Multi-scale Impacts of Dune Reactivation in the Southern Kalahari

Satellite data suggest that deserts and basins of internal drainage are the largest contributors of dust, and most sources are located in the Northern Hemisphere. The lower dust emissions in the Southern Hemisphere in part limit the supply of micronutrients to the lower deposition regions such as the Southern Ocean, thereby limiting its productivity. It has been suggested that overgrazing combined with regional climate change in the Kalahari portion of Botswana could lead to large-scale reactivation of stabilized Kalahari dunefields. The dependence of sediment fluxes and dust emissions on vegetation cover in the Kalahari dunelands remain poorly understood, which prevents a quantitative assessment of possible changes in aeolian activity in this region under different land use and land cover scenarios. In this dissertation, I show that vegetation loss and dune remobilization in the southern Kalahari can promote dust emissions comparable to those observed from major contemporary dust sources in the southern Africa. Dust generation experiments support the hypothesis that in the southern Kalahari sediments are capable of emitting substantial amounts of dust from interdune areas. The results indicate that dust from these areas is relatively rich in soluble iron, and commonly reaches the Southern Ocean. The results of an aeolian sediment sampling campaign allows for the quantification of potential rate of dust emissions under scenarios of vegetation loss in the southern Kalahari. This dissertation also investigates the resilience of duneland vegetation that is currently undergoing conditions of incipient degradation in areas closer to villages and boreholes. Observations at long term grazing exclosure experiments indicate that the perennial grasses reestablish on the recovery dunes, and ii bare dunes are poor seed banks. Thus, changes in grass cover and grass community composition, seed bank depletion, and decline in soil nutrient content can be used as indicators of imminent regime shifts from vegetated to bare dunes in the southern Kalahari. This dissertation also provides an estimate of the nutrient content (total nitrogen, total phosphorous, bioavailable iron and common anions) in dust that can be emitted both by currently active and potential dust sources in Southern Africa.

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