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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).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.
Lacustrine diatoms built their silicate frustules from dissolved elements and thus are an ideal substrate for registering the dissolved isotopic ratio of ) for the system studied here, we measured selected diatomites deposited in the north part of the basin during the last Holocene expansion of Lake Mega-Chad (32) (≈7 to 4.4 ka BP), obtaining a value of (2.54 ± 0.09) × 10Be ages were calculated with the inverse-variance weighted mean when more than two samples are considered and with the weighted mean when only two are.The green pelite level sampled at Koro Toro yields an authigenic Be age of 3.58 ± 0.27 Ma, whereas at Kollé, the sampled siliceous (diatomites and siltites) levels yield a mean age value of 3.96 ± 0.48 Ma (Table 2 and Fig. This is remarkably consistent with the mammal faunal age estimations, considering that at Koro Toro, the level sampled for this study is within the level from which the fauna and the Be ages obtained from four samples taken in a green pelite level (Table 2) interbedded within the discussed fossiliferous poorly cemented sandstones yield an inverse-variance weighted mean age of 5.26 ± 0.29 Ma, again remarkably consistent with the biochronological estimation.The demonstrated systematic strong agreement between the biochronological estimations and the calculated authigenic Be ages based on an initial authigenic ratio determined from sediments deposited in Holocene Lake Mega-Chad demonstrates that the sedimentary levels (clays, diatomites, argillites, siltites, and poorly cemented sandstones) deposited in the Chad Basin during wet periods (5) accompanied by major lacustrine extension in an area otherwise characterized by a recurrent desert climate since at least 8 Ma (8) have remained closed to gain or loss of beryllium other than by radioactive decay despite cycles of inundation and desiccation.This is unlike most other radiochronological methods (U-Th, Rb-Sr, Sm-Nd, K-Ar, Lu-Hf, Re-Os, etc.) that rely on parent–daughter ratios to determine ages.
In addition, because the absolute Be concentration depends also on parameters such as the scavenging efficiency and the specific surface area of the sedimentary particles, it cannot be used directly to determine an age.This opens up the possibility to date sedimentary deposits in the range of 0.2 to 14 Ma.