School of Earth Sciences - Theses
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ItemIsotope and trace element systematics in subduction systems and the concept of ambient mantle( 2018)This project employs Hf - Nd isotope ratio and trace element concentration data to investigate aspects of magmatism in subduction systems. The primary goal is to use the compositions of back-arc basin lavas as a means to investigate the nature of the pre-subduction mantle wedge ( ‘ambient mantle’). New Sr, Nd, Pb and Hf isotope data for the Lau Basin back arc spreading centres and Hf – Nd data for the Manus Basin back arc are reported. Both areas have been extensively studied using major, trace element and Sr-Pb isotope data but many of these elements are highly mobile in aqueous fluids released from the subducting slab and so are unfavourable for studying the mantle wedge itself; here the behaviour of the less mobile elements Hf and Nd can provide additional insights. Hf is a high field strength element (HFSE) and believed to be relatively immobile in subduction zone fluids until slab melting occurs whereas Nd is slightly mobile in aqueous fluids, and also mobile in melts. The new Hf and Nd data from both the Lau and Manus back-arc basins plot broadly in the Indian Mid-Ocean Ridge Basalt(MORB) domain with the exception of two lavas from the Eastern Lau Spreading Centre (ELSC). Two geochemical filters (Ba/Nb < 7 and Th/Ta < 3), used in a previous study of the back arc region to the Mariana arc, were modified slightly for each studied site and applied to the new data to remove major influences from subduction components in the mantle wedge. These filtered data for the Lau and Manus Basins confirm that the true mantle affinity in these regions is indeed Indian MORB. When compared to existing data from the Mariana Trough and Scotia Sea, it is clear that each mantle wedge has its own unique ambient mantle character and that these site-specific compositions should be used in preference to global average MORB compositions for the purpose of modelling. Despite the geochemical filtering methods employed, it is clear that all of the samples still retain a small subduction input in terms of their LILE contents. A subsidiary part of this project re-evaluates existing HFSE concentration data for lavas of the Tongan arc, previously determined by laser ablation methods. New solution ICP-MS data suggest that the older data are likely influenced by analytical artifacts, a conclusion that dramatically changes the genetic model required for their interpretation.
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ItemThe Flying Electric Generator: evaluating the claims of a largely ignored proposal for generating electricity from high-altitude winds( 2015)This thesis concerns the Flying Electric Generator (FEG), a technology proposed to generate electricity from winds several kilometres high in the sky. Airborne Wind Energy (the generation of electricity from high altitude winds) is an emerging field of research, with several technological approaches under development. High altitude winds are attractive for this purpose because they are generally much faster than surface level winds, and because power is a cubic function of wind velocity. Winds are fastest within the subtropical jet stream, located about 25–30 degrees north and south of the equator, at an altitude of 10–12 km. The FEG is a device consisting of multiple rotors attached to a frame, which is tethered to the ground. The rotors work as autogyros to provide lift; additional energy extracted from the wind is converted to electricity and conducted to the ground via the tether. The FEG would operate kilometres high in the atmosphere, up to jet stream levels. Papers about the FEG were first published in 1979, and in 2002 a company was founded to commercialise the FEG. So far, this has not happened, and many details of how the technology would operate remain uncertain, despite three decades of research literature. Only small test craft (rotors of up to 24 feet in diameter) have flown at low altitudes (up to 100 feet). Many of the claims in the literature, which are optimistic about the FEGs performance at high altitude, are experimentally untested. FEGs have never operated at the altitudes described in the corresponding literature, and the project has not been commercialised or attracted much if any recent research funding. Other, newer entrants to the Airborne Wind Energy field have seen success in research funding and commercialisation. This thesis addresses two problems: first, it tests some of the claims in the FEG literature and second, it attempts to fill in details not provided. The particular claims concern the power density available in high altitude winds over Australia and its seasonal variation, the amount of time a hypothetical FEG setup would be "grounded" due to insufficient wind speeds to keep it aloft, and expected capacity factors of a hypothetical FEG setup. Claims about the magnitude of the wind power resource were tested using reanalysis data (the ERA-40 dataset was used, and was validated against Bureau of Meteorology upper air statistics). Power density and wind speeds at different altitudes above Australia were calculated and analysed. The reanalysis wind data in conjunction with a model of FEG operation (based on lifting rotor theory detailed in the FEG literature) were used to calculate downtime and capacity factors. The results showed a clear seasonal variation in power density over Australia, which was most pronounced at 30 degrees south of the equator (although winds above Tasmania showed much less variation). Winter had the strongest winds, and summer the weakest. The highly skewed distribution of power density meant that median power densities (unreported in the FEG literature) were more appropriate than means. Downtime calculations showed that a particular FEG setup rated at 240 kW operating at a pressure level of 600 hPa would be landed for at least 20% of the year at all locations in Australia, and for at least 40% of the year north of 20°S. Annual capacity factors for the same FEG setup were calculated to vary between 0.1–0.4 over Australia, no different from conventional ground-based wind turbines. Capacity factors for the summer months were substantially lower than the annual values. These results support the main contention of the thesis, that the FEG is far more limited in its potential as source of energy from the wind than the literature claims.
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ItemInvestigating potential pathways to remediate thiocyanate-contaminated groundwater and wastewater at a Victorian gold mine( 2014)Thiocyanate (SCN-) is toxic to higher organisms, affecting the central nervous system, to cause irritability, nervousness, hallucinations, psychosis, mania, delirium and convulsions. This contaminant is commonly associated with gold mining activities and forms when cyanide, used for extraction of gold, reacts with reduced sulphur species in wastewater. Significant concentrations of thiocyanate have been detected in groundwater bores around the No. 2 Tailings Dam at Stawell Gold Mine in Victoria, with concentrations steadily increasing over time. Thiocyanate can be eliminated from groundwater and wastewater by oxidation to less harmful products such as ammonia and sulphate through chemical oxidation and biodegradation. Certain bacterial strains indigenous to thiocyanate-contaminated sites have demonstrated the ability to degrade thiocyanate and can be used to remediate contaminated land and water. Chemical and biological oxidation of thiocyanate was explored in this study through synthetic abiotic laboratory redox optimisation experiments and field-based injection experiments to determine the controls on thiocyanate degradation and potential pathways which could be implemented to remediate thiocyanate-contaminated groundwater and wastewater at the site. Analysis of site groundwater chemistry and aquifer properties revealed the presence of a plume of thiocyanate in acidic to near-neutral groundwater outside the tailings dam wall at monitoring bores SE12 and SE14 confined to the upper unconsolidated aquifer with low hydraulic conductivity (0.001-0.004 m/d) and low transmissivity (0.01-0.05 m2/d). The source of thiocyanate at the site was determined as the tailings dam. Inconclusive results from the nitrate reduction-thiocyanate oxidation experiments in synthetic and actual groundwater and wastewater suggests that nitrate may not form a redox couple with thiocyanate. However, in acidic solutions (pH 2) of Fe-EDTA and thiocyanate heated to 80°C, the Fe2+ was generated from thermal degradation of the Fe-EDTA compound, while thiocyanate was completely hydrolysed within 22 days. Catalysis of thiocyanate hydrolysis by iron reduction was not determined. However, solution pH and temperature were important factors, as thiocyanate hydrolysis did not proceed at pH 5.5 and 80°C. At pH 2, the hydrolysis of thiocyanate was faster at 80°C compared to 70°C. Thiocyanate-degrading microorganisms were not successfully stimulated in simulated injection (push-pull) experiments, as no appreciable decrease in thiocyanate concentrations was observed in groundwater or wastewater replicates.
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ItemThe palaeovegetation, palaeoclimate and biostratigraphy of South-eastern Australia during the Eocene Greenhouse to Oligocene Icehouse transition( 2014)The shift from the Eocene 'Greenhouse' to Oligocene 'Icehouse' conditions is a globally significant event. The Eocene – Oligocene boundary (33.9 million years ago) marks the onset of permanent large-scale glaciation in Antarctica and the onset of the current global climatic 'Icehouse‘ state. In the Austral-Antarctic region, the transition is well recorded in deep water marine carbon and oxygen isotopes, however in the terrestrial realm little is known of the nature of environmental change associated with this event. In Australia, this is partly due to a lack of precise methods of dating Paleogene terrestrial strata. The most common practice of dating terrestrial sediments is the application of Gippsland Basin‘s spore-pollen biostratigraphy. This research assessed the applicability of the Gippsland Basin‘s spore-pollen biostratigraphy to the Otway Basin. Results demonstrated Gippsland Basin‘s spore-pollen biostratigraphy provides inaccurate ages when applied to samples from the Otway Basin. In addition, results from pollen analyses combined with foraminifera, dinoflagellate, and calcareous nannofossils and other stratigraphic data were used to create a detailed climate history of this high palaeolatitude region (~60°S). The spore-pollen assemblage indicates the palaeovegetation of the Otway Basin was dominated by Nothofagidites spp., with an abundance of species from the families Myrtaceae, Casuarinaceae and Proteaceae. The Nothofagus-dominated mesothermal rainforest probably had an emergent layer of Podocarpaceae and Araucariaceae species and a diverse understorey of ferns. The palynological record of the Otway Basin shows no vegetation change occurred during the time prior to and across the Eocene-Oligocene boundary even though there is evidence of permanent continental–scale glaciation on Antarctica.
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ItemSubduction zone systematics: insights from high-precision ICP-MS radiogenic isotope analysis for the Mariana arc( 2012)Subduction zones play an essential role in the geochemical evolution of the Earth. They are sites where new continental crust is created, whilst oceanic crust and sediments are subducted and recycled into the mantle. Understanding these systems is therefore critical to our knowledge of both the crust and mantle, and the evolution of the planet as a whole. Radiogenic isotope ratios are a key tool in understanding these systems yet much of the existing literature is based upon relatively old data collected with methods that have now been superseded. The development of new analytical technologies, in particular MC-ICPMS, now provides an opportunity to re-evaluate many scientific questions in the light of ‘new generation’ analytical data with improved levels of precision and accuracy. To this end, a detailed Hf, Nd, Sr and Pb radiogenic isotope study has been performed utilising state-of-the-art analytical techniques to analyse 87 samples representing all nine islands of the active sub-aerial intra-oceanic Mariana arc. The quality of these data is shown to be a significant advancement over the bulk of previous literature data and they now represent the most comprehensive radiogenic isotope dataset available for the Mariana arc. The higher precision displayed by the current dataset allows for the observation of geochemical features that were not previously apparent. Here we show that individual islands in the arc form distinct trends in 206Pb/204Pb - 207Pb/204Pb space, a feature which has not been previously observed. It is shown that these trends may be attributed to either changes in the relative contributions of the two subducting sediment types (0.5 – 6.0% addition from a source composed of a mix of 5 – 25% pelagic and 75 – 95% volcanogenic sediment) or perhaps changes in the underlying mantle composition as was recently suggested to explain variations in Hf – Nd isotope space, or both. The new high precision data also allow for a re-examination of several key arc geochemical issues. While it is often suggested that the slab derived component dominates the Pb budget of arc lavas, it can be shown that the mantle wedge contribution is not completely overprinted, with ~ 15 % of the Pb in the arc lavas being mantle derived. There is also evidence to suggest that a sediment melt is potentially a key component in the transport of a sedimentary Pb signature from the slab to the mantle wedge. This study also casts further doubt over one of the fundamental assumptions made when using radiogenic isotopes to investigate the petrogenesis of subduction zone lavas, that of Hf immobility during subduction. It is demonstrated that a sediment derived Hf contribution most easily explains the Mariana-arc Hf isotope variation, with mixing models in Hf-Nd isotope space shown to require ~0.5 – 2.5 % sediment addition. These results demonstrate that significant new insights can be obtained from re-analysis of existing materials with improved analytical technologies.