Late Oligocene-Early Miocene Calcareous Nannofossil Biostratigraphy, Taxonomy, and Paleoecology in the Tropics

This dissertation presents biostratigraphic, taxonomic and paleoceanographic studies of sediment cores from low-latitude Ocean Drilling Program (ODP) Legs 154 (Site 929), 115 (Site 709), and 202 (Site 1237). Three major paleoceanographic events are featured: 1) the Oi-glacial events in the early-late Oligocene, 2) the Late Oligocene Warming Event (LOWE), and 3) the Oligocene/Miocene Boundary and Mi-1 glacial event. In Chapter Two, a new modified calcareous nannofossil zonation is proposed for the low latitudes that is composed of five major zones and three subzones between 24-30 Ma in age. The first occurrences of Sphenolithus avis, S. ciperoensis, Crassidiscus backmanii along with an increase in abundance of S. ciperoensis, and the last occurrences of S. predistentus, Crassidiscus backmanii and S. ciperoensis are synchronous and reliable throughout the tropics. The last occurrences of Sphenolithus distentus and S. avis are considered unreliable events due to a possible reworking of the former taxon and a diachronous occurrence of the latter. The last occurrences of Helicosphaera compacta and S. pseudoradians are only found in two out of three basins and thus are used as secondary events. The spatial and temporal distributions of calcareous nannofossil assemblages show major variations within the three tropical/subtropical sites. Sphenolithus taxa were found in low abundances in the Atlantic Ocean. Weak tolerance to low salinity is proposed since salinity declined in the Atlantic Ocean during the late Oligocene glacial events due to episodes of freshwater and terrigenous discharge from the Amazon River. This also lowered the δ18O values of seawater at Site 929 compared with the other sites. Furthermore, Site 929 was not far from its present-day position near the mouth of the Amazon River, thus, the low salinity levels best explains the observed major decline in Sphenolithus taxa. Calcareous nannofossil relative abundances (%) and bulk oxygen and carbon stable isotope data reveal a covariant response to glacial (Oi) events. During glacial events, temperate-, cool-water and eutrophic taxa dominated the tropical assemblage, whereas warm and oligotrophic taxa declined. This increase in temperate and cool-water taxa suggest that the tropical surface oceans witnessed a decrease in temperature that allowed cool-, and temperate-water taxa to extend their geographic range into the tropical zone. The Oligocene global glacial events (Oi-2*, Oi-2a, Oi-2b, and Oi-2c) were associated with accelerated upwelling in which eutrophic conditions prevailed and r-mode taxa dominated the total nannofossil abundance. Both Sphenolithus predistentus and Helicosphaera species show prominent increases during glacial (Oi) events. Unlike other Sphenolithus taxa that favored high salinity, S. predistentus increased in abundance during glacial events characterized by high δ18O and δ13C values. Similarly, Helicosphaera taxa show the same pattern with sporadic occurrences. This suggests that S. predistentus and Helicosphaera species were adapted to increased levels of nutrient (eutrophic or mesotrophic). Between 24-25 Ma, warm and oligotrophic taxa increased drastically and dominated the overall assemblage. However, bulk sediment δ18O and δ13C values at the three sites do not show a consistent decreasing trend similar to the global composite deep-sea isotopic record. Although the paleoecological group suggests oligotrophic conditions associated with dominate warm water, the paleotemperature of the sea surface during the LOWE was not as warm as during the Eocene hyperthermals. In Chapter Three, a new and significant late Oligocene Sphenolithus species is described from the tropical ODP materials. The new species, Sphenolithus avis, has a prominent short range spanning upper Zone NP23 to upper Zone NP25 with an age from ~29.9 Ma to 24.8 Ma. Sphenolithus avis is widely distributed and it differs from S. ciperoensis by more flaring proximal shields. In Chapter Four, high-resolution Oligocene/Miocene calcareous nannofossil biostratigraphy has revealed thirteen events, five of which are reliable and synchronous due to their similar ages in the Indian and Pacific Oceans. The synchronous events are: the last occurrences (LOs) of C. fenestratus and S. delphix, and first occurrence (FO) of S. disbelemnos and the beginning and ending of the S. conicus acme. The Sphenolithus conicus acme is documented in the Indian Ocean between 23.5 Ma and 22.8 Ma. This acme was first found in the Pacific Ocean across the Mi-1 glacial event which is characterized by high δ18O and δ13C values. This study supports a correlation of the S. conicus acme in the Indian Ocean with that of relatively similar age in the Pacific Ocean.