Abstract
There is growing interest in rock surface burial and exposure luminescence dating for use in Quaternary science and in archaeology. Such methods have enormous potential both in increasing the range of sedimentary contexts that can be dated, and improving the accuracy and the precision of dating within those contexts. Bleaching of the luminescence signal with depth into the rock surface is likely to vary with lithology. However, previous work on rock surface dating has not systematically studied the differences in light attenuation for rocks of different lithologies, or directly quantified the attenuation of light in different rock surfaces. This study investigates the attenuation of light in different rock types (greywacke, sandstone, two granites and quartzite) using two different approaches: 1) sunlight bleaching experiments, to assess the residual infrared stimulated luminescence signal measured at 50 °C (IRSL50) and the post-IR IRSL signal measured at 225 °C (post-IR IRSL225) at different depths within the rocks after different durations of exposure to daylight; and, 2) direct measurement of light attenuation in rock slices using a spectrometer. Data from the spectrometer shows that for all rocks, attenuation is greater for shorter wavelengths (∼400 nm) than longer ones. A consistent difference in attenuation coefficient is seen when comparing the IRSL50 and the post-IR IRSL225 signals; this is thought to reflect the different sensitivity of these two signals to infrared and visible light. Direct measurement using a spectrometer is much more rapid than undertaking a bleaching experiment, and also provides wavelength-resolved attenuation data. Comparison of the numerical values from the two approaches is complex, but they yield consistent results. For the samples analysed here, the rocks that appear lightest in colour show the least attenuation of light and the luminescence signals are bleached to the greatest depths, and are thus the most suitable for dating using luminescence.
Highlights
Luminescence dating is most commonly applied to 90–300 μm diameter sand-sized or 4–11 μm diameter silt-sized mineral grains extracted from sedimentary deposits, where such grain sizes have been produced by natural erosion and weathering of rocks
This study investigates the attenuation of light in different rock types using two different approaches: 1) sunlight bleaching experiments, to assess the residual infrared stimulated luminescence signal measured at 50 °C (IRSL50) and the post-IR IRSL signal measured at 225 °C at different depths within the rocks after different durations of exposure to daylight; and, 2) direct measurement of light attenuation in rock slices using a spectrometer
The saturation level of the sensitivity-corrected IRSL50 or post-IR IRSL225 signals (Ln/Tn) under natural irradiation conditions was calculated as the average of the lowermost slices, where there was no sign of the signal having been influenced by daylight exposure
Summary
Luminescence dating is most commonly applied to 90–300 μm diameter sand-sized or 4–11 μm diameter silt-sized mineral grains extracted from sedimentary deposits (e.g. see review by Rhodes, 2011), where such grain sizes have been produced by natural erosion and weathering of rocks. If the daylight exposure is sufficiently long, the mineral grains below the rock surface will have their luminescence signal bleached, and the depth of bleaching into the rock will increase over time. The potential for dating using rock materials has been studied for many years, especially with regard to its application in dating the construction of buildings using stone blocks (see review by Liritzis, 2011), and through attempts to directly date stone artefacts (Richards, 1994) In recent years these ideas have been developed further Simms et al, 2011; Sohbati et al, 2012a) In some settings, these two rock-dating methods can be combined to elucidate complex histories of exposure and burial These two rock-dating methods can be combined to elucidate complex histories of exposure and burial (e.g. Freiesleben et al, 2015)
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