LATE-CENOZOIC CLIMATE VARIABILITY FROM SEDIMENTARY ARCHIVES OF AFRICA’S RIFT VALLEY LAKES: IMPLICATIONS FOR FAUNAL EVOLUTION

New sedimentary records of tropical continental climate variability have been recovered recently from scientific drill cores and carefully located piston cores in Lakes Malawi (700 m maximum water depth) and Tanganyika (1450 m maximum water depth), in the western branch of the East African Rift. In both of these large, deep lakes, we observe evidence for profound water level changes over time periods of 10³-10⁵ years. In the case of Lake Malawi, lake levels were more than 500 m lower than present at ~100 ka, and results from core analyses indicate that at least 20 lowstands of comparable magnitude occurred in that part of the rift over the past 1.3 million years. This relatively high-frequency lake-level variability had significant implications for the radiation and speciation of haplochromine cichlid fishes, and probably other endemic organisms as well. For example biodiversity of modern of Lake Malawi cichlids is highest around rocky coastal habitats. The current shoreline configuration has more than 500 km of rocky coastline, compared to ~700 km and ~300 km of sandy and mixed mud/sand vegetated coast, respectively. Integrated analyses of scientific drill cores and seismic reflection data indicate that paleoshorelines during the most recent (~100 ka) -500 m lowstand included <100 km of total rocky coastline, with sandy and mixed mud/sand vegetated coastline lengths totaling ~200 and ~25 km, respectively. Many of the Malawi lowstands also produced a physical separation of the northern and central lake basins, further isolating populations and facilitating allopatric speciation. The lake-level variability described here would have promoted rapid speciation of cichlids by changing habitat availability, total lake surface area, and through geographic isolation of parts of the species flock. In the case of Lake Tanganyika, the physical separation of endemic populations during lowstands was likely even more severe, on account of structurally-controlled bedrock highs on rift-segment boundaries.