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ItemNo Preview AvailableAntarctic sea ice and its interactions with high latitude weather and climate( 1998)Antarctic sea ice plays a major role in the earth system by greatly influencing the high latitude exchanges of heat, moisture and momentum between the ocean and atmosphere, as well as profoundly effecting the salt budget of the ocean, and thus the production of Antarctic Bottom Water, one of the driving mechanisms of worldwide oceanic circulation. With such considerable and far reaching impact, it is important to document its climatology, understand its variability and quantify its influence. Climatologies and trends of the Southern Ocean sea ice pack are presented using the most recent satellite observations available from the Defense Meteorological Program’s (DMSP) Special Sensor Microwave Imager (SSM/I). The analysis of these data show that Antarctic sea ice is highly variable in both time and space. Statistically significant increases in the sea ice extent, open water and ice areas have been determined from the SSM/I data for the 9 year period 1987 to 1996, a result which differs from the Scanning Multichannel Microwave Radiometer (SMMR) observations (1978-1987). The increasing trend in the SSM/I observations can be attributed to the large increases in sea ice observed in 1994-1995, as confirmed by an analysis of data from the ERS-1 satellite. The mean season length during these years has remained relatively unchanged. Regional trends, both in the sea ice concentration and in season length, showed vast spatial inhomogeneity. SSM/I data displayed increasing season length in the central Weddell Sea, Bellingshausen Sea and Balleny Islands regions, with decreasing length in the Amundsen Sea, eastern Ross Sea and in the coastal areas off Wilkes Land. Similar trends are observed in the seasonal sea ice concentration. In most cases, these trends are opposite to those observed in the SMMR data, which may be linked to the shift observed in the Amundsen Sea low after 1990. Comparisons with historical data would suggest that no large scale anomalous change has occurred in the Antarctic sea ice limits over the course of human observation. Furthermore, the degree of variability suggests great care is needed in interpreting large scale changes in sea ice conditions, and hence atmospheric or oceanic change, from locally observed anomalies. Case studies of the effect of individual cyclones upon the sea ice concentration show small but definite modification of the ice conditions. To further diagnose aspects of the thermodynamic and dynamic forcing upon the Antarctic pack, detailed analysis of the sea ice concentration variability has been conducted using spectral techniques, and the spectra have been compared to those of the European Centre for Medium Range Weather Forecasts (ECMWF) temperature and wind data. In all cases, and with the seasonal cycle removed, the sea ice concentration shows a bias towards longer timescales of variability than either the wind stress or surface air temperature. This “red shift” in its frequency spectrum is strongest with the wind stress, and weakest with the temperature. For longer period waves, this may be due to the formation of new ice by surface cooling or the moderation of melting by the cold surface water, whereas for shorter period waves, where wind stress dominates temperature and ice concentration respectively, time is required for winds to draw in warmer or cooler air, as well as to overcome the ice masses inertia and keel friction to open or close leads. Strong intraseasonal variability of the sea ice concentration is observed in the 20-25 day period, reflecting similar timescales of the temperature variability, as well as that of the energetic eddies of the Antarctic circumpolar current. Examination of the latitudinal variation of the sea ice concentration, temperature and wind stress spectra showed not only the importance of the north-south temperature gradient in influencing the variability, but also the seasonal changes in the semi annual oscillation of the circumpolar trough. Regional spectra showed clear differences between location, and reflected the influences of the atmosphere and ocean upon the sea ice pack. This is clearly shown in the Weddell Polynya region and off East Antarctica, with high variability in the synoptic timescales, and in the western Ross Sea where changes occur in timescales of greater than 20 days. In order to determine if satellite derived, real time sea ice concentration and distribution would be of benefit to operational numerical weather prediction (NWP) schemes, the effect of sea ice concentration change upon the atmosphere in synoptic timescales was examined using a general circulation model in conjunction with the Australian Bureau of Meteorology’s GASP analyses. Experiments were conducted with a typical July sea ice concentration and distribution, as well as slab concentrations of 0, 10, 25, 50, 80 and 100%. Results from 5-day numerical weather forecasts show that the central pressure, structure and tracks of individual cyclones are sensitive to the ‘switch on’ of different sea ice conditions. Composites of all forecasts made with each concentration showed considerable, and mostly statistically significant, anomalies in the surface temperatures and turbulent heat fluxes over the sea ice. The magnitudes of these changes varied monotonically with the area of open water. The largest changes were simulated closest to the coast for all concentrations except for the typical July sea ice run, which displayed maxima over the outer pack. Significant westerly anomalies were induced over the ice in all cases, as were reductions in mean sea level pressure. The July sea ice runs displayed a distribution of the mean sea level pressure anomaly different from all others, with maxima occurring in the central to outer pack. All other forecasts displayed maxima at the coast. The results suggest that sea ice concentration does induce anomalies in the atmospheric parameters in timescales of less than five days. Further, the use of a realistic distribution of sea ice concentration produces results distinct from the constant concentration forecasts. Hence it is suggested that real time Antarctic sea ice data may be of considerable benefit to numerical weather prediction models.
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ItemNeoproterozoic to Permian evolution of the Cape Fold Belt, South Africa: Constraints on sediment provenance and orogenesis from high-precision 40Ar/39Ar dating of detrital and metamorphic micas( 2019)The Permian Cape Fold Belt extends 1300 km along the western and southern coastal margins of South Africa. It comprises complexly deformed rocks of the lower-Palaeozoic Cape Supergroup, and Mesozoic parts of the Karoo Supergroup. Large-scale thrusts expose portions of the underlying Saldania Belt; a low-grade metamorphic belt intruded by granites of the ca. 550-500 Ma Cape Granite Suite. Both the Cape Fold Belt and Saldania Belt are segments of ancient continent-scale orogenic systems. The former is thought to be a portion of the Permian Gondwanides Orogen that extended from the Sierra de la Ventana Fold Belt of Argentina, across southern Africa, and into the Falkland (Malvinas) Islands, and Ellsworth-Whitmore Mountains of Antarctica, whereas the latter is considered one of the many Neoproterozoic-Cambrian Pan-African/Brasilliano terranes developed during the amalgamation of west Gondwana. However, fragmentation of Gondwana and separation of the Cape Fold Belt from its neighbouring terranes during the Cretaceous has provided major challenges in understanding both the geodynamic evolution of the poorly exposed Saldania Belt and the mechanics by which the Cape Orogen formed within the Gondwana interior. Gondwanan tectonic models often rely on geochronological provenance studies to not only link sedimentary sources and sinks, but to also correlate sedimentary successions in separated terranes. Previous geochronological provenance studies on the Cape Fold Belt have utilised U-Pb dating of detrital zircons to suggest that sediments of the Saldania Belt and Cape Supergroup were largely sourced from Mesoproterozoic rocks of the Namaqua-Natal Metamorphic Belt to the immediate north and underlying the Cape Fold Belt, as well as undifferentiated Pan-African and/or Brasilliano terranes. However, as zircon is able to survive orogenic recycling and long-distance transport, U-Pb detrital zircon studies have been unable to identify the most recent and proximal sources of sediments. In addition to having only broadly defined sediment provenance, the timing and extent of orogenesis during the Pan-African and Permian periods are poorly constrained. The timing of deformation in the Saldania Belt is only defined relative to the Cape Granite Suite, whereas the Cape Orogeny has been dated in a handful of limited 40Ar/39Ar studies. Early studies attempting to constrain Cape deformation utilised bulk mineral aliquots that yielded largely discordant 40Ar/39Ar age spectra, which the authors interpreted to represent multiple phases of deformation. A more recent study performed 40Ar/39Ar dating of single mica grains and proposed a bi-model evolution for the Cape Orogeny; however, this is based on only eight analyses. In this study, high precision 40Ar/39Ar geochronology is used to constrain the ages of individual detrital and metamorphic micas from low-grade rocks, as well as localised zones of variable deformation intensity in the southern Cape Fold Belt branch. Fundamental to this study, was the collection of detailed structural observations, petrographic and mineral chemistry data used to delineate detrital and neocrystallised mica age populations, from partially reset, altered, and/or complexly intergrown micas. A total of 648 individual mica grains were dated from 57 samples representing a variety of relatively undeformed and deformed Cape Supergroup and eastern Saldania Belt sediments, including crenulated metasediments, axial planar cleavages, thrust planes in duplex structures, and major shear zones. Only seven samples, collected from zones of intense deformation and focused fluid flow, yielded reproducible mica ages indicating that Cape Orogenesis was most pronounced at 257-248 Ma. These samples were located along, or in close proximity to the Worcester and Kango Fault systems, which are considered major decollement structures responsible for thin-skinned deformation of the Saldania Belt and Cape Supergroup sediments inland from the Gondwana margin. Biotite fusion ages of 272-270 Ma from a metamorphosed mafic dyke in the Kaaimans Inlier provide possible evidence for an earlier onset of Cape Orogenesis, preserved only in the Saldania basement. Samples from less deformed zones contained partially recrystallised detrital mica grains, mixed detrital and neocrystallised mica grains, and/or complex micas hosting clay and chlorite intergrowths, resulting in a spread of apparent ages older than the Cape Orogeny. 40Ar/39Ar detrital mica age populations were defined for a number of Saldania Belt and Cape Supergroup samples; these data were integrated with published U-Pb detrital zircon ages for provenance analysis. Detrital muscovite and zircon ages for the Lime Bank sequence and possibly part of the Kleinrivier sequence (Gamtoos Inlier) suggest exclusive provenance from the Mesoproterozoic Namaqua-Natal Metamorphic Belt or the similar-aged Maud Belt of Antarctica. In contrast, detrital ages for the Groenefontein and Huis River formations (Upper Cango Caves Group, Kango Inlier) indicate a source that experienced early Pan-African Orogenesis (580-550 Ma), such as the Sor Rondane Mountains or the Dronning Maud Land sector of the East African-Antarctic Orogen. The Cango Caves Group was folded prior to deposition of the overlying Kansa Group, which hosts abundant 530-510 Ma zircons. This suggests that deformation of the Cango Caves Group is Pan-African in age (i.e. 550-530 Ma) - possibly related to tectonic loading of the Kaaimans Inlier to the south, or the western Saldania Belt and Gariep Belt to the west. Later deformation of the Kango Inlier folded both the Cango Caves and Kansa Group, after which the conglomeratic Schoemanspoort Formation was deposited. A tightly constrained detrital mica age population of 510-500 Ma in the Schoemanspoort Formation represents either cooling/exhumation of the source terrane during the late stages of Pan-African tectonism or a younger tectonic and thermal pulse in the source area. These events could have occurred in the proximal Kaaimans Inlier, and may have been responsible for the combined folding of the Cango Caves and Kansa Groups. In the Kaaimans Inlier, 40Ar/39Ar incremental step-heating of muscovite from a pegmatitic vein suggest that parts of the Saldania Belt were affected by a post-Pan-African Ordovician thermal overprint. This overprint, identified in other Cape Granite Suite intrusives by previous studies, may represent a raised geothermal gradient in the basement as a result of Cape Supergroup sediment load and mantle flow coupled to far-field subduction along the Proto-Andean margin, and/or asthenospheric upwelling in the southern East African-Antarctic Orogen to the east of the Cape Fold Belt. 40Ar/39Ar analysis of individual mica grains from the Cape Supergroup reveals a dominant Ordovician (490-465 Ma) detrital muscovite population, suggesting provenance from orogenic belts possibly associated with the aforementioned Ordovician thermal overprint in the Cape Fold Belt basement; i.e. the Famatinian Orogen of western Argentina and/or the East African-Antarctic Orogen. Lesser detrital muscovite populations of 650-500 Ma and >730 Ma corroborate previous zircon provenance studies suggesting Pan-African and Namaqua-Natal Metamorphic Belts sources, respectively. The sediment provenance investigations presented in this study provide insights into the evolution of late Neoproterozoic to Devonian sedimentation in the Cape Fold Belt, and enable correlation of orogenic terranes spanning South American, African and Antarctic. In addition, well-constrained deformation ages from intensely deformed metasediments and major shear zones permitted formulation of a tectonic model for the Cape Orogeny, considering the onset, duration of deformation, and structural development of the Cape Fold Belt along the southwest Gondwana margin.
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ItemThe origin, composition, and evolution of the Kimberley kimberlites (South Africa)( 2019)Kimberlites are deeply derived (i.e., >150 km), small-volume igneous bodies that have been emplaced on all continents throughout the last 2.8 billion years. The typical volcanic expression of kimberlites is a deep irregular root zone and/or feeder dyke system connected to a regular steeply dipping and outwardly tapering pipe-like diatreme, which may be overlain by a crater and extrusive material (when not removed by erosion). The crater and diatreme facies contain pyroclastic rocks, which transition into coherent (sub-volcanic) rocks in the root zone. Kimberlites are of economic value as the major host of gem quality diamonds at the Earth’s surface. They also hold great scientific significance, as the deepest derived melts to reach the surface, with entrained mantle material that provides some of our best information on the structure, composition, and evolution of the sub-continental lithospheric mantle. However, despite over a century of dedicated research, numerous aspects of kimberlite petrogenesis remain poorly understood and contentious. One central issue that this project has addressed is the composition and evolution of kimberlite melts. The composition of kimberlite melts remain poorly constrained because: (1) rocks emplaced near the surface are prone to deuteric and hydrothermal alteration; (2) they have been contaminated by the physical incorporation of xenocrystic and xenolithic material; (3) their parental magmas have been modified by interaction with and partial assimilation of mantle and crustal material; and (4) they undergo syn-emplacement differentiation. Therefore, in this study exceptionaly ‘fresh’ rocks from the well studied kimbeley cluster (the type locality) were examined to gain further insights into kimberlite melt compositions. Better constraints on kimberlite melt compositions are pivotal if we are to move toward a comprehensive understanding of the petrogenesis of these enigmatic rocks. The studied samples derive from the Kimberley cluster (South Africa), which lies within the Western terrane of the Kaapvaal carton. This cluster constitutes the type locality of kimberlites, containing five major kimberlite pipes (The Kimberley mine, De Beers, Dutoitspan, Wesselton, and Bultfontein), numerous smaller pipes, and abundant dyke/sill complexes (e.g., Benfontein, Wesselton Floors, Wesselton Water Tunnels). The Kimberley cluster has been dated by various geochronological techniques yielding emplacement ages of ~80-90 Ma. To provide new insights into the composition and evolution of kimberlite melts, a detailed petrographic study of sub-volcanic (hypabyssal) coherent kimberlites was conducted. This included the investigation of mineralogy, mineral zonation, inclusion populations (mineral, melt, and fluid), and textural relationships between phases, utilizing a range of microscopy techniques. This petrographic data formed the basis of targeted geochemical analysis by electron microprobe. A study of olivine compositions across multiple intrusions of the Kimberley cluster shows that olivine, more than any other mineral, provides the most complete record of kimberlite evolution. This study showed that pre-ascent metasomatism of the lithosphere by kimberlite melts is wide-spread, and that so-called ‘xenocrystic’ olivine is not directly representative of the wider lithosphere due to metasomatism of the conduit by previous pulses of kimberlite magmatism. The composition of kimberlitic liquidus olivine overlaps that of olivine from other mantle-derived carbonate-bearing magmas (orangeites, ultramafic lamprophyres, melilitites), with low Mn/Fe and Ca/Fe, and moderate Ni/Mg ratios. It is suggested that these compositions are typical of olivine in equilibrium with melts derived from carbonate-rich peridotite sources. Compositional zonation patterns indicte that olivine crystallises throughout magma ascent and that its crystallisation continues after emplacement into the upper crust. Magmatic olivine (i.e. crystallised from the kimbelrite magma) displays distinct generations of crystallisation, with increasing Mg, Ca, and Mn contents interpreted as the result of fractional crystallisation and increasing oxygen fugacity (fO2). The stability of olivine at sub-solidus conditions implies that secondary melt inclusions cannot trap primitive melts, but rather evolved residual fluids. Although olivine provides a wealth of information on early kimberlite melt evolution and metasomatism of the surrounding lithosphere, details about the later stages of kimberlite melt evolution are evident in other magmatic groundmass phases. Therefore, detailed petrographic and mineral chemical studies were undertaken on all mineral consistents from a suite of samples from the exceptionally fresh De Beers dyke to determine the crystallisation sequence. In turn, this data yielded insights into melt evolution from the perspective of multiple different magmatic phases. The early stages of kimberlite crystallisation (i.e., olivine, Cr-spinel, Mg-Ilmenite, rutile) are defined by decreasing Mg/Fe ratios. This subsequently reverses (i.e., increasing Mg/Fe) during later groundmass crystallisation, which is attributed to increasing fO2. Comparison with published data shows that the melt parental to early crystallising phases in this dyke are indistinguishable from those in the root-zone intrusions of the Kimberley cluster, meaning that not all dykes are the crystalisation product of magmas that underwent pre-emplacement fractionation. To gain additional insights into the very late stages of kimberlite melt evolution further detailed petrographic and mineral chemical studies were conducted on late-stage groundmass phases (i.e., apatite and mica) from samples of different root zone intrusions and dyke/sill complexes in the Kimberley area. Despite the early crystallising phases (i.e., olivine, Cr-spinel, Mg-ilmenite) being compositional indistinguishable in dykes/sills and root zone kimberlites, the compositions of apatite appear to be controlled by the style of magma emplacement. Apatite from dykes/sills is Si-rich and Sr-poor, whereas apatite in root zone intrusions show the opposite features. The high Si content of apatite in dykes/sills is attributed to the coupled incorporation of silica and a carbonate ion for phosphorus, reflecting higher CO2 contents in the melts parental to dykes/sills. The high Sr content of apatite in root zone intrusions likely requires crystallisation from, or overprinting by, hydrous fluids. These features indicate that dyke/sill kimberlites have higher CO2/H2O ratios than the magma that produced root zone intrusions. This is consistent with petrographic observations, whereby dykes/sills are enriched in carbonates, may contain dolomite, and have lower abundances of serpentine, mica, and monticellite than root-zone kimberlites. These differences in CO2/H2O ratios of the crystallised melt are attributed to differences in emplacement style, whereby a rapid decrease in pressure in root zone kimberlites leads to exsolution of a (CO2-rich) fluid phase, possibly caused by breakthrough to the surface. The knowledge gained through detailed petrographic and mineral chemical studies led to the development a new quantitative method for reconstructing the composition of kimberlite melts. This model allowed for constraints to be placed on the composition of primitive kimberlite melts and their evolution, as they incorporate and assimilate xenocrystic material, undergo fractional crystallisation, and post emplacement alteration. The results of this modelling indicate that the melt parental to the Bultfontein kimberlite was transitional between silicate and carbonate. This composition is consistent with experimental constraints on the amount of CO2 that can be dissolved into kimberlite melts in the upper crust. The reconstructed primitive melt composition is in equilibrium with asthenospheric source rocks. Based on constraints from experimental studies, Kimberley kimberlites could have been produced by ~0.5% melting of carbonated lherzolite in the upper asthenosphere (i.e., 6.0-8.6 GPa and ~1400-1500 degrees Celsius).
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ItemAn Investigation into the Potential of Pliocene Speleothems from the Nullarbor Plain, Southwest Australia, to Preserve Interpretable Palaeoclimate Signals( 2019)The Pliocene has been identified as a key time interval from which estimates of future climate scenarios can be made. At present there is a significant paucity of Pliocene climate data from the Southern Hemisphere, and moreover, from terrestrial sources therein. This thesis aims to address both of these scarcities through applications of both traditional and novel techniques of palaeoclimate analysis to speleothems from the Nullarbor Plain, southwest Australia. This thesis focuses on two primary stalagmites, as well as several additional speleothems, that grew during the Pliocene as revealed by U-Pb dating. It provides one of the first detailed studies of speleothems of a greater antiquity than the 500 kyr dating limit previously imposed by the U-Th method. This study has provided invaluable insights into the climate of the Nullarbor Plain during the Pliocene, through the application of conventional stable isotope analyses, trace element analyses, fluid-inclusion analyses, clumped-isotope analyses, and modern precipitation isotope analyses. For speleothems of such antiquity, a multi-proxy approach grounded in a site-specific modern isotope study, has provided the means for delineating the key drivers of the geochemical variations present. The use of both traditional and novel speleothem proxies provides key information regarding both temperature and precipitation dynamics in the Nullarbor region during the Pliocene. Conventional stable isotope and trace element records from the speleothems reflect changes in precipitation above the cave and the overlying vegetation dynamics. Modern precipitation analyses enable a comprehensive understanding of the influences on isotopic variation in current precipitation, with applications to the interpretation of the speleothem data. This study indicates that during the Pliocene, precipitation was significantly higher in the Nullarbor region, suggestive of an increase in moisture sourced from NCBs and the Southern Ocean. The increased precipitation resulted in significantly higher vegetation cover compared to present, as supported by both the carbon isotope and trace element signals. These conclusions are in agreement with both modelled estimates of vegetation and precipitation for the Nullarbor Plain, and also with studies of speleothem-derived pollen data from the region. Palaeotemperature estimates were derived from combined fluid-inclusion and clumped-isotope analyses; indicating a temperature range of 18.8 'C +/-1.8 'C to 21.4 'C +/-1.3 'C. While the individual methods provided several additional estimates, the uncertainties associated with each limit the reliability of the absolute temperatures. However, they signify significant temperature variations within the Pliocene, indicating that despite being a period of overall global warmth in comparison to the present day, the Nullarbor region experienced significant fluctuations in average temperatures. This study has identified several areas for focus in future research in order to further develop the novel application of speleothem research to the more distant past, while providing unique and important information regarding the climate dynamics of the Nullarbor Plain during the Pliocene.
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ItemAge, origin and extent of the Newer Volcanic Province in the Melbourne region, Australia( 2019)Abstract Intraplate basaltic volcanism is present on every continent, untethered to any specific tectonic setting. The ca. 4.6 Ma – 5 ka Newer Volcanic Province (NVP) in south-eastern Australia is a chemically and morphologically diverse intraplate basaltic province. Its diversity, preservation and accessibility make it an ideal natural laboratory for investigating the causes and evolution of intraplate magmatism worldwide. Previous studies in the NVP have put forward competing magmatic models, but their validity is dependent on a limited geochronological dataset. Furthermore, previous geochronology has seldom been supported by geochemical or geomorphological studies on the same volcanic products, such that there is often a disconnect between the absolute ages of NVP rocks and the extent and composition of their associated lava flows or eruption points. This study utilised a holistic approach to lava flow mapping, with a focus on the diagnostic petrographic and geochemical features of individual basaltic lava flows selected for geochronology. The current study area of Melbourne, which is located at the eastern margin of the NVP, was selected for its age range and geochemical complexity, rivalling those of the entire NVP. Drill core was utilised to trace lava flows at depths of up to 80 m, thereby unravelling complex flow networks and facilitating the construction of a detailed lava flow map for the Melbourne area. Trace element geochemistry was utilised to distinguish between petrographically similar flows, and to investigate the eruption of three chemically distinct magma batches erupted consecutively from Mount Fraser. This mapping and geochemical work is complemented by new, high-precision 40Ar/39Ar age constraints, spanning ~7.9 Ma – 0.8 Ma. The new geochronological constraints on Melbourne lava flows reveal that the earliest activity (~7.9 – 3.8 Ma) was dominated by small-volume eruptions that predominantly produced alkali basalts. From oldest to youngest, these included the Bald Hill, Mount Ridley, Tullamarine, Crowe Hill, Spring Hill, Summerhill Rd, Redstone Hill and Aitken Hill lava flows. After ~3.8 Ma, large-volume eruptions dominated, with Fenton Hill, Mount Kororoit, Tulloch Hill and Mount Fraser producing lavas generally of tholeiitic composition. This progression from alkali basalt to tholeiitic volcanism over time is contrary to the purported province-wide progression from tholeiitic to alkali-rich lavas, indicating either that geochemical evolution in Melbourne was distinct from that of the NVP, or that the conclusions of province-wide studies are based on an unrepresentative sample set. An age of 7.931 +/- 0.038 Ma for the Bald Hill Lava Flow far exceeds the generally accepted maximum age of NVP activity (ca. 4.6 Ma). This lava flow, along with those erupted from Pretty Sally, Green Hill and Mount Cooper, is also geochemically distinct from other Victorian NVP products, but is not dissimilar to lavas of the Cosgrove Leucitite suite, the purported products of a long-lived mantle plume. This raises the possibility that the ‘Cosgrove Plume’ traversed the latitude of Melbourne just after 7.9 Ma, casting doubt on its possible role in the initiation of NVP activity some 3.3 million years later. 11 of the 38 samples selected for 40Ar/39Ar dating produced concordant results, with the remainder exhibiting varying degrees of discordance. The underlying causes of this discordance and implications for accurate age determinations are examined and modelled in age spectra and inverse isochron space. A correlation is found between the proportion of radiogenic 40Ar (40Ar*) released and the nature of discordance exhibited by a sample. 39Ar recoil is suggested as the most likely cause of discordance in high-40Ar* samples, whereas low-40Ar* samples exhibit discordance consistent with the modelled effects of mass-dependent fractionation. Based on modelling results, isochron rotation is the main impact of isotopic disturbance on an inverse isochron plot (39Ar/40Ar vs 36Ar/40Ar), leading to a negative correlation between 40Ar/39Ar ages and (40Ar/36Ar)i values. A new framework for the treatment of 40Ar/39Ar data from basaltic rocks is submitted, optimising the interpretation of inverse isochrons and informing the allocation of age constraints. Finally, a new method of 40Ar/39Ar data treatment, here named the multi-isochron approach to 40Ar/39Ar dating, is introduced. A multi-isochron regression utilises the combined output of all possible inverse isochrons from data of a single aliquant to determine its ideal isochron and eruption age. This method has the potential of transforming 40Ar/39Ar data treatment, allowing eruption ages to be calculated even when isotopic disturbance is severe, and to be reported in cases where this was previously not possible.
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ItemInvestigation of air-sea fluxes over the Southern Ocean using an eddy-covariance technique and parameterization using stability functions( 2019)The Earth is an integrated system that consists of sub-systems that interact and influence each other. These interactions have an important influence on the understanding of weather and climate of the earth system. Air-sea interactions are one such interaction that affects the Earth's system — thus making it essential to understand the physical processes that affect the prediction and forecast of the weather and climate. The present state of art climate and numerical weather prediction models use bulk models which are based on Monin-Obukhov similarity theory and Charnock's relations to determine the fluxes across the air-sea interface. The COARE 3.5 model is the best performing model available, and it is seen that the model underestimates the fluxes at higher wind speeds. Hence, to avoid any assumptions and circular dependencies, we need to build a simple parameterization of coefficients of fluxes to determine fluxes. Eddy Covariance, the purest form of flux calculation, is used to develop the parameterization. Eddy covariance relies on high-frequency 3-D winds, which, on ships, are contaminated by platform motions. However, in the absence of reliable accelerometer data, or a failed collocated accelerometer, calculating these motions is difficult. Here, in this study, we studied if the ship's motion reference data can replace external collocated accelerometer data. We have characterized that for the anemometer mounted on the foremast of the R/V Investigator, and there is a lag of 1.4 sec in the ship's motion reference unit data. Hence, we can correct the wind speeds for platform motions using the ships' motion data after adjusting to the lag. The spectral speak due to the platform motions observed in the measured raw data by anemometer is removed after the corrections performed by the ship's data. Hence, achieving the redundancy of the external collocated accelerometer, GPS receiver, and heading sensors. The fluxes computed from the eddy covariance technique are used to get a simple parameterization to estimate fluxes. Here, we have developed the coefficients of drag, latent heat fluxes in terms of simple functions of Reynolds and bulk Richardson number, which are physically dependent on velocity and stability of the atmospheric boundary layer. The model proposed does not depend on any assumptions or does not have any circular dependencies. The coefficient of sensible heat flux could not be parameterized as we observed that there is no dependence on Reynolds number in the neutral, stable region. The proposed model is performing better compared to that of the COARE 3.5 model at higher wind speeds. Gas transfer across the air-sea interface is challenging to measure, and the existing relationships for the gas transfer velocity with wind speeds have a high variance at high wind speeds. It is essential to measure gas transfer velocities in the Southern Ocean as it is least sampled with the rough environment and high surface waves. It is estimated that the Southern Ocean is the largest sink of anthropogenic carbon dioxide, with about 40% of the total world ocean sink. Gas transfer velocities of CO_2 in the Southern Ocean are measured, and it is found that the results obtained are within the range that is reported by the previous researchers. However, there are no sufficient data points, and the variance in the data is high to get any conclusions from the results obtained.
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ItemTopics in Theoretical Metamorphic Petrology( 2019)The aim of this thesis is to progress research on three factors influencing phase equilibrium modelling of metamorphic rocks. The first involves updating the activity-composition model for the ilmenite-hematite solid solution so that the P-T conditions for the evolution of rocks containing ilmenite can be modelled across a wider range of composition. The second investigates how to model high-grade rocks that have equilibrated domainally. The third explores whether mechanical closure can be a primary control on mineral assemblage evolution in subsolidus rocks. A new thermodynamic model suitable for the ilmenite-hematite solid solution has been calibrated in the system FMTOMn (FeO-MgO-TiO2-Fe2O3-MnO). The activity-composition relationships are parameterised in the symmetric formalism, and the model now allows inclusion of long-range cation-order of Fe2+, Mg and Mn. Calibrated using relatively new cation-ordering data from Harrison, Becker and Redfern (2000), Harrison, Redfern and Smith (2000) and Harrison and Redfern (2001), the model is internally-consistent with the thermodynamic database of Holland and Powell (2011). As disorder in mineral solid solutions is a contributing factor to mineral stability at high temperatures, this model should be an improvement for use in calculations at higher temperature. Interpretation of the metamorphic history of a rock is commonly made through the application of P-T pseudosections which use the effective bulk composition (e.g. whole-rock XRF analysis) as a basis for modelling. This becomes difficult if the rock has equilibrated domainally post the metamorphic peak. Texturally-distinct microdomains can form when there are small-scale effective-composition variations in the rock where localised mineral reaction occurs at the grain scale. A method for interpreting the P-T history of a rock with domainal textures using a P-T grid and compatibility diagrams is investigated, using a texturally complex sapphirine-quartz bearing granulite from the Anosyen tectonic domain in southern Madagascar. Future studies that involve interpreting the conditions of formation of high-grade rocks may benefit from the proposed P-T grid and compatibility diagram method if effective bulk composition is inhomogeneous on a thin-section scale. The P-T history of the oxidised Anosyen terrane is further constrained using the traditional pseudosection approach on homogeneous samples in the same area. With ferric in the sapphirine model (Wheller and Powell, 2014), it is possible to explore the P-T conditions for the evolution of rocks across oxidation state. For the first time, rocks from highly oxidised to reduced sapphirine granulites can be modelled. The peak P-T conditions experienced in the south-east of the Anosyen tectonic domain attained at least 850C and pressures greater than 7.6 kbar. This also shows that the stability of sapphirine+quartz can occur below 900C and may not be a universal ultra-high-temperature (UHT) terrane indicator as commonly suggested. Finally, rocks with reactions textures that would normally be inferred to have formed due to sluggish kinetics may instead have formed through reaction along an isochor, the rock evolving at constant volume. In order to investigate the consequences of a constant volume process, a calculated volume--temperature (V-T) phase diagram is calculated for a granulite sample that has been reworked in subsolidus fluid-absent conditions. Constant volume paths show that reaction at equilibrium can be attenuated due to the reaction `vessel' maintaining constant volume, with the external environment unable to accommodate volume change. This has implications for the assumption that depth can be constrained by the mineral assemblage.
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ItemEnergy transition in liberalised electricity markets: Lessons from Australia( 2019)Renewable energy generation is being added to global electricity supply at a remarkable pace. A primary driver for this expansion has been the extraordinary cost reductions achieved by the sector, in large part in response to both explicit and implicit climate policy measures and objectives. While a globally consistent and coherent climate policy remains elusive, research and development efforts, deployment policies and industrial policy motivated by the need to dramatically reducing carbon emissions have all contributed to the recent success of the renewable energy industry. In December 2015, a historic global climate agreement was devised under the auspices of the United Nations Framework Convention on Climate Change at the 21 st Conference of the Parties in Paris. This agreement includes a global goal to hold average temperature increase to well below 2C and pursue efforts to keep warming below 1.5C above pre-industrial levels. According to the International Energy Agency, the average CO 2 intensity of electricity will need to fall from 0.411t/MWh in 2015 to 0.015t/MWh by 2050 (IEA 2016). Any future scenarios that meet this objective will necessarily involve significantly increased penetrations of renewable energy (Holz and Von Hirschhausen 2013). While many studies have concluded that such a substantial de-carbonisation of the electricity sector with renewable technologies is feasible, to date such studies have tended to focus on the technical viability of systems with high penetration of renewables to reliably supply electricity. In Australia alone 1 , there are now several such studies that show that 100% renewable energy systems are technically capable of meeting our electricity requirements (AEMO 2013a; Elliston, MacGill, and Diesendorf 2013; Wright and Hearps 2010). Challenges remain, but the questions are now shifting from technical viability to economic feasibility (Riesz 2012). Notwithstanding this, reaching high penetration of renewable energy requires more than technical or economic feasibility. There is a particular need for careful consideration of market design (IEA 2016). In this thesis, I explore the way market design issues are impacting the penetration of renewable energy in the context of the National Electricity Market (NEM) in Australia, with a particular focus on the experience in South Australia. Three different aspects of energy market design are considered in this thesis. Firstly, I consider how the current market design values energy arbitrage and storage. Secondly, I consider how specific market rules related to dispatch and settlement intervals impact the technology mix and capabilities, and effect the efficiency of the market itself. Finally, I consider how new technologies impact on secondary markets such as frequency control which have typically been provided by thermal generation. The thesis begins with a review of the energy-only market structure and design. This includes discussion of the ‘capacity cycle’ from both a historic and forward looking perspective, and how it is impacted by renewable technologies. The suitability of traditional ‘capacity overhang’ heuristics in a system where older technologies are displaced or replaced by fundamentally different technologies is discussed. I also explore the interaction with historical problems that remain contested and unresolved for energy-only markets, such as the so-called ‘missing money problem’, as well as the viability of energy-only markets in high penetration renewable energy systems. The second chapter specifically looks at the contextual settings of the Australian NEM, and in particular, the South Australian (SA) market region. Considered in isolation to the NEM, South Australia has an exceptionally high penetration of renewable energy, even by international standards, making this region a noteworthy case study in energy system transition. The historical and geographical context in which SA sits is nonetheless critical to understand both its place in the NEM and how it has responded to date. As such, this chapter includes an overview of the SA market, and important characteristics including energy mix, the size of the market, the impacts of the recent exit of coal generation and resultant changes in market concentration. The third chapter focuses on the way increasing renewable energy generation and price volatility give rise to opportunities for storage technologies in an energy-only market, using the NEM and SA as an example. In particular, I demonstrate that the main driver for storage options in an energy-only market is price volatility which, in turn, is dependant on capacity requirements to meet particular reliability settings. I show that storage technology economics are very similar to the economics of traditional forms of peak generation in this respect, and common hedging and contracting strategies used by peak generators would be suitable to finance storage assets. The analysis suggests that in energy only markets storage may also derive a competitive advantage over traditional peaking assets by deriving revenue from arbitrage opportunities, in addition to capacity payments. A case study involving Pumped Hydro Energy Storage (PHES) demonstrates that in certain circumstances, it may already have a competitive advantage over Open Cycle Gas Turbines (OCGT). The fourth chapter looks at a particular market-market design element, and investigates how this impacts the different technologies, and the efficiency of dispatch in the market. Specifically, I examine a particular characteristic of the NEM in which physical dispatch (which occurs on a 5-minute interval basis) is misaligned with financial settlement (which occurs on a 30-minute interval basis), and the incentives and outcomes that result. Drawing on the analysis and methods from the previous chapter, the effect and value of aligning these intervals is evaluated for storage technologies. The impact on other technologies, including demand side response and the costs to the system of ‘gaming’ this particular rule is also estimated. I find that market rules have a significant impact on the operation of an energy-only market under transition. The fifth chapter examines how, in a high penetration renewable market, storage technologies are able to contribute to security of supply, allowing for changes in traditional market management strategies. Specifically, I look at how lithium-ion batteries have participated in the existing frequency control framework, and the implications for the provision of system security in the longer term. The final chapter summarises and concludes the thesis, including a discussion of further avenues for analysis. A range of outputs have been generated as part of this thesis. In particular, chapter 3 was published as a journal article, while the analysis in chapter 4 was submitted and contributed to the Australian Energy Market Commission, as part of its assessment of a rule change. Finally, the analysis draws heavily on market data from the Australian Energy Market Operator (AEMO). In undertaking this analysis, substantial amounts of the publicly available data provided by AEMO have been curated in a large database that continues to be updated in real time. This database now exists as the ‘openNEM’ project - an online resource that is now both open access and opensource. Further details are provided in appendix F. Prior to this thesis I was the lead author of a journal article that investigated the merit order effect of rooftop solar in Australia (McConnell et al. 2013), and was co-author of a paper published in the Journal of Environmental Sociology (Haines and McConnell 2016) and another in the electricity journal (Sandiford et al. 2015). Whilst these are not included or part of this thesis, these work have informed my knowledge of the research area.
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ItemGeologic expressions of faulting and earthquake strong ground motions in intraplate bedrock terrains( 2019)Australian earthquakes offer unique opportunities to investigate environmental and landscape effects of reverse rupturing faults. All historic surface-rupturing earthquakes have occurred in arid, low-relief, bedrock dominated areas with little to no anthropogenic influence. Environmental earthquake effects identified following the 2016, reverse-mechanism, MW 6.1 Petermann earthquake in remote central Australia are categorised with the Environmental Seismic Intensity scale, the first application of this scale for an Australian earthquake. The intensity and distribution of environmental damage demonstrates strong asymmetry due to fault geometry, with damage increasing towards the surface rupture rather than epicentral region. The direction and distances of 1,437 co-seismically displaced rock fragments (chips) in the near-field of the Petermann earthquake provide a dense proxy-record of strong ground motions, both along- and across-rupture. Chips record preferred azimuths of displacement that are attributed to rupture fling effects. This unprecedented geological proxy-record of the distribution, directivity and intensity of strong ground motions has important implications for hazard analysis in the near-field of reverse earthquakes. Fine-scale mapping of the 2016 Peterman surface rupture and secondary fractures using field, drone-derived and remote-sensing datasets indicates surface rupture characteristics vary with changes in surface geology. Deformation zones are wider and less recognizable in granular materials (e.g. dunes, alluvium) compared with those in proximal bedrock. Kinematic analysis of bedrock fractures indicates sinistral-reverse faulting, consistent with published focal mechanisms, and a maximum compressive stress orientation generally consistent with the inferred regional SHMax orientation. Trenching and 10Be cosmogenic nuclide erosion rates provide preliminary evidence of absence for prior rupture on the Petermann faults within the last 200 to 400 kyrs. The 2016 earthquake is therefore hypothesized to be the first to rupture this fault in the near surface. Analyses of geological and geophysical data from ten moderate magnitude (MW 4.7 – 6.6) historical surface-rupturing earthquakes in cratonic Australia indicate that rupture likely propagated along pre-existing Precambrian bedrock structures. Six of seven events show evidence of multi-fault rupture across 2 to 6 discrete faults of greater than 1 km length, placing these events as some of the most structurally complex earthquake ruptures identified globally for this magnitude. No unambiguous geological evidence for preceding surface-rupturing earthquakes is present. This raises important questions regarding the recurrence behaviour of intraplate stable continental region faults, with implications for seismic hazard analysis. In summary, this thesis explores observational, seismic, and remote-sensing data of surface rupturing earthquakes in Australia to provide new (i) data regarding the recurrence patterns of Australian earthquakes (ii) insights into basement controls on these earthquakes (iii) and methods to quantify seismic directionality behaviour common to reverse earthquakes globally. These contribute to better understanding the why, what, when, where of intraplate earthquakes, and how seismic hazard varies across diverse tectonic and crustal environments.
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ItemOrigin(s) and evolution of granitic melts: theoretical considerations and field examples from Mount Isa, Australia( 2019)Felsic igneous rocks are formed from a variety of processes including: (1) partial melting and melt extraction from source regions in the crust and mantle, (2) host-rock assimilation, (3) magma mixing, and (4) fractional crystallisation. These processes do not generally occur in isolation, but often overlap in space and time. As a result determining the relative contributions of these processes to granite petrogenesis remains challenging. Here, a combination of geochemical proxies, mineral chemistry, and field data were used to assess the key parameters that have contributed to the origin and magmatic evolution felsic melts of the Proterozoic Mount Isa Inlier of northern Australia. An evaluation of the available geochemical data for the main granite suites of the western Mount Isa Inlier provides insight into the crustal evolution of the eastern margin of the proto-North Australian Craton (NAC). The 1860 Ma Kalkadoon Suite are geo- chemically similar to I-type granites of the Lachlan Fold Belt but exhibit anomalously low-Mg#, Na2O, as well as anomalously high-K2O, LREE, and Th/U for an I-type gran- ite. The modified I-type character of the Kalkadoon Suite reflects the high-geothermal gradient nature of the compressional 1890–1840 Ma Barramundi Orogeny. The 1740 Ma Wonga and the 1680–1650 Ma Sybella suites are compositionally similar to Lachlan Fold Belt A-type granites, but exhibit extreme enrichment in U and Th. The Sybella Suite is also extremely enriched in F and LREE. The anomalous geochemistry of these two suites reflects the interaction of mantle derived melts with an incompatible element-enriched ensialic crust. Part of the Sybella Suite, the zoned Queen Elizabeth Granite exhibits complex textural relationships which record incremental emplacement, differentiation, deformation, mix- ing and crystallisation of compositionally distinct magmatic phases during active rifting in the western Mount Isa Inlier. Emplacement of voluminous porphyritic to equigranular syenogranite was followed by emplacement of a leucogranite phase along the northern and eastern margins of the pluton. A high-strain zone developed contemporaneously with intrusion and crystallisation of an enclave-rich monzogranite phase in the paleo-floor zone of the pluton during continued extensional deformation. Intermingled doleritic, hybrid granitoid, and pegmatite lithologies along the western edge of the pluton highlight the bimodal character of the Queen Elizabeth Granite magmatism. Late-stage melt-drainage networks link the development of pegmatitic segregations in the marginal leucogranite phase to the accumulation of voluminous pegmatite bodies within the adjacent Mica Creek Pegmatite Field. Mineral compositional data highlight the mineral-scale variability of the main granite phases of the Queen Elizabeth Granite. Major minerals host most of the Rb, Ba, and Sr. Accessory minerals including allanite, titanite, zircon, and monazite are the major hosts of Y, Th, U, and REE. Hornblende contains a significant proportion of the HREE budget in the main syenogranite phase of the Queen Elizabeth Granite. Combined in situ zircon U-Pb-Hf-O isotopic analysis of pre-1600 Ma felsic igneous rocks from western Mount Isa Inlier provide insights into the magmatic evolution of the eastern margin of the proto-NAC during the Paleoproterozoic. Barramundi-aged granitoids are dominantly sourced from reworking of ca. 2.5 Ga, isotopically evolved continental crust. Zircon isotope data suggest that the Sybella Batholith was emplaced during two magmatic cycles at 1680–1670 Ma and 1660–1650 Ma with each cycle associated with a broad trend towards increasing juvenile mantle input with time. The geochemical, isotopic, and field data are consistent with the presence of a long-lived, mantle driven, transcrustal magmatic plumbing system active within the eastern proto- NAC crust during the late Paleoproterozoic. This mantle driven model contrasts with existing models that rely on derivation of A-type melts from protoliths within the crust and directly links bi-modal magmatism, crustal assimilation, sedimentation, and extensional tectonics within the western Mount Isa Inlier during the Paleoproterozoic.