Palaeolinmological evidence for widespread late 18th century drought across equatorial East Africa

摘要

The sediment records of three shallow climate-sensitive lakes in equatorial East Africa (Chibwera and Kanyamukali in western Uganda, and Baringo in central Kenya) contain clear lithological evidence of an episode of complete desiccation in the relatively recent past. Quantitative stratigraphic analyses of bulk sediment composition, texture (% sand and coarse organic detritus), the ON ratio of bulk organic matter, and magnetic susceptibility allowed us to define a common succession of sedimentary units in all three takes. This common stratigraphy includes a basal unit of dry and stiff low-organic clays capped by a horizon of peaty mud or peat, overlain by organic lake sediment. We interpret this stratigraphy to represent an episode of (nearly) complete desiccation of these lake basins. This was followed first by an early phase of modest lake refilling, during which time extensive littoral and submerged vegetation developed, producing peaty lake deposits. Mostly higher lake levels later on resulted in deposition of fine-grained lake muds. Using Pb-210 and Cs-137 activity profiles combined with four AMS C-14 dates on single terrestrial plant macrofossils or multiple charred grass particles indicate that in all three lakes the episode of most recent complete desiccation occurred in the late 18th and early 19th century. These results add to an accumulating body of palaeoenvironmental data revealing a prolonged period of extreme drought in equatorial East Africa about 200 years ago, also documented in the cultural history of the African Great Lakes region. Given the geographical spread of the study lakes across the East African plateau, these results indicate that the climatic anomaly of reduced rainfall responsible for this drought was at least sub-continental in scale. (c) 2007 Published by Elsevier B.V.