School of Earth Sciences - Theses
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ItemPleistocene to Holocene history of the Indo Pacific Warm Pool and its associated currents, the Kuroshio/Tsushima and Leeuwin currents( 2012)The history of the Indo Pacific Warm Pool (IPWP) can be determined by analysing IPWP sourced extra-tropical ocean currents in distal regions. Two such IPWP sourced ocean currents are the Kuroshio and Leeuwin currents. The Omma Formation in the Sea of Japan records the Early Pleistocene palaeoceanographic change of the Tsushima Current – an offshoot of the IPWP sourced Kuroshio Current. The Middle Pleistocene to Holocene activity of the Leeuwin Current was studied from the Delambre Formation from two boreholes in the Angel gas field on the North West Shelf of Australia. Several species of Indo Pacific benthic foraminifera migrate from the IPWP via ocean currents such as the Leeuwin and Kuroshio/Tsushima currents and can be used as biogeographic markers for extra-tropical current flow and IPWP evolution. The most common taxa are Asterorotalia spp., including A. gaimardii, A. milletti, A. concinna, and A. yabeii. The distribution of molluscs, planktic foraminifera and Asterorotalia spp. in the Omma Formation suggests Tsushima Current flow in every interglacial period between marine isotope stages (MIS) 56 and 20. The timing of initiation of Tsushima Current inflow into the Sea of Japan was determined in nearly every Early Pleistocene marine isotope stage based on the first appearance of warm water planktic and benthic foraminifera, and warm water molluscs. Horizons representing Tsushima Current maximum flow were identified for several marine isotope stages based on the maximum abundance of Asterorotalia spp., warm water planktic foraminifera and warm water molluscs. Data from this study was combined with previous bio-, tephro-, and magneto-stratigraphic data to produce an age-depth model for the Omma Formation for the first time. Radiocarbon dating, nannofossil, foraminiferal, and stable isotopic data in the Angel gas field boreholes show that the sections have a maximum age of ~280 ka, from MIS 8 to 1. However, MIS 2 and MIS 4 are not present. The presence of Asterorotalia spp. and “larger” benthic foraminifera suggests that the Leeuwin Current flowed during each marine isotope stage. The abundance of Asterorotalia spp. and larger benthic foraminifera increased during interglacial periods and reduced or became absent during glacial periods. This indicates that the Leeuwin Current flowed in an intensified form during interglacial periods and in a reduced or absent form during glacial periods. Fossil evidence from the Omma Formation and the North West Shelf of Australia Angel gas field boreholes clearly indicate strengthened flow of IPWP sourced extra-tropical flow during interglacial periods. Glacial periods see a reduction and/or disappearance of Indo Pacific foraminifera indicating reduced and/or absent extra-tropical current flow.
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ItemThe impacts of climate variability and change on severe thunderstorm environments in Australia( 2012)Severe thunderstorms present a relatively infrequent but significant threat to property and life in Australia during the spring and summer. These thunderstorms can produce hailstones over 2cm in diameter, winds in excess of 90kmh-1 and less frequently tornadoes. Any of these phenomena can result in localised high impact severe events. Recent examples of this potential are illustrated by damage caused by the 1999 Sydney Hailstorm, 2008 Gap Microburst and the 2010 and 2011 Melbourne Hailstorms. This risk makes the implications of a changing and variable climate on severe thunderstorms important to understand. Recent studies into the impacts of anthropogenic climate change on severe weather events, including thunderstorms, suggest a potential increasing trend in both frequency and intensity for Australia. While current convective parameterisations in both global and regional climate models limit direct assessments of future convection, the use of environmental parameters to estimate changes in severe thunderstorm environments has been successful in other geographical regions. This study seeks an answer to the question “Is the frequency and distribution of severe thunderstorm environments in Australia likely to change in the future?” A database of 1550 independent severe thunderstorm reports in Australia has been developed for the period March 2003 to April 2010. Severe thunderstorm reports are then used to identify relationships with their associated environments estimated using proximal soundings from a mesoscale numerical weather assimilation and prediction model (MesoLAPS). This proximity climatology of known severe thunderstorm environments has been successfully used to derive covariate discriminants that identify the potential of an environment to produce severe thunderstorms. These covariates use variables describing the potential for organised convection (deep-layer wind shear), and the potential for instability over the depth of the atmosphere (convective available potential energy). Applying these discriminants to a reanalysis dataset (ERA-Interim), a climatology of the frequency and spatial distribution of environments favourable to the development of severe and significant severe thunderstorms for Australia has been developed for warm seasons during the period 1979-2011. This climatology demonstrates that inter-annual variability in terms of both the frequency and spatial distribution of environments is influenced by El Niño- Southern Oscillation. La Niña conditions are typically associated with an increased frequency and an inland shift of favourable environments over eastern Australia, while El Niño typically results in fewer environments, particularly along the coastal fringe. Applying this climatology, the environments simulated by two climate models (CSIRO Mk3.6 and CCAM) for the 20-season period 1980-2000 are examined over Australia and tested against the reanalysis climatology. In particular, the ability of the models to resolve the intra-annual variability and both quantify and simulate the spatial distribution of convective variables are analysed, and are found to perform reasonably well, especially in the case of the higher resolution CCAM. Finally, future simulations of severe thunderstorm environments from high emissions projections for the period 2079-2099 are presented for both models. Comparing these simulations to the 20th century, a potential small increase in the frequency of severe thunderstorm environments appears likely for southeast and eastern Australia under a warming climate.
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ItemExperiencing, understanding and adapting to climate in south-eastern Australia, 1788-1860( 2012)This thesis explores a somewhat overlooked theme in Australian history - climate. Phenomena like drought, flood and bushfire continue to place a strain on Australian society, as we have clearly seen in the past few years alone. The Black Saturday bushfires of 2009, the 2010-2011 Queensland floods and the ‘Big Dry’ drought that stretched from 1997-2009 have had an enormous impact on modern Australian society. It is obvious that climate extremes frequently affect Australia today and this thesis asks: • How were societies in south-eastern Australia affected by weather and climate between 1788 and 1860? • Did European Australians adapt to the problems posed by weather and climate during this period? • Did the impact of rainfall variation in south-eastern Australia differ between and within colonies? An interdisciplinary approach has been taken to answer these research questions. Climate is ordinarily analysed and understood using scientific data like meteorological observations and palaeoclimate records derived from tree-rings, coral growth, ice cores and cave deposits. However, meteorological observations were not routinely kept prior to 1860, leaving gaps in our knowledge of early climate. This thesis fills these gaps by examining historical documents including letters, diaries, newspapers and government records. The information uncovered in these sources was then compared to available historical meteorological records and palaeoclimate data. This unique mix of historic and scientific data sheds light on Australia’s past climate. Australian history includes literature on a range of environmental themes but there remains very little on climate and weather. This thesis makes a substantial contribution to Australian climate history, deepening our understanding of colonial history and broadening our knowledge of historical weather and climate events in the southern hemisphere. This analysis reveals the way European settlers adapted to weather and climate challenges and also reveals the combined impact of climate extremes and colonisation on indigenous people in south-eastern Australia.
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ItemThe geochemistry and geochronology of Tanzanian kimberlites( 2012)The Tanzanian Craton is host to numerous kimberlite intrusions, although few have been studied in any detail. Due to its proximity to the East African rift zone, this region also provides a unique opportunity to investigate the effect of rifting on kimberlite volcanism. To date, however, no geochemical studies have been conducted on Tanzanian kimberlites, largely due to their altered and weathered nature, and only four intrusions have published age constraints. This thesis seeks to redress these deficiencies by the application of modern high precision analytical techniques to the study of kimberlites from this previously neglected region. The study focused on 30 separate kimberlite intrusions located on the Tanzanian Craton. Due to their highly altered and weathered character, perovskite was used as a proxy to obtain geochemical and geochronological information on the host kimberlites. Perovskite is a common accessory mineral in kimberlites and contains substantial U, Sr and Nd and lesser amounts of Pb and Hf. Consequently, perovskite provides the means to determine both age constraints and initial isotopic signatures for the host kimberlites. Perovskite extracted from the Tanzanian kimberlites was utilised for U-Pb geochronology and analysed for trace element and Sr-Nd-Hf isotopic compositions. Similar to kimberlites worldwide, these samples show extreme enrichment in large ion lithophiles and LREE, whereas they are heavy REE depleted. They are characterised by moderately radiogenic 87Sr/86Sri compositions (0.70268 – 0.70476) and ɛSr values (-14.56 to 4.46), restricted 143Nd/144Ndi compositions (0.51138 to 0.51277) and ɛNd values (1.8 to 3.8), and very restricted 176Hf/177Hfi compositions (0.282058 to 0.282966) and ɛHf values (0.8 to 8.2). All samples plot within the range previously defined for Group I kimberlites. This study provides compelling evidence for the utility of perovskite analysis in highly altered kimberlitic samples that would otherwise be intractable to study. The ages obtained for these samples define two periods of kimberlite volcanism for the Tanzanian Craton. Early Mesoproterozoic kimberlites include the Itanana kimberlite (1083 Ma) and the previously dated Bubiki kimberlite (1097Ma). All other kimberlites for which ages were obtained are significantly younger, with ages ranging from 44-80 Ma. The latter group includes some of the youngest reliably dated kimberlites known; the Kikhomango (44 Ma) and X073 (47 Ma) intrusions. Ages from both the older and younger Tanzanian kimberlites coincide with periods of known kimberlite activity worldwide. Previous studies of the Tanzanian kimberlites have focused on the thermobarometry of garnet macrocrysts derived from disaggregated mantle xenoliths. Garnet compositions vary with proximity to the East African Rift, suggesting that the SCLM in this region was affected by rift related heating prior to kimberlite emplacement. If the source of these kimberlites was within the SCLM, systematic geographic variation in the isotopic and trace element compositions in relation to their proximity to the rift would be expected. This has not been observed in this study, with the entire region displaying no significant geographic variation in geochemical composition. Therefore the current results suggest a deeper source for the Tanzanian kimberlite magmas, below the influence of rifting in the region.
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ItemThe timing and origin of orogenic gold mineralisation in the western Lachlan Orogen, southeast Australia: constraints from 40Ar/39Ar dating and halogen and noble gas geochemistry( 2012)The Ballarat East gold deposit (408t) is the second largest orogenic gold deposit in the Western Lachlan Orogen, southeast Australia. The western Lachlan Orogen is characterised by a thick package of Ordovician turbiditic sedimentary rocks overlying Cambrian oceanic volcanic sequences. The region was variably affected by multiple major deformation/metamorphism and magmatism events during the Cambrian to Devonian. The Ballarat East gold deposit is located in the Bendigo structural zone of the Western Lachlan Orogen and is hosted in Ordovician sediments of the Castlemaine Supergroup. Gold mineralisation in the Ballarat East deposit is sited in quartz and quartz-carbonate veins within goldfield-scale, west-dipping reverse faults. Two major lode types are present: 1) lode type ‘1’ is characterised by arsenopyrite-dominated quartz veins associated with early movement on reverse faults, whereas 2) lode type ‘2’ is related to structurally later, shallow east-dipping, pyrite-sphalerite-galena-white-mica dominated veins, emanating from reverse faults. Previous studies have suggested that gold mineralisation in the Western Lachlan Orogen occurred at ~440Ma, as a result of metamorphic devolatilisation reactions in the lower crust. However the age of mineralisation at the Ballarat East deposit is only broadly constrained to a period between 460 and 370 Ma, and the source of the gold-bearing fluids could include metamorphosed volcanic rocks, sedimentary rocks and/or granites. In order to provide a more robust chronological framework for gold mineralisation at the Ballarat East deposit, several samples of detrital and hydrothermal potassium-rich minerals were collected and analysed by 40Ar/39Ar dating. In addition, fluid inclusions in portions of quartz and quartz-carbonate veins were characterised by micro-thermometry and halogen/noble gas isotopic tracer methods to further constrain the source(s) of the gold mineralising fluids. The 40Ar/39Ar data obtained from detrital muscovite grains yield ages between 530 – 460 Ma and are concordant with previously published detrital ages. The vein muscovite/sericite ages fall into three age groupings as follows: 445 – 435 Ma (lode type ‘1’), 420 – 415 Ma (lode type ‘2a’) and 380 – 370 Ma (lode type ‘2b’). The gold-bearing quartz veins (from both lode types) contain low salinity (average 4 wt.% NaCl eq.) aqueous H2O inclusions and mixed H2O-CO2 fluid inclusions. Fluid inclusion 40Ar/36Ar values range from 322 (close to Air Saturated Water; ~296) up to a maximum of 4503, and 40Ar/36Ar is strongly correlated with Cl/36Ar. Fluid inclusions have variable Br/Cl values between 1.66 10-3 and 2.91 × 10-3 and I/Cl values between 153 × 10-6 and 501 × 10-6, with a strong correlation between Br/Cl and I/Cl. The fluid inclusion 84Kr/36Ar and 129Xe/36Ar values are variable but show a systematic enrichment in the heavier noble gases. The 40Ar/39Ar ages suggest gold mineralisation at the Ballarat East deposit occurred in three main episodes at ca. 445 Ma, ca. 420 Ma and ca. 380 – 370 Ma. All episodes of mineralisation are associated with fluid inclusions of similar composition. This fluid is suggested to reflect a deeply sourced fluid, possibly originating by devolatilisation of altered volcanic rocks (e.g. basalts). In this scenario, the fluid would have acquired additional noble gases and organic Br plus I by interaction with sedimentary rocks, including organic-rich shales that are found beneath and surrounding the deposit. The data are compatible with genetic models for orogenic Au in which gold mineralisation was initiated by metamorphic devolatilisation in the lower crust, linked to Lachlan Orogenesis at ca. 440 Ma.
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ItemCalibration of the cosmogenic 21Ne exposure dating technique for application to Quaternary volcanic chronology( 2012)The use of terrestrial cosmogenic nuclides to investigate surface exposure histories continues to gain popularity due to the applicability of the technique to many different minerals and rock types. However, errors associated with cosmogenic exposure ages are typically either poorly specified or relatively high compared to other more established dating methods. This is dominantly due to large systematic uncertainties. This study focused on the stable cosmogenic nuclides 21Ne and 3He, and investigated their production in olivine and clinopyroxene from young (<150 ka) basalt flows. The primary aims of this study were to (i) improve the cosmogenic 21Ne dating technique for application to these minerals, (ii) develop in-house methods for dating young basalt flows by the 40Ar/39Ar step-heating technique, and (iii) inter-calibrate cosmogenic 21Ne surface exposure ages with corresponding 40Ar/39Ar eruption ages, in order to test available cosmogenic nuclide production rate scaling models. Alkali basalt flows (~30 – 130 ka) from three mid-latitude localities were studied: Mount Porndon (Victoria, Australia), Amboy Crater (Mojave Desert, California, USA), and the Nave flow, Mount Etna (Sicily, Italy). Helium and neon isotopic compositions of olivine (n = 13) and clinopyroxene (n = 7) separates from the above localities were determined from two-step heating experiments (900 ̊C and 1600 ̊C) using a VG5400 mass spectrometer. All samples showed evidence of variable degrees of helium loss and isotope mass fractionation, which compromised the calculation of cosmogenic 3He (3Hec) ages. Although minor neon loss was also apparent in most samples, it did not affect calculation of cosmogenic 21Ne (21Nec) ages. Overall, olivine separates yielded more consistent cosmogenic 21Ne ages than corresponding clinopyoxene separates extracted from the same samples. Crushing experiments on selected samples demonstrated an atmospheric composition for the 21Ne/20Ne ratio of the trapped neon component. Apparent weighted mean 21Nec exposure ages were calculated at: 64 ± 10 (2σ, Mount Porndon), 105 ± 23 ka (2σ, Amboy Crater), 29.0 ± 7.6 ka (2σ, Nave flow), respectively. Prior to analysing the very young (<150 ka) basalts from the selected calibration sites by the 40Ar/39Ar technique, studies of slightly older samples (300-600 ka) collected from the Newer Volcanic Province of southeastern Australia (Mount Rouse basalt (n = 4) and Mount Warrnambool basalt (n = 2)) were carried out on a VG3600 mass spectrometer in order to establish appropriate sample preparation procedures and analytical protocols. Multi-aliquot 40Ar/39Ar furnace step-heating analyses of whole-rock samples from several (n = 4) Mount Rouse basalt flows yielded weighted mean ages of: 309 ± 20 ka (2σ, NVP19), 382 ± 24 ka (2σ, NVP06); 301 ± 27 ka (2σ, NVP20) and 280 ± 19 ka (2σ, NVP21). Samples from Mount Warrnambool (n = 2) yielded weighted mean ages of: 547 ± 23 ka (2σ, NVP03) and 535 ± 27 ka (95% CI. NVP04). The internal concordance of results and agreement with previous K-Ar ages enabled validation of the experimental protocols adopted. Basalt samples younger than 150 ka from Mount Porndon, Amboy Crater and Mount Eccles were analysed on a new generation multi-collector ARGUSVI mass spectrometer via laser step-heating. This system yielded significantly more precise isotopic measurements than was possible using the older VG3600 system. Two to four groundmass aliquots were analysed per sample (n = 4), giving weighted mean 40Ar/39Ar ages of: 105 ± 22 ka (2σ; Mount Porndon, NVP14), 121 ± 8 ka (2σ; Amboy Crater, 08010), 92 ± 8 (2σ; Mount Eccles, NVP18)), 84 ± 13 (2σ; Mount Eccles, NVP12) and 293 ± 7 ka (95% CI; Mount Rouse, NVP21). Although lava surfaces showing apparent primary flow textures were sampled for cosmogenic dating, comparison of the cosmogenic 21Ne and 40Ar/39Ar ages indicates that significant erosion has occurred at the outcrops sampled for exposure dating from the Mount Porndon lava flows. Erosion modelling assuming an eruption age of 105 ± 22 ka (2σ, 40Ar/39Ar) suggests an erosion rate ranging from 2 - 7 mm/ka, depending on the value assumed for the scaled 21Nec production rate. As estimated 21Nec production rates for Mount Porndon are at the lower end of the range predicted by latitudinal production rate scaling models, it is possible that available scaling models may require further refinement for application to Southern Hemisphere localities. For the Amboy Crater flow, an erosion rate of 1 mm/ka is estimated, consistent with previous studies. In the case of the Nave flow, the agreement of the 21Nec exposure age for sample 08025 with the weighted mean K-Ar age (32 ± 8 ka (2σ)) of Blard et al. (2005) is interpreted as evidence for minimal erosion at this outcrop. However, the other two samples collected from adjacent outcrops on the Nave flow yielded slightly younger results, suggesting that erosion cannot be excluded for this flow. In order to refine estimates for 21Nec production rates in olivine and clinopyroxene, studies involving step-heating of large (several grams) mineral separates from young (ca. < 30 ka) basalt flow surfaces that have undergone negligible erosion and have independently constrained eruption ages are recommended. For the purposes of investigating muon-induced production of cosmogenic neon in olivine, 21Nec concentrations were studied in samples spanning an 11 m vertical profile through an Eocene basalt flow located in the Monaro Volcanic Province, NSW, Australia. A 40Ar/39Ar age of 55.52 ± 0.02 Ma (2σ) is calculated for this flow from step-heating (n = 4) experiments on whole-rock samples. It is demonstrated that excess 21Ne in the deepest samples can be accommodated solely by nucleogenic neon production, meaning that muogenic neon production in olivine is insignificant, even over million year time-scales. This justifies the assumption implicit in previous studies that 21Nec production in olivine (and likely clinoproxene) is purely the result of spallation reactions. Preliminary modelling based on 21Nec concentrations in the studied profile suggests an appreciably low average Cainozoic erosion rate of ca. 1 m/Ma.
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ItemNeogene vertical motions in intraplate settings: case studies from Australia and Korea( 2012)The lithosphere is dynamically linked with the large-scale dynamics in the convective mantle beneath. While the horizontal lithospheric movements are now well understood in a framework of plate tectonics, it is still a challenge to explain the vertical motions of the Earth’s surface, especially in intraplate regions. Such movements may arise from a variety of causes, such as upper mantle flow and lithospheric responses to plate boundary forces. This thesis explores the unique records of vertical motion in two settings. In Southeast Australia, tectonic modes and rates during the Late Neogene are established from uplifted marine terraces and other geomorphic constraints. Uplift of southern Victoria has occurred at rates of up to ~55 m Myr-1 during the Quaternary, and the rate of uplift has increased progressively since the Middle Pliocene. Non-uniform uplift of coastal Tasmania varies from ~40 m Myr-1 to ~112 m Myr-1. Spatial variations of vertical motions in southern Victoria and Tasmania is order 100’s kms, and is largely explicable in terms of lithospheric flexure, in response to contemporary plate boundary forces coupled with localised lithospheric failures, superimposed on a longer wavelength dynamic topography. The eastern margin of NE Asia, including Korea, sits in the back-arc setting to the Western Pacific Subduction Zone, in the vicinity of a complex trench triple junction of the Philippine, Pacific, and Eurasian plate. Geochronology of the Quaternary marine terraces and fission track dating results of upper Cretaceous granites imply maximum uplift rates of 45.3±1.1 m Myr-1 during the Neogene and 168±16 m Myr-1 in the Late Quaternary. Maximum uplift rates correspond in a general sense with a belt of Quaternary mafic intraplate volcanism suggesting the uplift can be attributed to asthenospheric upwelling around the edges of the sinking Western Pacific slab. The edges of the subducting slab are currently located beneath the eastern margin of the peninsula at a depth of 600 km. In addition, plate boundary forces arising from interactions between the Pacific, Asian and Philippine plate contribute to lithospheric failure relating with faulting movements in the peninsula since the Late Neogene. This research on the long- term vertical deformation field of the continents provides new insights into the way in which the internal dynamics of the Earth modify the surface with the various scales driven by both lithospheric and sub-lithospheric processes.
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ItemThe influence and observability of overshooting convection in the tropical tropopause layer( 2012)Intense tropical convection that overshoots deep into the Tropical Tropopause Layer (TTL) is thought to play an important role in influencing the region’s moisture content. However, the net effect of such convection and the details of how it affects TTL humidity are still not well understood. Furthermore, the degree of this influence is dependent on the statistics of overshooting convection, such as its frequency, areal extent and depth, which may differ between different convective regimes. This thesis first examines the key physical processes that govern the net influence of overshooting convection on TTL water vapour when it penetrates into different observed TTL environments, one supersaturated and the other subsaturated (with respect to ice), using idealised three-dimensional cloud-resolving simulations. It then investigates the characteristics of overshooting convection during a convectively active monsoon regime and a ‘break’ period, using ground-based radar data from the Tropical Warm Pool-International Cloud Experiment (TWP-ICE). In addition, observational biases – particularly at TTL levels – are determined, guided by synthetic imagery derived from emulated radar scans of realistic, high-resolution numerical model simulations of the two TWP-ICE regimes. These are then corrected using a simple correction scheme. Overshooting convection is demonstrated to play a direct efficient role in driving the ambient local environment towards ice saturation through either net moistening (subsaturated TTL) or net dehydration (supersaturated TTL). However, the extent of dehydration in supersaturated conditions is greater than the moistening in subsaturated conditions. Ultimately, vapour-scavenging and sublimation processes from convectively-lofted ice play the most dominant role in defining the net TTL relative humidity tendencies, although transport and mixing are implicitly important. Analysis of gridded TWP-ICE radar data reveals that overshooting storms during the two regimes possess slightly different characteristics. ‘Break’ overshooting convection is relatively more intense, has a pronounced diurnal cycle, and reaches above 18 km more often than similar storms during the active monsoon phase. Intense storms during the ‘break’ period also exhibit much less variability in terms of mean overshooting area in the TTL than their active period counterparts. However, the active phase during TWP-ICE produced a larger mean overshooting area with height compared to the ‘break’, although this was significantly influenced by an atypical day of enhanced overshooting activity associated with the passage of a large mesoscale convective system. Due to geometrical constraints and sampling gaps at high altitude, ground-based radar is shown to overestimate certain echo-top heights while consistently underestimating the highest core intensities and the mean overshooting area with height across both TWP-ICE regimes (O(100) km^2). This implies that the radar is underestimating the overshooting flux of water into the TTL during both periods. Yet, corrected profiles of detected mean overshooting area indicate that the average area covered by convective overshoots only differs marginally between regimes. Based on these corrections, ‘break’ overshooting convection is concluded to typically transport slightly more (particulate) water mass into the TTL than archetypal overshooting convection during the active monsoon. This result suggests that each convective regime may exert dissimilar effects on the TTL over the course of one monsoon season.