Miocene climate transitions in the southwest Pacific, Tasmania: Interpretations based on calcareous nannofossils

The tectonic separation of Australia from Antarctica is a key constraint on the development of the Antarctic Circumpolar Current (ACC) during the Cenozoic. The ACC plays a critical role in global climate thermally isolating Antarctica and its initiation is hypothesized to have crossed a critical global climate boundary resulting in initial Antarctic ice sheet growth (Kennett, Houtz et al., 1975; Exon, Kennett, Malone et al., 2001). The relatively shallow Tasmanian Gateway is one of the few places in the Southern mid to high latitudes where fairly complete, carbonate-rich sequences can be drilled detailing development of the ACC. The upper Oligocene and Miocene sediments recovered by Ocean Drilling Program (ODP) Leg 189 are rare when compared to other Southern Ocean drilling sites in terms of biotic richness and continuous sedimentation through the Miocene (Exon, Kennett, Malone et al., 2001).; The Miocene has been characterized as a warm interval with significant cooling taking place in the middle Miocene with the permanent emplacement of the East Antarctic Ice Sheet (EAIS) (Shackleton and Kennett, 1975). Evidence for periods of ice sheet growth and decay prior to the mid-Miocene permanent emplacement of the EAIS include deep-sea oxygen isotope records (Miller et al., 1987) correlated with major sequence boundaries (Haq et al., 1987) and glacimarine sediments deposited on the Antarctic margin (Cape Roberts Science Team, 1999; Roberts et al., 2003). Climatic conditions fluctuated in the early Miocene peaking in warmth at the mid-Miocene Climatic Optimum (∼16Ma). A major shift in the stable isotope records at ∼14 Ma marks a significant phase of ice growth on Antarctica (Kennett and Shackleton, 1975). This cooling trend continued and by the middle/late Miocene the development of oceanic fronts was well established in the Tasmanian and New Zealand sectors of the Southern Ocean (Exon, Kennett, Malone et al., 2001; Nelson and Cooke, 2001).; This dissertation interprets the calcareous nannofossil assemblage data, coupled with robust age models, geochemical and sedimentological data, in order to reconstruct the paleoceanographic conditions in the Tasmanian Gateway during the Miocene. Comparisons between study sites in the Tasmanian Gateway, as well as with other Miocene sections, provides a detailed view of changing paleoceanographic conditions in the Tasmanian region as Australia and Antarctica continued to separate, the ACC and oceanic fronts in the Southern Ocean fully developed and ice volume on Antarctica waxed and waned.; A robust calcareous nannofossil biostratigraphy was constructed at ODP Sites 1168, 1170-1172, despite the lack of key marker species from the Discoaster, Sphenolith and Triquetrorhabdulus genera. Diversity was typical of a temperate-water setting, though Site 1168 had higher diversity than the other three sites. Several alternative markers not employed by the Okada and Bukry (1980) zonal scheme were tested and allowed for further subdivision of the stratigraphic column, though not all Miocene calcareous nannofossil zones could be identified. Core disturbance and bioturbation did complicate the interpretation of the sedimentary history, particularly in the upper Miocene and across the Miocene/Pliocene boundary. The robustness of the calcareous nannofossil biostratigraphy was further tested by comparison to biomagnetostraphic data from foraminifers, diatoms, dinocysts, and radiolaria. Second-order bioevents, used to increase the zonal resolution, were compared and found to represent reliable bioevents in the Tasmanian region.; Fluctuations in the relative abundances of Coccolithus pelagicus and Reticulofenestra perplexa indicate similar surface water conditions across the study area from ∼ 8-12 Ma. Site 1172 is apparently warmer than the other sites beginning at ∼ 8 Ma possibly caused by the inception of the proto-East Australia Current. Detailed analysis of calcareous nannofossil assemblage fluc...