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OCEANIC ANOXIA EVENT 2 (93.9 MA) IN THE U.S. WESTERN INTERIOR SEAWAY: HIGH RESOLUTION CALCAREOUS NANNOFOSSIL RECORD OF THE TROPIC SHALE FORMATION
Restricted (Penn State Only)
Master of Science
Date of Defense:
October 19, 2017
Timothy J. Bralower, Thesis Advisor
Mark E Patzkowsky, Committee Member
Michael A. Arthur, Committee Member
OAE2; Calcareous nannofossils; Paleoclimate; Paleoceanography
Oceanic Anoxia Event 2 (OAE2) occurred at the Cenomanian/Turonian Boundary (CTB; 93.9 MA) and had a duration of < 1 m.y.. This event 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. Organic C-rich sediment deposition is thought to have been triggered by volcanism that led to increased productivity and/or enhanced organic matter preservation. The temporal succession of volcanism, organic matter deposition, and changes in biota such as nannofossils, is crucial to understanding the dynamics of these major environmental perturbations during OAE2. Calcareous nannofossil assemblages during OAE2 in the WIS are marked by large shifts between taxa with eutrophic and oligotrophic affinities. Assemblages have the potential to qualitatively assess changes in nutrient and surface-ocean temperature conditions during OAE2. Here we study nannoplankton in an expanded section of the Tropic Shale in southern Utah near the western margin of the Western Interior Seaway to assess the relationships between anoxia, organic matter deposition, and planktic biotas during OAE2. Samples were collected from a 30-m section of a core that iv contains well preserved nannoplankton. Relative abundance data are complemented with Total Organic Carbon and Carbonate values to determine whether organic rich sediments were a response to high surface ocean fertility or water column stratification. Paleoecological/paleoceanographic interpretations of calcareous nannofossil assemblage change in this study suggest warm and oligotrophic conditions at the base of OAE2. Gradually, surface ocean conditions at the western margin cool and become eutrophic towards the end of the event as suggested by the increase in B. constans and Zygodiscus spp. Results from this study along with those from the Rebecca Bounds and Portland Core suggest a counterclockwise circulation of Tethyan and Boreal waters as modeled by Slingerland et al. (1996) in the WIS during the late transgression phase of the latest Cenomanian to early Turonian.
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