Paleoceanographic Significance of Calcareous Nannofossil Assemblages in the Tropic Shale of Utah during Oceanic Anoxic Event 2 at the Cenomanian/Turonian Boundary

Victoria Fortiz, Rosie Oakes, F. Garrett Boudinot, Matthew M. Jones, R. Mark Leckie, Amanda Parker, Bradley B. Sageman, Julio Sepúlveda, Timothy J. Bralower*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Scopus citations

Abstract

Oceanic Anoxic Event 2 (OAE2) at the Cenomanian/Turonian Boundary (CTB: 93.9 Ma) involved the global deposition of organic carbon-rich sediments, a distinctive positive shift in carbon isotope values, and significant species turnover, including changes in calcareous nannofossil assemblages. While it is thought that volcanism triggered organic C-rich sediment deposition during OAE2, it is unclear whether enhanced productivity, increased stratification, of some combination of the two increased organic matter preservation. Calcareous nannofossil assemblages have the potential to qualitatively assess changes in ocean nutrient and temperature conditions to disentangle such ecological dynamics during OAE2. Here we study an expanded section of the Tropic Shale in a drill core in southern Utah near the western margin of the Western Interior Seaway (WIS) to understand how circulation changed during the event and how this may have influenced primary productivity and organic carbon burial. Relative abundance data of well-preserved nannoplankton are complemented with measurements of trace metal, and organic carbon and carbonate concentrations to determine changes in temperature and water column structure, as well as controls on surface water productivity. Detailed statistical analysis helps refine species paleoecologies combined with information from planktic and benthic foraminiferal assemblages and organic biomarkers. Changes in calcareous nannofossil assemblages indicate that near the start of OAE2 the western WIS surface ocean actually cooled for a short time. Following this, surface waters became warmer and more stratified as a Tethyan water mass invaded the seaway. Assemblages suggest that warmth persisted for much of the OAE2 interval, while stratification waxed and waned. The local seaway cooled near the end of OAE2 as Boreal water masses streamed along the western margin. Variations, including the decrease in the abundance of Biscutum constans and short-lived peaks in the abundance of Eprolithus spp. are super regional or possibly global in extent. There is no correlation between calcareous nannofossil assemblages and trace metal concentrations, suggesting they were unaffected by volcanism-related nutrient inputs. Assemblages support other data that suggest increased stratification influenced organic carbon burial in the Western Interior Seaway, and possibly elsewhere, during OAE2.

Original languageEnglish (US)
Pages (from-to)205-224
Number of pages20
JournalMicropaleontology
Volume70
Issue number3
DOIs
StatePublished - 2024

Funding

We thank Matt Fantle, Lee Kump, Mike Arthur, and Mark Patzkowsky for discussions that helped shape our interpretations. We thank Jason Parlett and the USGS Denver drilling team for the superb coring at Smoky Hollow, Chris Lowery, Scott Karduck and Lindsey Victoria for help in the field and Matt Gonzalez and Scott Karduck for help in the laboratory. We are grateful to Elisabetta Erba, Jean Self-Trail, and David Watkins for extremely helpful reviews. We gratefully acknowledge funding from the National Science Foundation (EAR-1338316) and the late Rich Lane for his instrumental role in initiating the Earth-Life Transitions program.

Keywords

  • Calcareous nannoplankton
  • Cenomanian-Turonian boundary
  • Oceanic Anoxic Event

ASJC Scopus subject areas

  • Palaeontology

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