Luminescence Dating

General Principles of Luminescence Dating
OSL dating works on the principle that ionizing radiation—from U, Th, K, as well as from cosmic rays—ionizes atoms within silicate mineral grains like quartz and feldspar.
Figure 1. Ionizing radiation—from U, Th, K, as well as from cosmic rays—ionizes atoms within silicate mineral grains like quartz and feldspar.
These freed electrons become trapped at light sensitive crystal defects within the mineral. The number of trapped electrons increases over geologic time and is a direct measure for the energy deposited in the mineral by ionizing radiation. Exposure to heat or sunlight resets or “zeroes” the luminescence clock by releasing (“bleaching”) the electrons from the traps and the electrons recombine with charges of the opposite sign. The light emitted during this process by the minerals themselves is called TL (Thermally Stimulated Luminescence) or OSL (Optically Stimulated Luminescence). The luminescence signal is a direct measure for the number of trapped charges, and is proportional to the time elapsed since the mineral grains were last exposed to daylight (i.e. the time since burial). If the mineral grains are shielded from further sunlight by burial, trapped electrons begin to re-accumulate.
Figure 2. Exposure to heat or sunlight resets or “bleaches” the minerals. If the mineral grains are shielded from further sunlight by burial, trapped electrons begin to re-accumulate.
In the laboratory feldspar or quartz separates are prepared from the sediments. The measurements simulate the natural luminescence process. The sample is stimulated with light of one wavelength (e.g. blue or infrared), and the OSL emitted from the sediment is monitored at another wavelength (e.g. UV). The intensity of this “natural” luminescence signal N is proportional to the energy from ionizing radiation absorbed since burial. The amount of absorbed energy, the "equivalent dose" (De, unit: Gray; 1 Gy = 1 J/kg), is determined by comparing the natural luminescence signal with the signals obtained after known radiation exposures administered in the laboratory.
Figure 3. The equivalent dose De is determined by comparing the natural luminescence signal N with the signals obtained after known radiation exposures administered in the laboratory.
The rate of natural irradiation, the "dose rate" (in Gy/year), can be determined from the concentration of radioactive nuclides (U, Th, K) in the sediment. The age of the sample (i.e. the burial time in calendar years) is then derived from
Age = Equivalent Dose / Dose Rate
Several methods have been used to obtain the equivalent dose from the measured luminescence signal. The most revolutionary development of the last decade was the development of the single-aliquot regenerative-dose method (SAR; Murray and Wintle, 2000; Wintle and Murray, 2006). Measurements are carried out with sub-samples (aliquots) of only 1-2 mg (~ 100-1000 grains). A dose-value can be obtained for each single aliquot and the total equivalent dose is calculated as the average of all measured aliquots of a sample.
The range of OSL dating is from recent decades (Madsen et al., 2005) to 500,000 years ago (Olley et al., 2004; Watanuki et al., 2005). The lower age limit is determined by the lower detection limit of the OSL signal. For high doses the signal ceases to increase with increasing dose, which means that an upper measurable dose exists. The upper age limit therefore depends on the saturation region of the dose response as well as the natural dose rate. Higher ages can be obtained for lower dose rates. The uncertainty is usually about 10%. Detailed discussions of luminescence dating methods including equivalent dose and dose rate determination can be found in publications by Aitken (1985, 1998) and by Wintle (1997).
References
Aitken, M.J. An Introduction to Optical Dating, Oxford Univ. Press, Oxford, UK (1998).
Aitken, M.J. Thermoluminescence Dating, Academic Press, London (1985).
A.T. Madsen, A.S. Murray, T.J. Andersen, M. Pejrup, and H. Breuning-Madsen, "Optically stimulated luminescence dating of young estuarine sediments: a comparison with 210Pb and 137Cs dating," Marine Geology 214, 251 (2005).
A.S. Murray and A.G. Wintle, "Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol," Radiation Measurements 32, 57 (2000).
J.M. Olley, P. De Deckker, R.G. Roberts, L.K. Fifield, H. Yoshida, and G. Hancock, "Optical dating of deep-sea sediments using single grains of quartz: a comparison with radiocarbon," Sedimentary Geology 169, 175 (2004).
T. Watanuki, A.S. Murray, and S. Tsukamoto, "Quartz and polymineral luminescence dating of Japanese loess over the last 0.6 Ma: Comparison with an independent chronology," Earth and Planetary Science Letters 240, 774 (2005).
A.G. Wintle, "Luminescence dating: Laboratory procedures and protocols," Radiation Measurements 27, 769 (1997).
A.G. Wintle and A.S. Murray, "A review of quartz optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols," Radiation Measurements 41, 369 (2006).


Webmaster