Luminescence tutorial (next: Sedimentologic Context)

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OSL has emerged in the past decade as valuable Quaternary dating method for a variety of eolian, fluvial, marine and colluvial sediments. The OSL clock, like in TL dating is reset by exposure to sediment to sunlight prior to deposition. A key advantage of OSL dating is that the luminescence of quartz and feldspar grains is reduced to a low definable level after a few minutes of sunlight exposures versus hours for the corresponding thermoluminescence response. This level, called the residual level, is the point from which the geological luminescence accumulated after burial. OSL dating uses light of a particular wavelength or range of wavelengths, usually either blue, green or infrared light, releasing rapidly the most light sensitive trapped electrons from the crystal lattice. Astonishingly, the decrease in the OSL after a 20 second exposure to sunlight is equivalent to the reduction in TL after a 20 hour light exposure.

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There are currently two major analytical approaches that translate the time-stored luminescence to an equivalent dose, and thus to an age. The multiple aliquot additive dose method (MAAD), is used principally on the fine-grained polymineral or the quartz fraction. This method applies additional doses (beta or gamma) to the natural luminescence on separate aliquots of the sample effectively building a dose response curve, simulating future dose and interpolating an equivalent dose to the solar reset level. MAAD is a method that was original developed for TL dating and in largely unaltered form is currently used in OSL analysis. Numerous studies indicate the robustness of this procedure because it has yielded ages in agreement with other chronologic control for at least the past c. 100 ka.

The other method is the single aliquot regeneration method (SAR), which determines an age for each aliquot by matching a regenerated signal from the light reset level to the original natural level, while compensating for apparent sensitivity changes. This technique is used on both fine (4-11µm) - and coarse (100-300 µm)-grained fractions containing mono- or poly-mineral separates. The SAR method is particularly useful for sediments < 50 ka old and provides decadal scale resolution for dating sediments, particularly of eolian origin, that are <1000 years old.