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Aspects of Neogene palaeoceanography in the southern Tasman Sea (DSDP site 593)

Aspects of the Neogene palaeoceanographic history of the southern Tasman Sea are described in this thesis from DSDP Site 593 records of oxygen and carbon isotopes, and sediment texture and compositional data. Site 593 occurs in 1068 m water depth on the western flanks of the Challenger Plateau, west of central New Zealand. Through the last 40 Ma the plateau has accumulated up to 600 m of pelagic carbonate oozes. The modern physical oceanography of the region is reviewed to form a basis for describing the Neogene palaeoceanography. The Subtropical Front lies south of Site 593, and has done so throughout the Neogene, separating surface waters of tropical- and antarctic-origin. Cool Subtropical Waters (CSTW) at the modern surface are underlain by Subantarctic Mode Water (SAMW) sourced from the Southern Ocean, which in turn is underlain by Antarctic Intermediate Water (AAIW), which bathes the seafloor at Site 593. The Neogene equivalent watermasses are qualified as Southern Component waters, for example Southern Component Intermediate Water (SCIW), because the specific physico-chemical parameters used to define the modern watermasses are not known for their ancient analogues. The sediments are nannofossil ooze changing upcore to foraminifera-bearing nannofossil ooze. This change in texture has been attributed to intermittent winnowing at the site and experimentally determined to be due to selective removal of grains sized 3-13 ηm, which comprise nannofossil placoliths. The core log includes an unusual ‘orange unit’ dated from 18.5-15.1 Ma, previously speculated to be associated with faster bottom water flow rates that introduced highly oxygenated water over the site. The textural data from this study support this contention and show that an invigorated SCIW at this time caused pronounced winnowing, sufficiently intense to form a condensed sediment section from c. 18-14 Ma. Invigorated circulation has been attributed to climate cooling and ice build-up on the Antarctic continent through the Neogene. Stable oxygen and carbon isotopic analyses of both planktic and benthic foraminifera are used to establish a mid-latitude, southern hemisphere CSTW and SCIW record. This differs from previous Southern Ocean palaeoceanographic reconstructions which used tropical planktic data, being the only records then available, or Deep Water benthic data which describe conditions not representativeof Site 593. The SCIW and CSTW δ¹⁸O records from Hole 593 contain all of the globally-recognised major Early-Late Miocene glacial zones, namely Zones Mi1b toMi6. The Hole 593 benthic δ¹³C record also contains the major Miocene carbon events, including the Monterey Excursion, most of the carbon maxima CM1-7 events, and probably the Messinian Carbon Shift. This is the first time these glacial zones and carbon events have been recorded in intermediate-depth waters. Significantly, the SCIW glacial zones exhibit step-wise ice volume increases in contrast with the Deep Water records where they are identified as single-point events, and the higher than expected absolute δ¹³C values are interpreted to be due to thermodynamic tagging of the antarctic surface waters, from which SCIW is derived. The study documents the difficulties in establishing sea water δ¹⁸Ow values during the Neogene for use in palaeotemperature calculations, due to the unknown ice budget. A staggered value over time is established and applied to the Hole 593 δ¹⁸O data. Previous attempts at determining sea water palaeotemperatures often use microfossil assemblages described as ‘cool temperate’ or ‘subantarctic’ without actually specifying a temperature. SCIW palaeotemperatures of 8.5-12°C through the Early to Middle Miocene cool slightly to around 8-10°C during the glacial Mi Zones, but are consistently warmer than those of modern AAIW (3.5-7.5°C) through this interval. During the Late Miocene Zones Mi6 the SCIW palaeotemperatures reached those of modern AAIW for the first time in the record, and remain in the range 5-9°C until 5 Ma. Neogene surface water palaeotemperatures are not dissimilar to modern CSTW values in the range 10-17°C, interpreted to reflect a relative thermal stability of the southern Tasman Sea CSTW during the Miocene. Globorotalia (Globoconella) miotumida is a planktic foraminifera which exhibits distinct coiling excursions, where the dominant shell coiling direction changes from sinistral, which dominates the record, to dextral over very short intervals. Coiling direction changes in other species have been linked to water temperature. The δ¹⁸O data from both sinistral and dextral shells of Globorotalia miotumida do not support water temperature as a controlling factor. However, the δ¹³C record hints that dextrally coiled individuals secreted calcite slightly shallower in the water column than the sinistral individuals. Sediments from Hole 593 contain three coiling zones, all of which are associated with glacials, specifically Zones Mi5 and Mi6, and an unnumbered glacial interval. This co-occurrence is interpreted as reflecting oceanic reorganisation caused by the glacials, resulting in slight shallowing of the permanent thermocline. This shallowing impacted on the reproductive success of sinistral individuals by restricting juveniles returning to the surface waters to become adults. As a result, dextral individuals temporarily became the dominant contributors to the sea floor sediments. Recognition of bolboforms in the Late Miocene Hole 593 sediments resulted in an unexpected palaeoceanographic opportunity. By analysing many published core logs, isotopic data sets, and microfaunal assemblages it has been established that the bolboforms, now extinct, were associated with warm subantarctic conditions, and lived between the Subantarctic and Subtropical Fronts in the Southern Ocean. This has led to an assessment of the Cenozoic palaeoceanographic history of the Southern Ocean, including the evolution and migration of the various oceanic fronts. Transport via a subsurface route is proposed to explain the presence of bolboforms in the Hole 593 sediments, and their occurence in North Atlantic locations. Events are depicted on a series of palaeogeographical reconstructions of the Southern Ocean at eight time intervals during the Cenozoic. Finally, palaeoceanographic proxies have previously been used to establish orbitally-driven climate periodicities, specifically Milankovitch orbital frequencies. The standard method has been Fourier analysis, but this technique has problems when the data set is not continuous, such as the drilling-related gaps in Hole 593.Using several of the Hole 593 data sets, time-series analysis has been undertaken using Wavelets software which has successfully identified orbital frequencies in the data, and is the subject of continued study.
Type of thesis
Cooke, P. J. (2002). Aspects of Neogene palaeoceanography in the southern Tasman Sea (DSDP site 593) (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14018
The University of Waikato
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