Cenozoic paleoceanographic changes of the South Pacific from seismic reflection data

Michael.Horn [ at ] awi.de


The Thermohaline Circulation (THC) in the world oceans is closely coupled with earth climate. With reconstructions of past oceanic conditions and comparisons with past climate conditions this coupling can be better understood. This in turn allows to provide better boundary conditions on ocean-climate coupling for forecasting models. However, the paleoceanographic setup is far from being completely revealed. One of the largest areas of the world oceans poorly surveyed is the South Pacific Ocean. The present ocean currents that transport deep cold water into the South Pacific are the Antarctic Circumpolar Current (ACC) and two northward setting flows directly influenced by the ACC, the Deep Western Boundary Current (DWBC) or western limb close to New Zealand and an eastern limb above the western flank of the East Pacific Rise (EPR). Understanding changes of this current system is important for reconstructions of past climates and the Antarctic Glaciation history. At present the reconstructed paleoceanographic history of the South Pacific is limited to ACC and DWBC at the western margin of the Southwest Pacific Basin and only reveals changes back to the Eocene/Oligocene boundary. This thesis presents new multichannel seismic reflection data surveying two regions at the eastern and western margin in the Southwest Pacific Basin. The objective is the identification and characterization of deep current derived sediment deposits in the South Pacific. The investigation covers the whole Cenozoic and thus provides indications for paleoceanographic changes due to East Antarctic Ice Sheet build-up prior to 34 Ma. Furthermore the study helps to fill a data gap in the open Pacific Ocean between New Zealand and South America and presents the first evidence for paleoceanographic changes in this poorly explored area. East of New Zealand at the Bounty Trough mouth a sediment drift indicates the onset of a deep cold current originating from the South Pacific Ocean around the Cretaceous/Palaeocene boundary. This current prevailed until the establishment of the ACC around the Eocene/Oligocene boundary. A second sediment drift indicates a current modification due to climate cooling around 44 - 42 Ma occurs contemporaneously with first indications of an East Antarctic Ice Sheet. These results are the first hints on a deep cold current in the South Pacific Ocean before the Opening of the Tasmanian Gateway and thus support the hypotheses that an Antarctic Ice Sheet was present at that time. Since ~19 Ma the history of the ACC has been recorded by the sedimentary deposits at the western flank of the EPR and east of the Bounty Trough mouth. Close to the EPR (45 deg. S) the sedimentary cover suggests a weak bottom current flow allowing deposition that is followed by a bottom current intensification around 9 Ma. The stronger bottom current flow has continued until today. East of New Zealand seismic data and drilling results reveal a current that created a sediment drift between 19.5 Ma and 10 Ma followed by a 5.4 Ma long hiatus in deposition caused by bottom current intensification due to West Antarctic Ice Sheet build-up. Thus, both limbs of bottom current flow into the South Pacific show intensification at the same time that was probably caused by an intensified ACC. Although deposition close to New Zealand starts again around 5 Ma, the flow of the DWBC remained strong, which is also evident at the eastern limb. A higher sediment load due to upstream erosion of the DWBC caused further deposition. The sediment deposits seen in seismic profiles also suggest that a limb of the DWBC enters the Bounty Trough, which until present was only inferred from erosional structures at the Bounty Trough mouth. A new approach based on spectral analysis of Parasound sub-bottom profiler data was used to further quantify the extent of the DWBC in the Bounty Trough. Under the assumption that the 41 kyr obliquity cycle is related to DWBC changes, mapping of this cyclicity in Parasound data is used to show that the extent of the DWBC into the Bounty Trough is limited to 178.2 deg. E.

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Horn, M. (2015): Cenozoic paleoceanographic changes of the South Pacific from seismic reflection data , PhD thesis, Universität Bremen.

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