Carbon Concentrations and Stable Carbon Isotopes as Indicators for Wet and Dry Phases of Paleo-Lake Mababe, Botswana: Providing International Research Experience for Students

We analyzed sediments from Mababe depression for organic and inorganic carbon concentration and stable carbon isotopes ratios as part of our NSF-IRES project. The Mababe depression is a sub-basin of the Okavango basin and little is known about the environmental history of Paleo-Lake Mababe which occupies this depression. The historical area extent of Paleo-Lake Mababe is determined by a series of paleo-shorelines which suggest periodic wetting and drying phases that affected lake volume and productivity. Our study objective was to determine the causes of wetting and drying events and how they manifested in the stable carbon isotope ratios of lake sediments. We obtained continuous sediment samples at 5 and 20 cm from two trenches dug in the proximal and distal lake regions in the southwest portion of the Mababe depression. The sediments showed variations in organic carbon concentration with high carbon percent corresponding to enriched carbon isotopes ratios and low carbon concentrations concomitant with depleted carbon isotope ratios. The sediment intervals showing enriched carbon isotope ratios and lower organic carbon concentrations indicate lacustrine dominated conditions. Higher lake levels were likely supported by local rains in addition to discharge from the Okavango River. The more depleted carbon isotope ratios indicate drier conditions with diminished local rains and perhaps less recharge from the Okavango. Thus, increasing trends in the organic carbon concentration may be due to decrease in watershed input of carbon into the lake from the Angolan Highlands via the Okavango Delta. The inorganic carbon concentration is decreased during wetter periods due to lower lake productivity. Our results show that carbon isotopes in conjunction with major sediment chemistry indicate distinct wet and dry phases in the paleo-environmental history of the Mababe depression.