Abstract
Post-IR IRSL (pIRIR) signals from K-feldspar grains measured at elevated temperatures are increasingly being used for dating sediments. Unfortunately the pIRIR signal from K-feldspars bleaches more slowly than other signals (e.g. OSL from quartz) upon exposure to daylight, leading to concerns about residual signals remaining at deposition. However, earlier studies have not assessed whether the pIRIR signal bleaches at the same rate in all feldspar grains. In this study laboratory bleaching experiments have been conducted and for the first time the results show that the rate at which the pIRIR signal from individual K-feldspar grains bleach varies. To determine whether grain-to-grain variability in bleaching rate has a dominant control on equivalent dose (De) distributions determined using single grains, analysis was undertaken on three samples with independent age control from different depositional environments (two aeolian and one glaciofluvial). The De value determined from each grain was compared with the rate at which the pIRIR225 signal from the grain bleaches. The bleaching rate of each grain was assessed by giving a 52 Gy dose and measuring the residual De after bleaching for an hour in a solar simulator. There is no clear relationship between the rate at which the pIRIR225 signal of an individual grain bleaches and the magnitude of its De. It is concluded that variability in the bleaching rate of the pIRIR225 signal from one grain to another does not appear to be a dominant control on single grain De distributions.
Highlights
Stimulated luminescence (OSL) dating of single grains is beneficial in certain depositional environments to detect the partial bleaching of sedimentary grains (Duller, 2008)
Equivalent dose (De) values for the Post-IR IRSL (pIRIR) signal measured for modern analogues, or the residual De values remaining after laboratory bleaching of coarse-grained K-feldspar (Table 1) have been published for different pIRIR signals measured at different temperatures (e.g. Li et al, 2014)
Laboratory measurements of residual De values after bleaching in a solar simulator were used to investigate the variability in bleaching rates of the pIRIR225 and pIRIR290 signals for individual grains of K-feldspar from two aeolian dune samples (TC01 and GDNZ13)
Summary
Stimulated luminescence (OSL) dating of single grains is beneficial in certain depositional environments (e.g. glaciofluvial settings) to detect the partial bleaching of sedimentary grains (Duller, 2008). The pIRIR signal may be more stable over time than the IR50 signal, several studies of coarse-grain K-feldspar using multiple grains have obtained bleaching curves which show that the pIRIR signal bleaches more slowly in response to optical stimulation than the IR50 signal (e.g. Buylaert et al, 2012, 2013; Kars et al, 2014; Murray et al, 2012), which in turn bleaches more slowly than the quartz OSL signal (Godfrey-Smith et al, 1988). Investigating the grain-to-grain variability of bleaching rates of feldspars is important for single-grain dating as it has been suggested that the TL signal from different types of museum specimen feldspars bleaches at different rates in response to sunlight bleaching Spooner, 1994; Bailiff and Poolton, 1991) It is not clear whether the pIRIR signals from individual grains of K-feldspar in the density-separated fraction, composed of grains that have different internal K-contents, will bleach at different rates or not. Three samples of density-separated K-feldspars extracted from different depositional environments with independent age control are used for these investigations
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