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sur le développement des zones arides et semi-arides

Titre : Quaternary megapaleolake system in Northwest Botswana : evidence of Lacustrine deposition and geographical extent using multiple geochemical proxies

Auteur : Kgotlaebonywe, Charity

Université de soutenance : Botswana International University of Science & Technology (BIUST)

Grade  : MSc Geology 2020

Résumé
Reconstruction of climate dynamics in southern Africa has been immensely constrained by the absence of continuous proxies records. Sediments provide potential sources of studying past environments because their makeup is a direct response to variabilities in the environment and climate. In this study a multiple geochemical proxy approach was used to investigate two 30 m deep sediments cores from northern Botswana to construe their various geological processes and environments of deposition. Based on these proxy studies two distinct but dissimilar hydrological and climatic settings were identified on the two cores. For BH 11 lithological studies reveal a sediments sequence that has a bottom unit with several repetitive and alternating layers of sand-silt-clay, and a dominantly silty/clayey unit at the top. This arrangement of units likely indicates lake-related processes with input from regional rivers. Magnetic susceptibility measurements indicate a dual source of the sediments. Sediments in the lower unit (30 to 7 m depth) could have come from a distal sub-tropical source with rocks that are richer in magnetic minerals. Sediments in the upper section (from 7 m depth to surface) were strongly influenced by a more felsic source that was likely of local surrounding landscapes. Carbon dynamics in the middle portions of the lower unit of BH 11 suggests deposition under more humid conditions than in the upper section, within a periodically drying out fluvial system. Isotopically lighter δ 13Corg values (-26 to -28 ‰) for the lower section compared to the upper section ( -20‰) reflect difference in the vegetation cover, moisture conditions and probably climate between the two sections. For core BH 10, sediments in the lower unit were sourced locally from a less magnetic and humid landscape. The upper unit signifies continually drying up condition to present day. The two sites may have hosted major lakes at different times in the past. This study validates the effectiveness of sediments in unravelling environmental and climate change in fluvial-lacustrine depositional settings, and gives the possibilities of connecting the sediment record to regional stratigraphic markers.

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