| Summary: | <p>Ephemeral lake beds are prominent sources of atmospheric mineral dust aerosols globally, but emissions from these landforms are characterised by considerable spatial and temporal variability. It has proven extremely difficult to resolve the complex relationships between the climatic and surface drivers that control aeolian emissivity. Such complexity introduces uncertainty in model predictions of atmospheric dust loading with subsequent imprecision in estimates of its impacts and interactions with other global scale processes in the Earth system, which have far-reaching implications for climate, ocean fertilisation, nutrient transport, air quality and human health.</p>
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<p>A key challenge remains to collect high resolution, field-based measurements of the local scale controls and processes of aeolian entrainment from specific emitting surfaces and relating these to remote sensing observations of basin scale environmental drivers and surface erodibility dynamics, despite the promising potential for the generation and testing of dust emission models. This thesis presents a combined approach utilising in-situ field experimentation and remote sensing analysis techniques to determine the local and regional controls on the emission of mineral dust from Etosha Pan in Namibia, a major southern-hemisphere dust source.</p>
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<p>A range of satellite-retrieved (e.g., MODIS, MSG-SEVIRI and Landsat) measurements of lake inundation, mineral aerosols, surface geochemistry, and climate data (from meteorological stations and model reanalysis data sources) for the period 2000–2022 were used to determine the hydrological contexts, salt crust mineralogy, and climatic environments modulating dust emission dynamics from the pan. A discrete and sensitive set of significant associations were established between annual and seasonal variations in basin scale precipitation, ephemeral pan surface inundation, and the frequency and magnitude of dust emissions across the analysis period. In addition, a significant proportion of the observed variability in the dust and hydrological cycle of this source could also be attributed to El Niño-Southern Oscillation (ENSO) and Indian Ocean sea surface temperature (SST) anomalies.</p>
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<p>A twenty-year record of dust point source locations enabled the identification of clusters of dust sources (‘hot-spots’) on the pan surface. These were found to correspond to dynamic changes in sediment availability (i.e., surface erodibility) associated with variations in surface crust characteristics and moisture conditions, rather than a function of sediment supply driven by ephemeral inundation. Validated linear spectral unmixing analysis of the pan surface mineralogical composition from satellite image time-series revealed the varying development of evaporite mineral crusts through the process of salt efflorescence, influenced by the timing of seasonal precipitation and recession of surface flooding. It was established that the emergence of highly emissive dust ‘hot-spots’ on the pan surface were associated with the intermittent degradation of the evaporite crust by high winds in the dry season, which exposed a large supply of fine textured and highly erodible clay and rock-forming silicate subsurface sediments that were extremely susceptible to aeolian entrainment. Field-based investigations using a portable wind tunnel confirmed that such differences in surface crust morphology and geochemistry, dependent on antecedent hydrological and climatic conditions, offer a first-order control on sediment availability for dust emission from Etosha Pan.</p>
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<p>Evidence from the research presented in this thesis suggests that dynamic shifts in erodibility conditions associated with the development and degradation of an evaporite (salt) crust, as a function of seasonal surface moisture regimes, play a significant part in determining the spatiotemporal variability of dust emissions. Attempts to better represent the controls and seasonal variations in surface moisture dynamics and subsequent moisture-related effects on surface salt crust properties and sediment availability therefore represent a key endeavour towards improving large-scale model simulations of dust emissions from globally significant ephemeral lake bed sources.</p>
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