Application of U/Th and 40Ar/39Ar Dating to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic Site in Ardèche, France

Véronique Michel,Samir Khatib,Sylvain Gallet,Michel Manetti,Guanjun Shen,Marie-Hélène Moncel,Jean Combier,Chung-Che Wu,Chrystèle Vérati,Chuan-Chou Shen

doi:10.1371/journal.pone.0082394

Abstract

Refined radio-isotopic dating techniques have been applied to Orgnac 3, a Late Acheulean and Early Middle Palaeolithic site in France. Evidence of Levallois core technology appeared in level 4b in the middle of the sequence, became predominant in the upper horizons, and was best represented in uppermost level 1, making the site one of the oldest examples of Levallois technology. In our dating study, fourteen speleothem samples from levels 7, 6 and 5b, were U/Th-dated. Four pure calcite samples from the speleothem PL1 (levels 5b, 6) yield ages between 265 ± 4 (PL1-3) and 312 ± 15 (PL1-6) thousand years ago (ka). Three samples from the top of a second stalagmite, PL2, yield dates ranging from 288 ± 10 ka (PL2-1) to 298 ± 17 ka (PL2-3). Three samples from the base of PL2 (level 7) yield much younger U/Th dates between 267 and 283 ka. These dates show that the speleothems PL1 and PL2 are contemporaneous and formed during marine isotope stage (MIS) 9 and MIS 8. Volcanic minerals in level 2, the upper sequence, were dated by the 40Ar/39Ar method, giving a weighted mean of 302.9 ± 2.5 ka (2σ) and an inverse isochron age of 302.9 ± 5.9 ka (2σ). Both 40Ar/39Ar dating of volcanic sanidines and U/Th dating of relatively pure and dense cave calcites are known to be well established. The first parallel application of the two geochronometers to Orgnac 3 yields generally consistent results, which point to the reliability of the two methods. The difference between their age results is discussed.

Highlights

The Orgnac 3 site is located at a place called Mattecarlinque, at an altitude of 320 m, on the southwest fringe of an Urgonian karstic plateau, in southern Ardèche, central France [1,2,3,4] (Figure 1)
The uranium content ranges from 66 ppb to 148 ppb, and thorium from 250 to 32,495 ppt
Samples are mostly free from detrital contamination as indicated by 230Th/232Th activity ratios higher than 20

Summary

Introduction

The Orgnac 3 site is located at a place called Mattecarlinque, at an altitude of 320 m, on the southwest fringe of an Urgonian karstic plateau (lower Cretaceous), in southern Ardèche, central France [1,2,3,4] (Figure 1). The site was initially a cave with human settlement, later changed into a rock shelter, and became an open-air site [5] (Figure 1). The lower archaeological levels (8 to 4a) were deposited in a cave context while the upper levels 2-1 were accumulated in an open-air environment. The lower levels (8 to 3) are attributed to the Middle Pleistocene (MIS 9) and the upper levels 2 and 1 to the late Middle Pleistocene (MIS 8). Levallois debitage, marking the beginning of the Middle Palaeolithic, appears in the middle strata and becomes predominant at the top of the sequence, producing changes in tool kits, raw material procurement and subsistence strategies [1,5].

Objectives

Methods

Results

Discussion

Conclusion