School of Earth Sciences - Theses

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    Evaluation of aerosol representation by ACCESS-CM2 with increased aerosol-chemistry complexity in the Southern Ocean
    Wadlow, Imogen ( 2020)
    This thesis identifies the inherent biases of aerosol parameters within the next-generation Global Climate Model (GCM); the Australian Community Climate and Earth System Simulator Coupled Model (ACCESS-CM2). GCMs poorly represent clouds and aerosols over the Southern Ocean, resulting in systematic shortwave (SW) radiation biases with widespread global energy budget impacts. This research determines whether a more complex, physically-representative aerosol-chemistry scheme may reduce the Southern Ocean radiation bias, and the inherent aerosol biases established within ACCESS-CM2. Southern Ocean aerosols are dominated by sea-salt and biogenic products. This study ran a control ACCESS-CM2 simulation and three perturbation simulations, which altered either the representation of Primary Marine Organic aerosols, sea-salt, or implemented a fully interactive chemistry scheme respectively. A suite of ground-based and satellite observations was collected and compared against each simulation to establish model bias respective to key aerosol metrics including Aerosol Optical Depth (AOD), Cloud Con- densation Nuclei (CCN) and SW radiation. Simulation biases were explained through model-observation comparisons of aerosol chemistry, size and number parameters. Overall, ACCESS-CM2 exhibited a substantial positive bias in Aerosol Optical Depth (AOD) and SW radiation, and underestimated Cloud Condensation Nuclei concentration. This research suggests that increasing the complexity of aerosol schemes was able to provide a closer model agreement with observed aerosol metrics of AOD and CCN. Fully interactive chemistry provided the best reduction in both AOD and CCN bias. However, modified aerosol schemes have negligible effects upon the inherent SW radiation bias in ACCESS-CM2, suggesting further research into cloud schemes is necessary.
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    Antarctic sea ice and its interactions with high latitude weather and climate
    Watkins, Andrew Bruce ( 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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    The influence of a sub-lithospheric decoupling layer on subduction zone surface motion, mantle circulation and slab dynamics
    Carluccio, Roberta ( 2020)
    The lithosphere consists of a strong surface layer fragmented in a series of major and minor tectonic plates moving over a weaker and more buoyant layer of asthenospheric material. The boundary between the lithosphere and asthenosphere, the LAB, is a critical element of plate tectonics as it mechanically decouples the motion of strong tectonic plates from the ow in the underlying weaker mantle. Recent geophysical observations report an abrupt seismic velocity decrease and electrical conductivity increase beneath certain segments of the subducting Pacific plate, which may represent the channelised base of the subducting oceanic lithosphere. This inferred sub-lithospheric weakness is predicted to lubricate the base of subducting plates and enforce the decoupling at the LAB, with potential implications for our understanding of subduction geodynamics. However, little is known about the impact of this feature on subduction zone dynamics. This research work for the first-time compiles worldwide observations of a very thin and weak sub-lithospheric layer (SLL) embedded beneath the base of a descending oceanic lithosphere and computationally simulates SLL effects on subducting plate and mantle dynamics. In the last few decades, the rapid progress in technology has contributed to increasing complexity of geological observations and capacity of computational infrastructures. With direct geological observations limited in both time and space scales, computational modelling offers the extraordinary opportunity to test different hypotheses for Earth's evolution through geologic time and ongoing processes, whilst promoting the development of multidisciplinary approaches. This study, therefore, employs subduction numerical models and addresses the SLL impact on slab deformation style including subduction surface motions, induced mantle ow and patterns of seismic anisotropy. Our results show that the physical properties of the sub-lithospheric layer fundamentally control the partitioning of surface motions between trench migration and horizontal plate velocities, but also the partitioning of the induced mantle ow as it splits into its poloidal and toroidal components. A regional SLL mechanism firstly provides a novel explanation for the high convergence and low trench migrations rates observed globally at natural subduction zones. Then, an SLL also delivers an alternative answer for plate margins characterised by some degree of toroidal ow and for the apparent lack of a direct link between buoyantly driven convection (which drives only vertical and divergent motion) and the generation of toroidal energy (which corresponds to strike slip motion and plate spin). Additionally, a very thin and weak sub-lithospheric layer promotes the alignment of the maximum strain axes in the direction parallel to plate motion within the oceanic lithosphere and mantle directly beneath it, and in the direction parallel to the trench deeper in the sub-slab mantle. As a result, these findings contribute to the resolution of the slab anisotropy debate on whether an SLL can substantially influence three-dimensional ow around the subduction zone. These outcomes reveal that a very weak and thin sub-layer embedded in the Earth's uppermost asthenosphere impacts the complexity of current natural subduction systems, for which little predictive and holistic modelling exists. By acting as a slippery base for the motion of the subducting lithosphere, this layer sharpens the decoupling between the strong tectonic plates and the weaker mantle underneath. This has significant consequences on slab subduction style and patterns of seismic anisotropy as observed in several subduction regions, such as Izu-Bonin-Mariana, Central America and Kamchatka- Aleutian among others, where the presence of this layer is confirmed by geophysical studies. This thesis work ultimately aims to provide a novel conceptual framework to reinterpret the geological record of complex plate-margins, with broader implications for better understanding the evolution of the Earth's dynamic system.
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    Stress interactions in intraplate earthquakes
    Mohammadi, Hiwa ( 2020)
    Australia is a seismically active continent with an intraplate compressive stress field primarily driven by far-field plate boundary interactions. Major (magnitude >= 5) surface-rupturing earthquakes are primarily sourced from reverse faults. Major earthquakes perturb local-to-regional stress fields and influence the spatiotemporal properties of subsequent earthquakes. This thesis first contextualizes Australian earthquakes against global comparatives by investigating 260 finite-fault rupture models for 137 moment magnitude (Mw) 4.1 to 8.1 continental earthquakes worldwide. I find that: (i) Australian earthquakes are amongst the most kinematically and geometrically complex for their Mw, (ii) upper-bounds and variance of the number of faults that rupture co-seismically increase with increasing Mw, and (iii) multi-fault rupture populations show no dependency on strain rate or proximity to plate boundaries. The thesis then presents a suite of studies that model static and viscoelastic coulomb stress changes (dCFS) imparted by major Australian earthquakes on to receiver faults and in the surrounding crust. I find that static dCFS models provide an informative physical-statistical basis for characterising many seismic sequences in Australia, with some exceptions. Aftershocks occur predominantly within positive static stress lobes and close to the advancing viscoelastic positive stress lobes, especially over the first few decades after major earthquakes in these regions. Earthquake triggering appears to occur under stress perturbations as small as approx 0.001 to 0.01 bar, suggesting cratons contain regions of critically stressed lithosphere. The effects of varying source fault geometries and kinematics on dCFS fields and subsequent seismicity are used to show how progressive refinement of source fault models using emergent data can reduce epistemic uncertainties in the role of dCFS in earthquake triggering. In some instances, increased source model complexity does not significantly impact on dCFS results and possible relationships to aftershocks relative to simple source models. The thesis finally investigates the role of lithospheric-scale flexural bending due to eustatic sea level changes, and whether these impart dCFS perturbations on finite faults and in regions similarly to stresses imparted by preceding earthquakes. Preliminary age distributions of large paleo-earthquakes on these reverse faults are concurrent with dCFS peaks imparted by eustatic sea-level changes. dCFS of > 0.1 to > 1 bar on faults during the low-stand Marine Isotope (MIS) Stage 2 to Stage 4 interval (ca. approx 30 to approx. 70 ka) provides tentative evidence that earthquake clusters could be stimulated by sea-level lowstands. This thesis demonstrates the power and utility of dCFS modelling to improve understanding of intraplate earthquakes.
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    Modelling Australia’s transition to a low carbon electricity system with optimised transmission networks and renewable energy exports
    Wang, Changlong ( 2020)
    In order to meet the Paris Agreement and pursue efforts for a 1.5 degrees Celsius target, the electric power system worldwide must undergo a fundamental transformation as the main pillar for decarbonisation. The decarbonisation will almost certainly be driven largely by the massive adoption of variable renewables. The objective of this PhD project is to develop a co-optimisation model with the ability to perform comprehensive analysis to simultaneously explore the least-cost configurations of generation, storage and transmission over large geographic regions to facilitate the transition to a low carbon power system. Although applicable to any region, the capacity expansion model we developed focuses on Australia’s National Electricity Market. The capacity expansion calculated by the model satisfies prescribed demand projections and emission abatement targets from 2020 to 2050, subject to constraints including resource adequacy, system inertia and reliability, unit commitment, economic dispatch and transmission capacity. The model also takes into account the decommissioning of the existing fossil fuel generation fleets and explores a great range of renewable generation and storage options. The transmission model employs a Direct Current power flow approximation with 21 major load centres over the eastern states of Australia. We have used the model to examine various carbon abatement scenarios for Australia. We show that wind turbines and solar PV technologies combined with pumped hydro energy storage dominate the solutions, and that gas is unlikely to be a transition fuel towards a 100% renewable system by 2050. The 100% renewables systems simulated are not only technically feasible but also economically achievable. Australia’s vast wind and solar resources become increasingly valuable assets as the world decarbonises. Not only does this study consider meeting local requirements for electricity demand, but also the potential for export of green energy. Our research confirms the very real and great potential for Australia to produce cost-competitive hydrogen. In the pathways to carbon neutrality, we demonstrate there are substantial synergies between renewable hydrogen export and domestic energy transition in Australia. In particular, we show if the hydrogen industry co-locates within the National Electricity Market with shared renewable generation facilities (i.e. sector coupling), not only can the hydrogen industry produce cost-competitive hydrogen, but also that domestic electricity consumers could enjoy lower electricity costs, thereby benefiting the whole economy in the long run. Besides hydrogen export, we also conducted the first detailed least-cost optimisation studies of power system decarbonisation planning for both Australia and Indonesia that considers HVDC interconnections. We show direct electricity export to Indonesia could also benefit Australia’s domestic energy transition as any wind and solar energy not used by Indonesia could then be fed to the National Electricity Market. This could help both countries to achieve decarbonisation. Overall, our modelling suggests that a long-term system-wide perspective and orientation are critical for the design of energy policies that can secure broad energy system benefits. If planned well, Australia could become a major supplier of green hydrogen, direct renewable electricity and energy-intensive products as energy vectors in the global market. Particularly, this could be driven and strengthened by a robust commitment to “net-zero by 2050”.
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    Further development and application of the speleothem U-Pb geochronometer
    Engel, John Richard ( 2020)
    Speleothems are valuable archives recording information on past climates and processes of landscape evolution. In fact, the high preservation potential of speleothems and their ubiquity across the globe are key attributes and speleothems are now extensively used in palaeoclimate modelling as complements to the more traditional marine and ice core climate records. The real ’engine’ of speleothems science, however, lies in the ability to reliably and robustly place these archives into chronological context. Historically this has been facilitated by the highly successful U-Th geochronometer, which can accurately date speleothems that formed within the last 650 thousand years. However, many speleothems – and by extension their proxy archives – are far older than this 650 ka limit and so require a significantly longer-lived radiometric methodology: the U-Pb geochronometer has recently filled this void. The U-Pb geochronometer is well known in the geological sciences, but its application to the analytically-challenging medium of speleothems has been relatively recent. Modern mass spectrometry methods now allow the accurate and precise analysis of the extremely low-levels of U and Pb within speleothems, thus providing access to such ’deep-time’ (i.e. multi-million years and further) palaeoclimatic and landscape evolution archives. Despite these significant advances, there remain fundamental impediments to the whole-scale production of speleothem U-Pb ages. This thesis investigates some of these shortfalls with the ultimate aim of advancing the speleothem U-Pb methodology and application such that future speleothem-based studies may more rapidly and more reliably place their reconstruction models into a more robust chronology. The first half of this thesis addresses two fundamental limitations of the speleothem U-Pb method: the large expenditure of time and effort required for producing calcite U-Pb ages, and the potential limitations in the correction for initial isotopic disequilibrium effects in the U-Pb decay chain. To address the former, I provide new software for the rapid production of publication-quality U-Pb isochron figures and ages. Additionally, a new chemical separation (i.e. chromatography) procedure for generating high-purity U and Pb fractions is developed. This new ’stacked resin’ protocol can produce mass spectrometry-ready U and Pb sample aliquots in a single working day – roughly half the time of the previous method – and so accelerates the acquisition of U-Pb ages. In terms of subsequent data deconvolution, the largest hurdle to obtaining accurate speleothem U-Pb ages arguably relates to the uncertainty added from having to estimate an initial isotopic disequilibrium value after the system has returned to equilibrium. As a potential remedy to this problem, this research program develops a framework in which to calculate appropriate disequilibria values for the production of disequilibrium-corrected U-Pb ages. The second part of this thesis is designed to highlight new applications of the calcite U-Pb chronometer made possible by the methodological advances documented in the first half. This research program investigates the utility of speleothems as proxies for regional uplift rates in karst terranes. This is accomplished by producing chronologies for 120 speleothems from the Buchan karst along southeast Australia’s passive margin. The results of this study indicate that SE Australia experienced a renewed period of uplift occurring at a maximum rate of 76 +/- 7 m / Ma beginning at least 3.5 million years ago. This speleothem-derived uplift rate is consistent with other independent regional uplift estimates thus adding credibility to the utility of speleothems as proxies for uplift. The relatively straight-forward method developed here is likely applicable in many future karst studies. Additionally, this research program also investigated the potential applicability of U-Pb geochronology to another form of carbonate archive – stromatolites. The methodological advances described in this thesis were applied to stromatolites from Lake Turkana, Kenya, and the palaeo-lake Bungunnia in southern Australia. The results of this experiment suggest that stromatolites contain too much inherited Pb that simply overwhelms the radiogenic ’age signal’ and so are unlikely to be successful candidates for U-Pb dating, at least for stromatolites formed during much of the Cenozoic period.
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    The structure, rheology, and rupture mechanics of seismogenic faulting in continental lithosphere
    Yang, Haibin ( 2020)
    This thesis explores relationships between the structure, rheology, and rupture properties of seismogenic faulting in continental lithosphere from four different perspectives. First, I derive a scaling relationship that links the spacing between two nearly parallel strike-slip faults to the frictional strength, fault width, and lower crust viscosity for strike-slip shear zones. Based on the scaling law, I estimate a possible range of lower crust viscosity in the San Andreas Fault system (California), the Marlborough Fault Zone (New Zealand) and central Tibet (China). Second, using numerical modelling methods, I explore how lower crust rheology contrast may affect the long-term evolution of a major plate boundary fault. The case study is based on the San Andreas Fault, which is found to vary dipping angles (~50-90 degree) along strike. The moderately dipping strike-slip fault is not consistent with Anderson faulting theory. This inconsistency may be reconciled if there are lateral variations in the lower crustal rheology across the fault plane that decrease fault dip with time. Third, in addition to strike-slip faults in active tectonic settings, my study also includes reverse faults in stable cratonic regions, especially for the cratonic areas of western and southern Australia. I apply statistical methods to investigate the co-seismic slip distribution of 11 surface-rupturing earthquakes in Australian stable regions and provide a link between the shape and characteristics of co-seismic slip distributions and the geophysical properties of the host crust. Fourth, using a comprehensive geophysical survey with the co-located 13GA-EG1 and 12GA-AF3 seismic reflection profiles and magnetotelluric profiles and regional gravity and magnetic maps in the Nullarbor Plain (Australia), I find that faults initiated back to the Proterozoic could still be reactivated in a Cenozoic convergent setting, especially for those major faults cutting to deep crust. Those deep-penetrating faults at terrane boundaries could be a potential channel for fluids to pass through, and thus further weakened by the fluids, which is revealed by high-conductivity anomaly in magnetotelluric profiles. The last research chapter of this thesis addresses a technical issue in the particle-in-cell finite element method, which is widely used in geodynamic numerical modelling. As mixing materials with contrasting viscosity within one element results in stress fluctuations, I assess different smoothing methods to reduce the spurious stress in mixed-material elements.
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    A taxonomic revision of the tertiary echinoid genus monostychia
    Sadler, Tony ( 2020)
    For over 100 years the genus Monostychia (Echinoidea: Clypeasteroida) and its type species M. australis Laube, 1869 have been a taxonomic home for a wide range of genera and species with the commonality of a rounded to pentagonal, discoidal test and a submarginal periproct. The specimens comprising this group are all extinct and from the Tertiary strata of southern Australia. While there have been a few minor species identified beyond M. australis, notably M. etheridgei Woods, 1877 and P. loveni (Duncan, 1877), it has been clear to many researchers that the variability remaining in M. australis was representative of numerous other taxa awaiting discovery. Recent taxonomic works on the Clypeasteroida suggested that the number of interambulacral plates on the oral surface of the test of some species was a useful diagnostic character. Of interest were the plates that first come into contact with the periproct. However, there appeared little evidence in the literature that it had been established that the number of such plates remained constant with test length and age, or that the variability in each taxon, of those plate numbers, has been determined. Without understanding those two issues the utility of plate numbers was questionable. This study set out to resolve some of those issues for Monostychia and its relatives. It was found that the number of interambulacral and ambulacral plates on the oral surface was fixed and did not change with increasing test length and therefore there was potential utility for plate numbers as a taxonomic tool. However, there was substantial variability in the numbers. As a result, the use of plate numbers in the paired interambulacra, paired ambulacra, and ambulacrum III on the oral surface appears to have limited utility at genus level. At the species level, however, such numbers can be quite useful diagnostically, particularly when paired interambulacral, paired ambulacral and ambulacrum III plate numbers are used in combination. The plates that first come into contact with the periproct was shown to have little value taxonomically at the genus or species level within the monostychioids, largely because most species had the same plate number dominating, but also because of the variability. At subfamily level the taxonomic value of this feature is yet to be established. A previously unreported structural feature was identified in many of the specimens. This was a thin circumferential wall of stereom present on the right-hand side of the test, lying half way between the marginal and central buttressing. It was a form of intestinal buttressing referred to hereafter as the intermurum. Its presence enabled the establishment of a new subfamily, Monostychinae in the family Arachnoididae. Four genera have been placed in the Monostychinae; Monostychia Laube, 1869, Quinquestychia gen. nov., Rotundastychia gen. nov. and Deltoidstychia gen. nov. A key to these genera is provided. Several species have been established and others redescribed in this study. In Monostychia there are seven species; M. australis Laube, 1869, M. macnamarai Sadler et al. 2017, M. alanrixi Sadler et al. 2017, M. merrimanensis Sadler et al. 2019, M. etheridgei Woods, 1877, M. glenelgensis Sadler et al. 2019 and M. robheathi sp. nov. A new genus, Quinquestychia gen.nov., has also been established. It contains four species: Q. mannumensis sp. nov., Q. gigas sp. nov., Q. berylmorrisae sp. nov. and Q. robertirwini (Sadler et al. 2017). The last of these species was published as a Monostychia earlier in this study but reassigned later on the basis of further data. A second new genus, Rotundastychia gen. nov., has also been established. It contains three species: R. pledgei sp. nov., R. aquilaensis sp. nov. and R. eyriei sp. nov. A third new genus, Deltoidstychia gen. nov., has also been established. It currently contains a single species, D. erioaster sp. nov. In addition to the above, two other new genera were established but they do not belong in the subfamily Monostychinae. Instead they are tentatively placed in the subfamily Ammotrophinae. The first of these is Obscurostychia gen. nov. with two new species: O. spirographica sp. nov. and O. curtus sp. nov. Keys to all the genera discussed above that contain more than one species have been provided.
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    Radiocarbon dating of Kimberly rock art
    Finch, Damien Geoffrey ( 2020)
    Throughout the world, ancient rock art records some of the earliest attempts at complex human communication. However, constraining the age of older rock art has remained a largely intractable scientific problem thereby limiting our ability to integrate rock art into the rest of the archaeological record. Researchers have studied the globally significant Aboriginal rock art in the Kimberley region of Western Australia for more than 40 years and have comprehensively documented a sequence of rock art stylistic periods. It has long been thought that the oldest styles in this sequence date back to the Pleistocene but only two such dates, relating to identifiable motifs, have been published and both are problematic. The surviving pigment in paintings of all but the most recent Kimberley rock art style contains no material that can be radiometrically dated. There are, however, mineral accretions and mud wasp nests in the same Kimberley rock shelters that house rock art and, occasionally, these under or overlie paintings. This study explores the development of radiocarbon dating techniques to reliably date remnant mud wasp nests found to be in contact with rock art. Recently constructed mud wasp nests were collected and analysed to understand the source and age of carbon-bearing material they contain. Unburned plant material and charcoal were found in similar volumes, but charcoal is the carbon-bearing constituent most likely to provide a reliable radiocarbon age for old nests. Old wasp nests were analysed using a wide range of techniques to determine how taphonomic processes alter their physical and chemical composition. These results guided experimentation with pretreatment methods designed to remove sources of carbon contamination while preserving as much of the carbon in the original charcoal as possible. A total of 120 old mud wasp nests were prepared for radiocarbon dating of which 75 contained sufficient carbon for measurement. The distribution of the 75 ages indicated nests were built quasi-continuously over, at least, the last 20,000 years. The motifs in contact with the 75 nests were classified into one of the six main Kimberley rock art stylistic periods by two subject matter experts. Just 3 nests overlay motifs from each of the Cupules and Wanjina periods suggesting only that some motifs in these styles are older than 7,200 years and 500 years, respectively. The 4 dates available for each of the Static Polychrome and Painted Hand periods permit a very tentative hypothesis for their chronology while the 16 dates relating to Irregular Infill Animal Period (IIAP) motifs and the 20 dates for Gwion motifs provide a more secure estimate. The concise hypothesis proposed for the chronology of the Kimberley rock art styles is that the IIAP style was in use from at least 17,000 to 13,000 years ago. It was followed by the Gwion period from 13,000 – 12, 000 years ago and then the Static Polychrome period 11,000 to 9,000 years ago. The Painted Hand period followed at around 8,500 to 9,000 years ago.
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    Sub-meter scale lithological heterogeneity and its influence on the CO2 trapping capacity in CO2 storage reservoirs
    Mishra, Achyut ( 2020)
    Lithological heterogeneity at mm- to cm-scale exists in the form of sedimentary structures such as cross bedding and planar bedding in heterogeneous siliciclastic reservoirs. At this scale, sedimentary structures typically consist of two lithologies with significant differences in their porosity, permeability, capillary entry pressure and mineral composition. The lithology with high porosity, high permeability, low capillary entry pressure and a high abundance of quartz and feldspar characterizes the reservoir rock and is conducive to fluid flow. On the other hand, the lithology with low porosity, low permeability, high capillary entry pressure and a high abundance of clay and carbonate minerals characterizes the intraformational baffle and acts as flow barrier. The presence of both lithologies at mm- to cm-scale govern local fluid flow and geochemical reactions. This is especially important in CO2 storage reservoirs where CO2 flow and mineralization might be significantly affected. Hence, it is necessary to account for lithological heterogeneity at sub-meter scale in regional scale dynamic simulations. However, intraformational baffles are typically neglected in static reservoir models primarily because small scale intraformational baffles are not commonly resolved by wireline logs. Therefore, their lithological properties are not accounted for in reservoir models. This study addresses the characterization and incorporation of intraformational baffles in reservoir models so that the reservoir scale estimation of carbon mineral trapping capacity can be improved. Firstly, rocks from the CO2CRC’s Otway Research Facility were characterised using a range of sample and data analysis methods. The variability in porosity, permeability, capillary entry pressure, grain size and mineral composition was used to classify five homogeneous rock type classes: coarse sandstone, fine sandstone, siltstone, mudstone and carbonate-cemented sandstone. A new workflow was developed where homogeneous rock types and composite rock types consisting of two lithologies characteristic of intraformational baffles were represented. Rock properties for composite rock types were derived so that mm-scale heterogeneity can be upscaled for integration in coarser discretised reservoir models. Lithotype logs of composite rock types were derived and used to build a high resolution 2D static reservoir model of the site. Multiphase fluid flow and reactive transport simulations were run on 120 and 8 realizations, respectively, to develop a detailed understanding of CO2 flow and geochemical reactions at mm-scale. It was found that the capillary entry pressure characteristic of the reservoir rock significantly governed the range of turning point saturations while the amount of clay and carbonate content of the baffle governed the amount of carbon mineralized. Reactive transport simulations were also run on two 2D reservoir models to quantify the error associated with the estimation of carbon mineral trapping at reservoir scales if intraformational baffles are not accounted for in dynamic simulations. The results show that the neglection of intraformational baffles in reservoir rocks might lead to an underestimation of mineral trapping capacity by nearly 2.5 times.