Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by browse.metadata.advisor "Eckardt, Frank"

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    Southern 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.