Mid Miocene orbital climate variability and biotic response in the Pacific Ocean

During the Miocene, the Earth's climate transitioned from an extended phase of global warmth (Miocene climatic optimum) into a colder mode with the establishment of a permanent and stable East Antarctic Ice Sheet (EAIS). The mechanisms which drove this extreme climate shift are still poorly understood, because continuous, well-dated Miocene sedimentary archives are still scarce. Reliable sea surface temperature estimates are crucial to any reconstruction and modelling of past ocean salinity and density, water column stratification, thermohaline circulation, and ice volume. Despite extensive studies of benthic foraminifera, existing planktonic foraminiferal records for this interval are extremely scarce and of low resolution. Consequently, the impact of global warming and cooling on tropical surface waters and the propagation of orbital cycles in the Earth System are unknown. The overarching aim of this thesis is to investigate the nature and variability of early-middle Miocene climate and the relationship to orbital variations in solar insolation, in order to better understand the extent and magnitude of the global middle Miocene Climate Transition (MMCT) and the subsequent cooling/EAIS events. Furthermore, this study aims to investigate changes in the thermal structure of the Pacific Ocean during the development of MMCT to examine Pacific Ocean circulation across the middle Miocene climatic events. This is achieved through high resolution planktonic foraminiferal stable isotope analysis, spectral analysis and wavelet transform analysis. The first ever high-resolution (3 kyr) astronomically-tuned record of δ18O and δ13C from planktonic foraminifera for the eastern equatorial Pacific Ocean (15.56–13.35 Myr) is presented here. These data provide vital new information on sea surface temperatures and primary productivity changes at the tropics during the middle Miocene, at a resolution not achieved in any previous study, which sheds new light on the extent and magnitude of the MMCT and associated carbon-isotope excursion. In order to assess the reliability of these new records this thesis also goes on to document the taxonomy and palaeobiology of Miocene tropical planktonic foraminifera and their response to times of climatic stress. Finally the data from Site U1338 is compared to Site 1146 in the western equatorial Pacific Ocean, to reconstruct bottom and surface water conditions and changes in ocean dynamics across the equatorial Pacific during this highly complex interval of climate history.