School of Earth Sciences - Theses
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ItemThe Variation of atmospheric carbon dioxide,methane and nitrous oxide during the holocene from ice core analysis(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).
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ItemThe geology, petrology and geochemistry of the granitic rocks of Victoria(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.
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ItemPetrogenesis of the Melba Flats Ni-Cu-PGE Deposit in Western Tasmania: Insights from a Geochemical and Geochronological Investigation( 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.
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ItemNo Preview AvailableAntarctic sea ice and its interactions with high latitude weather and climate( 1998)Antarctic sea ice plays a major role in the earth system by greatly influencing the high latitude exchanges of heat, moisture and momentum between the ocean and atmosphere, as well as profoundly effecting the salt budget of the ocean, and thus the production of Antarctic Bottom Water, one of the driving mechanisms of worldwide oceanic circulation. With such considerable and far reaching impact, it is important to document its climatology, understand its variability and quantify its influence. Climatologies and trends of the Southern Ocean sea ice pack are presented using the most recent satellite observations available from the Defense Meteorological Program’s (DMSP) Special Sensor Microwave Imager (SSM/I). The analysis of these data show that Antarctic sea ice is highly variable in both time and space. Statistically significant increases in the sea ice extent, open water and ice areas have been determined from the SSM/I data for the 9 year period 1987 to 1996, a result which differs from the Scanning Multichannel Microwave Radiometer (SMMR) observations (1978-1987). The increasing trend in the SSM/I observations can be attributed to the large increases in sea ice observed in 1994-1995, as confirmed by an analysis of data from the ERS-1 satellite. The mean season length during these years has remained relatively unchanged. Regional trends, both in the sea ice concentration and in season length, showed vast spatial inhomogeneity. SSM/I data displayed increasing season length in the central Weddell Sea, Bellingshausen Sea and Balleny Islands regions, with decreasing length in the Amundsen Sea, eastern Ross Sea and in the coastal areas off Wilkes Land. Similar trends are observed in the seasonal sea ice concentration. In most cases, these trends are opposite to those observed in the SMMR data, which may be linked to the shift observed in the Amundsen Sea low after 1990. Comparisons with historical data would suggest that no large scale anomalous change has occurred in the Antarctic sea ice limits over the course of human observation. Furthermore, the degree of variability suggests great care is needed in interpreting large scale changes in sea ice conditions, and hence atmospheric or oceanic change, from locally observed anomalies. Case studies of the effect of individual cyclones upon the sea ice concentration show small but definite modification of the ice conditions. To further diagnose aspects of the thermodynamic and dynamic forcing upon the Antarctic pack, detailed analysis of the sea ice concentration variability has been conducted using spectral techniques, and the spectra have been compared to those of the European Centre for Medium Range Weather Forecasts (ECMWF) temperature and wind data. In all cases, and with the seasonal cycle removed, the sea ice concentration shows a bias towards longer timescales of variability than either the wind stress or surface air temperature. This “red shift” in its frequency spectrum is strongest with the wind stress, and weakest with the temperature. For longer period waves, this may be due to the formation of new ice by surface cooling or the moderation of melting by the cold surface water, whereas for shorter period waves, where wind stress dominates temperature and ice concentration respectively, time is required for winds to draw in warmer or cooler air, as well as to overcome the ice masses inertia and keel friction to open or close leads. Strong intraseasonal variability of the sea ice concentration is observed in the 20-25 day period, reflecting similar timescales of the temperature variability, as well as that of the energetic eddies of the Antarctic circumpolar current. Examination of the latitudinal variation of the sea ice concentration, temperature and wind stress spectra showed not only the importance of the north-south temperature gradient in influencing the variability, but also the seasonal changes in the semi annual oscillation of the circumpolar trough. Regional spectra showed clear differences between location, and reflected the influences of the atmosphere and ocean upon the sea ice pack. This is clearly shown in the Weddell Polynya region and off East Antarctica, with high variability in the synoptic timescales, and in the western Ross Sea where changes occur in timescales of greater than 20 days. In order to determine if satellite derived, real time sea ice concentration and distribution would be of benefit to operational numerical weather prediction (NWP) schemes, the effect of sea ice concentration change upon the atmosphere in synoptic timescales was examined using a general circulation model in conjunction with the Australian Bureau of Meteorology’s GASP analyses. Experiments were conducted with a typical July sea ice concentration and distribution, as well as slab concentrations of 0, 10, 25, 50, 80 and 100%. Results from 5-day numerical weather forecasts show that the central pressure, structure and tracks of individual cyclones are sensitive to the ‘switch on’ of different sea ice conditions. Composites of all forecasts made with each concentration showed considerable, and mostly statistically significant, anomalies in the surface temperatures and turbulent heat fluxes over the sea ice. The magnitudes of these changes varied monotonically with the area of open water. The largest changes were simulated closest to the coast for all concentrations except for the typical July sea ice run, which displayed maxima over the outer pack. Significant westerly anomalies were induced over the ice in all cases, as were reductions in mean sea level pressure. The July sea ice runs displayed a distribution of the mean sea level pressure anomaly different from all others, with maxima occurring in the central to outer pack. All other forecasts displayed maxima at the coast. The results suggest that sea ice concentration does induce anomalies in the atmospheric parameters in timescales of less than five days. Further, the use of a realistic distribution of sea ice concentration produces results distinct from the constant concentration forecasts. Hence it is suggested that real time Antarctic sea ice data may be of considerable benefit to numerical weather prediction models.
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ItemSynthesising uncertainties of transient sea level rise projections( 2017)Global sea levels increased by around 0.2 m over the 20th century and will continue to rise during the 21st century and far beyond. This has profound implications for coastal populations, infrastructure and ecosystems around the globe. Efforts to assess future impacts on low-lying coastal areas need to be based on robust projections capturing the latest physical understanding of sea level drivers. This PhD research project provides an efficient and robust modelling tool that more consistently links the future sea level response to plausible emission scenarios and allows for extensive uncertainty assessments of long-term sea level projections until 2300. The new MAGICC sea level model is consistent with the Fifth Assessment Report (AR5) of the IPCC. It has been extended to also account for more recent research suggesting additional Antarctic discharge dynamics. In the IPCC AR5 consistent setup, global mean sea levels in 2100 are projected to rise between 0.4 and 0.6 m (66% range) under RCP 2.6 and between 0.7 and 1.0 m under RCP8.5, relative to 1986-2005. Global Mean Sea Level Rise (GMSLR) projections for the year 2300 yield median responses of around 1.1 m for RCP 2.6, 1.8 m for RCP 4.5, 2.4 m for RCP 6.0, and 4.8 m for RCP 8.5. If additional Antarctic rapid dynamics are included, we project 2300 median GMSLR of around 1.0 m under RCP 2.6, 3.3 m under RCP 4.5, 5.3 m under RCP 6.0, and 13.4 m under RCP 8.5. For the new Shared Socioeconomic Pathways (SSPs) without dedicated climate mitigation, 2100 GMSLR is projected to range between 1.0 and 1.9 m (66% range) for a 21st century storyline of high fossil-fuel use and energy demand. SSP pathways staying below 2 degC of warming relative to pre-industrial levels with a likely chance yield 2100 median GMSLR between 0.3 and 0.8 m. 2100 median SSP GMSLR could be limited to around 0.5 m if 2050 cumulative CO2 emissions since pre-industrial stayed below 850 GtC and the global coal phase-out was nearly completed by that time. The analysis of GMSLR under Paris Agreement climate targets clearly points to the need for early and stringent CO2 emission reductions between 2020 and 2035 for limiting 2300 GMSLR to around 1 m relative to 1986-2005. The Antarctic ice sheet represents the most uncertain but also potentially largest future sea level contribution, followed by the Greenland ice sheet and ocean thermal expansion. Due to its great scenario flexibility and robust 2300 projection capability, the MAGICC sea level model would be well suited to feed into regional sea level rise and coastal impact assessments.
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ItemReactive barrier formation as CO2 leakage mitigation technology( 2017)Global climate change driven by the emission of carbon dioxide (CO2) to the atmosphere is one of the grand challenges of our time. Energy from the use of fossil fuels is the main source of anthropogenic CO2 emissions. While renewable energies are emerging, fossil sources are expected to continue providing a large portion of our energy needs into the foreseeable future. One way to mitigate emissions from fossil fuel usage is through Carbon Capture and Storage (CCS). This involves capturing CO2 by engineered methods and storing it in a high porosity-high permeability geological formation overlain by a low permeability shale (or caprock). It is expected that these formations will be able to hold CO2 for thousands of years. However, in some instances CO2 leakage through the caprock cannot be entirely precluded, e.g. through undetected zones of higher permeability including natural fractures. This thesis proposes the use of a chemical reactive barrier formation as a technology to mitigate and remediate CO2 leakage successfully. The proposed technology consists of injecting an alkaline sodium silicate solution that reacts with the leaking CO2 or CO2-saturated water, leading to silica gel formation. This work was conducted in three main research phases. Firstly, experimental and modelling studies were undertaken to evaluate the properties of the sodium silicate solution and geochemical viability of the chemical reaction. The second phase involved experiments in a flow-through unconsolidated sand-packed column at ambient conditions. During this phase, variables such as flow rate, temperature and incubation times were evaluated under different scenarios. Finally, core-flood experiments were carried out at reservoir conditions. CO2saturated fluid and supercritical CO2 (scCO2) were injected into a sandstone saturated with silicate at high pressure and temperature. Reduction in permeability and the retention of silica in the column were used as measures of barrier formation in the last two experimental phases. The first phase of work showed that 7.15 wt% sodium silicate solution proved to be the most practical for applications in subsequent experiments. The modelling also showed that at this concentration the reaction is predicted to happen at different geochemical reservoir conditions. During the second phase, the formation of the silica barrier was found to be controlled by the mixing gradient of the two reactants, where the reaction resulted in reduction of permeability by at least one order of magnitude for mitigation and remediation scenarios. Moreover, the results of the core-flood experiments demonstrated that the formation of the silica barrier under CO2 reservoir conditions is possible and viable for a mitigations scenario. These results showed a significant reduction in permeability of two or three orders of magnitude and that the barrier was still strong a month after completing the test. In conclusion, results of this thesis suggest that the silica barrier formation is a promising technology to abate CO2 leakage from a geological carbon storage reservoir and provides useful findings for further research.
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ItemClimate justice: can we agree to disagree? Operationalising competing equity principles to mitigate global warming( 2017)With the Paris Agreement, the international community has agreed to limit global warming to well below 2 °C and to pursue efforts to stay below 1.5 °C (UNFCCC 2015a) to avoid dangerous climate impacts. Staying within these boundaries requires important emissions mitigation efforts from all countries (Rogelj et al 2015). Equitable distribution across countries of mitigation efforts, or equivalently of emissions rights, consistent with global mitigation objectives is a contentious issue that involves divergent interpretations of distributive justice (Winkler and Rajamani 2014a). The latest Intergovernmental Panel on Climate Change (IPCC) report categorises equity approaches from the scientific literature in five groups (Clarke et al 2014). At climate negotiations, most countries tend to support the approach that requires the least efforts on their behalf (Fleurbaey et al 2014, Lange et al 2010). With the absence of consensus on an effort-sharing approach, current negotiations under the United Nations Framework Convention on Climate Change (UNFCCC) follow a self-interested, or ‘bottom-up’, approach to target setting (Andresen 2015, Bodansky 2016) where each country decides its own effort following its understanding of fairness. As a result, the sum of all parties’ announced contributions is not consistent with limiting global warming to 2 °C, let alone 1.5 °C (Rogelj et al 2016a). Under the Paris Agreement, countries committed to increase the ambition of their post-Kyoto climate pledges through a ratcheting-up process that begins in 2023. With the disagreement on effort-sharing approaches, the international community relies on diverging metrics to evaluate the adequacy of national pledges with the global warming thresholds. Since the beginning of climate negotiations under the United Nations, a rich literature has modelled allocations of emissions rights to countries using various effort-sharing approaches with uncoordinated parameterisation. At the start of this PhD work, no study modelled the effort-sharing categories presented in the last IPCC report under a common parameterisation. Additionally, the literature on the combination of effort-sharing approaches remained thin and consisted of averaging the emissions allocations of multiple effort-sharing approaches. This PhD thesis addresses these gaps with the modelling of a new emissions allocation framework, the ‘PRIMAP-Equity’ framework, and with the suggestion of a new combination of effort-sharing approaches. Firstly, this thesis quantifies allocations of emissions rights to countries in a manner that reflects the existing literature on distributive justice. An emissions allocation framework is developed to derive national emissions allocations that reflect the five equity categories of the fifth IPCC report. This modelling framework is applied to derive emissions allocations, under each of the five equity categories, consistent with the emissions mitigation goals of the G7 Elmau agreement signed in June 2015. The allocation framework is then used to derive national emissions trajectories aligned with the recent Paris Agreement goals of both well below 2 °C and 1.5 °C, consistently with the five equity categories . This work represents the first quantification of equitable national trajectories to achieve 1.5 °C goal and informs scientists and government experts in the preparation of the IPCC Special Report on 1.5 °C (IPCC 2017). The Nationally Determined Contributions (NDCs), countries’ national pledges, of 171 Parties are then evaluated in order to determine which, if any, categories of equity they are consistent with. As well, the thesis highlights the consistency of G20 countries’ pledges with equity allocations. This is discussed in the context of the statement on fairness contained in each pledge. This PhD thesis then addresses the apparent incompatibility between the global warming thresholds and countries’ self-interested visions of effort-sharing by suggesting a new quantitative approach. Doing so, this PhD thesis provides a new metric, inclusive of all international positions, to assess the ambition of the NDCs under the Paris Agreement. This new ‘hybrid’ allocation method reconciles the ‘bottom-up’ approach of equity with the ‘top-down’ climate threshold that they commonly agreed. Under this ‘hybrid’ approach, each country follows the least stringent effort-sharing approach – out of the five that reflect the equity categories presented in the last IPCC report – to achieve the Paris Agreement. The aggregation of current national pledges is found to align with such a ‘bottom-up’ combination of approaches and lead to a warming of up to 2.3 °C in 2100 (with a 50% chance). Conversely, an enhanced ‘bottom-up’ approach – ‘hybrid’ – of global emissions scenarios leading to 1.1 °C and 1.3 °C warmings results in the achievement of the Paris Agreement mitigation goals of 1.5 °C and well below 2 °C, respectively. Ultimately, this study quantifies a compromise where each country can choose an equity approach to determine its effort, but does directly use that approach to assess other countries’ pledges. Finally, the application of this ‘hybrid’ approach provides a temperature assessment for all countries’ climate pledges, indicating the consistency of countries’ ambition in light of the global temperature goals. The NDCs of India, the EU, the USA and China are in line with global ‘bottom-up’ situations leading to warmings of 2.6 °C, 3.2 °C, 4 °C and over 5.1 °C, respectively. The results of this thesis can inform public opinions and decision makers through the ratcheting-up process on what constitutes fair and ambitious pledges to achieve the Paris Agreement following a range or combination of equity approaches. Additionally, the assessments of the adequacy of countries’ pledges with international agreements can inform courts when ruling ‘climate cases’ where governments are sued for their lack of ambition in mitigating emissions (Sabin Center for Climate Change Law 2018).
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ItemThermochronological insights into the morphotectonic evolution of Zimbabwe, southern Africa( 2017)The Zimbabwe Craton and surrounding mobile belts that make up Zimbabwe form the north-eastern part of the Southern African Plateau, which is of great scientific interest due to its anomalous elevation. The Phanerozoic history of Zimbabwe is largely unresolved and is difficult to unravel using conventional field methods due to the fragmentary nature of the preserved geological record and lack of structural controls in the dominantly granitic lithologies. Low-temperature thermochronology systems provide an invaluable toolkit for understanding upper crustal processes and in turn deciphering cryptic morphotectonic histories. Despite their value, thermochronology studies within Zimbabwe are considerably lacking, especially within the cratonic interior. In this work, a multiple low-temperature thermochronology approach— including the first apatite and zircon (U-Th)/He data and a more spatially extensive apatite fission track dataset—is employed together with inverse thermal history modelling to unravel the Phanerozoic histories of the different tectonic provinces of Zimbabwe. The data reveal that structural reactivation, largely caused by stress transmission and associated with uplift and denudation of different crustal blocks, has played a major role in the morphotectonic evolution of Zimbabwe, albeit spatially and temporally variable. The new dataset allows for a more clearly defined spatial and temporal structural reactivation pattern and suggests that the cratonic interior experienced reactivation in the Paleozoic but has since remained tectonically stable. Cratonic Zimbabwe preserves a Pan-African signature associated with Gondwana amalgamation, whereas the eastern cratonic margin and neighbouring mobile belts are dominated by Jurassic Gondwana breakup signals. The spatial extent and trend of the Pan-African rejuvenation signature suggest that the anomalous topography of Zimbabwe may have an ancient component. The regional dataset suggests unroofing of a previously more extensive sedimentary cover over the craton that began in the Paleogene. The zircon (U-Th)/He dataset in this work provides significant methodological insight. The unexpectedly recurrent ‘inversion’ of low-temperature thermochronology ages suggests that moderately-extremely radiation-damaged zircons can, in certain geological settings, act as ultra-low-temperature thermochronometers and provide insight into the more recent morphotectonic history. However, at present, the current zircon α-radiation damage accumulation and annealing model (ZRDAAM) does not adequately capture the He diffusion behaviour of the majority of the dated zircons. The exact source of this issue in the zircon (U-Th)/He system is uncertain, but could be associated with a ZRDAAM calibration issue, an unaccounted source of error and/or a currently unrecognised factor affecting He diffusion and retentivity within zircon.
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ItemUncertainties in runoff projections in southwest Western Australia and central Chilean catchments( 2017)Important runoff reductions have been reported in mid-latitude, Mediterranean-like climate catchments of the Southern Hemisphere (SH), in particular in the southwest of Western Australia (SWA) and in central Chile (CC). These changes have been driven by decreases in rainfall since the mid-1970s. Despite regional rainfall and runoff projections from Global Climate Models (GCMs) indicating that the observed trends are expected to continue during the 21st century, the projections are affected by large uncertainties that limit their utility to decision makers. The main source of uncertainty in runoff projections are the GCMs used to produce future climate projections. However, uncertainties arise from the observations of the climate variables, the statistical methodology to downscale the GCM simulations to the catchment scale and the hydrological model used to simulate runoff. In particular, the short length (<50 years) and poor spatially distributed observed climatological variables in mountainous catchments, characterized by steep topography, hampers a deep analysis of runoff trends and runoff variability, such as the case of CC mountainous catchments. The impact of the GCM uncertainty on runoff projections has mainly been assessed through comparison of multi-model runs of future climate with little exploration of uncertainties inside the models due to different parameterisations. This thesis seeks to investigate the uncertainty response of projected runoff due to both: perturbed physics parameter variations within a GCM using a novel 2500-member ensemble from the HadCM3L model, the climaprediction.net data (CPDN), termed the within-GCM uncertainties, and from a multi-model ensemble of different GCMs collated by the CMIP5 project, termed the between-GCM uncertainties. The impact of GCM uncertainties on runoff modelling for pluvial regimes in southwest Western Australian and Central Chilean catchments was assessed. Both regions share similar trends and climatic features, with major decreases in winter precipitation and runoff since the mid-70s that have been related to a displacement of the Southern Hemisphere storm track. Nonetheless one important difference between SWA and CC catchments, is the presence of nivo-pluvial regimes located at the foothills of the Andes in CC, whose hydrology is poorly understood mainly due to the lack of well distributed and long gauge records that represent its variability. The results presented in this thesis show that the impact of within-GCM uncertainties on runoff projections in SWA catchments is very large; larger than previous estimates of within-GCM uncertainties impact on runoff. The perturbed physics approach indicates that current water management assessments underestimate uncertainties in runoff projections. Regarding the comparison of the impact of between-GCM and within-GCM uncertainties on runoff projections in SWA catchments quantified as the difference between the 5th and the 95th percentile of simulations, the impact of within-GCM uncertainties on runoff projections range between 39% and 65%. Whereas the impact of between-GCM uncertainties on runoff projections range between 44% and 83% for the Representative concentration pathway 4.5 (RCP4.5) scenario and about 38% and 72% under the RCP8.5 for the period between 2050-2080 compared to 1970-2000. Regarding CC catchments, between-GCM uncertainties of about 55% and 51% in runoff projections using the RCP4.5 and the RCP8.5 scenarios were found. The results here reported indicate that the impact of within-GCM and between-GCM uncertainties in SWA catchments runoff projections is very similar. The results also indicate that because some GCMs in the CMIP5 ensemble have multiple runs, using different initial conditions, CMIP5 gives some insight into within-GCM uncertainty as well. For these reasons and because CMIP5 provides runs that represent all regions of the world, it is recommended for use in hydrological assessments of climate change impact and the uncertainties around the projections.
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ItemThe composition of altered oceanic crust: Implications for mantle evolution( 2017)The geochemical characterisation of oceanic crust, in its dual role as a mantle-derived melt and a subducted material, is an essential component in understanding the evolution of the Earth’s mantle. Interaction of newly formed crust with seawater, however, producing ‘altered oceanic crust’ (AOC), provides a degree of complexity in this undertaking. This study aims to conduct a global survey of trace element and Sr-Nd-Hf-Pb isotopic compositions of AOC by compiling high quality literature data and adding new analyses where appropriate to address identified gaps in the record. Ten representative DSDP/ODP/IODP sites were selected for the new analyses. In total 93 samples for homogeneous portions and 19 samples for heterogeneous portions of AOC were analysed. These results are combined with all available data for these ten sites as well as geochemical data from other locations that also cored AOC samples. Prior to any assessment of alteration effects, variations due to magmatic processes are considered. Comparison of alteration-insensitive elements and isotope ratios of AOC, grouped by rock type, crustal age and ocean basin (i.e. spreading rate) reveals the relative importance of these parameters on AOC composition. The results show that rock type provides the first order control on trace element variations. Alteration-insensitive trace element compositions are not correlated with either crustal age or ocean basin/spreading rate, whereas Nd and Hf isotope ratios do show trends with crustal ages, mirroring the evolution of the mantle source over the past 170 Ma. In addition, the compositions of lower crustal rocks from slow-spreading ridges are significantly more variable than those from fast-spreading ridges. This is because the lower crust at slow-spreading ridges is generated from multiple magma pulses compared to that at fast-spreading ridges that generally forms more homogeneous magmas derived from larger magma chambers In terms of alteration effects compositional variations in the upper and lower crust are considered separately because of their very different nature. The alteration in homogeneous parts of the upper AOC varies with crustal age. This is consistent with the correlation between the oxidation state, which plays an important role in upper crust alteration, and crustal age. The alteration in homogeneous parts of the lower AOC, however, is better linked with spreading rate. In heterogeneous parts of AOC, such as breccias and veins, compositional variation is principally dependent on the phases themselves instead of crustal age or spreading rate. The compiled results, with the controls on compositional variation revealed, are used to calculate the global compositions of eight AOC types, namely fast-upper AOC, slow-upper AOC, fast-lower AOC, slow-lower AOC, fast-total AOC, slow-total AOC, global AOC and global AOC-plume free. Site medians are calculated first and the crustal age- and spreading rate-weighted averages are determined for upper and lower crust, respectively. The global AOC compositions are then calculated using the proportion of fast- and slow-spreading crust worldwide. Also, the compositions of plume-free global AOC are also estimated by removing some of plume-influenced sites. The results are then compared with previous estimates of AOC composition revealing some differences. The results of this investigation encompass a greater number of drill sites with a more detailed understanding of the controls on variation, and are therefore likely a more accurate representation than previous estimations. The newly derived AOC compositions can be used to test models for the involvement of subducted AOC in generating Ocean Island Basalt (OIB) compositional variation, in particular the origin of the ‘high-Mu’ (HIMU) signature. As an example a preliminary test was performed using source and recycling ages of 3 and 2 Ga, respectively. The model results show that 1) subduction modification is essential for AOC to generate HIMU signals and 2) the upper crustal components are most likely to generate HIMU signals in all isotope systems after subduction modification (dehydration) with adjusted mobility of Th. However, a more detailed determination of element mobility is required in order to obtain more reliable modelling results. Intriguingly, four analysed samples have extremely unradiogenic Pb isotope compositions, unlike anything previously recovered from the ocean floor. These are compared with similar signals observed in peridotites but show different trends, indicating a different source or magmatic origin. They might also be caused by the occurrence of secondary sulfides, but this aspect requires further investigation.