LATE PLEISTOCENE LAKE-LEVEL HISTORY OF PALEOLAKE MABABE

The Mababe Depression is part of a large network of river and lake basins spanning the Middle Kalahari Desert region in southern Africa. Despite its arid climate today, geomorphic evidence indicates that the Mababe, Ngami, and Makgadikgadi basins, and much of the area that surrounds them, were once joined by a single large paleolake system, commonly referred to as Megalake Makgadikgadi. Recent chronometric data from beach ridges has demonstrated that this paleolake system has undergone repeated changes in lake level, joining and isolating the lake basins at least seven times in the last 130,000 years. Because of the tectonic complexity of the region, attempts to reliably determine the depth and volume of this megalake system from geomorphic evidence have been confounded. The absence of long, continuous paleolimnological records from this region and considerable hydrological sensitivity of the region has also hindered a unified reconstruction of its long-term history.

By analyzing fossil diatom assemblages and sediment geochemistry from a 5.6-m trench of lacustrine sediments in the southeastern region of the Mababe Depression, we have reconstructed ~60,000 years of lake-level fluctuations for Megalake Makgadikgadi. Fossil diatom assemblages indicate repeated changes in lake level, evidenced by substantial variability in the dominant species. Multiple transitions between planktic and benthic flora as well as significant changes in the dominant flora from freshwater to saline taxa occur to substantiate fluctuating lake levels.