Developing a SAR TT-OSL protocol for volcanically-heated aeolian quartz from Datong (China)

Quaternary Geochronology 10 (2012) 308e313

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Research Paper

Developing a SAR TT-OSL protocol for volcanically-heated aeolian quartz from Datong (China) Jinfeng Liu a, *, Andrew S. Murray b, Mayank Jain c, Jan-Pieter Buylaert b, c, Yanchou Lu a, Jie Chen a a

State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Dewai Qijia Huozi, Beijing 100029, China Nordic Laboratory for Luminescence Dating, Department of Geoscience, University of Aarhus, Risø DTU, DK-4000 Roskilde, Denmark c Center for Nuclear Technologies, Denmarks Technical University, DTU Risø campus, DK-4000 Roskilde, Denmark b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 October 2011 Received in revised form 1 February 2012 Accepted 2 February 2012 Available online 8 February 2012

The thermally-transferred optically stimulated luminescence (TT-OSL) responses of chemically-purified fine-grained quartz from a lava-baked aeolian sediment from Datong (China) are presented. Our main focus is to examine the suitability of the test dose TT-OSL and OSL response to monitor sensitivity changes during SAR measurements. It is found that, in contrast to the test dose OSL, the TT-OSL response to a test dose can successfully monitor sensitivity changes, and a high-temperature blue-light bleach (600 s at 260
C) in the middle of each SAR cycle is necessary to minimize interference with the test dose TT-OSL signal. The revised SAR TT-OSL protocol was tested by dose recovery tests on two very young (quartz OSL De < 10 Gy) sediment samples (one heated by human activity and one of aeolian origin). Dose recovery tests using different test doses indicate that it is more appropriate to use larger test doses (>20% of the given dose) in this revised SAR TT-OSL protocol. Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.

Keywords: TT-OSL Fine-grained quartz Dating Heated quartz

1. Introduction The dating range of quartz OSL dating is generally limited by saturation in the dose response of the fast component OSL signal. The thermally-transferred optically stimulated luminescence (TT-OSL) signal has the potential to be used as the basis for a luminescence dating method that can be applied to samples about one order of magnitude older than with the quartz fast component OSL signal (Wang et al., 2006a,b, 2007; Duller and Wintle, 2012). Previous studies using the TT-OSL signal observed a residual signal remaining after repeated measurements of the TT-OSL signal; this was termed the Basic Transferred OSL (BT-OSL) signal following the terminology of Aitken (1998). This signal was light insensitive, thermally stable, and independent of dose (Rhodes, 1988). Some authors have suggested that the BT-OSL signal needs to be measured and subtracted from the TT-OSL signal in order to isolate the main component of the TT-OSL, termed the recuperated OSL (ReOSL) signal (Wang et al., 2006a,b, 2007; Jacobs et al., 2011). On the contrary, comparative analysis indicates that although BT-OSL accumulates with dose, the intensity is rather small in comparison to TT-OSL (e.g. Kim et al., 2009). Thus, rather than * Corresponding author. Tel./fax: þ86 10 62009038. E-mail address: [email protected] (J. Liu).

making long, cumbersome measurements to quantify BT-OSL, it seems more appropriate to remove it as fully as possible at the end of each SAR measurement cycle and thus avoid a ‘build-up’ effect. Several researchers have taken this approach to avoid potential residual charge carry-over either by heating at a high temperature (Porat et al., 2009) or by using a high-temperature blue (470  10 nm, typically w50 mW/cm2) bleach at the end of each measurement cycle (Tsukamoto et al., 2008; Stevens et al., 2009; Adamiec et al., 2010). Stevens et al. (2009) suggested the insertion of an additional high-temperature optical bleach before the test dose to remove charge carry-over between regenerated and test dose TT-OSL measurements. In addition, there are various views on the most reliable test dose signal for sensitivity correction. Some protocols used the OSL signal to monitor the sensitivity change, and thus were able to use a small test dose (Wang et al., 2006a,b, 2007; Tsukamoto et al., 2008; Kim et al., 2009; Jacobs et al., 2011). However other studies have argued that the source of the electrons giving rise to the TT-OSL signal was different from that giving rise to the fast component OSL (Adamiec et al., 2008; Pagonis et al., 2008), and therefore using the test dose OSL response for correction may not be entirely appropriate. Stevens et al. (2009) showed that using an OSL test dose response did not provide a satisfactory sensitivity correction in their samples; in contrast the TT-OSL response to a test dose did perform satisfactorily.

1871-1014/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2012.02.008

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There is clearly considerable debate over the most appropriate heat treatment during and at the end of each SAR cycle that will remove any charge carry-over and avoid interference with the sensitivity correction. The aim of the current study is to investigate, using a naturally heated loess sample from China, two of the important questions in the TT-OSL dating method: what is an appropriate correction for sensitivity changes during each SAR TT-OSL measurement cycle and how does one minimize any residual TT-OSL signal remaining at the end of a measurement cycle? The potential accuracy of the protocol is assessed using dose recovery experiments, and suggestions on sensitivity correction when using a SAR TT-OSL protocol are made.

The luminescence signals were measured using Risø TL/OSL systems (models DA-12 and 20) at Risø DTU (Denmark) and at the Institute of Geology (China), using blue-light emitting diodes (LEDs) for stimulation (470  10 nm, w80 mW/cm2). The luminescence emission was detected with EMI-9235QA photomultiplier tubes filtered with w7 mm of Hoya U-340 glass filter (Bøtter-Jensen et al., 2003). The different protocols used in the study are summarized in Table 1. The luminescence intensity was calculated using the sum of the photons detected in the first 0.625 s of stimulation (1e5 channels) minus an early background sum between 1.875 s and 2.5 s (16e20 channels) (e.g. Ballarini et al., 2007). Typical TT-OSL decay curves are given in Fig. S2.

2. Samples and measurement facilities

3. Monitoring sensitivity change using TT-OSL

The Datong group of volcanoes is an important landform type in eastern China. In most of this volcanic region, the originally aeolian (loess) deposits were baked by lava-flows. Field observations clearly indicated that the layers baked by the lava-flow had a distinct clear brick-red color compared to the underlying unbaked layer (Fig. S1). A sample (08DT-1) was taken from the brick-red baked layer under the volcanic basalt. Based on laboratory heating experiments carried out on the original loess sediment we deduce that this type of sample must have reached a temperature >700
C during contact with the lava-flow; zeroing of the luminescence signal was thus clearly the result of heating (see Supplementary Information S1). The sample was processed at the Institute of Geology, China Earthquake Administration. After the treatment with HCl and H2O2 to remove carbonates and organic material, the fine grain-size fraction was then immersed in 30% hydrofluorosilicic acid to purify fine-grained (4e11 mm) quartz (Lu et al., 2007). The OSL IR depletion ratio method (Duller, 2003) and the 110
C thermoluminescence (TL) peak method (Aitken, 1998) were used to confirm the purity of the quartz fraction. Two very young samples were used in dose recovery experiments: a heated sample from a buried fireplace found in the Leigu paleoearthquake trench (08XY10-2, 4e11 mm quartz, OSL De w5.8 Gy) (Liu et al., 2010) and a desert sample from the Hunshandake Sandy Land in China (SY-23-2, 90e180 mm quartz, OSL De w2 Gy). For the desert sample, the conventional coarse grain preparation technique was used.

Using protocol A (Stevens et al., 2009; Adamiec et al., 2010) in Table 1, linearity tests between regeneration (Ltt) and test dose signals (Tosl, Ttt) (Murray and Wintle, 2000; Wintle and Murray, 2006) were carried out to check the reliability of the sensitivity correction of OSL and TT-OSL signal response to test dose. Three groups of three aliquots were prepared from the naturally heated quartz (08DT-1). These were measured for eight SAR cycles using a fixed regenerative dose except in the 5th cycle when no regenerative dose was given. Group 1 used the steps from 1-1 to 2-5 only, whereas group 2 included steps from 1-1 to 2-5 and step 3-1 (bleaching at the 280
C for 300 s) to avoid progressive signal accumulation between cycles. Group 3 included steps from 1-1 to 2-5 and step 3-2, thermal treatment at 350
C for 200 s (Fig. 1). Only the data in Fig. 1d (TT-OSL test dose) show regressions that pass through the origin, and so give a constant L/T ratio (Fig. 1c); the data obtained using the OSL test dose signal show linear correlations (Fig. 1b), but with significant intercepts. As a result the L/T ratio (Fig. 1a) is not independent of sensitivity change, as is required by SAR. These results indicate clearly that the TT-OSL response to a test dose is a better monitor of sensitivity change than the OSL test dose response, whether or not a high-temperature bleach or a thermal treatment was applied at the end of each cycle. Although the quartz TT-OSL response to a test dose appears to provide an accurate correction for the sensitivity changes in the regenerated TT-OSL signal, caution should be exercised in interpreting these results. Either the TT-OSL is really monitoring the

Table 1 SAR TT-OSL protocols used in this study. Step

A

B

C

1_1 1_2 1_3 1_4 1_5 1_6 1_7 1_8 1_9 2_1 2_2 2_3 2_4 2_5 2_6 2_7 2_8 2_9 3_1 3_2

Dose (300 Gy) Preheat at 260
C for 10 s OSL at 125
C for 100 s Preheat at 260
C for 10 s OSL at 125
C for 100 s (Ltt)

Dose (210 Gy) Preheat at 260
C for 10 s OSL at 125
C for 100 s Preheat at 280
C for 10 s OSL at 125
C for 100 s (Ltt) OSL/Anneal at 260
C for T s (Empty) OSL at 125
C for 100 s Preheat at 280
C for 10 s OSL at 125
C for 100 s (Lttc-check) Test dose (105 Gy) Preheat at 220
C (or 260
C) for 10 s OSL at 125
C for 100 s Preheat at 280
C for 10 s OSL at 125
C for 100 s (Ttt) OSL/Anneal at 260
C for T s (Empty) OSL at 125
C for 100 s Preheat at 280
C for 10 s OSL at 125
C for 100 s (Tttc-check) OSL at 280
C for 300 s (Clean)

Dose Preheat at 260
C for 10 s OSL at 125
C for 100 s Preheat at 280
C for 10 s OSL at 125
C for 100 s (Ltt) OSL at 260
C for 600 s (Empty)

Test dose (100 Gy) Preheat at 220
C for 10 s OSL at 125
C for 100 s (Tosl) Preheat at 260
C for 10 s OSL at 125
C for 100 s (Ttt)

OSL at 280
C for 300 s (Clean) Thermal treatment of 350
C for 200 s (Clean)

Test dose Preheat at 260
C for 10 s OSL at 125
C for 100 s Preheat at 280
C for 10 s OSL at 125
C for 100 s (Ttt)

OSL at 300
C for 600 s (Clean)

310

J. Liu et al. / Quaternary Geochronology 10 (2012) 308e313

a

b

1.2

25000 Group1

20000

0.8

Group2 Group3

Ltt/counts

Normalized Ltt/Tosl

1

Group1

0.6

Group2

0.4

15000 10000

Group3

5000

0.2

0

0 0

1

2

3

4

5

6

7

8

0

9

1000000 2000000 3000000 4000000

Cycle No

c

Tosl/counts

d

1.20

20000

Group1 Group2

0.80 0.60

Ltt/counts

Normalized Ltt/Ttt

1.00

25000

Group1 Group2

0.40

Group3

15000 10000

Group3

5000

0.20

0

0.00 0

1

2

3

4 5 6 Cycle No

7

8

9

0

5000 10000 15000 20000 25000 Ttt/counts

Fig. 1. Results of the measurements made using protocol A from Table 1. a) and c) show the sensitivity-corrected TT-OSL (normalized to the value in the first regenerative cycle) as a function of cycle. In a) the test dose OSL response (Tosl, step 2-3 in protocol A in Table 1) is used for sensitivity correction; in c) the test dose TT-OSL response (Ttt, step 2-5 in protocol A) is used. b) and d) show the linearity tests. b) TT-OSL Ltt signal (step 1-5 in protocol A) plotted against the OSL test dose response (Tosl) and in d) against the TT-OSL test dose response (Ttt). The regenerative and test doses used in this experiment were 300 and 100 Gy, respectively. Group 1 had no clean-out at the end of the SAR cycle, group 2 was subjected to a high-temperature blue-light stimulation (step 3-1 in protocol A) and group 3 had a purely thermal cleaning (step 3-2 in protocol A).

sensitivity change or the majority of the signal following the test dose is simply a residual signal proportional to regeneration dose (Stevens et al., 2009). The size of any residual signal derived from the natural or regenerative dose was investigated using repeated heat/OSL cycles (steps 1-2 to 1-5 from protocol A). In Fig. S3 it can be seen that even after 4 cycles of TT-OSL measurements the remaining signal is still about 20% of its initial intensity. 4. Removing residual TT-OSL signals by annealing We next undertook investigations to determine appropriate conditions for reducing this residual signal to acceptable proportions. We chose to undertake these investigations using a fairly low anneal temperature of 260
C (steps 1-6 and 2-6 in protocol B in Table 1) in the expectation that this would reduce the likelihood of significant additional thermal transfer. To determine the effect on the residual TT-OSL signal of storing at 260
C for different durations we added an extra thermal treatment and TT-OSL measurement after each primary measurement of Ltt and Ttt (steps 1-7e1-9 and 2-7e2-9 in protocol B). The results with and without blue-light stimulation are shown in Fig. 2a. After blue-light bleaching for 600 s at 260
C the residual signal (Lttc-check/Ltt, protocol B) is reduced to a negligible level (w2%) (Fig. 2a) (note: corresponding check data from the test dose, namely step 2-9 Tttc-check, is not

used). Based on this observation, we decided to add one step in the middle of each SAR cycle of optical stimulation for 600 s with the sample held at 260
C (see step 1-6 in protocol C). The impact of two different test dose preheat temperatures (220
C and 260
C for 10 s, step 2-2 of protocol B) on the residual TT-OSL signal (Lttc-check/Ltt) was also investigated. Although all the Ltt/Ttt lines pass through or close to the origin (Fig. 2c and d) and although the different test dose preheat temperatures have no significant impact on the residual TT-OSL (Fig. 2a), the slope of the Ltt against Ttt correlation increases from w1.4 to w2.0 when the test dose preheat temperature changed from 220
C to 260
C with the blue-light stimulation for 600 s (Fig. 2b and d). In this experiment, the test dose used in step 2-1 (105 Gy) is half of the regeneration dose in step 1-1 (210 Gy), and so the Ltt/Ttt ratio would be expected to be close to 2 had there been no carry-over from the previous dose, and the same signal was used for correction. Accordingly we selected a test dose preheat temperature of 260
C for 10 s for our final protocol shown as protocol C in Table 1. At the end of each measurement cycle, a high-temperature bleach (blue-light stimulation for 600 s at 300
C) was used to clean-out the sample (e.g. Stevens et al., 2009). Compared with protocol A, to maximize the extent of thermal transfer, a preheat temperature of 280
C instead of 260
C was used in steps 1-4 and 2-4 of protocols B and C (Wang et al., 2006a).

J. Liu et al. / Quaternary Geochronology 10 (2012) 308e313

b

0.16

2.1 2

0.14 no light,CH220 0.12

no light,CH220

1.9

light on,CH220 light on,CH260

0.1 0.08 0.06

light on,CH220

1.8 Ltt/Ttt slope

Fractional residual TTOSL(Lttcheck/Ltt)

a

light on,CH260

1.7 1.6 1.5

0.04

1.4

0.02

1.3 1.2

0 0

0

300 600 900 1200 1500 1800 2100 2400

300 600 900 1200 1500 1800 2100 2400 Anneal time/s

Anneal time/s

c

d

8000 y = 1.4327x R2 = 0.9245 y = 1.5866x R2 = 0.9559

8000 y = 2.0361x R2 = 0.9456

y = 1.3487x R2 = 0.975

6000

y = 1.227x R2 = 0.9876

4000

y = 1.4128x R2 = 0.9822

y = 1.8772x R2 = 0.8851

6000

Ltt

Ltt

311

y = 1.2914x R2 = 0.9694

4000

300s,light on,CH 220 300s,no light,CH220 600s,no light,CH220 1200s,no light,CH220 2400s,no light,CH220

2000

0 0

2000

4000

6000

2000

300s,light on,CH 260 600s,light on,CH 220 600s,light on,CH 260

0 8000

0

2000

4000

Ttt

6000

8000

Ttt

Fig. 2. Results of the measurements made using protocol B from Table 1. Several groups of three natural aliquots were subjected to at least five cycles of protocol B using a dose of 210 Gy and a test dose of 105 Gy. The data of the first (natural) measurement cycle was discarded. The different groups underwent different combinations of annealing treatments (steps 1-6 and 2-6: hold aliquot for time T at 260
C with or without blue light) and test dose preheat temperatures (step 2-2) (see legends for treatment combinations). a) Percentage of TT-OSL remaining (Lttc-check, step 1-9) after annealing at 260
C for various times T as a proportion of the first TT-OSL Ltt (step 1-5). Uncertainties represent standard errors (n ¼ 5). b) The Ltt/Ttt ratio for various anneal times T at 260
C (with or without blue light) and different test dose preheats (220
C and 260
C for 10 s). c) and d) Ltt/Ttt plots for individual aliquots measured as described in a) and b). Dashed lines have a slope of unity.

5. Constructing TT-OSL dose response curves Using protocol C three dose response curves were constructed, each measured with a different test dose (Fig. 3). It can be seen that these dose response curves have different curvatures and useful dose ranges for different test doses. With a test dose of 105 Gy, the D0 is w1700 Gy, implying a useful range of only

16 12 8

Test dose=105 Gy D0=1680 Gy

4 0

c

6 5 4 3 Test dose=1050 Gy

2

D0=5880 Gy

1

5000 10000 Lab dose/Gy

15000

2.0 1.5 1.0 Test dose=4200 Gy

.5

D0=7560 Gy

0.0

0

0

Normalized TTOSL

b

20

Normalized TT-OSL

Normalized TT-OSL

a

w3.5 kGy (Fig. 3a). When larger test doses were used (1050, 4200 Gy), the D0 values increased to w5900 Gy and 7600 Gy, implying dose ranges of w12 kGy and 15 kGy (Fig. 3b and c). Recycling was generally good (see also Fig. 3). The recycling ratios of one 840 Gy point in Fig. 3a and two 10,080 Gy points in Fig. 3b and c are within 10% of unity; only one 840 Gy point in Fig. 3c has a rather poor ratio of 1.19.

0

5000 10000 Lab dose/Gy

15000

0

10000 5000 Lab dose/Gy

15000

Fig. 3. Dose response curves measured with different test doses using protocol C (Table 1). One aliquot was measured for each dose response curve. The data were fitted with a saturating exponential of I ¼ Imax  (1  exp(D/D0)).

312

J. Liu et al. / Quaternary Geochronology 10 (2012) 308e313

1.2 1.1

Dose recovery ratio

1 0.9 0.8 0.7

08XY10-2,given dose 570Gy

0.6

SY-23-2,given dose 570Gy SY-23-2,given dose 1140Gy

0.5 0.4

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Test dose/Given dose Fig. 4. Dose recovery ratio using protocol C (Table 1) for the very young samples as a function of test dose size (expressed as a ratio to the given dose). At least three aliquots were measured per data point and error bars represent one standard error.

These results show that in these samples the D0, and so the apparently useful dose range appears to be dependent on the size of the test dose. This may still be an artifact of insufficient removal of residual signal (carry-over). One would then expect a lower D0 for small test doses, because the residual from the previous dose would increase up the growth curve (as the regenerative dose increased), the test dose response would then be overestimated more with increasing regenerative dose, and so the corrected signal depressed at higher doses, reducing the apparent D0. While this is clearly undesirable, it is important to realize that it does not necessarily introduce any inaccuracy (assuming satisfactory recycling), although of course the precision will deteriorate as apparent saturation is approached.

suitable conditions for applying corrections for sensitivity change and for reducing any residual TT-OSL to acceptable levels were developed and tested. We have shown that residual TT-OSL signals left after incomplete emptying of natural and regenerative doses can be a significant fraction of test dose induced signals, and so adversely affect the accuracy of a test dose TT-OSL measurement. In order to reduce any residual TT-OSL signal to acceptable levels, we find it is necessary to apply a long optical bleach at elevated temperature (260
C for 600 s) before administering the test dose. With this extra treatment to reduce residual signals from prior doses, our data confirm that the TT-OSL response to a test dose is likely to be a more accurate correction for sensitivity changes in the natural and regenerated TT-OSL signal than the OSL response. The dose response curves of the sensitivity-corrected TT-OSL were found to have increasing values of D0 as larger test doses were used. Larger test doses also provided more accurate dose recovery results. We conclude that, in general, the test dose should be >20% of the dose under investigation. Future work will use this protocol to date known age samples, and to determine whether these differences in growth curve shape observed in the laboratory are indeed reflected in the behavior of natural samples. Acknowledgments We are grateful to Frank Preusser and an anonymous reviewer for the critical reviews that helped to improve the manuscript. The authors thank the staff of the Research Laboratory of Luminescence Dating, Institute of Geology, China Earthquake Administration, Risø National Laboratory for Sustainable Energy, Technical University of Denmark and the Nordic Laboratory for Luminescence Dating, Aarhus University. Dr. Xudong Fu is thanked for providing desert sample SY-23-2. We also would like to express thanks to Ann G Wintle and Xulong Wang for discussion on TT-OSL dating. This work was financially supported by the National Natural Science Foundation of China (No: 40802040), the Special Fund of Seismic Research (200808015), and the Project of the State Key Laboratory of Earthquake Dynamics (LED2008A02). Editorial handling by: F. Preusser

6. Dose recovery tests

Appendix. Supplementary material

To further test our protocol C, it was used with two very young samples: a desert sample (SY-23-2) and a human-baked sample (08XY10-2). The use of very young samples is desirable in dose recovery tests because it avoids the bleaching step prior to dosing the sample; bleaching of TT-OSL signals in the laboratory has proven to be difficult (e.g. Tsukamoto et al., 2008). Because of the low TT-OSL signal of the very young samples, the natural TT-OSL signal was not detectable, and so we did not subtract a dose from the measured SAR TT-OSL De. The dose recovery test was carried out for a range of given dose to test dose ratios and the results are shown in Fig. 4. These data suggest that the best dose recovery is obtained when the test dose is >20% of the given dose. This observation is again consistent with an unacceptably large residual contribution to test dose signal at low test doses, as discussed above.

Supplementary data related to this article can be found online at doi:10.1016/j.quageo.2012.02.008.

7. Conclusions The volcanically-heated sample in this study shows TT-OSL behavior similar to that reported by Stevens et al. (2009); we support their conclusion that OSL signals are unlikely to be suitable measures of sensitivity change in TT-OSL signals. As part of the development of a SAR protocol appropriate to the TT-OSL signal,

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