(The circle on the localization map indicates the studied area; red numbers, number of the sample in Table 2.) The Toumaï cranium is precisely located in the TM 266 section.

(Toumaï) (1, 2) have changed substantially the understanding of early human evolution in Africa (1–4).

It is first necessary to find some method of normalizing Be is from detrital input, of which a small fraction is dissolved (12, 13)], it has been shown that, in suitable instances, the dissolved beryllium isotopes are homogenized before deposition and that their ratio is a useful chronologic tool.

cosmogenic nuclide dating ppt-79cosmogenic nuclide dating ppt-67

Be concentrations were measured by graphite furnace atomic absorption spectrophotometry (at CEREGE on a Hitachi Z-8200) using Zeeman effect background correction.

The reproducibility of standard addition absorptions and the fit of standard addition lines were then used to determine Be ratios measured by accelerator MS at the Tandetron facility in Gif/Yvette (France).

Ages were determined at two hominid localities from the Chad Basin in the Djurab Desert (Northern Chad).

In the Koro Toro fossiliferous area, KT 12 locality (16°00′N, 18°53′E) was the site of discovery of (Toumaï).

Because all of the calculated MSWD were close or lower than 1.0, the uncertainty associated to the inverse-variance weighed mean ages σ We thank Profs.

Paul Rene, Edouard Bard, and Robert Finkel for advice and discussions; Prof. This manuscript benefited significantly from a thorough review by Francis H. This work was supported by the Ministère Tchadien de l'Enseignement Supérieur et de la Recherche (l'Université de N′Djamena and the Centre National d'Appui de la Recherche), the Ministère Français des Affaires Etrangères (Ambassade de France à N′Djamena, Service de Coopération et d'Action Culturelle, projet Fonds de Solidarité Prioritaire and Direction de la Coopération Scientifique, Universitaire et de la Recherche, Paris), the Ministère Français de l'Education Nationale et de la Recherche (Centre National de la Recherche Scientifique, Environment et Climat du Passé: Histoire et Evolution, the Agence National de la Recherche, Université de Poitiers), Région Poitou Charentes, and the National Science Foundation Revealing Hominids Origins Initiative.Sequential leaching procedures have been developed to selectively extract this authigenic component for dating purposes (14).It has effectively been demonstrated in marine systems that the can be accurately estimated, and the second is that the selected samples must have remained “closed” to entry or loss of the cosmogenic isotope and its normalizing isotope.The base of the mapped sections consists of a well developed, thick, aeolian facies (8). U.) is composed of poorly cemented sand and argillaceous sandstone alternation characterized by dense networks of root tubules/root molds (palaeosols) and termite nests (9, 10). The uniform stratigraphy at the TM localities allowed us to use absolute ages from both TM 266, where Toumaï was discovered, and TM 254 to assign an age to Toumaï.It was hoped that the tephra layer would contain material datable by the Be associates with continental particles, where it decays with a half-life of ≈1.4 million years.At both localities, the evolutive degree of the associated fossil mammal assemblages allowed a biochronological estimation of the hominid remains: early Pliocene (3–3.5 Ma) at KT 12 and late Miocene (≈7 Ma) at TM 266. This chronological constraint is an important cornerstone both for establishing the earliest stages of hominid evolution and for new calibrations of the molecular clock. Localities investigated in this area include TM 266 and TM 254 (Fig. The Kollé (KL), Kossom Bougoudi (KB), and Koro Toro (KT) fossiliferous areas were specifically studied to calibrate the authigenic Be dating method by direct comparison with biochronological estimations of co-located mammalian assemblages.