PRELIMINARY GEOCHEMICAL ANALYSES OF SEDIMENTS FROM THE OKAVANGO DELTA, NORTHWESTERN BOTSWANA TO FURTHER CONSTRAIN THE PALEOLIMNOLOGICAL HISTORY OF THE PALEOLAKE MABABE SYSTEM

The middle Kalahari of northwestern Botswana is currently a semiarid environment, but has experienced wetter climates in the past. Notably, over the past 65 kya, the region was home to a series of large lakes, which are now represented by a series of salt pans and flats. Paleolake Mababe existed in what is now the dry Mababe Depression. Previous geochemical and sedimentological analyses of samples recovered from a trench in the now-dry Mababe Depression have been used to develop a model whereby lake levels were controlled by the interactions between climate variations in the Angolan highlands and tectonic events, which resulted from the southwestward propagation of the Okavango Rift Zone. However, the relative effects of climatic versus tectonic processes in the development and ultimate demise of the lake remains unknown.

To address this issue, we are conducting an integrated sedimentological and geochemical study of sediments sampled from a series of quarries in the distal region of the Okavango Delta in July, 2010. Petrographic descriptions, grain-size analyses and geochemical analyses of the sediments will be used to determine the processes responsible for lake conditions. For example, during times characterized by locally dry conditions, variations in the K, Na and Ca concentrations provide constraints on evaporation rates, while variations in C:N ratios are indicative of shifts in local vegetation patterns as swamps develop around a stable lake. Sediments derived from the Angolan highlands, during time of wet tropical climates are characterized by changes in metal (e.g., V, Fe, U and Th) concentrations relative to Si and inorganic carbon concentrations. Taken together, our results will be used to deconvolve the effects of changing climatic regimes driven by shifts in the ITCZ and the tectonic reorganization of the local hydrologic regime. Ultimately, understanding the coupled roles of such processes will be critical for future predictions of the water balance in the Okavango Delta, and future planning of increasingly scarce water resources.