Scholarly article on topic 'Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: Implications for single-grain dating'

Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: Implications for single-grain dating Academic research paper on "History and archaeology"

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Radiation Measurements
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Feldspar / Luminescence / Single grains / Infrared stimulated luminescence / pIRIR / Residual De values / Bleaching rate

Abstract of research paper on History and archaeology, author of scientific article — R.K. Smedley, G.A.T. Duller, H.M. Roberts

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.

Academic research paper on topic "Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: Implications for single-grain dating"

Accepted Manuscript

Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: implications for single-grain dating

R.K. Smedley, G.A.T. Duller, H.M. Roberts

PII: S1350-4487(15)30034-2

DOI: 10.1016/j.radmeas.2015.06.003

Reference: RM 5432

To appear in: Radiation Measurements

Received Date: 12 June 2013 Revised Date: 18 May 2015 Accepted Date: 5 June 2015

Please cite this article as: Smedley, R.K., Duller, G.A.T., Roberts, H.M, Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: implications for single-grain dating, Radiation Measurements (2015), doi: 10.1016/j.radmeas.2015.06.003.

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18th May 2015

Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: implications for singlegrain dating

Smedley R.K.*, Duller, G.A.T. and Roberts, H.M.

Department of Geography and Earth Sciences, Aberystwyth University, Ceredigion, SY23 3DB, UK Corresponding author (rks09@aber.ac.uk)

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.

Keywords

Feldspar; luminescence; single grains; infrared stimulated luminescence; pIRIR; residual De values; bleaching rate

1. Introduction

Optically 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). A major challenge for single-grain measurements using quartz is that commonly only 5 % or fewer of the grains emit a detectable OSL signal e.g. Duller (2006) detected as few as 0.5 % of quartz grains in glaciofluvial sediments from Chile. In contrast to quartz, a larger proportion of K-feldspar grains are reported to emit a detectable OSL signal and the signals are also typically brighter (e.g. Duller et al., 2003). However, a major drawback for luminescence dating of feldspars is that the infrared stimulated luminescence signal measured at 50 °C (IR50) is prone to anomalous fading over time, which some workers claim to be a ubiquitous phenomenon (e.g. Huntley and Lamothe, 2001). Currently there are two single aliquot regenerative dose (SAR) protocols commonly used for K-feldspar dating, (1) IR50 measurements, (e.g. Wallinga et al. 2000), and (2) post-IR IRSL measurements typically performed at 225°C or 290°C, giving rise to the pIRIR225 and pIRIR290 signals (e.g. Thomsen et al. 2008, 2011). Since the development of pIRIR measurement protocols, they have been widely applied to coarse-grained K-feldspars (e.g. Buylaert et al. 2009, 2012) as the pIRIR signals are thought to access more distal donor-acceptor pairs than the IR50 signal and are therefore more stable over geological time, minimising the effects of anomalous fading on the pIRIR signal used for dating (Jain and Ankj^rgaard, 2011).

Although 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). More recently, Colarossi et al. (in press) have directly compared the bleaching rates using multiple grain measurements of feldspars and quartz, confirming previous findings, and showing that the pIRIR290 signal bleaches more slowly than the pIRIR225 signal. Equivalent dose (De) values for the 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). The smallest residual De values reported for multiple grains (< 1 Gy) are measured using procedures with the lowest preheat and pIRIR stimulation temperatures (e.g. pIRIRi50 and pIRIR180 protocols). It has therefore been suggested that lower temperature pIRIR protocols may be more appropriate for dating young sediments (e.g. Madsen et al. 2011; Reimann et al. 2011; Reimann and Tsukamoto, 2012). However, the model proposed by Jain and Ank^rgaard (2011) suggests that higher temperature pIRIR protocols access signals that are more stable over geological time. Thus, the pIRIR225 and pIRIR290 signals have the potential to provide more accurate and precise single-grain K-feldspar ages by further minimising the influence of fading beyond that of the pIRIR signals measured at lower temperatures.

Feldspars form a solid-solution series, ranging from anorthite (CaAl2Si2O8), to albite (NaAlSi3O8), to orthoclase (KAlSi3O8). Density separation is routinely used for luminescence dating of sedimentary grains to isolate the K-feldspar fraction. However, geochemical measurements have demonstrated that density-separated K-feldspar fractions can be composed of different types of feldspar grains, which are

chemically variable (e.g. Smedley et al. 2012). Thus far, bleaching curves ^ have not been reported for single

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 (e.g. Robertson et al. 1991). However, it has also been reported that the IRSL signal of different types of museum specimen feldspars bleaches at similar rates in response to a range of monochromatic wavelengths from 400 to 1065 nm (e.g. Spooner 1994; Bailiff and Poolton 1991). Thus, 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. The aim of this study is to investigate the bleaching of the pIRIR225 and pIRIR290 signals from single grains of K-feldspar and to examine whether any difference in bleaching rate may influence the De determined. Three samples of density-separated K-feldspars extracted from different depositional environments with independent age control are used for these investigations.

2. Equipment and measurement protocols

All luminescence measurements were performed using a Ris0 TL/OSL DA-15 automated single-grain system equipped with an infrared laser (830 nm) fitted with an RG-780 filter (3 mm thick) to remove any shorter wavelengths (B0tter-Jensen et al. 2003, Duller et al. 2003), and a blue detection filter pack containing a BG-39 (2 mm), a GG-400 (2 mm) and a Corning 7-59 (2.5 mm) filter placed in front of the photomultiplier tube. The inclusion of the GG-400 filter is to ensure removal of the thermally unstable UV emission centred on 290 nm seen during IR stimulation of feldspars (e.g. Balescu and Lamothe, 1992; Clarke and Rendell, 1997). The system was equipped with a 90Sr/90Y beta source delivering ~0.04 Gy/s.

Single aliquot regenerative dose (SAR) pIRIR225 and pIRIR290 protocols were used for dose-recovery and residual dose experiments (Table 2). A high temperature bleach was used at the end of each SAR cycle (step 9, Table 2) to remove any remaining charge arising from the test-dose and prevent charge transfer from the Tx measurement through to the subsequent Lx measurement which may affect the accuracy of the dose determinations. The IRSL signal was summed over the first 0.3 s of stimulation and the background calculated from the final 0.6 s. Regenerative doses of 0, 24, 48, 96 Gy and 0, 2, 4, 8, 20 and 40 Gy were used for dose-recovery and residual-dose experiments, respectively. A second 0 Gy dose was repeated after the largest regenerative dose as a second test for recuperation, which was then followed by a dose of 48 Gy (dose-recovery tests) or 8 Gy (residual-dose tests) used for recycling ratio tests.

Four rejection criteria were applied throughout the analyses unless otherwise specified; (1) whether the response to the test dose was less than three times the standard deviation of the background, (2) whether the uncertainty in the luminescence measurement of the test dose was greater than 10 %, (3) whether the recycling ratio was outside the range 0.9 to 1.1, taking into account the uncertainties on the individual recycling ratios, and (4) whether recuperation was greater than 5 % of the response from the largest regenerative doses, which were 96 Gy and 40 Gy for dose-recovery and residual dose experiments, respectively. Following the method of Thomsen et al. (2005) the instrument reproducibility of the singlegrain measurement system was assessed for the protocols used in this study, giving values of 4.6 % and 4.5 % (per stimulation when the signal is summed over the initial 0.3 s) for the pIRIR225 and pIRIR290

measurements, respectively (Smedley and Duller, 2013). These instrument reproducibility values were

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3. Sample descriptions

Three samples of density-separated K-feldspar grains were used in this study. Sample TC01 was collected from an inland dunefield in eastern Argentina and has a multiple grain quartz OSL age (20 ± 5 years) indicating very recent deposition. Sample GDNZ13 was taken from a Late Glacial dune sand from North Island, New Zealand and is overlain by the Kawakawa tephra, which has been dated by radiocarbon to 25.36 ± 0.16 cal. ka BP (Vandergoes et al. 2013). Sample LBA12F4-2 was extracted from glaciofluvial sediments in Patagonia, directly linked to a moraine ridge dated to 25.2 ± 0.4 ka using cosmogenic isotope dating (10Be) of moraine boulders (Kaplan et al. 2011).

Prior to measurement the samples were all treated with a 10 % v.v. dilution of 37% HCl and with 20 vols H2O2 to remove carbonates and organics, respectively. Dry sieving isolated the 180 - 212 ^m diameter grains, and density-separation with sodium polytungstate provided the < 2.58 g cm-3 (K-feldspar-dominated) fractions. The K-feldspar grains were not etched in hydrofluoric acid because of concern about non-isotropic removal of the surface (Duller, 1992). Details of the calculation of the alpha dose-rates for these samples are described in the caption of Table 3. Finally, the K-feldspar grains were mounted into 10 x 10 grids of 300 ^m diameter holes in a 9.8 mm diameter aluminium single-grain disc for analysis.

Dose-rates were calculated for the K-feldspar dominated fractions of all three samples using thick source alpha and beta counting on Daybreak and Ris0 GM-25-5 measurement systems, respectively (Table 3). The K-content of each feldspar separate was measured using a Ris0 GM-25-5 beta counter to analyse 0.1 g sub-samples of the separated material; this gave values of 6.5 % K (TC01), 6.2 % K (GDNZ13), and 3.9 % K (LBA12F4-2). To calculate the internal dose rate arising from K within the feldspar grains a value of 10 ± 2 % was used following the work of Smedley et al. (2012) and Smedley (2014) who showed that the K-content of the majority of grains from these samples which emitted detectable pIRIR signals was 10 ± 2 %.

4. Determination of De remaining in a recently-deposited sample

The recently-deposited dune sand sample, TC01, was used to assess the residual De values for the pIRIR225 and pIRIR290 signals, using the protocols outlined in Table 2. Two hundred grains were measured using each signal but after applying the rejection criteria only 14 and 10 grains provided residual De values for the pIRIR225 and pIRIR290 signals, respectively. These single-grain residual De values are presented as histograms to show the populations of grains measured using the pIRIR225 (Fig. 1a) and pIRIR290 (Fig. 1b) signals. Although there was variation between the residual De values measured for individual grains, 12 of the 14 grains (86 %) measured using the pIRIR225 protocol and 6 of the 10 grains (60 %) measured using the pIRIR290 protocol gave residual De values of < 2 Gy. The central age model (CAM) De values were calculated from the pIRIR225 and pIRIR290 single-grain populations, giving values of 1.0 ± 0.3 Gy and 1.7 ± 0.4 Gy, respectively.

Multiple-grain dating using the OSL signal from quartz gave a luminescence age for sample TC01 of 20 ± 5 years. For comparison, luminescence ages determined from single grains were also calculated

using the CAM De values of the pIRIR225 (CAM De value of 1.0 ± 0.3 Gy) and pIRIR290 (CAM De value of 1.7 ± 0.4 Gy) signals, and the dose-rate in Table 3. The ages calculated for sample TC01 using the pIRIR225 and pIRIR290 signals for single-grains of K-feldspar were 325 ± 100 years and 550 ± 130 years, respectively.

The CAM De value calculated for the pIRIR225 signal from the recently-deposited dune-sand sample (1.0 ± 0.3 Gy) is comparable to the published residual De values of < 1 Gy for other samples using the pIRIRi80 signal shown in Table 1. However, the CAM De value (1.7 ± 0.4 Gy) calculated in this study using the pIRIR290 signal was larger than 1 Gy. When a synthetic aliquot is derived by summing the signal emitted from all the grains on the single-grain disc, the mean De values calculated from two synthetic aliquots per signal were 1.4 Gy (pIRIR225 signal) and 2.6 Gy (pIRIR290 signal). These De values are consistent with the smallest residual dose values published for the pIRIR signals from multiple-grain aliquots of K-feldspar (Table 1), but slightly larger than the values derived from measurements using single grains.

Dose-recovery experiments were performed on a suite of 200 fresh grains from sample TC01 using both the pIRIR225 and pIRIR290 signals to assess the suitability of each measurement protocol. A 52 Gy dose was added to the small natural dose as measured above and the resultant De was assessed using the pIRIR protocols outlined in Table 2. The CAM De for the pIRIR225 and pIRIR290 signals gave residual-subtracted dose-recovery ratios of 0.98 ± 0.02 and 0.97 ± 0.04, and overdispersion values of 9.6 ± 0.4 % (n = 37 grains) and 17.9 ± 0.4 % (n = 45 grains), respectively, demonstrating the appropriateness of both of these protocols for determining De values.

5. Measurement of De remaining after laboratory bleaching

The measurements from the naturally-bleached sample, TC01 (Section 4), demonstrate the degree of variation in residual De values expected in a well-bleached environment. However, single-grain dating is typically used to analyse sediments in environments where the opportunities for bleaching are limited (Duller, 2008). Thus, an investigation of the residual De values observed in response to different bleaching times was conducted to assess the grain-to-grain variability in the rate of bleaching of the pIRIR signal.

5.1 Experimental design

Eight hundred grains that had previously been analysed to determine the natural De value (400 grains from sample TC01, and 400 grains from sample GDNZ13) were used for these experiments to assess the residual De values measured following different laboratory bleaching durations. For each sample, half of the 400 grains were measured using a pIRIR225 protocol, and the other half were measured using the pIRIR290 protocol. Prior to these measurements, the grains were given a 52 Gy dose and then bleached at a distance of ~50 cm from the bulb of a SOL2 solar simulator for different periods of time. Lx/Tx measurements were performed after each bleaching interval and interpolated on to a dose-response curve previously constructed for each individual grain. Replicate measurements were performed on the same grains after different intervals of 1, 4, 8, and 20 hours to monitor the depletion of the pIRIR signals for individual grains.

5.2 Laboratory bleaching of an Argentinean dune sand

The CAM was applied to residual De values obtained from single grains of TC01 which passed the rejection criteria (Section 2) for the pIRIR225 (Fig. 2, closed diamonds; n =15 grains) and pIRIR290 (Fig. 2, open circles; n = 19 grains) signal, measured after the different laboratory bleaching times. The pIRIR225 and pIRIR290 CAM De values determined for the naturally-bleached grains of TC01 (Section 4) are also marked as dashed lines in Fig. 2 for comparison. Neither the pIRIR290 nor the pIRIR225 signal deplete to the natural residual De value, even after a prolonged 20 hour bleach in the SOL2, which is equivalent to ~ 5 days of natural sunlight exposure. Instead, the pIRIR225 and pIRIR290 CAM De values reduced to only 5.0 % and 6.6 % of the 52 Gy given doses, respectively. The pIRIR225 signal is shown in Fig. 2 to bleach more rapidly after 1 hour of bleaching (5.6 ± 0.8 Gy residual De; 11 % of the given dose) than the pIRIR290 signal (9.2 ± 1.0 Gy residual De; 18 % of the given dose). However, beyond 4 hours of bleaching, the residual De values for both signals are similar to one another, and after 20 hours both the pIRIR225 and pIRIR290 signals gave a CAM De value ~5 % of the 52 Gy given dose

The bleaching rate of the pIRIR225 signal from single grains of sample TC01 is shown in Fig. 3, and demonstrates that different grains bleach at different rates. Three grains that bleach at different rates (fast, moderate, slow) are highlighted in Fig. 3a (denoted grains a, b and c). Grain (a) bleaches rapidly to a residual De value of 2.4 ± 1.0 Gy (4.6 % of the given dose) after 1 hour of bleaching and remains at ~2 Gy for the prolonged bleaching times. Grain (b) has a moderate bleaching rate, reaching a residual De value of 7.6 ± 0.8 Gy (15 % of the given dose) after 1 hour of bleaching and reduces to a value of 2.1 ± 0.3 Gy (4.1 % of the given dose) after the prolonged 20 hour bleach. Grain (c) bleaches the slowest, giving a residual De value of 15.4 ± 2.0 Gy (29.6 % of the given dose) after 1 hour of bleaching with a SOL2 but reaches a value of 3.5 ± 0.6 Gy (6.6 % of the given dose) after a 20 hour bleach. Although the bleaching rates of individual grains varies, all three of the grains (a, b and c) have residual De values of < 10 % of the given dose after 20 hours of bleaching. The implication of this is that for samples from environments where grains are exposed to long periods of sunlight, the variability in residual De value from one grain to another at deposition will be small, and hence would be expected to contribute little to scatter in De distributions determined from single grains. Whilst the variability in De from one grain to another may be small, the average residual De remaining even after 20 hours in the SOL2 (Fig 3e) is 2.7 ± 0.3 Gy, which would be significant when dating young samples.

The residual De values of grains (a), (b) and (c) relative to the rest of the single-grain population are also shown as histograms in Fig. 3, representing the different bleaching times used, namely 1 hour (Fig. 3b), 4 hours (Fig. 3c), 8 hours (Fig. 3d) and 20 hours (Fig. 3e). The single-grain population has a large range of residual De values after the shorter, 1 hour bleach (2 - 15.5 Gy) and a smaller range in residual De values after the 20 hour bleach (0 - 5.5 Gy). Moreover, there is an identifiable population of grains that bleach more rapidly (e.g. grain a). After a short 1 hour bleach ~20 % and ~ 50 % of the grains bleach to < 5 % and < 10 % of the given dose, respectively. The grain-to-grain variability of bleaching of the pIRIR signal demonstrates that a population of grains (e.g. grain a) bleaches more rapidly in response to optical stimulation than others (e.g. grains b and c); these rapidly-bleaching grains may be preferable for single-

grain analysis of the pIRIR signal from partially-bleached sediments as they might be expected to have the smallest residual De values upon depositic

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5.3 Laboratory bleaching of a New Zealand dune sand

The same experiment as that discussed in Sections 5.1 and 5.2 was undertaken for the Late Glacial dune sand sample from New Zealand, GDNZ13. The CAM De values calculated from the single-grain populations of sample GDNZ13 measured after the different SOL2 bleaching times are presented in Fig. 4a for the pIRIR225 (n = 45 accepted grains; closed diamonds) and pIRIR290 (n = 38 accepted grains; open circles) signals. The CAM pIRIR225 De of GDNZ13 is 6.5 ± 0.5 Gy (12.6 % of the given dose) after 1 hour and 3.7 ± 0.3 Gy (7.1 % of the given dose) after 20 hours of bleaching (Fig. 4a). However, the pIRIR290 signal of GDNZ13 bleaches comparatively slowly giving a residual De value of 12.4 ± 1.1 Gy (23.8 % of the given dose) after 1 hour and 5.3 ± 0.5 Gy (10.2 % of the given dose) after 20 hours in the SOL2.

The distribution of De values for individual grains of sample GDNZ13 measured using the pIRIR225 signal was similar to that seen for sample TC01 in Fig. 3 (b - d). Histograms of the residual De values of the single-grain population measured for GDNZ13 are presented in Fig. 4 for 1 hour (Fig. 4b), 4 hours (Fig. 4c), 8 hours (Fig. 4d) and 20 hours (Fig. 4e) bleaching with the SOL2; highlighted on each graph is the CAM De value calculated for each bleaching time (dashed line). Although the data are not shown here, the grain-to-grain variability in bleaching was larger for the pIRIR290 signal in comparison to the pIRIR225 signal for this sample. No grains bleached to residual levels < 5 % of the given dose after a 1 hour SOL2 bleach using the pIRIR290 signal for GDNZ13, however, 11 % of the grains did bleach to < 10 % of the given dose after a 1 hour bleach. The pIRIR225 and pIRIR290 data from GDNZ13 and TC01 demonstrates that different grains bleach at different rates.

5.4 Dependence of residual De on prior dose

Sohbati et al. (2012) measured the dose-dependence of pIRIR225 residual De values using multiple-grain aliquots of K-feldspar for samples from southeast Spain. Larger residual De values were obtained following a 4 hour SOL2 bleach for the samples with the larger natural De values (up to ~1000 Gy). The dataset was extrapolated to derive an estimate for the residual De value at deposition of 0.98 ± 0.8 Gy, which is similar to the residual De value determined for the recently-deposited aeolian dune sand sample, TC01, in this study (Section 4). In the current study, the impact of prior dose on the residual De of individual grains was assessed using given doses of different magnitudes prior to bleaching. One hundred grains of sample GDNZ13 that had been previously analysed to determine a natural De value (similar to the grains in Sections 5.2 and 5.3) were given a 52 Gy beta dose, bleached for 8 hours in the SOL2 and the Lx/Tx ratios were measured. This procedure was repeated twice more following given doses of 102 Gy and 202 Gy, and the Lx/Tx values were interpolated on to a dose-response curve constructed for each individual grain to determine the residual De values.

The residual De values obtained for the pIRIR225 signal of sample GDNZ13 are shown in Fig. 5a as a function of the given dose (i.e. 52 Gy, 102 Gy and 202 Gy). The CAM residual De value (Fig 5b - d, dashed lines) increased with increased given dose prior to bleaching in the SOL2, and is comparable to the

residual De values measured by Sohbati et al. (2012). In the present study, the residual De values after different given doses for grains characterised by a fast, moderate or slow bleaching are highlighted in Fig. 5a (denoted grains x, y and z) for the pIRIR225 signal of sample GDNZ13. Fig 5a shows that all three of the grains (x, y and z) give larger residual De values with larger given doses prior to bleaching. In the natural environment the dose each grain has received prior to the event being dated is unknown and so any variability in the rate at which the pIRIR225 signal of the different grains bleaches can further complicate single-grain dose-distributions.

6. Grain-to-grain variability in bleaching rates of the pIRIR signal

Thus far, this study has demonstrated that the pIRIR signal of individual grains of K-feldspar have the potential to bleach at different rates in response to light. Previous studies have suggested that slow bleaching rates of the pIRIR signal may restrict the use of the pIRIR signal for dating of K-feldspar in partially-bleached environments (e.g. Blombin et al. 2012; Trauerstein et al. 2012). However, the influence that grain-to-grain variability in bleaching rates of the pIRIR signal has on single-grain De distributions has not yet been investigated for natural sedimentary samples.

The observation that the pIRIR signal of different grains bleaches at different rates suggests that dating of partially-bleached sediments may be optimised by trying to preferentially select for analysis those grains that bleach most rapidly. To test this idea, the bleaching rates of individual grains of K-feldspar were assessed by measuring the residual De values after a short 1 hour bleach in order to force the largest divergence in behaviour between the more- and less-rapidly bleaching grains in the dataset (e.g. Fig. 3b). These short laboratory bleaching tests involved (1) a given dose of 52 Gy, followed by (2) a 1 hour bleach in the SOL2 solar simulator, and (3) single-grain Lx/Tx measurements, which are then interpolated on to the original dose-response curves constructed for dating. The residual De values measured during these bleaching tests give an indication of the relative bleaching rates of the individual grains that form the singlegrain De distribution.

Short bleaching tests were performed using the pIRIR225 and pIRIR290 signals on a further suite of single grains of K-feldspar extracted from sample GDNZ13. The single-grain data were first ranked from the smallest to the largest by the residual De values, and then the cumulative percentage of grains (y-axis) were plotted against the residual De values as a percentage of the 52 Gy given dose (x-axis). Fig. 6a compares the bleaching rates measured for sample GDNZ13 using the pIRIR225 and pIRIR290 signals. There was more variability in the single-grain residual De values measured after a 1 hour SOL2 bleach using the pIRIR290 signal in comparison to the pIRIR225 signal; ~80 % of the grains reduced to residual De values that were < 31 % of the given dose (i.e. < 17 Gy) for the pIRIR290 signal whilst for the pIRIR225 signal the same proportion of grains had residual De values of < 19 % of the given dose (i.e. < 10 Gy). This reinforces the view that the pIRIR290 signal bleaches slower than the pIRIR225 signal and this is reflected by the larger and more variable single-grain residual De values.

To assess whether the grain-to-grain variability in bleaching rate of the pIRIR225 signal is a dominant control on the single-grain De distributions, the residual De values for the pIRIR225 signal after the 1 hour bleach in the SOL2 solar simulator were compared to the single-grain De values for three

sedimentary samples from different depositional environments. The three samples tested were from different depositional settings and are constrained by independent age control (Section 3). Sample TC01 is a recently-deposited aeolian dune sand, sample GDNZ13 is a Late Glacial aeolian sand, and sample LBA12F4-2 is a glaciofluvial sample deposited during the Last Glacial period. The short laboratory bleaching tests were performed for all three samples after the measurement of the pIRIR225 signal to determine the natural De values. Fig. 6b compares the bleaching rates measured for the three different samples and demonstrates that there was little difference between the samples in the behaviour of the pIRIR225 signal. After the 1 hour bleach in the SOL2 solar simulator, the typical behaviour shown by all three samples is that the measured De values of ~80 % of all the grains reduced to < 20 % of the given dose (i.e. < 10.4 Gy) (Fig. 6b). The bleaching tests and the De values were assessed using exactly the same grains to permit direct comparison between the inferred bleaching rates and the natural De values (Fig. 7). If bleaching rates were a dominant control on the single-grain De distribution then there would be a relationship between the residual De values measured after the short laboratory bleaching tests and the natural De values. The results in Fig. 7 for samples TC01 (a), GDNZ13 (c), and LBA12F4-2 (e) shows that there is no direct relationship between the inferred bleaching rates and the De values for single grains from any of the three samples.

The individual grains included in Fig. 7 were also ranked from smallest to largest according to the size of the residual De value measured after the short 1 hour bleaching tests and binned into five groups (0 -2.6 Gy, 2.7 - 5.2 Gy, 5.3 - 7.8 Gy, 7.9 - 10.4 Gy and > 10.4 Gy). The number of grains included in each bin is shown in the histograms in Fig. 7 (b, d, f). The CAM De value was calculated for each bin of all three samples (Figs 7b, 7d and 7f). MAM De values were also calculated for each bin of the glaciofluvial sample LBA12F4-2 (Fig.7f) as the large overdispersion value calculated for single-grain De values of this sample (71.6 ± 0.1 %; n = 260 grains) suggested that it was partially bleached upon deposition. Since these samples have independent age control, expected De values could be calculated using the dose-rates (Table 3). The CAM and/or MAM De values calculated for all the grains of each sample are plotted in Fig 7 (b, d, f), in addition to the expected De value for each sample (Table 3).

If bleaching rates are a dominant control on the single-grain De distributions then the bins containing the grains with the pIRIR225 signals that bleach most rapidly in response to exposure to the SOL2 solar simulator should give rise to the lowest CAM and MAM natural De values. For sample TC01 (Fig. 7b) the CAM De values calculated using the grains with the most rapidly-bleaching pIRIR225 signal (230 ± 30 years) do not give ages in agreement with the OSL age obtained from quartz (20 ± 5 years). The results for sample GDNZ13 (Fig. 7d) show lower CAM natural De values for the binned grains that gave the lowest residual De values, but the bin representing residual De values of 0 - 2.6 Gy contains only one grain, and the difference between the CAM De value calculated for the 2.7 - 5.2 Gy bin and the bins > 5.2 Gy is small.

The opportunity for bleaching in the natural environment is likely to be less in a glaciofluvial setting in comparison to an aeolian setting, and so differences in bleaching behaviour of individual grains (e.g. Fig. 3e) is likely to have a larger influence in a glaciofluvial setting. Fig. 7f presents the CAM and MAM natural De values calculated for the bins of grains for the glaciofluvial sample LBA12F4-2. The results show no trend between the CAM or MAM De values and the inferred bleaching rate of the grains. It is concluded that although differences are observed in the inferred bleaching rates of the pIRIR225 signals of single grains,

these variable bleaching rates are not a dominant control on the single-grain De distribution of these samples. Note that the bleaching rates of individual grains are not related to the extent of bleaching in the natural environment and so the two factors will likely impact samples taken from different depositional settings to different extents. Presumably for samples from well-bleached settings (e.g. aeolian) where the opportunity for resetting of the pIRIR signal is high, other factors such as internal geochemistry (K, Rb, U or Th), external microdosimetry and anomalous fading are a more dominant control on single-grain De distributions. This is in contrast to environments where the opportunity for bleaching is low (e.g. glaciofluvial or fluvial) and the extent of bleaching in the natural environment is the dominant control on the De distribution; this highlights why single-grain analysis is important for providing accurate ages for sedimentary samples taken from poorly-bleached settings.

7. Conclusions

A naturally-bleached dune sand from Argentina (TC01) that gave an age of 20 ± 5 years using the OSL signal of quartz, gave ages of 325 ± 100 years and 550 ± 130 years using single-grain measurements of the pIRIR225 and pIRIR290 signals, respectively. Laboratory measurements of residual De values after bleaching in a solar simulator were then 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) . These bleaching experiments demonstrated that some grains bleach more rapidly than others in response to laboratory bleaching, regardless of the prior dose.

Although the pIRIR signals from individual grains bleach at variable rates, this variation appears to have little impact upon the natural De values determined for K-feldspar grains from the samples measured in this study (Fig 7). For the two aeolian samples it is likely that prior to deposition the grains experienced prolonged periods of sunlight bleaching and so all the grains, regardless of the potential rate of bleaching, reached low residual De values (c.f. 20 hour bleach in Fig. 3a). The extended exposure to sunlight in an aeolian environment reduces the impact of variable bleaching rates on the natural De distributions (e.g. Fig. 7b, d). In contrast, the probability that individual grains have experienced prolonged periods of sunlight exposure in a glaciofluvial setting is low. It is likely that some grains experienced shorter exposure to sunlight than other grains and that the difference in bleaching rates would result in variable residuals in natural De distributions (e.g. Fig. 3a). However, the glaciofluvial sample shown in this study suggests that the influence that the bleaching rates of individual grains had on the natural De distributions (Fig. 7f) was minimal in comparison to other factors, including the variation due to the stochastic nature of the exposure of individual grains to sunlight.

The pIRIR signal from individual grains of K-feldspar bleaches at different rates, but analysis of the samples described here suggests that these differences in rate are not sufficiently great to have any discernible impact upon the De distribution obtained using single grains.

Acknowledgements

Financial support for the laboratory work contributing towards this paper was provided by a NERC PhD st UUCIIloll ip to RKS (NE/1152/845/1). Prof. Joanne Bullard (Loughborough University) is thanked for collecting the aeolian dune sand from Argentina (TC01). Aberystwyth Luminescence Research Laboratory (ALRL) benefits from being part of the Climate Change Consortium for Wales (C3W). Two anonymous reviewers are thanked for their comments that helped to improve the manuscript.

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Table 1. A summary table of published pIRIR residual De values obtained for coarse-grained K-feldspar from multiple-grain aliquots in various studies, and one single-grain study (*Reimann et al. 2012).

Table 2. Experimental details for the single aliquot regenerative dose (SAR) pIRIR dose-recovery and residual-dose experiments performed throughout this study. Note that the regenerative- and test-doses (shown in brackets) were smaller for the residual-dose experiments than those of the dose-recovery measurements.

Table 3. Dose-rates calculated for the density-separated K-feldspar fractions of the samples used in this study. Water contents are expressed as a percentage of the mass of dry sediment and were based on measurements of the field water contents and saturated water contents. An internal K-content of 10 ± 2 % was applied for the K-feldspar fraction after Smedley et al. (2012). External dose-rates were determined using thick source alpha and beta counting. Dose-rates were calculated using the updated conversion factors of Guerin et al. (2011). An a-value of 0.11 ± 0.03 was used after measurements performed by Balescu and Lamothe (1993). Alpha and beta dose-rates were attenuated for grain size after Bell (1980) and Guerin et al. (2012), respectively. Attenuation for moisture content was calculated after Zimmerman et al. (1971). The cosmic dose-rate was calculated based on Prescott and Hutton (1994).

Fig. 1. Histograms of the single-grain population of natural De values obtained for the recently-deposited sample TC01 using the pIRIR225 (a) and pIRIR290 (b) signal. The dashed line marks the central age model (CAM) De value calculated for the single-grain population.

Fig. 2. Central age model (CAM) residual De values calculated from the single-grain population of TC01 after a 52 Gy given dose followed by SOL2 bleaching intervals of 1, 4, 8 and 20 hours. Data are presented for the pIRIR225 and pIRIR290 signals. The horizontal dashed lines mark the CAM De values calculated from the naturally-bleached grains from Fig. 1.

Fig. 3. Grain-to-grain variability in residual De values obtained for sample TC01 using the pIRIR225 signal. a) Examples of individual grains that have a rapid (a; open triangles), moderate (b; closed circles) and slow (c; closed triangles) bleaching rate. Histograms are also presented showing the single-grain population of De values following exposure to the SOL2 for (b) 1, (c) 4, (d) 8 and (e) 20 hours. Residual De values of grains a, b and c within the single-grain populations are indicated. The CAM residual De value calculated from the grains of each histogram is represented by the vertical dashed line in Figs b - e.

Fig. 4. Central age model (CAM) residual De values calculated from the single-grain population of GDNZ13 after SOL2 bleaching of 1, 4, 8 and 20 hours. a) Data are presented for the pIRIR225 and pIRIR290 signal. Histograms of single-grain residual De values obtained for the pIRIR225 signal after SOL2 bleaching intervals of (b) 1, (c) 4, (d) 8 and (d) 20 hours. The CAM De values calculated from the grains of each histogram are represented by the vertical dashed line in Figs b - e.

Fig. 5. Grain-to-grain variability in residual De values obtained using the pIRIR225 signal for sample GDNZ13 after 8 hours exposure to the SOL2. a) Examples of individual grains that have a rapid (x; open triangles), moderate (y; closed circles) and slow (z; closed triangles) bleaching rate. Note that the x-axis shows the given dose on a log scale. Histograms are also presented showing the single-grain population following given doses of (b) 52 Gy, (c) 102 Gy and (d) 202 Gy; residual De values of grains x, y and z within the single-grain population are indicated. The CAM residual De value calculated from the grains of each histogram is represented by the dashed line in Figs b - e.

Fig. 6. Cumulative percentage of grains with residual De values expressed as a percentage of the 52 Gy given dose after a short 1 hour SOL2 bleach. The single-grain data presented in this plot were ranked from the smallest to the largest by the residual De value, and the cumulative percentages of grains (y-axis) were then plotted against the residual dose as a percentage of the 52 Gy given dose (x-axis). Data are presented to compare (a) the pIRIR225 and pIRIR290 signals of sample GDNZ13 and (b) the pIRIR225 signals of samples TC01, GDNZ13 and LBA12F4-2.

Fig. 7. Comparisons of the residual De value, measured in the laboratory after a 1 hour bleach following a 52 Gy given dose, with the natural De value. All data were measured using the pIRIR225 signal and are shown for samples TC01 (a), GDNZ13 (c) and LBA12F4-2 (e). The grains in (a), (c) and (e) were then ranked according to the size of the residual De measured after the 1 hour SOL2 bleach following the 52 Gy given dose, and binned into five groups. The CAM and/or MAM De values for each bin were then calculated and plotted (b, d, f). For the MAM analysis a ob value of 0.3 was used. The De value expected for these samples is calculated from the dose-rate and independent age control. Fading tests of the pIRIR225 signal were undertaken for samples GDNZ13 (g-value 3.5 ± 0.7 %/decade, n = 5) and LBA12F4-2 (g-value 2.1 ± 1.3 %/decade, n = 5) using multiple grain aliquots. The data presented here have not been corrected for fading because it is the pattern of relative change in De for grains with different bleaching rates that is primarily of interest. The number of grains used to calculate each De value is shown in the histograms (b, d and f). Where bins contained fewer than three grains, the mean and standard deviation De values were calculated for comparison.

Table 1.

Reference Depositional environment Bleaching method Protocol Residual

Reimann and Tsukamoto (2012) Coastal 17 hours SOL2 bleach PIRIR150 0.4 Gy

1 week of daylight exposure PIRIR150 0.7 Gy

Madsen et al. (2011) Beach Modern analogue PIRIR150 0.05 ± 0.01 Gy to 2.66 ± 0.06 Gy

Reimann et al. (2011) Coastal 4 hours SOL2 bleach PIRIR180 ~1 Gy

Reimann et al. (2012)* Coastal Modern analogue PIRIR180 0.6 ± 0.03 Gy

4 hours SOL2 bleach PIRIR180 0.9 ± 0.04 Gy

Thomsen et al. (2008) Beach sand Modern analogue PIRIR225 2 Gy

Buylaert et al. (2009) Beach sand Modern analogue pIRIR225 1.4 ± 0.1 Gy

Alappatt et al. (2010) Deltaic core 4 hours SOL2 bleach pIRIR225 ~3 Gy

Thiel et al. (2012) Shallow marine Modern analogue pIRIR290 ~2 Gy

Buylaert et al. (2012) Coastal Modern analogue pIRIR290 5 ± 2 Gy

Reimann et al. (2011) Coastal 4 hours SOL2 bleach pIRIR290 6.4 ± 1.2 Gy

Alexanderson and Murray (2012) Glaciofluvial 5 hours SOL2 bleach** pIRIR290 12 ± 0.6 Gy

**In this experiment the distance of each aliquot from the SOL2 light source was ~40 cm.

Table 2.

Step Treatment_

1 Dose

2 Preheat 250°C or 320°C

3 SG IRSL 2 s at 60°C

4 SG IRSL 2 s at 225°C or 290°C

5 Test dose (52 Gy or 4 Gy)

6 Preheat 250°C or 320°C

7 SG IRSL 2 s at 60°C

8 SG IRSL 2 s at 225°C or 290°C

9 SG IRSL 3 s at 330°C

Table 3.

Sample Grain size Water K (%) U (ppm) Th (ppm) Cosmic dose- Dose-rate

(^m) content (%) rate (Gy/ka) (Gy/ka)

TC01 180 - 250 5 ± 2 1.72 ± 0.08 1.89 ± 0.19 4.38 ± 0.61 0.18 ± 0.02 3.08 ± 0.14

GDNZ13 180 - 212 30 ± 5 1.02 ± 0.07 2.26 ± 0.19 5.25 ± 0.61 0.14 ± 0.02 2.23 ± 0.14 LBA12F4-2 180 - 212 5 ± 2 2.35 ± 0.11 1.99 ± 0.24 6.37 ± 0.80 0.09 ± 0.02 3.75 ± 0.46

a) CAM 1.0 ± 0.3 Gy I OD = 0.90 Gy n = 14 grains

b) CAM 1.7 ± 0.4 Gy I OD = 0.97 Gy n = 10 grains

0 2 4 6 8 Residual De (Gy)

0 2 4 6 8 10 Residual D (Gy)

Time (hours)

1 10 20

pIRIR225 CAM De = 1.0 ± 0.3 Gy

Time (hours)

10 0 10 Residual Dp (Gy)

10 0 10 Residual (Gy)

Time (hours)

1 10 20

0 20 0 20 0 20 0 20 Residual De (Gy) Residual De (Gy)

Given dose (Gy) 100

b)52 Gy

c) 102 Gy

d)202 Gy

5 10 15 Residual Dp (Gy)

5 10 15 Residual D (Gy)

5 10 15 Residual (Gy)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Residual De measured after 1 hour SOL2 bleach (Gy) Residual De measured after 1 hour SOL2 bleach (Gy)

b) TCOl

Mean ± St. dev.

Expected D

0 5 10 15 20 25 30 1 hr S0L2 bleach residual D value (Gy)

Binned 1 hr S0L2 bleach residual D value (Gy)

d) GDNZ13 • CAM

■ One grain

Expected De 1 •

50 40 30 20 10

0 5 10 15 20 25 30 1 hr SOL2 bleach residual D value (Gy)

ofo ^ ^ ^ ^

Binned 1 hr SOL2 bleach residual D value (Gy)

f) LBA12F4-2

• CAM O MAM

Expected De 0

0 5 10 15 20 25 30 1 hr SOL2 bleach residual D value (Gy)

Binned 1 hr SOL2 bleach residual D value (Gy)

Highlights

1. The bleaching rate of the pIRIR signal from single K-feldspar grains was assessed

2. The PIRIR225 signal bleaches more rapidly than the pIRIR290 signal

3. pIRIR225 and pIRIR290 signals reach the same level after 4-20 hours of bleaching

4. The bleaching rate of the pIRIR signal varies from one grain to another

5. Variable pIRIR bleaching rates do not control single-grain De distributions