Constraining the Relative Influences of Watershed and Authigenic Processes Using An Integrated Sedimentological-Geochemical Approach in Paleo Lake Mababe, Botswana

Sediment samples were obtained from two trenches in the presently dry Lake Mababe, a graben within the Okavango Rift Zone, NW Botswana. Samples from each trench, MabX and Mab6, were analyzed for TOC and inorganic carbon (IC) concentrations to examine changes within the paleo-lake and how such variations reflect both internal processes as well as processes in the surrounding watershed.

MabX, located near the center of the lake, was sampled every five cm to a depth of 5.6 meters. TOC results indicate that conditions within the lake were relatively constant from 400 cm depth to 560 centimeters. Small fluctuations where there were periods of increased organic matter production within the lake are discernable between 135 cm and 400 cm. These instances of increased organic carbon content indicate possible dry periods in the watershed area where sediment input to the lake was reduced and in-lake processes led to conditions that favored higher productivity and/or TOC preservation.

Analysis of sediments from 135 cm to the surface reveal an overall trend of increasing organic carbon production, which indicates a significant change took place in the history of the lake, whereby, the lake was potentially cut-off from the catchment area and a reduction in sediment supply led to the dominance of in-lake processes that favored an increase in organic carbon production and/or preservation. These results are consistent with those observed in the Mab6 core, located closer to the shoreline. Additional analyses of trace metals and phosphorous concentrations will be combined with grain-size analysis to further constrain the paleolimnology of Lake Mababe. This approach will allow further refinement of the conceptual models concerning the preservation of signals from watershed and in-lake processes, and may shed light on interactions between the surface and the lithosphere in the development of faulting on the incipient Okavango Rift Zone.