| Popis: |
The late Pliocene is marked by the end of an interval of warm, relatively stable global climate and a secular shift into a bipolar glaciated world. The intensification of northern hemisphere glaciation (iNHG) central to this climatic transition was accompanied by a decrease in atmospheric carbon dioxide concentrations and commenced around 3.5 million years ago. The climate forcing and response mechanisms involved in the iNHG are subjects of ongoing debate in the palaeoclimate literature. In this thesis, I reconstruct palaeoproductivity and suborbital climate fluctuations during this important interval, with particular focus on the first three consecutive, large, obliquity-paced glacial-interglacial cycles (marine isotope stages, MIS, 101-95), using proxy methods applied to deep-sea sediments in the equatorial oceans and the North Atlantic. In this way, I evaluate the forcing mechanisms, biogeochemical cycles and climate implications during the late Pliocene. In Chapter 2, palaeoproductivity is reconstructed in the western and eastern equatorial Pacific Ocean. Data reveal that productivity fluctuations are in phase between east and west and are obliquity-paced. This implicates high-latitude rather than local forcing of export productivity and no apparent role for east-west thermocline tilting on these timescales, as previously proposed. In Chapter 3, multi-proxy palaeoproductivity reconstructions in the eastern equatorial Pacific and Atlantic Ocean upwelling zones are considered in terms of proxy applicability and export productivity. Results suggest that alkenone accumulation may be a useful indicator of export productivity and that the late Pliocene biological pump was stronger during glacials than interglacials. In Chapter 4, an inferred secular productivity shift is investigated using calcareous nannofossil assemblages. Assemblage shifts at an equatorial Pacific and a North Atlantic Ocean site support the interpretation of an increase and a decrease in export productivity, respectively. Implications of a strengthened tropical biological pump at this time are considered. In Chapter 5, high-resolution climate records are used to investigate suborbital variability at North Atlantic Integrated Ocean Drilling Program Site U1313. Data indicate that only smallamplitude suborbital variability occurs during the late Pliocene, with no amplification within the boundary conditions and inferred ice-volume variations of MIS 103 to 95. |
