NOAA/WDS Paleoclimatology - Tropical Atlantic ODP1260 Cretaceous Calcareous Nannofossil Data

STUDY NOTES: Calcareous nannofossil abundances estimated from smear slide counts of middle Cenomanian to late Turonian age (~95 - 88 Million Years BP) from Ocean Drilling Program Leg 207, Site 1260. This dataset was used to constrain nannofossil biostratigraphic events for this time period as well as analyze nannofossil assemblage fluctuations across OAE 2 at the Cenomanian/Turonian boundary. Raw abundance counts of 500 were made on an Axioskop2 microscope and converted to percent abundance (relative to 100% of the total nannofossil populatio
ABSTRACT SUPPLIED BY ORIGINATOR: Calcareous nannofossils are used as biostratigraphic indicators integral for age control in both petroleum industry and academic settings. The Oligocene and Late Cretaceous are especially problematic for nannofossil biostratigraphers due to low diversity and species' concepts of gradually evolving lineages (Oligocene) and an absence of carbonate-rich deep-water sections (Upper Cretaceous). Sediments from Ocean Drilling Program (ODP) Leg 199 (eastern Equatorial Pacific) and Leg 207 (Demerara Rise) provide a unique opportunity to analyze nannofossil bioevents from complete, nearly continuous sections Oligocene and Upper Cretaceous in age. Chapter two analyzes calcareous nannofossil bioevents across the Oligocene-Miocene transition. Established nannofossil biostratigraphies for the Oligocene-Miocene transition exist; however, sections in which nannofossil bioevents can be finely tuned to the Geomagnetic Polarity Time Scale are rare. Ocean Drilling Program Leg 199, Site 1218, located in the eastern Equatorial Pacific recovered a near continuous Oligocene-Miocene boundary section with complete magnetostratigraphic and orbitally-paced stable-carbon and 'oxygen isotope records. This study examines calcareous nannofossils from ODP Site 1218 at 30-cm intervals across the Oligocene-Miocene transition. Seventeen nannofossil datum events between 24.65 and 21.69 Ma are identified and calibrated to the Site 1218 chronostratigraphic age model. Sphenolithus delphix is still the most precise nannofossil datum for the Oligocene/Miocene boundary; its top calibrated just ~30-ky below the Aquitanian Stage base at Site 1218 (23.06 Ma). Two robust acme events of Sphenolithus conicus (22.87-23.57 Ma) and Triquetrorhabdulus carinatus (22.26-22.87 Ma) also characterize the Oligocene-Miocene transition and are interpreted here to result from fluctuations in nutrient input across the Mi-1 cooling event. In addition, nannofossil preservation exhibits a strong correlation with the 400-ky and larger 1.2-Myr eccentricity and obliquity cycles. Chapters three and four analyze calcareous nannofossils from a rare Middle Cenomanian-Upper Turonian section from ODP Leg 207, Site 1260 (Demerara Rise, South America). This study observes 56 nannofossil bioevents spanning the Middle Cenomanian to uppermost Turonian providing a high-resolution calcareous nannofossil framework for this Upper Cretaceous interval: six Middle Cenomanian bioevents, 10 Upper Cenomanian bioevents, 14 Lower Turonian bioevents, 18 Middle Turonian bioevents, and 8 Upper Turonian bioevents. Thirty-two of these 56 nannofossil bioevents have been previously described in the literature. Nannofossil bioevents were calibrated to the 2008 timescale of Ogg et al. (2008) utilizing carbon isotopes (Cenomanian-Turonian transition) and two nannofossil datums from 'traditional' nannofossil schemes (E. rarus, M. furcatus). Calcareous nannofossil assemblages from Site 1260 show evidence for dynamic surface-water conditions in response to the global Oceanic Anoxic Event 2, most notably, a pronounced nannfossil assemblage shift from a Placozygus howei-dominated assemblage in the Upper Cenomanian to a Zeugrhabodtus moulladei-dominated assemblage in the lowermost Turonian. The acme of Zeugrhabdotus moulladei at the onset of OAE 2 supports evidence for an increase in surface-water paleoproductivity during this anoxic event. This study also examines four Turonian nannofossil lineages: Stoverius-Cylindralithus, Radiolithus-Eprolithus-Lithastrinus, Quadrum, and Rhomboaster-Liliasterites-Marthasterites. Three of these lineages show gradation between evolving genera, a phenomenon not commonly observed in calcareous nannofossils. Evidence of iterative evolution was shown in the Turonian Rhomboaster-Liliasterites-Marthasterites lineage, equivalent to the evolutionary history of the Paleogene Rhomboaster-Tribrachiatus lineage. Nine new species of calcareous nannofossils are described from these lineages: 'Cylindralithus' antarius, Liliasterites arkellii, Liliasterites salfeldii, 'Lithastrinus' novenarius, Quadrum kathetos, Quadrum trikathetos, Quadrum dodrans, Quadrum tricuspis, and Rhomboaster demerarensis. These forms evolved during a transition from compact, dense nannofossil morphotypes to elongated, sleeker nannofossil morphotypes, possibly in response to changing paleoceanographic conditions during two brief cooling intervals in the Middle Turonian.