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The Cenozoic evolution of the Southern Ocean : impact on sedimentation, ocean circulation and global climate

Sauermilch, I ORCID: 0000-0003-4639-6699 2020 , 'The Cenozoic evolution of the Southern Ocean : impact on sedimentation, ocean circulation and global climate', PhD thesis, University of Tasmania.

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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, high-resolution
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 >6

Item Type: Thesis - PhD
Authors/Creators:Sauermilch, I
Keywords: Southern Ocean, tectonic gateways, Antarctic Circumpolar Current, Greenhouse-Icehouse, seismo-stratigraphy, IODP, MITgcm, Cenozoic
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