EXTREME HYDROCLIMATE VARIABILITY IN KENYA'S BARINGO BASIN DURING THE LATE PLIOCENE: THE DIATOM RECORD

The Hominin Sites and Paleolakes Drilling Project has obtained sediment cores from six important hominin fossil and artifact sites in Kenya and Ethiopia in an effort to investigate the role of environmental forcing in shaping human evolution. One of those sites, Kenya's Tugen Hills, features extensive exposures of the Chemeron Formation, which contains ~100 fossil vertebrate localities including three hominin sites. A 227-meter drill core, dating from ~3.4 to 2.6 Ma, was recovered from the Tugen Hills. The core is characterized by fluvio-lacustrine sediments, including multiple diatomites. Before ~3 Ma, diatoms were present only in very low abundance, consistent with a fluvial or lake marginal setting. Five diatomites and three additional diatom-bearing intervals were observed after ~3 Ma. Two of these date to between 2.98-3.04 Ma. From 2.66 to 2.58 Ma, we observed four diatomites. Species of planktonic Aulacoseira and Stephanodiscus dominated all intervals, suggesting a fresh lake with a moderate to high supply of nutrients. Benthic diatoms were present in these intervals at very low relative abundances, typical of sediments deposited in relatively deep water. Transitional assemblages were largely absent. Instead, we observed a pattern of increasing diatom frustule dissolution at the tops of diatomite units, indicating lake waters with increased alkalinity. This pattern of preservation also occurred in the phytolith record for these intervals. A Na-bearing zeolite (analcime) indicative of saline waters was observed in intervals between diatomites after 2.66 Ma, suggesting this period was characterized by extreme climate variability. Planktonic Cyclotella menegheniana was observed at the base of two diatomites. This species is tolerant of rapidly fluctuating salinities indicative of lake-filling episodes. Between 2.66-2.98 Ma, diatoms were intermittently present at very low abundances and typically poorly preserved. The lack of diatom deposition/preservation in this interval suggests an alkaline lacustrine or wetland environment. This period was also characterized by a progression from K- and Ca-bearing zeolites to Na-bearing zeolites dominating the record, which supports inferences of increasingly saline conditions and suggests a long-term trend of increasing aridity.