CHANGES IN BIOLOGICAL PRODUCTIVITY IN RESPONSE TO TECTONIC AND CLIMATE PROCESSES RECORDED IN THE SEDIMENTS OF PALEOLAKE MABABE, NORTHWESTERN BOTSWANA

The Mababe Depression in northwestern Botswana, Africa is in an arid environment, yet previous studies indicate the persistence of lacustrine conditions over the last ~70 ka. Paleolake Mababe is characterized by changes in water balance, which, in turn, should affect biological productivity. To test this hypothesis, we have analyzed sediments recovered from a trench dug in the Depression. Overall, sediments are characterized by accumulations of diatom-rich intervals that decrease in thickness and abundance from the base of the trench (560 cm) to the surface. Sediment leach chemistry (Ca, Mg, Na, K), concentrations of inorganic and organic carbon (C_org), phosphorus and the isotopic composition of C_org were used to develop a conceptual model for the biological response to climate shifts and the propagation of faulting in the Okavango Rift Zone (ORZ).

Geomorphological evidence indicates that fault movement on the ORZ changed regional hydrology by interrupting and diverting flow from the Okavango and Kwando Rivers, and altered basin morphology through northward tilting. Sediments show major changes at 240 cm, indicating a major alteration in water and sediment budgets from this tectonic activity. Sediments below 240 cm indicate that the lake was relatively deep and stable. The sediments are dominated by diatom-rich intervals that vary in response to conditions of increased evaporation and nutrient input from the local watershed. C_org concentrations are generally low in this section, and have C:N ratios that reflect an aquatic source. Sediments above 240 cm have low diatom abundances, with C_org concentrations and C:N ratios that generally increase toward the surface. The organic matter is characterized by isotopic compositions that reflect differential mixing of material derived from C3 and C4 plants. Such shifts in vegetation likely reflect variations in local climate between relatively wet and more arid times. Overall, our results support a model whereby the lowermost (below 240 cm) sediments were deposited from a large, stable lake, with overlying sediments reflecting the draining of Paleolake Mababe to form a shallower lake, surrounded by a swamp or marsh. Thus, indicators of biogeochemical processes provide important constraints of climatic and tectonic process operating on both a regional and local scale.