This paper aims to provide an overview concerning the optically stimulated luminescence OSL dating method and its applications for geomorphological research in France. An outline of general physical principles of luminescence dating is given. A case study of fluvial sands from the lower terrace of the Moselle valley is then presented to describe the range of field and laboratory procedures required for successful luminescence dating.
The paper also reviews the place of OSL dating in geomorphological research in France and assesses its potential for further research, by focusing on the diversity of sedimentary environments and topics to which it can be usefully applied.
Hence it underlines the increasing importance of the method to geomorphological research, especially by contributing to the development of quantitative geomorphology. They are now largely used to date not only palaeontological or organic remains, but also minerals that characterise detrital clastic sedimentary material. The most common Osl dating limits and fits applied to minerals are cosmogenic radionuclides, electron spin resonance ESR and luminescence techniques.
The latter were first applied to burned minerals from archaeological artefacts [thermoluminescence TL method].
Improvements of this technique led to the development, for more than twenty years, of the optical dating method [commonly referred to as Optically Stimuled Luminescence OSL ] which is now applied to sediments from various origins Wintle, The aim of this paper is to provide people involved in geomorphological research a global overview about Osl dating limits and fits principles and procedures of optical dating, from the field sampling to the age interpretation.
Most of the publications actually focus on one part of either the method e. The general principles of the method are described first. The paper then explains how OSL dating is applied to obtain a depositional age, through the field and laboratory procedures employed.
These procedures are described as clearly as possible in order to provide useful information for geomorphologists interested in the method, and illustrated by a case study that has involved luminescence dating of fluvial sands samples LUM and LUM from the lower alluvial terrace of the Moselle River M1 terrace as defined by S. However, it is essential to keep in mind that the protocols preparation of the sediments, measurements may vary from one laboratory Osl dating limits and fits the other: Finally the paper reviews the recent applications of OSL dating in France, and assesses the potential of applying the luminescence dating technique to a range of geomorphic research.
The main minerals studied are quartz and K-rich feldspar, which can be found in almost all sedimentary environments. They lead to the emission of electrons which are subsequently trapped in crystalline lattice defects.
On the contrary, defects situated deeper inside the lattice have a higher thermal lifetime. The total amount of trapped electrons within a crystal is proportional to the total energy and retained by the crystal or dosehence the time it was exposed to radiation. As soon as the mineral is exposed to sunlight, for example during its transport, trapped electrons absorb the photon energy from the Sunand are released from the traps.
The recombination generates the emission of light the luminescence signal which can be measured in laboratory through heating for TL or through light stimulation for OSL Huntley et al. "Osl dating limits and fits" intensity of the signal is proportional to the amount of released electrons. The wavelength of the signal is allocated to the nature of the mineral: These latter wavelengths are however less used to avoid interactions with the stimulating light. Feldspars mainly emit in the nm wavelength Huntley et al.
It is exposed again to radiation and accumulates trapped electrons. Much later the grain is sampled in the field and stimulated in the laboratory using either visible light for quartz OSL stricto sensutypically performed with blue or green LEDsor infrared light for feldspars Infrared Stimulated Luminescence IRSL.
The observed OSL signal from quartz is however made up of several exponentials related to different levels of traps Bailey et al.
This reflects the existence of a fast component associated with the emptying of the most light-sensitive trapsa medium component, and several slow components corresponding with less bleachable traps; Bailey, The analysis of the OSL signal makes it possible to estimate the time elapsed since burial, i.
This age is derived using the following equation:.
It is estimated in the laboratory by the determination of the equivalent dose D ei. D r is the dose rate the rate at which the sediment is exposed to natural radiations. However, the measured signal does not always reflect the time elapsed since the burial of the sediment fig.
The mineral becomes saturated, and its equivalent dose will consequently underestimate the event of interest. Quartz generally saturates at lower doses than feldspars. On the contrary, it is difficult to date quartz beyond ka except when the dose rate is low, i. Recent research focusing on the recuperated signal of quartz Re-OSL however opens the way to overcome satisfactorily the Osl dating limits and fits of quartz saturation Wang et al.
This phenomenon, which might be explained by a quantum mechanic tunnelling process, is associated with a spontaneous eviction of electrons in Osl dating limits and fits traps, at room temperature, and without exposure to light.
Anomalous fading generates an underestimate of the equivalent dose, and consequently of the burial age estimate. It is highly recommended to apply measurement procedures in the laboratory to detect the fading. A successful mathematical model was proposed by D. Lamothe to compensate for anomalous fading and get more reliable age estimates see also M. This decay is not a single exponential curve as several levels of traps are involved, some of them being more rapidly bleached fast component than the others medium and slow components.
The choice of these sizes is justified by the procedures used for Osl dating limits and fits dose rate determination see next section. The best sampling strategies are those that involve communication between a geomorphologist and luminescence dating expert prior to and during the field sampling process. The sampling is best undertaken from a pit or a natural sectionor using cores.
In a pit, samples should be taken from an homogeneous layer more than cm thick, away from coarse deposits, which may complicate the dose rate calculation. Sampling should also be avoided for sedimentary layers affected by bioturbation or pedogenic activity, as it may generate in situ bleaching and variations in the dose rate through time fig.
The sampled sediment should be cleaned first by removing the outermost 10 cm. It is possible to operate by night especially for consistent and hardened sediments which can be dug with a shovel but also by day since the outer parts of the sample will be removed before the measurements.
Once the sample has been removed the remaining hole can be used for an in situ determination of the dose rate see below. The dose rate may also be determined in the laboratory. When the profile presents several sedimentary units, further samples should also be taken so that the luminescence ages can be compared. This comparison is however not sufficient to assess the accuracy of the age, which can be only obtained using an independent technique see below.
A second possibility is to collect luminescence samples from sedimentary cores.