Paleolimnological evidence for the onset and termination of glacial aridity from Lake Tanganyika, Tropical East Africa
Résumé
Geochemical and sedimentological data in a continuous 60,000-year sediment core record from the Kalya horst region of central Lake Tanganyika provide a detailed history of paleoclimate-mediated weathering and overflow events from upstream Lake Kivu. Univariate (elemental profiles), bivariate (elemental ratios) and multivariate analyses of chemical trends show variations between the dry Late Pleistocene (3218 ka cal yr BP) and the wetter conditions that both preceded and post-date that interval. This record places important new constraints on the timing of aridity in East Africa during the high-latitude Last Glacial Maximum (LGM) based on significant decreases in magnetic susceptibility and soluble cation concentrations coinciding with biogenic silica. Elemental indicators in the oldest portion of the sedimentary record (6050 ka cal yr BP) characterize this interval as a comparatively wet period, similar to modern conditions. Our record demonstrates that the ensuing transition toward arid conditions in tropical Africa during high-latitude glaciation was a two staged event with intermediate levels of aridity occurring from 5032 ka cal yr BP followed by intense aridity from 3218 ka cal yr BP. The initiation of inflow from upstream Lake Kivu into Lake Tanganyika is evidenced at 10.6 ka cal yr BP through its influence on both elemental profiles (Mg, Ca) and through its effect on 87Sr/86Sr. Increases in elemental (Mg, Ca, Sr) concentrations coincide with the timing of the Lake Kivu overflow. Metal geochemistry suggests that the overflow from Lake Kivu into Lake Tanganyika may have ceased between 8 and 6 ka cal yr BP, suggesting a period of Middle Holocene aridity in East Africa.