Kenya's Baringo-Tugen Hills-Barsemoi drill site is one of six localities across Kenya and Ethiopia from which the Hominin Sites and Paleolakes Drilling Project has obtained sediment cores in an effort to investigate the role of environmental forcing in shaping human evolution. The Baringo Basin site features extensive exposures of the Chemeron Formation, which contains >100 fossil vertebrate localities including five hominin sites. The 228-m drill core, dating from ~3.29 to 2.56 Ma, is characterized by fluvio-lacustrine sediments, including multiple diatomites, with evidence of variable degrees of later pedogenic modification. In the lower part of the core (~3.29–3.04 Ma), diatoms were preserved only in very low abundance, consistent with predominantly fluvial or lake marginal environments. In contrast, five diatomites and two additional diatom-rich intervals were deposited after ~3.04 Ma, reflecting a major shift in the basin hydrology. Planktonic freshwater species dominated these diatom-rich intervals, whereas periphytic taxa were present in proportions less than 2%, suggesting that these intervals represent open-water deposition during lake highstands. Littoral or saline assemblages are largely absent throughout the core. Instead, we observed a pattern of increasing diatom frustule dissolution at the tops and bottoms of diatomite units, indicating increased alkalinity during the transgressive/regressive phases. A Na-bearing zeolite (analcime) indicative of saline waters precipitated in clastic-dominated intervals between diatomites, suggesting extreme environmental variability between lake highstands and lowstands. Diatom assemblages were consistently dominated by a few species belonging to the genera Aulacoseira and Stephanodiscus, which were at times co-dominant. We infer that assemblages dominated by Aulacoseira represent a well-mixed lake with abundant supply of silica. When Stephanodiscus was dominant, which occurred more frequently in the later freshwater phases, we infer incomplete mixing and reduced silica flux to the epilimnion (upper water layer).
Bibliographical noteFunding Information:
This research was conducted as part of the Hominin Sites and Paleolakes Drilling Program (HSPDP). Initial core processing and sampling was carried out at the National Lacustrine Core Facility (LacCore) which houses the cores. Funding for this work was provided by National Science Foundation (NSF; United States) grants EAR-1123942 , BCS-1241859 , EAR-1338553 , and the International Continental Scientific Drilling Program (ICDP) . M.S. acknowledges support from the Swiss National Science Foundation grant P300P2_158501 (SNF ; Switzerland ). We are grateful for discussions with HSPDP colleagues Anne Billingsley, Alan Deino, Daniel Deocampo, Rachel Lupien, and Karim Minkara. We thank Robert Edgar for providing microscope slides from the Barsemoi diatomites for comparison. We thank Sabrina Brown, Kendra Reininga, Ellie Gaskill, Emily Brana, Helena Bierly, and Erika Smith for assistance in subsampling, processing samples and preparing slides. We also thank two anonymous reviewers for valuable comments that improved the manuscript. This is publication #19 of the Hominin Sites and Paleolakes Drilling Project.
© 2019 Elsevier B.V.
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- Alkaline lakes
- Biogenic silica
- East African rift
Continental Scientific Drilling Facility tags