Doctoral Degrees (Earth Sciences)
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- ItemMagmatic and metamorphic evolution of the Stolzburg Block, Barberton Granitoid-Greenstone Terrain(Stellenbosch : Stellenbosch University, 2022-12) Muhlberg, Moritz; Stevens, Gary; Moyen, Jean-Francios; Kisters, Alex; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: The processes that formed and shaped the first stable continents in the Archean are poorly constrained for various reasons, including the scarcity of well-preserved, unaltered rock sequences and the lack of modern equivalents of lithologies that are typical for the Archean. This has led to the emergence of various geodynamic models that try to explain the unique features of the Archean crust, but too often rely on assumptions on the thermal state of the crust, in particular of the felsic crust. This thesis is an investigation of the igneous and metamorphic history of the Stolzburg Block, an area comprised of tonalite-trondhjemite-granodiorite (TTG) and amphibolite-facies supracrustal rocks, located in the Barberton Granitoid Greenstone Terrain (BGGT) in South Africa. The thermal history of a well-preserved piece of felsic Archean crust is studied through a combination of field work and geochemical, geochronological and petrological analyses on TTG and related rocks, with the aim of providing robust constraints on the thermal state of the Archean crust to improve geodynamic models. Uranium-lead (U-Pb) thermochronology on apatite grains has revealed four distinct groups of 207Pb/206Pb ages that are present throughout the Stolzburg Block. The three oldest groups of U-Pb apatite ages overlap with the time of emplacement of TTG plutons (~3450 Ma), of regional metamorphism (~3230-3200 Ma) and of intrusion of granitic plutons (~3105 Ma), while the ~2820 Ma group indicates a previously undiscovered thermal event. The preservation of the ~3450 Ma apatite ages indicates that the Stolzburg Block has not been heated above ~400- 500 °C after the emplacement of TTG plutons – at least not for a prolonged period. Trace-element and strontium isotope analyses of the dated apatite grains show homogenous compositions regardless of age and 87Sr/86Sr ratios that are within uncertainty identical to the initial 87Sr/86Sr ratio of the respective bulk rock, and these features are being interpreted as primary igneous signatures. This indicates that the different groups of U-Pb apatite ages are not the result of new growth but rather of partial resetting of the U-Pb systematics through heating. Zirconium-in-titanite and titanium-in-zircon thermometry gives temperatures of ~700 °C for magmatic titanite and ~650 °C for metamorphic titanite and zircon from TTG samples, which is in the same range as the reported metamorphic conditions recorded by the associated supracrustal rocks at ~3230-3200 Ma. While no chronological constraints are available for the temperature estimates of the TTG rocks, a shared history between TTG plutons and greenstones can be inferred from the intrusive relationship between the two. The preservation of ~3450 Ma U-Pb signatures in apatite grains shows that, regardless of the timing of heating to ~650 °C, the heating was very short-lived (< 1 million years). This work demonstrates that the felsic Archean crust – at least the section preserved in the BGGT – was relatively cool and stable, and that any heating of the TTG plutons was short- lived. The results of this study argue against a prolonged radiogenic heating of TTG crust and a partial convective overturn of the Archean crust, as proposed by some geodynamic models.
- ItemUnderstanding Southern Ocean phytoplankton ecophysiological response to iron availability(Stellenbosch : Stellenbosch University, 2022-04) Singh, Asmita; Ryan-Keogh, Thomas; Fietz, Susanne; Thomalla, Sandy; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Over the past century, climate change has been of growing concern, due to its ecological and economic global impact. The Southern Ocean buffers the impacts of climate change by accounting for a significant proportion of the total oceanic uptake of CO2. Phytoplankton primary production and carbon export (the biological carbon pump) plays an important role in the Southern Ocean carbon cycle and despite their ability to remove a significant amount of the global organic carbon flux each year, it is often constrained by the availability of light and nutrients. The micronutrient iron is particularly important in the production of key metabolic and photosynthetic proteins in phytoplankton and its scarcity in the Southern Ocean dictates its role as a key driver of variability in Southern Ocean productivity. A greater understanding of the response of Southern Ocean phytoplankton to seasonal and regional variability to their environmental drivers (with a focus on iron in particular) is thus required in order to increase the accuracy in assessing and predicting the impact of climate change. Active chlorophyll-a fluorescence is a non-invasive, powerful instantaneous tool, which can assess the phytoplankton photosynthetic efficiency in response to potential environmental drivers and in particular under stressful growth conditions, i.e. under iron limitation. However, our understanding of the seasonal cycle of photophysiological responses of phytoplankton to iron and other biogeochemical drivers remains limited, primarily due to the prevalence of experiments and measurements only being conducted in summer. In this research, three individual studies of phytoplankton photophysiology across all seasons of the Atlantic Southern Ocean were used to investigate physical and biogeochemical drivers of inter-zonal, inter-annual and intra-seasonal variability in phytoplankton photophysiology (Fv/Fm) in summer. Results from this study point to a combination of drivers (notably sea surface temperature, macronutrients and community structure) that elicit simultaneous and oftentimes antagonist responses in Fv/Fm, making it difficult to ascertain one dominant driver over another. In addition, the degree of iron stress in the Sea-Ice Zone in autumn, and the full zonal extent of the open Atlantic Southern Ocean in spring and winter, was determined using a series of short-term (24 hrs) in situ iron addition experiments. Key results suggest that phytoplankton in the Sea-Ice Zone of Dronning Maud Land are not iron-limited in autumn, and presumably have a sufficient year-round supply of iron potentially from shallow topography. However, both winter and spring showed some positive responses to iron addition with zonal variability being linked to the timing of the incubation experiments relative to the seasonal cycle of the mixed layer depth, highlighting the importance of convective overturning as a dominant seasonal iron supply mechanism. The research presented in this thesis contributes to a greater understanding of the complex interplay of multiple drivers of phytoplankton photosynthesis across the seasonal cycle. It is recommended that future research continues to address seasonal and regional variability in phytoplankton photophysiology but with a focus on resolving the relationship between multiple drivers in line with anticipated climate-mediated adjustments in environmental conditions.
- ItemInsights into early winter Southern Indian Ocean dissolved iron distributions and remineralisation using excess barium (GEOTRACES GIPr07 cruise)(Stellenbosch : Stellenbosch University, 2022-04) Van Horsten, Natasha; Roychoudhury, Alakendra N.; Sarthou, Géraldine; Mtshali, Thato; Bucciarelli, Eva; Planquette, Hélène; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: The Southern Ocean is of central importance to the global ocean, connecting the Atlantic, Pacific and Indian basins via the Antarctic Circumpolar Current, which carries large amounts of heat, carbon and nutrients. Thus, this oceanic province partly regulates the Earth's climate, present and future. One of the factors influencing the biological carbon pump of the Southern Ocean is the availability of iron, which is an essential nutrient for phytoplankton. However, observations in this region have mainly been conducted in spring and summer, leaving significant gaps in our knowledge of the biogeochemical cycle of iron, in particular on the seasonality of internal processes such as mesopelagic remineralisation. During this study, the distributions of dissolved iron and excess barium (Baxs), a mesopelagic remineralisation proxy, were measured throughout the water column, at seven stations along 30 ° E in the Southern Indian Ocean during early austral winter of 2017. To our knowledge, this is the first winter study on these two parameters and in this sector of the Southern Ocean. A more in-depth evaluation of the Baxs proxy was conducted by comparing integrated remote sensing primary production data and the Baxs signal, for all observations available in the Southern Ocean, on an annual timescale. Our results suggest that remineralization is maintained at comparable levels as observed during summer, well after the bloom decline, resulting in the mesopelagic Baxs signal, at the onset of winter. Regarding dissolved iron, the winter concentrations were not significantly different to summer observations in the western Southern Indian Ocean, remaining at sub-nanomolar concentrations, from 0.02 to 0.73 nmol L-1. Concentrations in surface water were depleted, resembling late summer observations, indicating that in July there has not yet been a replenishment of the dissolved iron stock in surface waters. The distributions of dissolved iron and Baxs exhibited peaks in intermediate waters on the water column profiles, indicating iron remineralization. Using the apparent oxygen utilization proxy, we estimated that a large fraction (58 ± 1 to 160 ± 2%) of the measured mesopelagic dissolved iron stock was provided by remineralisation. Iron remineralisation fluxes were largest in the subantarctic zone and decreased to a minimum in the oligotrophic Subtropical zone. This study contributes to the emerging understanding of winter trace metal distributions and internal cycling, giving insight into the replenishment of the winter dFe reservoir, as well as obtaining better estimates of export and remineralisation on annual and basin scales, within the Southern Ocean.
- ItemSouthern African dust characteristics and potential impacts on the surrounding oceans(Stellenbosch : Stellenbosch University, 2021-12) Kangueehi, Kaukurauee Ismael; Fietz, Susanne; Eckardt, Frank; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: This study supports understanding the potential impact of dust aerosols from southern Africa have on the proximal ocean ecosystems. Dust can release essential nutrients and thus fertilize the ocean, which affects the food-web and the carbon dioxide concentrations in ocean waters, i.e. climate. Dust that fertilizes the phytoplankton communities in open oceans stimulates the drawdown of carbon dioxide from the atmosphere through the process of photosynthesis. Key here is that the nutrients must be released from the aerosol particles, as they can only be uptaken by biological organisms in soluble form. However, mineral dust emitted from arid environments differ from dust emitted through industrial activities; as a result the solubility and, by extent, potential impact on the ocean may also differ. In this study, I investigated mineral characteristics and solubility of dust from three distinct regions, one with a strong human footprint (Saldanha Bay), one with mostly natural mineral dust (Namib Desert) and one off shore over the Southern Ocean. The first study targeted Saldanha Bay, a town that hosts the largest port in South Africa, with exports of up to 60 million tons of iron and manganese ore annually and is home of a steel plant and a smelter. Satellite images and photos from the area have shown extensive dispersion of dust from the area. Solubility leaching experiments revealed that dust collected in this town is highly soluble (bioaccessible) for trace metals such as Fe (up to 28%), Cu (up to 33%), Pb (up to 45%) and Zn (up to 38%). Phytoplankton communities in open oceans are sometimes depleted in these trace metals, and thus, such high solubility of dust from Saldanha Bay can prove to be an important nutrient supplier to surrounding oceans. In addition, air mass trajectories revealed that this readily available dust most likely affects the southeast Atlantic and Indian Ocean. The major implication was that harbour towns can be essential sources of trace metals to proximal oceans. Secondly, I present results from the largest non-playa environment in the Namib Desert. Some of the prominent dust emitters located in the Namib Desert provide, feed and source of hundreds of tons of mineral dust to proximal surface ocean waters. The solubility of the mineral dust, in contrast to the mixed (anthropogenic and natural sourced) dust mentioned above, is much lower (e.g. up to 2 % for Fe, up to 7.8 % for Cu, up to 16% for Zn, up to 9% for Mn, 8% for Pb). These solubilities are in similar range to other semi-arid to arid major global dust sources such as the Sahara Desert in the Northern Hemisphere. Differences in mineralogy and particle size distribution at the sampling stations are the main drivers of the solubility. For example, the solubility of trace metals such as Fe, Zn and Pb is higher at the inland station than at the coastal station. The aggregated particles along the coastal stations are less soluble probably because of persistent foggy conditions and less sunlight. The inland stations had less foggy conditions and more fine-grained particles which are rich in FeO oxides. Air mass trajectory modelling indicated that this mineral dust, with its low solubility, typically travels towards the southeast Atlantic Ocean, but can also reach the nutrient poor areas in the Southern Ocean in some cases. This study highlighted the role of non-playa environments as important dust sources and that mineralogy coupled with particle size are closely related to trace metal solubility. In addition, to assessing the solubility and potential impact of dust with natural and anthropogenic imprint, we investigated the solubility of aerosols collected over the Southern Ocean south of South Africa on board on the research vessel SA Agulhas II. The solubility of the trace metals over the Southern Ocean ranges from 0.5 to 41% Fe, 9 to 48% Al, 0.1 to 88 % Mn, 25 to 72 % Zn and 3.3 to 39 % Co. This study found large variability in trace metal composition and fractional solubility in dust aerosols collected from three distinct environments in southern Africa and south of southern Africa. Our study highlighted the importance of southern African dust of both anthropogenic and natural sources as a nutrient supplier to surrounding coastal and open oceans.
- ItemAn investigation of archean and paleoproterozoic metamorphic processes in the Limpopo belt via detailed analysis of reaction textures in metapelites from the Bandelierkop Formation, Southern Marginal Zone of the Limpopo belt(Stellenbosch : Stellenbosch University, 2021-12) Madlakana, Nonkuselo; Stevens, Gary; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: The well exposed granulite facies rocks in the Southern Marginal Zone (SMZ) of the Limpopo Belt (LB), South Africa, represent an exceptional window into the lower crust where partial melting processes can be directly observed and investigated. Previous studies have demonstrated that these rocks experienced a single granulite facies metamorphic event at ca. 2.71 Ga under conditions of ~850 ᵒC and 11 kbar along with partial melting via a sequence of biotite incongruent melting reactions, which produced melt together with peritectic garnet, orthopyroxene, cordierite, sillimanite and plagioclase. In rare cases, peritectic garnets host an abundance of euhedral plagioclase inclusions with variable compositions. Despite their potential that they present in understanding details of the melting processes they have not been a focus of a previous study. This thesis focuses on these inclusions in order to understand the anatectic processes responsible for the generation of the metasedimentary migmatites in the SMZ, and importantly also characterises their retrograde metamorphic history. Specifically, this work integrates geochemical, geochronological and petrological investigations of metapelitic granulites from two localities in the Bandelierkop Formation (the Bandelierkop quarry and the Brakspruit quarry), where Neoarchean high-grade metamorphic textures have been preserved. Electron beam microanalysis show that some of the euhedral plagioclase inclusions have high anorthite content (up to An83) compared to the majority of plagioclase inclusions, as well as the matrix plagioclase, in these rocks. This is interpreted to reflect disequilibrium melting of plagioclase in the source due to slow diffusion in plagioclase. The variable composition of these euhedral plagioclase crystals suggests their entrainment by the melt from different localised domains and the entrapment at the sites where peritectic garnet was growing from the melt. The preservation of these garnets must be due to rapid and efficient melt loss from the residual source, possibly aided by short residency of these granulites at high temperature due to fast exhumation. The peak metamorphic assemblage is partially retrogressed under amphibolite facies conditions. The detailed petrological investigations of the retrogressed metapelitic granulites, combined with LA-ICP-MS dating on rutile suggests that the SMZ of the LB has experienced a previously unrecognised Paleoproterozoic regional amphibolite facies event. This Paleoproterozoic event is 600 Myrs younger than the age of peak metamorphism in the SMZ. Within the Kaapvaal Craton, it relates to the ca. 2.15 Ga U-Pb age from metamorphic monazite and titanite in the Ghaap group, Chuniespoort Group and the Pretoria Group of the Transvaal Supergroup and the ca. 2.1 monazite age from the Central Rand Group of the Witwatersrand, all of which have been interpreted to indicate craton-scale fluid flow associated with thrusting and folding in the Kaapvaal Craton. Globally, the ca. 2.15 Ga event correlates to the ca. 2.22–2.15 Ga tectonic driven fluid flow and the Ophthalmain orogeny in the Pilbara Craton in Western Australia. It can be concluded that the SMZ of the LB, is a polymetamorphic terrain, with an older granulite facies anatectic metamorphic event at ca. 2.71 Ga and a younger amphibolite facies Paleoproterozoic overprint at ca. 2.1 Ga.