Environmental instabilities in the Suguta Valley, Northern Kenya Rift, during the 'African Humid Period'

At least three episodes characterized by the occurrence of large, but fluctuating lakes have been identified in East Africa during the last three million years: at ~2.7-2.5 Ma, ~1.9-1.7 Ma, and ~1.1-0.9 Ma. These three episodes of a wetter and more variable climate seem to correlate with key stages in hominin evolution. Little is known, however, about the short-term climate variability during these episodes. We therefore studied the most recent dry-wet-dry cycle in the Suguta Valley where a 300-m-deep lake has formed during the so-called African Humid Period (14.8-5.5 ka BP). Today, the extremely arid basin hosts the shallow and highly-alkaline Lake Logipi. We reconstructed a paleo-lake level record for the last ca. 14 ka from up to 40 m thick lake-sediment sequences at three locations in the ~2,500 km2 paleo-lake area. The sediments have been investigated for sediment characteristics such as grainsize distributions, detrital and authigenic mineral phases, geochemical properties and microfossil assemblages. The stratigraphy for the sequences is based on 38 AMS 14C ages of biogenic carbonate and charcoal samples. Parallel dating of charcoal and snail-shell samples revealed by 1,600-2,200 year older carbonate ages suggesting a remarkably high, but well defined reservoir age for paleo-Lake Suguta. The lake-level record of mega-Lake Suguta suggest significant fluctuations between 14 and 5 ka BP, in particular during the later stages of the wet episode between 9.5 ka and 5 BP. Together with 14C dated sequences of paleoshorelines, the lake fluctuated by ca. 100 m from the highest level in relatively short periods of time. This observation suggests a highly nonlinear response of mega-Lake Suguta to sinusoidal insolation forcing with relatively stable conditions during maximum northern hemisphere insolation and flickering hydrological conditions during decreasing insolation until a certain threshold is reached and the lake disappeared. The results of the analysis will provide new information about the sensitivity of East African Rift Valley lakes to climate change. Moreover, it might help testing hypothesis of climate-evolution linkages.