A Brief Description of Optically Stimulated Luminescence Dating
Please reference: Mallinson, D., 2008. A Brief Description of Optically Stimulated Luminescence Dating, http://core.ecu.edu/geology/mallinsond/OSL, if use of Figure 1 is desired.
Optically stimulated luminescence is a method of determining the age of burial of quartz or feldspar bearing sediments based upon principles of radiation and excitation within crystal lattices, and stems from the fact that imperfections in a crystal lattice have the ability to store ionizing energy (Aitken, 1998; Botter-Jensen et al., 2003; Lian, 2007). Radiation within sediments comes from alpha, beta, and gamma radiation emitted during the decay of 235U, 238U, 232Th, 40K, and 87Rb, and their daughter products, both within the mineral grains and in their surroundings (Lian, 2007), and from cosmic rays (Figure 1).
Radiation is absorbed by the crystal lattice upon sediment burial, and over time, excites electrons causing them to migrate within the crystal and become stored in “traps” resulting from crystal lattice defects. This energy is then released as photons in visible wavelengths (luminescence) upon photon irradiation either by exposure to sunlight or artificial light, or by heating (~500°C) thus resetting the clock. Under controlled laboratory conditions, assuming the sample was collected under light-restricted conditions, controlled exposure of the sample to photons yields a luminescence response (the equivalent dose, De), the intensity of which is a function of the dose rate within the sediment, and the length of time the sample was exposed to the background radiation. In order to measure the age, two factors must be known; 1) the environmental dose rate, and 2) the laboratory dose of radiation that produces the same intensity of luminescence as did the environmental radiation dose (the equivalent dose). Dividing the equivalent dose by the dose rate yields time. Although the fundamental concept is straight-forward, there are many caveats that must be accounted for stemming from partial bleaching of grains during burial, mixing of grains by bioturbation, and pedogenic (soil formation) processes that alter the dose rate over time (Bateman et al., 2003; 2006).
Samples for OSL analysis are typically collected from opaque core tubes (aluminum or black pvc tubes) that are pushed into the sediment using coring equipment (vibracore, geoprobe, etc.), or by manual insertion into sediment exposures along natural bluffs or man-made pits. Samples are then extracted for processing under dark-room conditions. Typical processing of a sample for OSL analysis includes treatment with HCl and H2O2 to remove carbonate and organics. This is followed by sieving, heavy liquid (Li- or Na-polytungstate) separation, and (sometimes) magnetic separation to concentrate quartz sands of the appropriate size. Finally, etching with HF is performed to remove the outermost “rind” of the quartz grain. All of the processing must be done under dark-room conditions.
The main component of an OSL laboratory is the “Reader” (Figure 2). This device facilitates the determination of De, and the creation of a luminescence “growth” curve, which plots luminescence intensity versus laboratory dose rates (beta dose), for a particular sample aliquot (one sample containing ~100 grains). The single aliquot regeneration (SAR) protocol (Murray and Wintle, 2000) is the technique of choice for a variety of applications, and was used for analyses associated with this USGS investigation. This is done by first exposing the sample aliquot to a known quantity of photons (blue wavelength) and determining the luminescence that occurs in response. The sample is then irradiated with increasing radiation levels (beta), and re-exposed to determine the luminescence that occurs at each irradiation level. The equivalent dose is then determined by applying a regression to the data, and determining the radiation dose that corresponds to the initial luminescence signal. Determining the age is then a simple function of dividing the paleodose by the dose rate that is measured on the surrounding sediments. (This is a much simplified explanation – there is more involved; e.g. preheating treatments and sensitivity tests).
Figure 1. Generalized processes that produce the luminescence signal (steps 1 and 2), and the sampling and analytical procedure to determine the age of deposition (steps 3 through 6).
Figure 2. Equipment that comprises the “reader”, which is necessary for measuring the paleodose, irradiating the sample, heating the sample, and deriving a “growth” curve (from Lian, 2007).
Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford Science Publications, Oxford, UK. 267 p.
M., Boulter, C., Carr, A.,
Botter-Jensen, L., McKeever, S., Wintle, A., 2003.
Optically Stimulated Luminescence Dosimetry. Elsevier,
Lian, O.B., 2007. Luminescence Dating, in: Encyclopedia of Quaternary Science. Elsevier. 3576 p.
Murray, A.S. And Wintle, A.G. (2000) Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements, 32, 57-73.