title: The Cenozoic evolution of the Southern Ocean : impact on sedimentation, ocean circulation and global climate
Creators: name: Sauermilch, I
Publisher: name: University of Tasmania
Year: 2020

description: Global ocean circulation is strongly controlled by the formation and closing of oceanic basins and gateways. Together with changing atmospheric carbon dioxide concentrations, these processes are key long-term drivers of the global climate. During the Cenozoic, Earth’s climate underwent one of the most fundamental transitions known in geological history: from hot “Greenhouse” conditions in the Late Cretaceous, through the warm early Eocene, to cold “Icehouse” conditions with Antarctic-wide glaciation around the Eocene-Oligocene boundary. Coevally, a large-scale oceanographic reorganization occurred, from gyres dominating the subpolar Pacific and Indo- Atlantic to the onset of the Antarctic Circumpolar Current (ACC). Also, during this period, Gondwana breakup reached its final stages, with the opening of the Australian-Antarctic Basin and deepening of both Southern Ocean gateways, the Tasmanian Gateway and the Drake Passage. Fundamental questions remain regarding the links between these tectonic, oceanographic and climatic phenomena. In recent years, the tectonic formation of the Southern Ocean basins and gateways, enabling the onset of the thermally isolating ACC, have been attributed to play a secondary role during this prominent climate transition, while long-term declining atmospheric CO2 concentrations have been favored as the key driving mechanism. Controversies around the timing of gateway deepening, compared to the timing of the onset and strengthening of the ACC, and long-term hiatuses in Southern Ocean sedimentary records, have weakened support for a tectonic driver of climatic change. This interdisciplinary thesis combines observations from geophysical data and drill cores, tectonic and bathymetric modelling, and ocean circulation simulations to re-examine the impact of Southern Ocean tectonic processes on ocean circulation and ultimately climate. Four chapters investigate the tectonic, sedimentary, bathymetric, and oceanographic evolution of the Southern Ocean during the Cenozoic climate transition, and show that the role of tectonic processes on Cenozoic oceanographic conditions has been severely underestimated. The first chapter is an analytical study, investigating the uncertainties in seismic correlations with sediment core data, in relation to petrophysical and depositional properties of the drilled material, using the comprehensive global scientific ocean drilling datasets. It shows that time-depth relationship functions, which are essential tools for such correlations, vary up to 55% depending on the velocity data that has been used. Drill sites with high carbonate content and coarse grain textures are particularly affected. The outcomes of this analytical work provide important constraints for addressing seismo-stratigraphic interpretations and seismic-core-log integrations and emphasize the importance of conducting downhole logging velocity measurements during drilling expeditions. The second chapter focuses on understanding the oceanographic evolution of the Australian sector of the Southern Ocean, in the context of the tectonically driven formation of this ocean basin from the Cretaceous as Australia moved northwards away from Antarctica. I undertake a stratigraphic analysis of a comprehensive network of more than 500 seismic reflection profiles, and geological data from numerous drill sites along the Australian and Antarctic conjugate margins. This framework enables detailed investigation of the patterns and characteristics of the sedimentary architecture, which is strongly controlled by Cretaceous-Cenozoic tectonic, oceanographic and climatic processes. I find that the opening Australian-Antarctic Basin underwent a transition from large deltaic sediment deposition during the Late Cretaceous “Greenhouse”, to a marine environment in the early Cenozoic dominated by clockwise bottom currents, which strengthened and progressed farther east through the Eocene culminating in major sediment hiatuses along both continental slopes. The third part of this thesis focuses on Cenozoic bathymetric evolution, with a particular focus on the formation of the basins of the Southern Ocean. Using the sedimentary architecture developed in the previous chapter, and extending globally using global total sediment thicknesses, highresolution paleobathymetry grids for two key geological time slices (Late Eocene (38 Ma, 0.25° resolution) and Late Cretaceous (~67 Ma, 0.5° resolution) are reconstructed. A modeling approach reconstructing backwards in geological time and backstripping sediments from the present-day bathymetry is used. Corrections for changing sediment decompaction, isostatic adjustment, crustal thermal subsidence, mantle dynamic topography and global sea level are implemented. The resulting paleobathymetry grids are compared to previous studies, in terms of slope gradients in the oceanic basins and along the continent-ocean transition zones. The high-resolution bathymetries presented in this study contain realistic small-scale seafloor roughness and continental slopes patterns, which are closest to the present-day observations. Using these grids as boundary conditions for high-resolution paleo-ocean models is key to accurately unravelling the influences of bathymetry and tectonic processes. The fourth chapter focuses on understanding the impact of the Tasmanian Gateway and Drake Passage deepening on ocean circulation patterns and temperature distribution in the Southern Ocean. My high-resolution (0.25°) general ocean circulation model with realistic Late Eocene bathymetry (from the previous chapter) demonstrates that Southern Ocean circulation changes and &gt;6° C of sea-surface water cooling can be caused by progressive deepening of both gateways. When at least one gateway is shallow (≤ 300 m), the subpolar Pacific and Indo-Atlantic ocean gyres transport warm (up to 20 °C) equatorial waters to the Antarctic coast. The subsidence of the second gateway below 300 m causes the gyres to weaken and the Antarctic waters to dramatically cool by &gt;6°C. The eastward circumpolar current strengthens and dominates the Southern Ocean once both gateways are deeper than 600m. These model results are consistent with available paleo sea surface temperature, microfossil and isotope records from sediment cores. These results imply that tectonically driven deepening of gateways, in conjunction with decreasing CO\(_2\) concentrations, likely played a fundamental role in causing climate cooling and the transition into a global “Icehouse” world at the Eocene-Oligocene boundary. The results presented here demonstrate the importance of tectonically-driven processes in the changing oceanographic and climatic conditions of the Southern Ocean. These outcomes need to be more carefully assessed, especially in conjunction with other climate drivers like the declining atmospheric CO2 changes.