Diachronous dawn of Africa's Middle Stone Age: New ⁴⁰Ar/³⁹Ar ages from the Ethiopian Rift

The Middle Stone Age (MSA) of Africa, like the Middle Paleolithic of Europe, is thought to represent a time period wherein toolmakers acquired significant increases in cognitive abilities and physical dexterity. Existing data fail to resolve whether the MSA emerged gradually, abruptly, or discontinuously, and whether this industry reflects the activity of Homo sapiens. Here we present new 40Ar/39Ar geochronological data revealing that advanced MSA archaeology at two sites in the main Ethiopian Rift is older than 276 ka, much older than technologically comparable MSA archaeology from elsewhere. An age of 183 ka for a unit farther upsection, along with the technological stasis observed throughout the section, indicates that similar technology was used here for ~93 ka. These results suggest that MSA technology evolved asynchronously in different places, and challenge the notion of a distinct time line for either the appearance of the MSA or the disappearance of the earlier Acheulean. These and other recent results indicate that the oldest known MSA consistently predates fossil evidence for the earliest Homo sapiens

IUPAC-IUGS recommendation on the half life of 87Rb

The IUPAC-IUGS joint Task Group “Isotopes in Geosciences” recommends a value of (49.61 ± 0.16) Ga for the half life of 87Rb, corresponding to a decay constant λ87 = (1.3972 ± 0.0045) × 10-11 a-1

Re–Os geochronology of a Mesoproterozoic sedimentary succession, Taoudeni basin, Mauritania: Implications for basin-wide correlations and Re–Os organic-rich sediments systematics

The exceptionally well-preserved sedimentary rocks of the Taoudeni basin, NW Africa represent one of the world's most widespread (> 1 M km2) Proterozoic successions. Hitherto, the sedimentary rocks were considered to be Mid Tonian based on Rb–Sr illite and glauconite geochronology of the Atar Group. However, new Re–Os organic-rich sediment (ORS) geochronology from two drill cores indicates that the Proterozoic Atar Group is 200 Ma older (1107 ± 12 Ma, 1109 ± 22 Ma and 1105 ± 37 Ma). The Re–Os geochronology suggests that the Rb–Sr geochronology records the age of diagenetic events possibly associated with the Pan African collision. The new Re–Os geochronology data provide absolute age constraints for recent carbon isotope chemostratigraphy which suggests that the Atar Group is Mesoproterozoic and not Neoproterozoic. The new Re–Os ORS geochronology supports previous studies that suggest that rapid hydrocarbon generation (flash pyrolysis) from contact metamorphism of a dolerite sill does not significantly disturb the Re–Os ORS systematics. Modelled contact conditions suggest that the Re–Os ORS systematics remain undisturbed at 650 °C at the sill/shale contact and ≥ 280 °C 20 m from the sill/shale contact. Moreover, the Re–Os geochronology indicates that the West African craton has a depositional history that predates 1100 Ma and that ORS can be correlated on a basin-wide scale. In addition, the Re–Os depositional ages for the ORS of the Taoudeni basin are comparable to those of ORS from the São Francisco craton, suggesting that these cratons are correlatable. This postulate is further supported by identical Osi values for the Atar Group and the Vazante Group of the São Francisco craton

Preliminary dating of the Viluy traps (Eastern Siberia) : eruption at the time of Late Denovian extinction events ?

International audienceIn this short note, we report new age determinations from four samples of the Middle-Paleozoic Viluy Traps in Siberia, east of the more famous Permo-Triassic Siberian Traps. These samples, which were collected from three drill cores, have been analyzed in parallel and independently in the Orsay (France) and Berkeley Geochronology Center (BGC; USA) geochronology laboratories, using respectively the Cassignol-Gillot K-Ar and the Ar-40/Ar-39 techniques. Dating these samples as a concerted effort in two independent laboratories working jointly on their interpretation is a rather rare yet very valuable exercise. With the K-Ar technique, ages ranging from 338 to 367 Ma with uncertainties on the order of 5 Ma were obtained. With the Ar-40/Ar-39 technique, integrated ages range from 344 to 367 Ma, with uncertainties on the order of 1 Ma, and two samples yielded plateaus, i.e. the best determined ages, at 360.3 +/- 0.9 and 370.0 +/- 0.7 Ma. Three out of four ages yielded by the two separate methods are in agreement within uncertainties. One sample yields incompatible ages and could be from a later, altered dyke event. The Ar-40/Ar-39 plateau age of 370.0 0.7 Ma (conventional calibration) or 373.4 +/- 0.7 Ma (recalculated per Renne et al., 2010), the most reliable age obtained in this study, is compatible with recent determinations of the Late Devonian extinction events at the end-Frasnian (similar to 376 +/- 3 Ma). These results underscore a need for further work, in progress. (C) 2010 Elsevier B.V. All rights reserved

Preliminary dating of the Viluy traps : eruption at the time of Late Devonian extinction events ?

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