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Luminescence Dating

1. Introduction

Luminescence dating is based on the process of a time-dependent accumulation of electrical charge at light or heat sensitive traps that are associated with imperfections in the crystal lattice of common minerals such as quartz and feldspars. These structural defects and charge transfers are induced by ionizing radiation from naturally occurring radiative processes in the sediment and a contribution from cosmic rays. 

vergrößern: Figure 1: Schematic representation of luminescence signal accumulation and resetting in nature
Figure 1: Schematic representation of luminescence signal accumulation and resetting in nature

2. Signal accumulation and resetting in nature

When a quartz grain is buried, the ionizing radiation results in an increased trapping of charge at structural defects (Fig. 1). When the grain becomes exposed to sunlight again, the solar energy induces the release of the electrical charge. This process takes places under the emission of energy as light which is known as luminescence.

 

 

3. Age calculation

If this light emission from quartz or feldspar is measured in the laboratory (Fig. 2), it can be translated into dose values through calibration with known doses of radiation. To derive the age information from this so called “equivalent dose”, it must be divided by the amount of radioactivity of a sample and its surroundings per unit of time. This “dose rate” is assessed by radiometric methods such as Gamma-Spectrometry.

vergrößern: OSL - dose response curve
Figure 2: (upper) A luminescence signal decay in response to laboratory stimulation. (lower) Luminescence response to laboratory irradiation, signal intensities are integrated and plotted relative to the given dose, the natural luminescence signal can then be interpolated onto the “dose response curve” to obtain the equivalent dose (De). This can then be divided by the environmental radiation dose rate to determine the sample age

4. Application

Aeolian sediments, in the form of dune sands or loess, are best suited to luminescence dating as complete resetting of the signal during transport can be expected. Depending on the bleaching characteristics, fluvial and glaciolacustrine deposits, as well as colluvial and tsunami deposits can also be dated. Because the luminescence signal is also thermally sensitive, it has great potential as a low-temperature thermochronometer, which is currently undergoing development. The time span covered by this dating method ranges from a few tens of years up to about 300 ka. The maximum age limit can vary considerably and is highly dependent on the material characteristics of the measured mineral grains and the amount of natural radioactivity in the surrounding sediment.