U-TH DISEQUILIBRIUM DATING OF LAKE MAGADI CHERTS

Early formed “syndepositional” cherts in sediment cores from Lake Magadi, Kenya, have been dated by the Uranium-Thorium disequilibrium technique. Lake Magadi was cored to bedrock in June 2014 as part of the Hominin Sites and Paleolakes Drilling Project (HSPDP), which aims to provide a high-resolution paleoenvironmental context for human evolution in East Africa. The cores (1A, B and C: total depth of 137 meters and 2A: total depth of 198 meters) are composed mainly of zeolitic mud, chert, trona, and calcite, and contain a Pleistocene and Holocene record of closed-basin paleoenvironments and paleoclimate in a tectonically-active rift basin.

Cherts in the Magadi cores formed by replacement and cementation of siliceous gels, magadiite, evaporites (trona), diatomaceous deposits, and carbonate shell beds (ostracodes and gastropods). Early chertification is indicated by randomly-oriented plant and insect fragments, and “labyrinth” patterns that indicate lithification of uncompacted siliceous gels, in which sediment grains were displaced by air. Magadi cherts incorporate significant amounts of uranium but little initial thorium as shown by their high ²³⁰Th/²³²Th and ²³⁴U/²³²Th ratios. Ages for such samples can be calculated from measurement of ²³⁰Th/²³⁴U and ²³⁴U/²³⁸U. Ten cherts in core 1A, from 24 m to 88 m in depth, range in age from 11.8 to 285 ka. Nine cherts from core 2A, from depths of 36 to 107 m, range in age from 12.4 to 267 ka. Three correlated stratigraphic intervals from core 1A and 2A have U-Th ages that overlap. The HSPDP Magadi cores confirm a shift from an early shallow dilute lake stage, evolving to a narrow, at times anoxic, stratified, saline lake. The suite of chemical sediments (zeolites, chert, magadiite, and trona) indicates alkaline saline lake conditions for much of the history of the Magadi Basin. Chert dates from trona layers show the first evaporites were deposited in the Magadi basin ~119 ka, much earlier than previously thought.