School of Earth Sciences - Theses

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The Variation of atmospheric carbon dioxide,methane and nitrous oxide during the holocene from ice core analysis

MacFarling Meure, Cecelia. (University of Melbourne, 2004)

Recent studies have demonstrated that the atmospheric concentrations of radiatively important greenhouse gases, including methane (CH4), carbon dioxide (CO2), nitrous oxide (N2O) and carbon monoxide (CO), have significantly increased during the past 200 years due to anthropogenic emissions. Analysis of air trapped in polar ice cores allows for past atmospheric variations due to natural climate conditions to be investigated, placing recent changes in a historical context. In this thesis new high- precision, multispecies measurements of atmospheric trace gas concentrations during the Holocene have been produced by analysing the air trapped in the ice at Law Dome, East Antarctica (66�46'08"E, 112�48�28�S). The ice core records are well-dated, have high age resolution and overlap with modem instrumental records due to the high accumulation rate at the drilling sites. The combination of high age resolution, precise dating and high precision measurements allows for subtle, decadal-scale variability to be detected. The multispecies measurement technique allows for biogeochemical causes of variations to be identified. The first part of this study focused on the late Holocene period (AD 0 to 1975). New high-precision records of CH4, CO2, N2O and CO have been produced for this period. The CH4 and CO2 measurements are used to build upon the existing Law Dome records of these gases during the last 1000 years, to validate and further define previously observed variations. The new measurements extend the records of these gases by another 1000 years. As a consequence of the multispecies measurement technique it has been possible to also measure N2O and CO during this period. These new measurements highlight the atmospheric response to the Little Ice Age (LIA) cooling (AD 1550 to 1800), particularly a 10 ppm decrease in atmospheric C02 between AD 1550 and 1600. A stabilization of CO2 during the 1940s was also confirmed in the Law Dome record. Increased data density during this period shows that the atmospheric CO2 mixing ratio stabilized at ~310 ppm between 1937 and 1955. New signals were observed in the extended records, including a 100 ppb increase in the CH4 concentration between AD 0 and 1800, which is probably the result of increasing pre-industrial anthropogenic emissions. The second part of this study focussed on the CO2 and CH4 response to a rapid, abrupt cooling at 8,200 years BP. The Law Dome (DSS) measurements are complemented by four measurements of NorthGRIP (Greenland) ice core. A decrease of at least 52 ppb CH4 is observed in the DSS record, and a decrease of at least 62 ppb is observed at NorthGRIP during the same period. A smaller CO2 response of 4 to 5 ppm is seen in both the records. The CH4 signal is used to improve the chronologies of these ice cores by synchronising with other well-dated CH4 records, specifically GRIP (Greenland) and Dome C (Antarctica).
The geology, petrology and geochemistry of the granitic rocks of Victoria

Rossiter, Allan G. (University of Melbourne, 1973)

In the field the granitic rocks of Victoria show a wide variety of characteristics but chemically all are related and are the result of similar processes that occurred cyclically during the Palaeozoic. The granites may be divided into two groups on the basis of whether or not primary hornblende is present in the more basic members of the suite. Rocks of the hornblende-free series concentrate mainly in central Victoria and usually contain biotite that is red-brown in colour. In the granites of the hornblende-bearing group green-brown biotite is generally present. The two different colours appear to indicate unlike oxygen fugacities in the magmas from which the micas crystallized. The rocks of the hornblende-bearing and hornblende-free provinces also vary slightly in chemistry - the most important difference being that the basic members of the first group contain slightly more Ca than those of the second having comparable silica content. The compositions of the Victorian granites is consistent with their derivation by anatexis of a mixture of basic igneous rocks and sedimentary material. The conclusion is drawn that a combination of Cambrian basalts and dolerites and Cambro-Ordovician sediments constitute the source. The explanation of the fact that two chemical trends are observed in the granites may lie in the possible existence of two chemically distinct groups in the Cambrian igneous rocks - the one having lower Ca predominating under central Victoria.
Petrogenesis of the Melba Flats Ni-Cu-PGE Deposit in Western Tasmania: Insights from a Geochemical and Geochronological Investigation

Phua, Marcus ( 2016)

Since its discovery in 1893, the Melba Flats Ni-Cu-PGE deposit has produced 10,000 tons of Ni and Cu at an average grade of 9.7% and 4.7% respectively. It is a magmatic sulphide deposit located 8 km north-east of the township of Zeehan, along the eastern margin of the Dundas Trough in Western Tasmania. The deposit is associated with a suite of bifurcating mafic intrusions hosting magmatic Ni-Cu-PGE sulphides intruded into a sequence of volcaniclastic lithic greywackes, which are correlated to the Crimson Creek Formation. U-Pb detrital zircon geochronology was utilized to show that the Melba Flats sediments have a maximum depositional age of c. 582 Ma. The Melba Flats mafic intrusions were formed by primitive magmas with 13 to 16 wt% MgO and a sub-alkaline tholeiitic affinity. 40*Ar/39Ar hornblende geochronology was employed to establish that the mafic intrusions were emplaced at c. 568 Ma, along an attenuated continental margin characterized by a transitional rift setting, analogous to the early Paleogene break-up margin of East Greenland. Melba Flats Ni-Cu-PGE sulphides are characterized by massive-to-semi-massive sulphides that possess high Ni, Cu and PGE tenors and mantle-like δ34S values and S/Se ratios and disseminated sulphides that have low Ni, Cu and PGE tenors, along with crustal δ34S values and S/Se ratios. Geochemical data indicates that the massive-to-semi-massive sulphides were formed at depth before being transported to their current sites, whilst the disseminated sulphides were formed during transport as the primitive magma interacted with the S-bearing crustal rocks.
Nonlinear, statistical and stochastic dynamics of geophysical fluids with application to state estimation and prediction

O'Kane, Terence John ( 2021)

Many important results underpinning our understanding of variations in the climate have been achieved by linearizing the deterministic equations of motions or through Markovian and Gaussian assumptions on the statistical representation of the systems probability distribution. With the accrual of several decades of ocean and weather observations of sufficient spatial density and homogeneity to resolve the climate modes of variability, it has become evident that the climate system is highly nonlinear, non-stationary and non-Gaussian with transitions between persistent or metastable regimes ubiquitous across all scales. As a result, our ability to make skilful predictions of the state of the various weather systems and climate teleconnections and their causal relationships requires a foundational understanding of the dynamics and life cycles of instability processes that determine predictability. While state of the art operational numerical weather prediction systems now routinely apply aspects of mathematical methods from dynamical systems to characterise the impact of small-scale growing disturbances (error modes) on the predictability of the large scale quasi-stationary structures of the synoptic weather, these approaches are largely empirical. Ideally, a modern mathematical representation of the dynamics of the Earth’s weather and climate must necessarily be cast in terms of a nonlinear, stochastic and multiscale system. However, the sheer complexity and dimensionality of the problem makes development of a foundational theoretical basis for climate variability and predictability a distinct challenge. With the development of novel data driven stochastic optimization methods, high dimensional weather and climate data can now be reduced to low dimensional systems of stochastic differential equations that retain the essential dynamics but make tractable the extraction of regime behaviours of persistent states in systems where the signal to noise ratio is low. The body of work contained in this thesis represents a sustained effort to advance a deeper understanding of the role of nonlinear processes in geophysical fluids with specific application to the development and analysis of weather, ocean and climate prediction systems. Chapter 1 develops the statistical mechanics and dynamics of inhomogeneous turbulent flows with application to modelling unresolved (subgrid) processes associated with the interactions between eddies and topography, and methods of ensemble numerical weather prediction and data assimilation. Mathematically, this is the foundational problem of strong interactions across scales in systems with a quadratic nonlinearity. In essence, one is required to find tractable and accurate representation of an infinite hierarchy of moment equations. The work presented in this chapter develops theoretical approaches borrowed from modern physics and novel computational methods that achieve this goal. This is one of the most technically and intellectually challenging problems in mathematical physics manifesting in areas as diverse as quantum electro- and chromo-dynamics and plasma physics. The theory is validated against ensemble direct numerical simulations of the primitive equations and is shown to enable deep insights into a range of problems of direct societal relevance and in particular weather prediction and state estimation. Chapter 2 describes the development of weather (p228), mesoscale ocean (p242), cyclone (p258) and climate (p296-399) prediction systems. The ensemble weather prediction system described on pages 228-241 forms the basis of the current operational system at the Australian Bureau of Meteorology. The Climate Analysis Forecast Ensemble (CAFE) system described on pages 296-399 is currently operational at CSIRO and is the first system developed in the Southern Hemisphere to contribute to the official World Meteorological Organization near term climate predictions. These systems implement state of the art data assimilation methods to constrain models to observations to generate initial forecast conditions. For the climate, this requires observations of the atmosphere, ocean, land and sea ice to be assimilated and a large ensemble (order of 100) of model forecast simulations employed to generate covariances between the respective domains. The insights gained from statistical dynamics provides additional insights to tackling these challenges. A severe challenge to the analysis of the aforementioned ensemble prediction systems is the dimensionality of the state estimation and forecast data. To underpin the development of such systems, a deeper understanding of the processes and timescales whereby long-term predictive skill resides in the climate system is required. This is a massive challenge given the short observational record in the ocean and large ensembles of initialized forecasts necessary to identify predictive skill. Despite the huge data sets generated, the number of data instances is far exceeded by the dimensionality of the problem, hence the challenge is a classically “small data” issue. The work in Chapter 3 describes the development and application of novel advanced methods from across applied mathematics (dynamical systems, optimization, numerical methods) applied to better interrogate both model and observational data by extracting, not simply correlations between the respective climate modes of variability, but their (conditionally) causal relationships, including the anthropogenic factors that drive their changing relationships and regime transitions over time. This work reveals a detailed understanding of the abrupt climate regime shift that occurred in the Southern Hemisphere climate during the late 1970s. This change in the large-scale circulation precipitated the prolonged periods of reduced rainfall and heat extremes experienced in recent decades over the Australian continent and changes in the statistics of persistent synoptic scale weather patterns. Low frequency variability in the climate system occurs principally via the coupling of the ocean to the atmosphere. The work in Chapter 4 is primarily concerned with understanding the ocean’s response to the constituent components of the atmospheric forcing. Specifically, this work shows how information is communicated via coherent waves in internal oceanic pathways that are analogous to the storm tracks of the atmosphere. The specific role of baroclinically unstable Rossby waves and density compensated salinity anomalies and the mechanisms by which they are excited are examined revealing the importance of coherence resonance processes whereby the synoptic scale atmosphere excites coherent oceanic disturbances on inter-annual timescales. In the final Chapter 5, a range of approaches to deriving reduced order models of the climate modes of variability are presented. Dynamic Bayesian Networks and directed graph methods are employed to quantify causal relationships between the major climate teleconnections in reanalysis data. A minimal skeleton model specific to the Madden Julian Oscillation is identified based on coherences between Kelvin and Rossby waves from a normal mode decomposition of observations. Finally, stochastic linear inverse models are applied to construct reduced order models of the tropical-extra tropical South Pacific Ocean with the synoptic scale Pacific South American (PSA) mode of tropospheric variability identified as the key stochastic forcing.
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.
Marine Carbonates as Paleoredox Proxies: Links between Ocean Redox and Key Neoproterozoic and Paleozoic Evolutionary Events

Shuster, Alice Mary ( 2021)

The emergence of complex life on Earth is associated with a long-term rise in oxygen concentrations in Earth’s atmosphere and oceans. Fluctuations of surficial oxygen levels are suggested to have played a large role both in evolutionary diversification and in mass extinctions. It has become increasingly evident that the mid-Palaeozoic was not as oxic as previously thought, and it was not until the Late Palaeozoic or even Mesozoic that Earth’s surface systems reached present oxygen levels. This thesis has utilised marine carbonates to develop complexity in our understanding of ocean oxygenation, marine conditions and the evolution of life in the Late Neoproterozoic and early Phanerozoic. Marine chemical conditions may be reflected in the record of marine carbonate mineralogy. The reason for the abundance of dolomite in Earth's early geological record compared to modern environments remains contentious, but could be linked to seawater composition. This study provides new insight into this Precambrian “dolomite problem” by revisiting one of the most controversial dolomite localities, the Beck Spring Dolomite, of Death Valley, USA. Consistent with some previous studies, petrographic evidence indicates that although the Beck Spring Dolomite now consists almost entirely of dolomite, it was originally precipitated largely as aragonite and high-Mg calcite. Depositional constituents (microbialites and ooids) were likely originally aragonitic, and early marine length-fast cements (now dolomite) are suggested to have precipitated as high-Mg calcite then replaced syntaxially by dolomite. Based on petrographic and geochemical evidence, marine dolomitization was the dominant synsedimentary diagenetic process in the unit, and for the most part, involved syntaxial and mimetic replacement. A length-slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement. The trace metal geochemical composition of well-preserved marine components, especially dolomite marine cements, reveals information about redox conditions in this Tonian shallow seawater. In terms of rare earth element geochemistry, the Beck Spring Dolomite has no significant Ce anomaly, and a ubiquitous positive Eu anomaly, consistent with widespread oceanic anoxia during deposition. Furthermore, the relatively low levels of iron and chalcophile elements ii Marine Carbonates as Paleoredox Proxies: Links Between Ocean Redox and Key Neoproterozoic and Palaeozoic Evolutionary Events Co, Cu, Pb and Zn in marine components compared to other Neoproterozoic carbonates suggest euxinic conditions (both anoxic and sulphidic) prevailed during deposition of the Beck Spring Dolomite. This petrographic and geochemical evidence suggests ocean anoxia plays a significant role in promoting marine dolomite precipitation. Methods established in the Beck Spring Dolomite case study were utilised to explore Paleozoic ocean redox. Rare earth and trace element compositions of well-preserved marine cements from the Mid-Late Devonian Canning Basin Reef Complexes record three intervals of marine anoxia, with Ce anomalies ranging from present day oceanic values to almost a negligible anomaly over an approximately 15 My period (from Late Givetian to Middle Famennian), linked to episodes of the drawn-out Late Devonian Mass Extinction. Fluctuating, shallow-water redox conditions during the Late Devonian in the Canning Basin could be related to the interaction of two distinct water masses: a shallow, oxygenated water body, and a deep, anoxic (possibly ferruginous and/or euxinic) water mass. Related to the unstable marine redox and mass extinction events in the Late Devonian, the Canning Basin presents a diverse range of stromatolites from the mid- Frasnian until the mid-Famennian including deep-water varieties and the Fe microstromatolite Frutexites. Petrographic analysis shows that these deep-water stromatolites consist partly of organisms which actively precipitated Fe oxides. SEM microscopy reveals a variety of Fe-oxide-rich skeletal structures, including sheaths, elongated gelatinous crusts, aggulated spheroids and Fe-rich walls and tubes which make up Frutexites. Non-skeletal laminated varieties consist predominantly of microbially precipitated micrite and spar but also have Fe-oxide rich laminae, possibly mediated by another Fe-precipitating bacterium or underdeveloped Frutexites. It is likely that these organisms precipitated Fe-oxide and trapped minor detrital phases possibly as a result of an iron-oxidising metabolism. Trace metal and rare earth element work in marine cements shows that anoxic and possibly ferruginous deep waters upwelled onto the Lennard Shelf during the same interval as the deep water stromatolite development, suggesting ocean anoxia as the source of the iron.
Stratigraphic and structural evolution of the Gippsland Basin, Late Cretaceous to Miocene, Australia

Mahon, Elizabeth ( 2021)

Deposition in the Gippsland Basin from the Late Cretaceous to the Miocene was characterised by extensive, wave-dominated shorelines in front of lower coastal plain peatlands. Using depositional architecture evident on seismic data, these deposits have been interpreted to consist of 23 discrete packages. Shoreface morphology ranges from progradational beaches to large, aggradational beach-barriers. While some of these beach-barrier-coastal plain units are progradational, on a multi-million-year timescale they retrograde. Transgression occurred from the Late Cretaceous to the Oligocene, with the main driver for this transgression likely basin subsidence. Despite large changes in paleoclimate, basin tectonics and ocean chemistry, the depositional style remains remarkably consistent. The Gippsland Basin experienced compressional tectonics which resulted in large anticlines forming across the basin. The timing of onset of compressional tectonics in the basin has been revised based on measurements of syn-tectonic sediment thickness changes across structures. These measurements indicate extensional growth faulting was occurring from the Late Cretaceous until the Late Eocene, and compressional structures did not begin growing until the Eocene-Oligocene transition. This research has brought the previously interpreted date for the onset of compressional tectonism in the Gippsland Basin forward approximately 10-20 Ma, from the previously interpreted early to mid-Eocene, to the Eocene-Oligocene transition. From the Palaeocene-Eocene transition to the Eocene-Oligocene transition a series of large channels incised into the top Latrobe Group - the Tuna and Marlin Channels. These channels have previously been interpreted as forming via fluvial processes associated with tectonic uplift. However, this research has shown that tectonic uplift occurred after channel incision, indicating uplift did not contribute to channel down- cutting. Additionally, well data reveals a marine origin for channel fill sediments, and seismic data indicates channels are located seaward of coeval palaeoshorelines. This suggests these channels formed in a submarine environment, with the close proximity of channel heads to the shorelines indicating they were shelf-incising. A second pulse of compressional tectonics occurred in the mid Miocene, which measurements across structures indicate primarily affected the present-day onshore area. This episode of tectonic uplift corresponds to the previously documented unconformity at ~10 Ma. From the early Miocene onwards, shorelines become progradational and regressive. This is interpreted to be the result of compressional tectonics and global icehouse conditions.
Novel proxies study of long-term climate variability and karst evolution using speleothems from southern Australia

Weij, Rieneke Petronella ( 2021)

This research program aimed to produce robust and reliable speleothem chronologies for the Naracoorte Caves in South Australia which could then be used in studies of palaeoclimate and karst evolution. The thesis is structured around three sub-themes and the main findings of each are outlined below. The first component evaluates the utility of speleothem age frequency distributions for palaeoclimate assessment. Age distributions of this type can be used as proxies for past climate change, where the peaks in age density are linked to a controlling climatic parameter (e.g., temperature or precipitation). There remain, however, considerable gaps in our understanding of how best to use speleothem age frequency distributions in this way. To address these issues, a synthetic age dataset was modelled by randomly generating U-Th ages based on a known climatic forcing, followed by sampling from this dataset under varying conditions. The model shows that periodic Quaternary climate fluctuations can be recovered from age frequency distributions with a minimum sample size of 120–150 radiometric ages. This study provides a much-needed statistical framework for the use of age frequency distributions relevant to speleothem palaeoclimate studies — and one which is also beneficial for the radiocarbon and zircon dating communities. The second theme concerns the antiquity of the Naracoorte Cave Complex (NCC) in southern Australia. Caves are unique archives of past environmental and climatic conditions and may also act as important fossil repositories, as is the case for the World Heritage listed NCC. In these circumstances, understanding the timing of initial cave development and opening can shed light on the potential antiquity of the fossil deposits (and thus guide excavation), but these geomorphological processes remain challenging to constrain. This study places robust temporal constraints on the onset of cave and entrance development of the NCC by utilising an extensive campaign of U-Th and U-Pb dating of speleothems. Additionally, speleothem charcoal and pollen concentrations were used as novel indicators of cave openness. The key finding is that caves can be twice as old as their surface expression. These techniques provide important new tools for a range of disciplines interested in the timing and extent of cave opening, e.g., palaeontology, palaeoanthropology and archaeology. The final theme concerns the palaeoclimatic history of semi-arid southern Australia. Changes in the hydroclimate during the Quaternary remain poorly constrained for the southern Australian semi-arid subtropics. In this study, changes in southern Australia’s hydroclimate were reconstructed for the Late Quaternary using an age frequency distribution and pollen-based climate reconstruction from U-Th dated speleothems. This study represents the largest geochronological dataset from a single cave province in the Southern Hemisphere. Collectively, these reconstructions demonstrate orbitally-paced speleothem growth within the 100-ka period linked to changes in moisture availability that consistently lag maximum interglacial temperatures by ~25 ka over the last three glacial-interglacial cycles. The results show that times of highest moisture availability occurred during parts of the glacials, rather than interglacials periods, which implies that, in the semi-arid subtropics, temperature and moisture availability were strongly decoupled.
Recent Trajectories in Urban Emissions from Remote Sensing Proxies

Luqman, Muhammad ( 2021)

The United Nations Framework Convention on Climate Change (UNFCCC) motivates monitoring fossil fuel emission variations. This thesis studies the trends in urban fossil fuel emissions of Carbon Dioxide (CO2) and their drivers by developing an algorithm for delineating urban boundaries to generate masks for population and CO2 emissions and applies bottom-up techniques to approximate on-road emissions. The algorithm named BUNTUS (Built-up, Nighttime Lights, and Travel time for Urban Size) incorporates three remote sensing datasets; built-up area extracted from Landsat imagery, nighttime lights datasets, and travel time rasters. These three datasets are combined through a ruleset to produce an urban boundary. BUNTUS made 125 city boundaries, out of which 91 cities are studied. There are two opinions about the impact of urbanization on fossil fuel emissions intensification. First, some scientists consider that developing cities produce more fossil fuel emissions due to the migration of the rural population into cities. On contrary, the second opinion says that once a city develops economically, it becomes a low-emissions city. To test the role of urbanization in emissions intensification, this study quantified the urban emissions of 91 cities by taking ODIAC (Open-source Data Inventory for Anthropogenic CO2) as CO2 emission proxies and LandScan’s raster for population count. We find that urban CO2 emissions are increasing everywhere but that the dominant drivers differ according to development. Urban emissions are increasing along with urban populations, but per capita emissions are often growing more slowly than their national counterparts. A cluster analysis of factors shows that developing countries are dominated by cities with the rapid area and per capita CO2 emissions increases. Cities in the developed world, by contrast, show slow area and per capita CO2 emissions growth. The thesis also updates an existing global CO2 emissions inventory (FFDAS), improving its resolution and sectoral detail. FFDAS employs a data assimilation approach and yields globally gridded datasets of CO2 emissions. This study can serve as a foundation to study emissions trends in cities and on-road emissions inventory development.
Exploring the source attribution of environmental Pb – development of a continental-scale Pb isotope regolith map of Australia

Desem, Candan Ulgen ( 2021)

The element Pb is a particularly useful tracer as it is both a toxin of environmental concern and is often associated with mineralisation. As a result, Pb concentration data are often collected as part of environmental contamination and mineral exploration studies. Pb isotopic analysis, however, offers an important advantage in discriminating between different sources of Pb. The Pb-isotopic signature of regolith is a function of contributions from bedrock geology (including mineralisation) and exogenous inputs such as windblown dust, groundwater and anthropogenic contamination (industry, mine wastes, paints, transport, agriculture, waste handling and residential inputs). Pb-isotopic analysis allows us to estimate the relative contributions from each of these components and therefore provides an important tracing tool in both mineral exploration and environmental contamination studies. The spatial distribution of Pb in the regolith profile is also a function of the source of Pb (exogenous versus endogenous), depth and grain-size providing further diagnostic tools. Finally, Pb isotopic signatures in the continental crust show large natural variations which are transferred to the regolith and thus provide bedrock signatures that can be easily distinguished. Naturally occurring Pb has four stable isotopes with the following crustal abundances: 204Pb (1.36%), 206Pb (25.42%), 207Pb (21.11%) and 208Pb (52.10%). 206Pb is the decay product of 238U, 207Pb is the decay product of 235U and 208Pb is the decay product of 232Th. In contrast 204Pb is non-radiogenic and is often used as a stable reference isotope relative to which the accumulation of the other radiogenic isotopes over time can be quantified. Importantly, in mineralised systems, once an ore rich in Pb (e.g., galena, PbS) forms, the isotopic composition of its contained Pb does not evolve any further, as it does not contain significant amounts of U or Th. The Pb signature is thus ‘frozen in’ to the ore and will reflect the time at which the Pb was incorporated. As a consequence of these geological processes most ore deposits, and the products derived from them (leaded petrol, paint etc) have distinct Pb isotopic signatures that enable ‘fingerprinting’ of anthropogenic Pb contamination in the environment. In order to determine whether Pb-isotope signatures in the regolith are anomalous, an understanding of background Pb-isotopic signatures is critical. The use of large Pb-isotope datasets, however, has until recently been limited due to constraints associated with the availability and accessibility of lower cost, high-precision and high-throughput analyses. The advent of instrumentation such as Sector-Field Inductively Coupled Plasma Mass Spectrometers (SC-SF-ICP-MS) allowing rapid Pb-isotope analysis, now enables us to revisit the utility of Pb isotopes in this regard. Exploring the use of this new technology in the Australian context is a direct aim of this research. To this end, a continental-scale Pb isotope regolith map of Australia has been produced as part of this study, using catchment outlet (~ floodplain sediment) samples from Geoscience Australia’s National Geochemical Survey of Australia (NGSA) dataset. The dataset contains samples from 1300 locations across Australia obtained at a sampling frequency of approximately 1 sample/5200 km2 and ultimately covering ca. 81% of the continent. The Pb-isotope map is based on the coarse grain-size fraction (less than 2mm) of the top outlet sediment (0 – 10 cm depth) samples. Leaching techniques have been employed to separate loosely bound Pb (e.g. anthropogenic contamination) from Pb structurally bound in minerals (i.e. from underlying geology, mineralisation, or their weathering products). An analytical framework has also been established to achieve ‘fit-for-purpose’ data quality, with the elimination of the Pb-separation chemistry step required by conventional Multicollector-Inductively Coupled Plasma-Mass Spectrometer (MC-ICP-MS) or Thermal Ionisation Mass Spectrometer (TIMS) analysis combined with the high-throughput required in order to process large sample sets (> 1500 samples in this case). A sequential leach protocol (‘P618’) originally developed at CSIRO (Carr et al., 2011) has been utilised in which an ammonium acetate leach is followed by an aqua-regia (HNO3-HCl) digest. Pb isotope signatures were acquired using a Nu Instruments Attom single-collector sector-field ICP-MS (SC-SF-ICP-MS) with data quality comparisons also made with MC-ICP-MS and Quadrupole-ICP-MS, in which this research has demonstrated that both precision and accuracy achieved on a SC-SF-ICP-MS is greater than that achievable on a Quad-ICP-MS. The aims of this program were to (i) compare Pb signatures released using the different leaches, (ii) examine whether soil Pb isotope signatures can identify bedrock geology and metallogenic provinces as well as (iii) investigate the presence and extent of anthropogenic contamination across the continent. The data, in particular across older terranes, reveal a dominant contribution of Pb to the regolith from the underlying geology with major crustal elements well defined. A variety of exogenous Pb inputs from point sources of pollution are also apparent. The continent-wide Pb isotope regolith map of Australia demonstrates that the top coarse aqua-regia digested fraction of regolith samples in Australia, are dominated by Pb from the catchment bedrock geology. The influence and prominence of bedrock geologic Pb signatures are more easily visible in older terranes, such as the Archaean Yilgarn Craton and Proterozoic North Australia Craton of Australia where radiogenic signatures have had more time to establish. Apparently meaningful Pb-Pb isochrons on these top coarse fraction regolith samples demonstrate not only the data quality obtained from the Attom SC-SF-ICP-MS but also that the dominant bedrock geologic Pb-isotope signature is carried to shallow depths within the regolith profile in these areas. Visibly distinct Pb-isotope patterns which correlate with the major crustal elements and geological regions of Australia, further support the idea that the Pb-isotope signatures at shallow depths across Australia are governed by large-scale Pb reservoirs. At the smaller scale, variation in grain-size and depth of Pb-isotope signatures provide information as to how the Pb-isotopic signature varies within the regolith profile. Throughout this study, variation in Pb isotopic signatures with depth and grain-size have been found to be more pronounced where there is a distinct exogenous Pb input. In areas where a significant exogenous Pb input has not been identified (by means of an anomalous Pb isotopic signature), it appears there is some level of homogenisation of Pb-isotopic signatures within the regolith profile. Finally, this analytical program has revealed areas of anomalous Pb isotopic signatures in the regolith which would be useful targets for future investigation as the source of these signatures has not yet been determined.

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