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- ItemOpen AccessNitrogen cycling in the subtropical southeast Atlantic and southwest Indian Oceans as recorded by the nitrogen isotopes of modern planktic foraminifera(2023) Granger, Robyn; Fawcett, SarahDespite the importance of nitrogen (N) for ocean productivity, and the long history of using fossil foraminifera to reconstruct past ocean conditions, it is only in recent years, due to methodological advances, that the nitrogen isotope ratio (δ15N) of foraminifera has become a viable proxy for past marine nutrient cycling. Organic N trapped within planktic foraminifer shells is protected from bacterial degradation, with its δ15N recording the processes acting on the upper- ocean N pool. This thesis examines the relationship between local biogeochemi- cal cycling and foraminifera tissue- and shell-bound δ15N in the greater Agulhas Current system and southeast Atlantic Ocean, focusing on the implications for reconstructing Agulhas leakage (i.e., the transfer of Indian Ocean waters into the Atlantic). Past fluctuations in this important component of the Atlantic Meridional Overturning Circulation, whereby warm, saline Agulhas waters are transported to the North Atlantic along its upper limb, have been tied to global glacial-interglacial cycles, highlighting the region's sensitivity to large-scale cli- mate change. The work detailed in this thesis includes the first foraminifer- bound δ15N ground-truthing studies from the southeast Atlantic and the Agul- has Current regions and examines the extent to which the unique δ15N signature of Indian Ocean nitrate is preserved in the tissue and shells of foraminifera living in Agulhas leakage features (e.g., eddies). The isotopes of several forms of N, including nitrate, particulate organic N, size-fractionated zooplankton biomass, living foraminifera tissue and shell N, and fossil foraminifera, were measured and interpreted in the context of coincident hydrographic measurements to deter- mine the controls on the δ15N of foraminifera and their potential food sources. The data presented here reveal that mixed layer nitrate δ15N was noticeably lower within an Agulhas eddy than it was for the surrounding Cape Basin wa- ters, a characteristic that was likely inherited from low thermocline nitrate δ15N produced in the region of leakage origin, the Agulhas Current System. Simi- larly, the δ15N of foraminifera inhabiting the Agulhas eddy was found to be low relative to foraminifera under background southeast Atlantic conditions, despite foraminifera in the Agulhas Current System displaying on average a higher δ15N than was recorded by foraminifera inhabiting the eddy. The data therefore sug- gest that anticyclonic eddies “leaking” into the region from the Indian Ocean maintain a low-δ15N environment that sustains the growth of foraminifera for several months, and that N2 fixation and/or recycling of low-δ15N ammonium within the eddy environment likely contributed to lowering of foraminifer-δ15N. That foraminifer-δ15N is on average 2-3‰ lower in Agulhas leakage than in the southeast Atlantic suggests that enduring periods of increased leakage could result in relatively low-δ15N material being transferred to the sediment and recorded. A comparison of data from the southeast Atlantic and Agulhas regions to previous ground-truthing studies from the Sargasso Sea and Southern Ocean reveals similarities in both foraminifer tissue-shell δ15N relationships and inter- species δ15N differences. For instance, symbiont-hosting foraminifera are consis- tently lower in δ15N than deeper-dwelling, symbiont-barren individuals at the same location due to the symbiont's ability to recycle low-δ15N ammonium. Also consistent with previous studies is the positive correlation observed be- tween fossil foraminifera from core tops and modern shell- and biomass δ15N in the Atlantic, despite sediment being derived from multiple locations within the Cape Basin. This study adds to burgeoning efforts to ground-truth the foraminifer-δ15N palaeo-proxy and supports the argument that the δ15N of liv- ing foraminifera, which is set by both the local N supply and N-cycling processes, can be deduced from foraminifera shell-bound δ15N in the sediment record. Fur- thermore, the work detailed in this thesis examines how the unique δ15N of the nitrate and biological community of a particular water mass might be leveraged to reconstruct past variations in Agulhas leakage.
- ItemOpen AccessStrategies for people with intellectual disability to engage in social policy-related self-advocacy in South Africa(2023) Goldberg, Nicole; Kleintjes, SharonAim: This study investigated strategies for people with intellectual disability to self-advocate for inclusion of their priorities in social policy processes in South Africa. Method: Self-advocacy strategies were identified through a scoping literature review, a review of self-advocacy toolkits and semistructured interviews with people with intellectual disability and other stakeholders working at nongovernmental and disabled people's organisations. The data were triangulated to identify common strategies which can best support self-advocacy for people with intellectual disability. In addition, these findings informed the development of a local self-advocacy toolkit which was reviewed by a group of young adults with intellectual disability. The toolkit provides an evidence-based compilation of strategies for self advocacy that can be used by self -advocates with intellectual disability and their supporters in the local context. Results: Data triangulation identified three core strategies for selfadvocacy, specifically, (1) strategies for in-person self-representation in public spaces, (2) written communication strategies, and (3) engagement through social and other forms of media. These were incorporated into the self-advocacy toolkit. The experiential review of the toolkit yielded positive feedback from participants. As part of the experiential review, participants related to the idea of social media advocacy by choosing to produce a video as their self-advocacy activity, aimed at sharing their perspectives on the way they experience society and social policy. Discussion: Inclusion of people with intellectual disability in civic and political life is crucial and will only be achieved if self-advocates are accepted into the policy-making arena. Strategies are available for people with intellectual disability to challenge the status quo of society in which prejudice and stigma continue to impact on their inclusion in social discourse. By using these self-advocacy strategies, people with intellectual disability can have a say in policy decisions which impact on their lives. Conclusion: Self-advocacy to enhance policy participation can promote the inclusion of the voices of people with intellectual disability to influence public social policy decisions. Adopting strategies which enable the inclusion of the voices of people with intellectual disability in civic activities holds potential for diversifying perspectives brought to public participation in policy development and implementation, which is currently primarily the domain of citizens without disability.
- ItemOpen AccessStrategies for people with intellectual disability to engage in social policy-related self-advocacy in South Africa(2023) Goldberg, Nicole; Kleintjes, SharonAim: This study investigated strategies for people with intellectual disability to self-advocate for inclusion of their priorities in social policy processes in South Africa. Method: Self-advocacy strategies were identified through a scoping literature review, a review of self-advocacy toolkits and semistructured interviews with people with intellectual disability and other stakeholders working at nongovernmental and disabled people's organisations. The data were triangulated to identify common strategies which can best support self-advocacy for people with intellectual disability. In addition, these findings informed the development of a local self-advocacy toolkit which was reviewed by a group of young adults with intellectual disability. The toolkit provides an evidence-based compilation of strategies for self advocacy that can be used by self -advocates with intellectual disability and their supporters in the local context. Results: Data triangulation identified three core strategies for selfadvocacy, specifically, (1) strategies for in-person self-representation in public spaces, (2) written communication strategies, and (3) engagement through social and other forms of media. These were incorporated into the self-advocacy toolkit. The experiential review of the toolkit yielded positive feedback from participants. As part of the experiential review, participants related to the idea of social media advocacy by choosing to produce a video as their self-advocacy activity, aimed at sharing their perspectives on the way they experience society and social policy. Discussion: Inclusion of people with intellectual disability in civic and political life is crucial and will only be achieved if self-advocates are accepted into the policy-making arena. Strategies are available for people with intellectual disability to challenge the status quo of society in which prejudice and stigma continue to impact on their inclusion in social discourse. By using these self-advocacy strategies, people with intellectual disability can have a say in policy decisions which impact on their lives. Conclusion: Self-advocacy to enhance policy participation can promote the inclusion of the voices of people with intellectual disability to influence public social policy decisions. Adopting strategies which enable the inclusion of the voices of people with intellectual disability in civic activities holds potential for diversifying perspectives brought to public participation in policy development and implementation, which is currently primarily the domain of citizens without disability.
- ItemOpen AccessNonlinear vibration of beams and plates resting on elastic foundations having nonlinear stiffness properties(2023) Feulefack, Songong Edwige; Zingoni, AlphoseBeam elements are used to model components in which one dimension (the length) is significantly greater than the other two dimensions. On the other hand, a plate is a structural element char- acterised by its thickness being very small compared with the other two dimensions. Beams and plates find civil and mechanical engineering applications and are mostly used to construct build- ings, bridges, floors, pavements, and runways. Investigating the vibration of beams and plates is an ongoing topic, and researchers are becoming more interested in it. Because the structure foundation is the most important part of the building process, engineers and designers dedicate more attention to that part of the structure. This contributes to the desire to build strong, safe, and economic structures for a sustainable environment. Beams and plates on elastic foundations (BPoEFs) also find many applications in civil engi- neering. They are frequently used to design structural members of buildings, railroads, airports, highways, and railway tracks. For example, when studying railway track behaviour, a beam on an elastic foundation (BoEF) theory is often employed, and the study can be extended to track dynamics, noise, and vibration. Winkler (1867) was the first to introduce the topic of BPoEFs and developed these to analyse railroad tracks. Since then, many other researchers have extended the concept, giving rise to other foundation types. The foundation can be linear or nonlinear, depending on the purpose for which the structure will be used. For example, to model the interaction between the beam or plate and an elastic foundation, the entire beam or plate and the foundation are modelled without modelling the foun- dation itself. Then, the modelling can become easy or difficult depending on the foundation type and property (linear or nonlinear). Therefore, linear analysis is appropriate for easier cases, while nonlinear analysis is left for other complex cases. Nevertheless, it is important to note that linear analysis is limited to simple structures and is only valid for design. As a result, nonlinear analysis is recommended because it can be used for design and realisation purposes. Beams and plates on linear elastic foundations (BPoLEFs) have been extensively studied, whereas beams and plates resting on nonlinear elastic foundations (BPoNEFs) have been ne- glected and constitute the interest of the current thesis. A beam simply supported at both ends and a rectangular plate also simply supported are considered. Linear and nonlinear foundations are considered to capture the beam and plate behaviour and response. The foundation is assumed to be an assembly of discrete linear or nonlinear springs or connectors. First, the beam on a linear foundation is studied as the basis and then extended to the beam on a nonlinear foundation. Three cases of nonlinearities are discussed: quadratic, cubic, and the combination of quadratic and cubic. The straightforward expansion method (SEM) is used to find solutions to the governing nonlinear differential equation. Finally, the natural frequencies and the corresponding mode shapes of the system beam-foundation are derived. We note that the overall thesis only focuses on free vibration analyses. A rectangular plate on an elastic foundation (PoEF) is studied to extend the research on beams. Here also, linear and nonlinear foundations are considered. The same analyses as those of the beam are carried out on the plate to cover all structure types. Both studies on BoEFs and PoEFs revealed that the natural frequencies of the system increase with the increase of the nonlinear stiffness parameter of the foundation. Interestingly, the same increase in the nonlinear stiffness of the foundation tends to decrease the beam and the plate temporal vibration amplitude. Comparing linear to nonlinear results reveals that neglecting the nonlinearity of the foundation produces differences which are more significant at lower modes. This confirms the fact that, if the model of a beam or a plate resting on a nonlinear elastic foundation is simplified to a linear one, the results obtained from the linear analysis might not accurately represent the real system. Finite Element Analysis (FEA) in Abaqus is used for the numerical analysis. Here also, the natural frequencies and corresponding mode shapes of the beam and the plate with foundation are derived. Extensive parametric studies conducted on the beam and plate vibration reveal that the foundation's nonlinear stiffness is indispensable because it lowers the beam and plate vibration amplitude. Having validated the results from Abaqus, they are used as a benchmark to validate the analytical results. Satisfactory agreement is found between the results of the two methods, with details of each method shown. The results from this thesis show that the nonlinear foundation can accurately be used to control the vibrations of the beam and plate model. The findings further reveal that adding nonlinear quadratic or cubic or mixed parity stiffness properties in the foundation when studying BPoEFs is beneficial and should be adapted for design and realisation purposes.
- ItemOpen AccessInfluence of environmental parameters on atmospheric aerosol size distributions in a South African coastal zone(2023) February, Faith; Altieri, KatyeAerosols are microscopic solid or liquid particles suspended in the atmosphere, which impact and are impacted by many physicochemical processes related to climate. By scattering and absorbing solar radiation and acting as cloud condensation nuclei, they have both a direct effect and indirect effect on Earth's radiative budget. The Intergovernmental Panel on Climate Change (IPCC) recognizes aerosols as an important component of climate change, and as such the estimation of their impacts on climate remains an important scientific challenge. Atmospheric aerosol sources include natural and anthropogenic emissions, and these are both present in coastal areas. Marine aerosols are a major contributor to total aerosols in coastal regions, and these aerosols can interact with other gases and molecules, and influence biogeochemical cycles, air quality, and human health. Due to the complex nature of aerosols in coastal areas and the importance of aerosols to climate change, there is a need for in-situ observational aerosol data from data poor regions, which is the case for most of the Southern Hemisphere. To that aim, this thesis presents an extensive series of measurements of aerosol properties (concentrations, sizes, and types) at the coastal location of Simon's Town in False Bay, to characterize these properties for the first time in a coastal region of South Africa. The study site provides the opportunity to measure these aerosol properties in pristine marine conditions exclusively, as well as mixed conditions typical of a coastal site. With these geographical advantages and long-term, unbiased measurements, the aerosol processes of generation, transport, dispersion, mixing, and deposition have been investigated under unique natural conditions. The data analyses focus on evaluating changes in the average particle size distributions with changes in meteorological parameters. An increase in aerosol concentrations with an increase in wind speed during clean marine conditions resulted in the identification of sea spray generation and long-range transport as the dominant natural aerosol processes with the type of aerosol being predominantly marine. For the NW conditions, a decrease in aerosol concentrations with an increase in wind speed led to the identification of dispersion as the dominant process. A mixture of aerosol types emerged, but the higher aerosol concentrations for the pure continental conditions compared to the pure marine conditions indicated that non-marine aerosols dominate around 0.4 μm and 10 μm. The main conclusions of this thesis focus on the behavior of natural sea spray aerosols in the absence of continental or anthropogenic sources. This highlights the pure marine influence of the Southern Ocean at this location and the unique opportunity to explore understudied natural aerosol processes absent from anthropogenic influence.