Contribution of cathodoluminescence to the characterization and selection of quartz for ESR dating

International audienceElectron Spin Resonance (ESR) applied to optically bleached quartz grains extracted from sediment provides an age estimate for the last exposure of sediment to sunlight. This method has been increasingly used in archaeological, geological and geomorphological contexts for the last 30 years. However, its successful application is highly dependent on the geological context, the type and quality of the sampled material, but also its preparation. In most of the ESR dating studies, Equivalent Dose calculation uses the multiple ali-quot additive dose (MAAD) approach. Accuracy of the ED is the slightly correlated with the dose response of different aliquots of quartz grains to irradiation. In most of the measured quartz samples, a single saturating exponential dose response curve or a two components combining an exponential with a linear term are classically used in ESR dating to fit the experimental ESR data points derived from the aluminum (Al) or the titanium (Ti-Li, Ti-H) centers in quartz. However, large deviation and large associated errors may occasionally be observed. Heterogeneous bleaching (incomplete resetting) of the ESR dating signal can sometimes explain such deviations. Scattering of the dose response to irradiation may also be due to heterogeneous dose response of the different aliquots related to various nature and/or origin of the quartz grains constituting the dating sediment. Quartz grains extracted from several sediments were measured by ESR and observed using cathodoluminescence (CL) technique. The CL colours of quartz are weak, compared with emission of other minerals such as carbonates or phosphates, but are highly variable and can be related to genetic conditions of quartz formation. Hence, lumines-cence microscopy can be used to reveal internal structures, growth zoning and lattice defects in quartz crystals not discernible by means of other analytical techniques, but also to reveal heterogeneity of the quartz grains within the sample, or even the presence of other minerals, not removed during the chemical process of quartz purification. The cathodoluminescence gave evidence of several types of situation within our quartz samples. Some of them are made of monocrystalline quartz grains, emitting a uniform CL within the grain but with a light heterogeneity of the wavelength emission between the grains. Some are made of polycrystalline quartz grains, displaying a strong heterogeneity of the luminescence within and between the quartz grains. Finally we also observed some CL attributed to plagioclase and K-feldspar, which may contaminate, even strongly, the quartz samples. The presentation will report this variability and discuss about its effect on the dose response curve when using the ESR dating of quartz grains

Centre de la France : les formations fluviatiles du bassin de la Loire moyenne

International audienc

Géochronologie et Préhistoire des nappes fluviatiles fossiles du Loir vendômois à Naveil, Villiers, Thoré et Lunay (Loir-et-Cher).

National audienc

Evaluation of ESR residual dose in quartz modern samples, an investigation on environmental dependence

International audienceLuminescence and ESR dating methods of quartz sediment are based on the natural resetting of the signal by light exposure (optical bleaching). When the bleaching is incomplete, a residual dose (DeR) is added to the post-depositional dose accumulated since the deposit and hence the age is overestimated.Insufficient bleaching is usually linked to the environment and conditions of transport/deposition of the quartz grains affecting the light exposure duration. Indeed, each transportation mode – fluvial, marine or aeolian – is associated to specific conditions of light exposure, depending mainly to the location of grains in the transport agent during the transport phase, the opacity of the transport environment and the velocity of the transport.The present study attempts to discriminate the modes of transport/deposition providing a satisfying reset of the ESR signals of quartz grains. For this purpose, we investigated bleaching rates and ESR residual doses of aluminum centers from “present-day” aeolian, fluvial and marine sediments sampled in various sedimentary environments. The bleaching efficiency evaluation in these different environments may help for a better understanding of the resetting phenomenon for quartz signals which represents presently the main difficulty for ESR dating.The results show that the residual doses are small enough to allow an ESR dating of the main part of the sediment transported in almost all the context examined in this study. The smallest residual doses are obtained from quartz grains within the range of 100–200 μm and transported in clear water. Some limits for the application of optically bleached quartz ESR dating appears nevertheless, mainly when the residual dose and the dose accumulated after the deposit are quite similar, i.e. for Upper Pleistocene samples

Defining minimum reporting requirements for ESR dating of optically bleached quartz grains

More than 30 years after the first Electron Spin Resonance (ESR) dating application to optically bleached quartz grains by Yokoyama et al. (1985), the absence of standardization for reporting methodology and age results remains an obstacle for the development and recognition of the ESR dating method within the Quaternary scientific community. To overcome this issue, the present work proposes some basic guidelines which should hopefully be useful not only for the ESR dating community, but also for any potential reviewers who may not be familiar with the specificities of this field.Mathieu Duval, Jean-Jacques Bahain, Melanie Bartz, Christophe Falguères, Verónica Guilarte, Davinia Moreno, Hélène Tissoux, Miren del Val, Pierre Voinchet, Lee J. Arnol

Lower and Middle Pleistocene human settlements recorded in fluvial deposits of the middle Loire River Basin, Centre Region, France

This paper records the findings from c. 80 prehistoric sites that have been discovered in the alluvial deposits of the rivers Creuse, Cher, and Loir, tributaries of the middle Loire River, over the period since 1981. These deposits comprise river terrace aggradations formed during successive glacial-interglacial cycles which have recorded climate and environment during Quaternary time. The systematic dating of these river deposits by Electron Spin Resonance (ESR) applied to optically bleached sedimentary quartz has resulted in the establishment of a chronological framework for the evolution of these rivers during Lower and Middle Pleistocene (between 1.7 Ma and 130 ka). Evidence for Early Palaeolithic (Mode 1) industries with an in situ context (workshops, soils) in the highest aggradations indicates that Hominins were present in the study area, near the geographical centre of France (47°N), around 1.1 Ma. Examination of the sites indicates that Human occupations were located along valley bottom sites during temperate episodes. Subsequently, after a gap of several hundred thousand years industries with handaxes appear in the Middle Loire Basin in the interval between 700 and 600 ka, and then continuously from 400 ka. These two phases of settlement produced industrial assemblages with clear differences in their responses to the supplies of raw materials and in the modes of making flake

Lower and middle Pleistocene human settlements in the Middle Loire River Basin, Centre Region, France

none16Evidence of earliest human settlements observed in the alluvial formations of the Middle Loire Basin during systematic surveys organised since 1981. Many stepped fluvial terraces deposited during the successive interglacial–glacial Quaternary cycles have been identified in three tributary valleys of the Loire River: the Creuse, Cher and Loir Valleys. These alluvial remnants were systematically dated by the Electron Spin Resonance (ESR) method applied on optically bleached quartz, in order to construct a chronostratigraphical framework for the fluvial systems and the associated archaeological localities. Four localities containing Early Palaeolithic industries with Mode 1 technology, occupied during the Lower Pleistocene, and ten sites characterized by assemblages with handaxes, discovered in the Middle Pleistocene alluvial formations, are described in this paper. These two sets of localities are characterized by lithic industries with many differences in supply of raw materials and in technical behaviours. The results indicate that this area was reached by two separate phases of human settlement (one in relation to Mode 1, the other to Mode 2) with an interval around 400 ka, at the end of the Lower Pleistocene, during a period characterized by major climatic degradations.mixedDESPRIEE J.; VOINCHET P.; TISSOUX H.; MONCEL M-H.; ARZARELLO M.; ROBIN S.; SALA R.; BAHAIN J-J.; FALGUERES C.; COURCIMAULT G.; DEPONT J.; GAGEOONET R.; MARQUER L.; MESSAGER E.; ABDESSADOK S.; PUAUD S.Despriee, J.; Voinchet, P.; Tissoux, H.; Moncel, M. H.; Arzarello, Marta; Robin, S.; Sala, R.; Bahain, J. J.; Falgueres, C.; Courcimault, G.; Depont, J.; Gageoonet, R.; Marquer, L.; Messager, E.; Abdessadok, S.; Puaud, S