The nature of pan sediments: A case study on dust supply from the Makgadikgadi pans, Botswana

Doctoral Thesis


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University of Cape Town

Understanding dust sources is essential, as aerosols from arid basins have widespread effects on atmospheric, biological and terrestrial processes on regional to global scales. This thesis presents the results of chemical and morphological analysis for sediments from Sua Pan, Botswana, which forms part of the Makgadikgadi Basin, southern Africa's most productive dust source. The analysis was performed in order to evaluate pan sediment characteristics and erosivity controls including the analysis of weather and climatological conditions. We analysed 41 grab samples consisting of crusts, fluff, and soil from 12 locations within a 144 km² grid (centred on 25.959°E and -20.5754°S) as well as 6 airborne samples collected using frisbees, BSNE traps and PI-SWERL exhaust, between July and October 2011. Dry sieving and laser diffraction for all 41 samples revealed a dominance of uni-modal, sand sized sediments, with the particle size distribution controlled by evaporite minerals, inferred from sample digestion. XRD and ICP analysis on 8 and 22 samples respectively, identified evaporite minerals including halite, thenardite, mirabilite and trona, in addition to carbonate minerals, which represent the young transient mineralogy of the pan. Older metal oxides were also present, originating from parent substrates. The addition of QEMSCAN analyses performed on 5 airborne and 1 surface sample, identified feldspar and pyroxene, along with quartz in its amorphous state, which was further confirmed through SEM analysis. QEMSCAN evaluated 255 minerals, many of which were below the detection limits of the 20 minerals identified through XRD analysis. QEMSCAN further attributed mineralogy and morphological detail to 27 000 individual grains per sample, positively linking airborne samples with their surface source. Seven years of twice daily MODIS images were analysed to identify the emission signature of the Makgadikgadi complex, revealing a number of synoptic states associated with emissions, peaking in July and driven largely by ridging anticyclones. The emission signature of the pans revealed that they respond to diurnal, seasonal and inter-annual controls, moisture dynamics and crust conditions with anthropogenic stresses also being evident. This study has interpreted the driving mechanisms and limitations associated with both the micro and macro scale of dust entrainment, advancing the current understanding of the Makgadikgadi as a dust source through interpreting these controls at the local scale.

Includes bibliographical references.