data allows for a comparison of carbonate preservation across the EOT including identification of the main post-depositional processes that impact the calcareous nannofossil ooze at Site 1209. The main aim of this work is to understand the extent to which the bulk δ18O and δ13C records and their sources (mainly calcareous nannofossils) are altered by diagenesis. Our detailed SEM study highlights some differences before, during and after the EOT, suggesting local diagenetic dynamics. At Site 1209, a distinctive change, both in nannofossil assemblage composition and preservation state, is observed from the pre-EOT phase to the Late Eocene Event (LEE), with a shift in the dominant process from dissolution to recrystallisation. Surprisingly, despite the overall poor preservation, only the interval between 141 and 142.4 (adj. rmcd) was compromised in term of isotopic values and assemblage diversity and abundance. This interval, recorded in the upper Eocene, was characterized by severe dissolution, concomitant with deposition of secondary calcite on solution-resistant forms. Diagenetic processes have strongly biased the δ18O isotopic signal, resulting in a positive oxygen isotope anomaly through the upper Eocene that is difficult to reconcile with other published trends. For the remaining time intervals, diagenesis seems not to have altered the bulk δ18O profile, which closely resembles that of other sites across the world, and is particularly consistent with other data from the Pacific Ocean. In summary, the impact of diagenesis on nannofossil preservation even if clearly visible both in SEM and optical microscope observations does not always cause a pervasive alteration of the primary isotopic signal and can instead provide important clues on local depositional dynamics.

Original languageEnglish
Article number111778
Number of pages32
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Volume629
Early online date22 Aug 2023
DOIs
Publication statusPublished - 1 Nov 2023

Bibliographical note

Funding Information:
A. V. and C. A. were supported by University of Padova .

The authors are grateful to the Integrated Ocean Drilling Program (IODP) for providing samples and data used in this study. The IODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under the management of Joint Oceanographic Institutions, Inc.

Publisher Copyright:
© 2023 The Author(s)

Keywords

  • Biostratigraphy
  • Calcareous nannofossils
  • EOT
  • IODP Hole U1411B
  • ODP Site 1209
  • Preservation

ASJC Scopus subject areas

  • Oceanography
  • Ecology, Evolution, Behavior and Systematics
  • Earth-Surface Processes
  • Palaeontology

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