Eocene-Oligocene paleoceanography of the subantarctic South Atlantic: Calcareous Nannofossil reconstructions of temperature, nutrient, and dissolution history

Extremely warm 'Greenhouse' climates of the early Eocene came to a gradual end during a prolonged interval of global climatic deterioration in the middle-to-late Eocene (~49 to 34 Ma), resulting in the eventual initiation of an 'Icehouse' climate state in the early Oligocene. This global climatic transition represents one of the most significant climate changes of the Cenozoic and is punctuated both by prominent cooling steps and by several transient warming and cooling phases superimposed on the long-term cooling trend. This dissertation focuses on two key periods of accelerated climatic change during this interval: the Middle Eocene Climatic Optimum (MECO) (~40 Ma) and the Eocene-Oligocene Transition (EOT) (~34 Ma). The MECO is a transient (500 kyr) warming event recorded worldwide by foraminiferal oxygen isotopes and is associated with deep-ocean acidification. In contrast, the EOT is associated with expansion of Antarctic ice sheets, global cooling, sea level fall, marine and terrestrial biotic turnover, and deepening of the calcite compensation depth (CCD). Calcareous nannofossil assemblages are used to investigate both surface-water environments and deep-sea dissolution across these events at drill sites in the South Atlantic Ocean. Paleoecological study of the nannofossil assemblages is employed to reconstruct temperature and nutrient changes at the sea surface, and analysis of preservation state of individual nannofossil taxa is used to constrain the history of CCD fluctuations. In Chapter 2, the MECO interval was studied at Ocean Drilling Program (ODP) Site 702 (50°S; Islas Orcadas Rise) using quantitative analysis of calcareous nannofossil assemblages in the time interval between 43.5 and 39.5 Ma. Biostratigraphic analysis shows that the MECO event corresponds to significant nannofossil turnover, with five biostratigraphic events occurring in conjunction with warming. Paleoecological interpretation of the assemblages also indicate that temperature and nutrient conditions of surface waters at this site varied frequently and rapidly during the MECO, and a shift from oligotrophic to eutrophic conditions occurred in correspondence to peak warming. Cooling at the termination of the MECO marks the return to oligotrophic conditions following the warming event. In the long-term record prior to the MECO, nannofossil assemblages also record a marked sea-surface warming between 41.5 and 42.5 Ma (designated as the 'C19r warming event'), which is not identified in the bulk oxygen isotope record. To verify that assemblages were not biased by dissolution within the studied section at Site 702, selected taxa were used to calculate dissolution and preservation indices. This analysis suggests that strong sea-floor dissolution did not affect the assemblages which therefore faithfully record surface-water changes at this site. In Chapter 3, high-resolution quantitative nannofossil assemblage analysis was carried out on late Eocene†"late Oligocene (35.5†"26.5 Ma) sediments at ODP Site 1090 (42°S; Agulhas Ridge), with the primary goal of reconstructing paleoceanographic changes through the Eocene-Oligocene transition. Eleven biostratigraphic events and several distinct climatic phases are recognized within the study interval. Surface waters initially cooled between 35.5 and 34.25 Ma and then warmed between 34.15 and 33.8 Ma. Following this warming interval, marked changes in nannofossil assemblages are recorded both at 33.6 Ma, interpreted to indicate an increase in nutrient availability, and at ~30 Ma, when the opportunist species Cyclicargolithus floridanus became dominant. Throughout the study interval at Site 1090, climatic variations recorded by nannofossil are interpreted as a response to changes in the influence of different water masses sourced from the Indian Ocean or the Pacific Ocean, or to the influence of latitudinal movements of frontal positions. An intense dissolution level, previously identified at other Southern Ocean sites, is interpreted between 28.0 and 28.8 Ma, which is may be related to a regional shoaling of the CCD in the South Atlantic. One of the primary characteristics of the EOT interval is a worldwide deepening of the CCD, interpreted based on the increase in carbonate accumulation at deep-sea sites. In the South Atlantic, the CCD is interpreted to have deepened by ~1 km, but a detailed CCD history has not been developed across the EOT interval in this region. For this reason, quantitative analysis of calcareous nannofossil assemblages was carried out across the EOT at ODP Site 1090 (42°S). These results are reported in Chapter 4. The goals of this work were to: (1) to assess the degree of dissolution affecting nannofossil assemblages; (2) to use the nannofossil dissolution signal as proxy for CCD variation; and (3) to characterize surface-water temperature and nutrient changes. Indices of dissolution were calculated using the preservation state of two common taxa (Coccolithus pelagicus and Reticulofenestra umbilicus group) and the characteristics of the entire assemblage. Comparison between these indices and carbonate content shows a striking correspondence, indicating that dissolution was a major factor controlling carbonate sedimentation and assemblage composition in the EOT interval at this site. Additionally, a good correspondence is noted between carbonate content and Blackites and Clausicoccus abundance, suggesting that dissolution is also determinant in controlling the stratigraphic distribution of these taxa. Variation in the calculated indices and carbonate content are interpreted to reflect CCD fluctuations. Several high-magnitude CCD oscillations are recorded in the latest Eocene, which are followed by a CCD deepening in the earliest Oligocene in correspondence with oxygen isotope Step 2. An intense dissolution interval is observed in the latest Eocene immediately prior to oxygen isotope Step 1, and an interval of carbonate dilution is interpreted just prior to oxygen isotope Step 2. A selection of well-preserved samples was used for the paleoecological interpretation of the assemblages, and a major change is observed near the E/O boundary (~33.6 Ma), which most likely reflects an increase in sea-surface nutrient availability. This event is followed by a gradual increase in fertility associated with cooling that culminates at Step 2 in the earliest Oligocene. Chapter 5 of this dissertation reports the preliminary results of Integrated Ocean Drilling Program (IODP) Expedition 317, 'Canterbury Basin Sea Level (New Zealand), Global and Local Controls on Continental Margin Stratigraphy', in which I participated as a nannofossil micropaleontologist. One of the central objectives of the expedition was to understand the Eocene to Recent climatic evolution of the southern Pacific Ocean †" a theme that complements the general topic of paleoclimatology in the high southern latitudes treated in my other doctoral projects. Shipboard biostratigraphic analysis of nannofossil assemblages enabled the construction of detailed age models for the drilled sequences, and changes in assemblage characteristics, such as abundance and preservation, provide new information for interpreting the sedimentary records and sequence stratigraphy of the New Zealand continental margin.