Cosmogenic dating ppt

This as well as the associated uncertainties is fully discussed in the Materials and Methods section.

In this first attempt of calibration in a continental context, the calculated authigenic Be ages obtained in or close to the fossiliferous levels were compared with the ages estimated by using the evolutive degree of fossil mammal assemblages.

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.

Much as is the case with B, which is undistinguishable from common boron.

The best way to do this is to measure recent samples deposited in an environmental context similar to that in which the sediments of interest have been deposited, that is, in our case, during periods of major lacustrine extension.

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.

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

The arid environment punctuated by wet periods during which a large shallow lake formed both provided Holocene sediments with which to determine the (Abel, KT 12) sampled for calibration purpose indicates that the Koro Toro fossiliferous locality is at least 3.3 Ma old, an age that is in the range of the biochronological estimation by using the mammal evolutive degree (3, 4, 36).

This new absolute dating constraint shows that , the earliest known hominid (1, 2, 40), is probably very close in time to this divergence contrary to the unlikely “provocative explanation,” which recently suggested a “possible hybridization in the human-chimp lineage before finally separating less than 6.3 Ma” (41).

(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).

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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.

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