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Testing the use of an OSL standardised growth curve (SGC) for De determination on quartz from the Chinese Loess Plateau
Radiation Measurements 41 (2006) 9 – 16 www.elsevier.com/locate/radmeas
Testing the use of an OSL standardised growth curve (SGC) for De determination on quartz from the Chinese Loess Plateau ZhongPing Lai∗ Centre for the Environment, University of Oxford, Mansfield Road, Oxford OX1 3TB, United Kingdom Received 19 November 2004; received in revised form 4 June 2005; accepted 13 June 2005
Abstract In the Standardised Growth Curve (SGC) method of De determination, once a SGC has been constructed, a De can be determined by measuring the natural OSL and the OSL response to a test dose only, instead of constructing a growth curve for each aliquot. This is a major advantage when a large number of samples from the same section, or the same geographical area, have to be dated, and the samples are well bleached before deposition. In the present study, the use of a SGC for De determination is tested using quartz extracted from loess samples from the Chinese Loess Plateau. It is demonstrated that: (a) A common growth curve exists for samples collected from four different sections in the Chinese Loess Plateau; (b) The scatter in De for samples younger than about 270 ka is dominated by the scatter of the natural signal levels, not that of the growth curve; (c) There is a slight difference between the shape of the growth curves of young samples (< 270 ka) and those from old samples (c. 0.65–2.5 Ma). The cause is not known, but may be due to the difference in ages (total natural irradiation dose) or in the dust sources (origins of quartz); (d) Up to a De of c. 200 Gy, the single aliquot regenerative-dose (SAR) De and the SGC De are in agreement. The ratio of the sum of SAR De s to the sum of SGC De s is 1.01, close to unity. It is therefore concluded that SGC is an alternative procedure for De determination for loess samples from the Chinese Loess Plateau. © 2005 Elsevier Ltd. All rights reserved. Keywords: Quartz OSL; Standardised growth curve; Luminescence; Chinese Loess
1. Introduction A standardised growth curve (SGC) in optically stimulated luminescence (OSL) of quartz has been reported by Roberts and Duller (2004) when using the single aliquot regenerative-dose (SAR) protocol for equivalent dose (De ) determination (Murray and Wintle, 2000). In SAR procedures, the OSL signal from the test dose (Tx ) is employed to correct for sensitivity changes over a series of regeneration measurements (Lx ) (Murray and Wintle, 2000). Roberts and Duller (2004) argued that such a test dose response can ∗ Fax: +44 1865 271929.
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potentially be used as an inter-aliquot normalisation step, and that normalising the luminescence signal (Lx ) with the test dose response (Tx ) not only corrects for changes in luminescence sensitivity, but should also compensate for differences in natural signal intensity (LN ). When the normalised luminescence signal (Lx /Tx ) is multiplied by the size of the test dose (TD ), i.e. ((Lx /Tx )TD ) which they termed ‘standardised luminescence signal’, they found that a SGC exists for different samples from different continents. Attempts to construct a common growth curve for luminescence dating date back to the 1980s (Smith, 1983), but in general such efforts were not successful before. Sensitivity change in both feldspar and quartz after laboratory treatments (e.g. bleaching, irradiation, or preheat treatment) are commonplace (e.g., Li and Wintle, 1992; Zhou and Wintle,
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Z. Lai / Radiation Measurements 41 (2006) 9 – 16
1994; Wintle and Murray, 1999). This complicates the construction of a universal growth curve unless the sensitivity changes can be corrected for. To avoid sensitivity changes due to laboratory bleaching in the regeneration method, Smith (1983) attempted to provide a general growth curve for thermoluminescence (TL) dating using an additive approach on coarse-grained sedimentary quartz. He firstly constructed an additive growth curve in the laboratory using the youngest sample from a sedimentary sequence, and then obtained the De s for older samples by matching their normalised natural TL signal levels to this growth curve. The validity of this method assumes that for all the samples dated: (a) the TL residual level is similar; (b) the quartz TL has the same growth curve during burial. This was the first attempt to use a general growth; such an approach has difficulties in practice, as the assumptions are not universally valid. Shlukov and Shakhovets (1987) also proposed a general growth curve for samples from the Russian Plain. The studies by Shlukov and Shakhovets (1987) and Shlukov et al. (1993) found that laboratory irradiation can cause quartz TL signal loss. They showed that such “irradiation-induced fading” was even greater than the thermal fading, and proposed that the use of laboratory irradiation to reconstruct a growth curve should be abandoned. To obtain a growth curve without the use of laboratory irradiation, Shlukov and Shakhovets (1987) and Shlukov et al. (2001) suggested a method for quartz TL dating based on the assumption that all quartz in a specific geographical area has the same growth curve. The parameters that define the growth curve can be measured on a sample with known age and another saturated sample. To obtain an age for a sample, only the natural signal and the residual signal after laboratory bleaching are measured, in addition to the dose rate. However, it is difficult to accurately define the parameters for a growth curve in this way, and this method has not been widely adopted. A universal growth curve has been constructed by Porat and Schwarcz (1994) in ESR dating. They showed that the radiation sensitivity of all tooth enamel is quite similar, and have formed a universal growth curve for equivalent dose estimation. In a further test, using a saturating exponential equation of the form I = Imax (1 − exp −D/D0 ) to fit the data, Rink and Schwarcz (1994) found that both the fitting parameters (Imax and D0 ) and the sensitivity were similar for modern and fossil enamel from various sites. As a result, they concluded that the universal growth curve is a good predictor of equivalent dose. The development of a SAR protocol for quartz OSL (Murray and Wintle, 2000) made it easy to compare luminescence sensitivity for different aliquots or samples. The crucial step incorporated in SAR is the use of the OSL response to a test dose for correction of sensitivity changes. The implication of this is that the test-dose-normalised growth curve can be compared for many different samples (Roberts and Duller, 2004). Consequently, the SGC can be tested and constructed.
Some results suggest that the existence of a SGC is unlikely as different quartz grains have different luminescence characteristics. Based on single grain measurements, it has been shown that for quartz it is common for a small proportion of the brightest grains to be responsible for a large proportion of the total OSL signal (Duller et al., 2000; Mc Coy et al., 2000; Thomsen et al., 2002; Jain et al., 2002). In two aeolian samples from coastal dunes in southern Africa, Duller et al. (2000) found that 95% of the total OSL light sum of all grains was dominated by the signal from approximately 5% of the grains, and that there were large variations in the dose saturation characteristics of individual grains within a sample. Yoshida et al. (2000) also showed that different grains of sedimentary quartz had different D0 values characterising the saturating exponential function used to fit the data. Based on single aliquot measurements, Murray and Wintle (2000) also demonstrated that quartz from different samples had different saturation doses. However, in multiple-grain (coarse-grained) aliquots, in which there are about 1000 grains in an aliquot (Roberts and Duller, 2004) and the OSL measurements (e.g., intensity, growth curve) are the sum of all grains which may have a variety of different luminescence characteristics, Roberts and Duller (2004) found that the ratio of the SAR and SGC De values is consistent with unity for 22 samples from different continents. They have also obtained similar results for the quartz dominated post-IR OSL from fine-grains of 14 loess samples from three different sites along an east–west transect spanning 700 km in the Chinese Loess Plateau. They concluded that the SGC was applicable, and provided a rapid and accurate method for De determination for samples from the same section, or from a geographical area where the lithology and geological history are the same. Their finding is important as it opens a new door for De determination for quartz OSL. Loess from the Chinese Loess Plateau (CLP) is largely homogeneous due to the effect of long distance transport by wind on grain size sorting, and thus this sediment is ideal for further testing of the SGC. As most of the samples in Roberts and Duller (2004) were young and within linear fitting range, belonging to the Holocene epoch (out of their 36 quartz OSL samples, three samples had a De > 30 Gy, and one sample had a De > 40 Gy), the present work is a further test of the SGC in the non-linear dose range up to a De of c. 200 Gy. 2. Samples and measurement techniques Samples with ages spanning c. 10 ka to c. 2.5 Ma were collected from four different sections in the CLP; the Luochuan section (109◦ 25 E, 35◦ 45 N) from Shaanxi Province; the Lingtai section (107◦ 34 E, 34◦ 59 N) in Gansu Province in the central part of CLP; the Jiuzhoutai section near Lanzhou city in Gansu Province in the western part of CLP; and the Yuanbao Section (103◦ 08.8 E, 35◦ 38.4 N) in Gansu Province on the western edge of CLP. These sections have
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
given regeneration dose showing a standard error of within 1.7% (except for the zero dose). This pattern is seen for all 12 samples with a standard error of within 4.3%. It is worth noting that for those younger samples (LC/1 with an De of c. 27 Gy, LC/14 with an De of c. 140 Gy, LT/0.8 with an De of c. 60 Gy and LT/6.6 with an De of c. 228 Gy) the scatter in the signal observed from the natural dose is higher than that of any regeneration dose (except the zero dose), implying that the scatter in De s for each sample is mainly from the scatter in the natural signal, not from that of the growth curve.
5 Normalised OSL (L x / Tx )
11
4 3 2 1 0 0
50
100 150 200 Regeneration Dose (Gy)
250
Fig. 1. Growth curves of six aliquots from sample LC/1. The growth curves are identical for all 6 aliquots.
been well studied for their palaeoclimatic records (e.g., Liu, 1985; An et al., 1990; Balsam et al., 2004; Chen et al., 2003). In the laboratory, samples were treated with HCl and H2 O2 to remove carbonates and organics. The grain size from 45 to 63 m or 38 to 63 m was separated by dry sieving and treated with 35% fluorosilicic acid for about two weeks (Berger et al., 1980) to isolate pure quartz whose purity was tested by IR stimulation, and there is no obvious contamination of feldspar for all samples. The quartz grains were then deposited on stainless steel discs using silicone oil to make a multiple-grain aliquot. Experiments were carried out using a RisZ DA-15a automated TL/OSL system incorporating a Strontium-90 beta source (BZtter-Jensen, 1997). The OSL was detected using two U-340 filters. The SAR protocol (Murray and Wintle, 2000) was employed for De determination, in which the OSL of the first 0.4 s stimulation (background subtracted) was integrated to isolate the fast component (Bailey et al., 1997).
3.2. The comparison of growth curves of individual samples from the same section For each of the 12 samples, six aliquots were used to construct growth curves using the same SAR conditions. The growth curve for each sample is the average of the data from the six aliquots. Fig. 2a shows the growth curve of each of the three samples from the Lingtai section. The normalised signal levels for a given regeneration dose are identical with a standard error of 2% (except for the zero dose) (see Table 3). Similar results were observed in sections from Jiuzhoutai (three samples) (Fig. 2b), and Yuanbao (two samples) (Fig. 2c). In the Luochuan section (four samples) the scatter is larger with the normalised signal level for a given regeneration dose showing a standard error of 7% (except for the zero dose) (Fig. 2d). This may be related to the large difference in the ages of the samples. When the four samples were divided into two groups, with the two young samples LC/1 and LC/14 in one group, and the old samples L13/UP and CH02/20/1 in another group, the two growth curves in each group are very similar and over-lap (Fig. 2e and f). This will be discussed further in the following section. 3.3. Growth curves of samples from four sections in the Chinese Loess Plateau
3. The possibility of a common growth curve For this experiment, twelve samples from the four sections were selected (Table 1). Six aliquots were used for each sample, and the same SAR conditions (preheat at 260 ◦ C for 10 s, cut-heat at 220 ◦ C for 10 s, test dose 25.9 Gy) were applied to all aliquots. Seven regeneration doses (from 6.9 Gy to 207.4 Gy) were used. 3.1. The comparison of growth curves of individual aliquots from the same sample Table 2 shows the average normalised OSL signal level (Lx /Tx ) of each dose point from six aliquots for each of the 12 samples. Fig. 1 shows the 6 growth curves of six aliquots from sample LC/1. The growth curves of all 6 aliquots are very similar, with the test-dose normalised signal level for a
Fig. 3a shows the growth curves of all 12 samples. The normalised signal levels for a given regeneration dose have a standard error of within 5% (except for the zero dose) (Table 3). When the data are divided into two groups, 7 young samples with ages < 270 ka (LT/0.8, LT/6.6, LT/19.2, LC/1, LC/14, CH02/4/5 and CH02/4/10) and 5 old samples with ages from c. 0.65 to c. 2.5 Ma (CH02/5/1, CH02/5/2, CH02/5/3, L15/UP and CH02/20/1), within each group the growth curves are very similar with the normalised signal level for a given regeneration dose showing a standard error of within 3% (except for the zero dose) (Fig. 3b, c and Table 4). For practical reasons, the group that is of interest for De determination is that containing the younger samples (< 270 ka), where the charge trapping centres have not become saturated during the period of burial. For these samples the forms of the growth curves are very similar. The
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Z. Lai / Radiation Measurements 41 (2006) 9 – 16
Table 1 Sample information Sample ID
Section name
Collected from loess/soil layer
Expected agea (approximate) (ka)
LC/1 LC/14 L15/UP CH02/20/1 LT/0.8 LT/6.6 LT/19.2 CH02/5/3 CH02/5/2 CH02/5/1 CH02/4/10 CH02/4/5
Luochuan Luochuan Luochuan Luochuan Lingtai Lingtai Lingtai Jiuzhoutai Jiuzhoutai Jiuzhoutai Yuanbao Yuanbao
S0 L1 middle L15 bottom of Wucheng Loess L1 top L1 bottom L3 top L7 L8 L8 S1 middle S1 middle
8 40 1000 2500 15 70 260 650 750 750 110 120
a For those younger than 70 ka, the expected age is based on luminescence dating, and for those older than 70 ka it is based on the loess stratigraphy (Liu, 1985).
Normalised OSL (L x / Tx )
(a)
5
5
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4
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3
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1
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50 100 150 200 Regeneration Dose (Gy)
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(c)
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(d)
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0 0 (e)
0 (b) 0
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250 (f)
Fig. 2. Growth curves for samples from the same section. (a) Three samples from Lingtai section. (b) Three samples from Jiuzhoutai. (c) Two samples from Yuanbao section. (d) Four samples from Luochuan section. (e) The two young samples (LC/1 and LC/14) from Luochuan section. (f) The two old samples (L15/UP and CH02/20/1) from Luochuan section.
Table 2 The average normalised OSL (Lx /Tx ) obtained by SAR (6 aliquots each) Normalised OSL
Std. error (%)
Normalised OSL
N 6.9 16.1 38 57.5 103.6 149.8 207.4 0 16.1
LC/1 1.22 ± 0.04 0.35 ± 0.01 0.78 ± 0.01 1.53 ± 0.01 2.03 ± 0.01 2.81 ± 0.02 3.34 ± 0.03 3.80 ± 0.05 0.02 ± 0.001 0.78 ± 0.01
3.3 1.7 1.7 0.9 0.7 0.8 0.9 1.2 4.3 1.0
LC/14 3.06 ± 0.07 0.38 ± 0.003 0.79 ± 0.002 1.51 ± 0.01 1.98 ± 0.02 2.74 ± 0.03 3.24 ± 0.03 3.69 ± 0.06 0.02 ± 0.001 0.77 ± 0.004
N 6.9 16.1 38 57.5 103.6 149.8 207.4 0 16.1
LT/19.2 4.83 ± 0.17 0.38 ± 0.004 0.76 ± 0.01 1.49 ± 0.03 1.94 ± 0.06 2.71 ± 0.11 3.22 ± 0.14 3.67 ± 0.16 0.02 ± 0.001 0.76 ± 0.01
3.6 1.3 1.6 2.3 2.9 3.9 4.3 4.3 5.8 1.5
CH02/5/1 5.46 ± 0.03 0.28 ± 0.01 0.56 ± 0.01 1.14 ± 0.02 1.56 ± 0.03 2.38 ± 0.05 3.04 ± 0.05 3.74 ± 0.07 0.04 ± 0.004 0.56 ± 0.01
Std. error (%)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
2.4 0.9 0.3 0.5 0.9 1.0 1.0 1.5 2.9 0.6
L15/UP 5.15 ± 0.11 0.32 ± 0.004 0.63 ± 0.01 1.24 ± 0.03 1.69 ± 0.04 2.50 ± 0.05 3.31 ± 0.07 3.13 ± 0.10 0.04 ± 0.003 0.63 ± 0.01
2.1 1.5 1.9 2.2 2.7 2.0 2.4 2.7 6.1 1.7
CH02/20/1 5.50 ± 0.17 0.33 ± 0.01 0.64 ± 0.01 1.28 ± 0.02 1.74 ± 0.01 2.58 ± 0.03 3.16 ± 0.04 3.85 ± 0.06 0.03 ± 0.003 0.64 ± 0.01
3.1 2.1 1.5 1.3 0.8 1.2 1.2 1.5 5.2 1.1
LT/0.8 1.84 ± 0.06 0.37 ± 0.01 0.78 ± 0.01 1.51 ± 0.01 1.99 ± 0.02 2.76 ± 0.05 3.23 ± 0.07 3.66 ± 0.10 0.02 ± 0.003 0.78 ± 0.01
3.4 1.5 1.0 0.8 0.9 1.8 2.3 2.7 6.1 1.3
LT/6.6 3.94 ± 0.12 0.39 ± 0.01 0.80 ± 0.01 1.52 ± 0.01 2.01 ± 0.02 2.80 ± 0.05 3.36 ± 0.08 3.85 ± 0.10 0.02 ± 0.004 0.78 ± 0.01
3.1 1.5 1.5 0.7 0.8 1.8 2.4 2.0 2.0 1.1
0.6 3.4 2.1 2.1 1.9 2.2 1.6 1.9 13.0 2.1
CH02/5/2 5.2 ± 0.11 0.3 ± 0.004 0.57 ± 0.01 1.17 ± 0.01 1.64 ± 0.02 2.45 ± 0.05 3.15 ± 0.04 3.80 ± 0.06 0.04 ± 0.003 0.57 ± 0.01
2.2 1.5 1.9 0.9 0.9 2.0 1.4 1.5 7.1 1.2
CH02/5/3 5.54 ± 0.13 0.28 ± 0.01 0.58 ± 0.01 1.14 ± 0.04 1.61 ± 0.05 2.47 ± 0.07 3.08 ± 0.07 3.79 ± 0.08 0.05 ± 0.004 0.57 ± 0.02
2.3 2.1 1.5 3.1 2.9 2.8 2.2 2.2 8.2 2.9
CH02/4/5 4.47 ± 0.18 0.39 ± 0.004 0.78 ± 0.01 1.49 ± 0.01 1.98 ± 0.01 2.76 ± 0.02 3.29 ± 0.05 3.79 ± 0.07 0.02 ± 0.001 0.76 ± 0.01
4.1 1.0 1.2 0.8 0.7 0.9 1.4 1.8 5.5 1.0
CH02/4/10 4.33 ± 0.20 0.38 ± 0.01 0.76 ± 0.01 1.46 ± 0.01 1.95 ± 0.02 2.73 ± 0.04 3.29 ± 0.07 3.79 ± 0.10 0.02 ± 0.001 0.75 ± 0.01
4.5 1.7 1.2 0.9 1.0 1.5 2.2 2.7 3.2 1.7
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
Dose (Gy)
13
14
Table 3 The average normalised OSL (Lx /Tx ) obtained by SAR for all samples from the same section Regeneration dose (Gy)
Lingtai section (3 samples)
Jiuzhoutai section (3 samples)
Yuanbao section (2 samples)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
0.36 ± 0.01 0.79 ± 0.01 1.52 ± 0.01 2 ± 0.03 2.78 ± 0.03 3.29 ± 0.05 3.25 ± 0.05 0.02 ± 0.001 0.78 ± 0.004
3.9 0.7 0.6 1.3 1.2 1.5 1.4 4.9 0.5
0.32 ± 0.01 0.64 ± 0.01 1.26 ± 0.02 1.71 ± 0.02 2.54 ± 0.04 3.15 ± 0.02 3.78 ± 0.07 0.03 ± 0.003 0.64 ± 0.01
1.7 1.2 1.5 1.4 1.5 0.5 1.9 10.8 1.0
2.0 1.1 0.9 1.6 1.1 1.0 0.5 6.8 0.5
0.38 ± 0.004 0.77 ± 0.01 1.48 ± 0.01 1.97 ± 0.01 2.75 ± 0.02 3.29 ± 0.003 3.79 ± 0.002 0.02 ± 0.001 0.76 ± 0.01
1.2 1.3 1.0 0.7 0.6 0.1 0.04 2.9 0.9
0.34 ± 0.01 0.71 ± 0.04 1.39 ± 0.07 1.86 ± 0.09 2.66 ± 0.07 3.22 ± 0.05 3.76 ± 0.04 0.03 ± 0.004 0.71 ± 0.04
3.9 6.1 5.4 4.6 2.7 1.4 1.0 14.5 5.7
(c)
Normalised OSL (L x / Tx )
5
4
3
2
1
0
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Regeneration Dose (Gy)
0
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
0.29 ± 0.01 0.57 ± 0.01 1.15 ± 0.01 1.6 ± 0.03 2.43 ± 0.03 3.09 ± 0.03 3.78 ± 0.02 0.04 ± 0.003 0.57 ± 0.003
5
1.9 1.3 0.6 1.0 0.9 1.3 1.7 4.4 1.0
4
0.38 ± 0.01 0.78 ± 0.01 1.50 ± 0.01 1.98 ± 0.02 2.76 ± 0.03 3.27 ± 0.04 3.72 ± 0.06 0.02 ± 0.001 0.77 ± 0.01
3
69 161 380 575 1036 1498 2074 00 161
Std. error (%)
2
Normalised OSL
1
Std. error (%)
0
Normalised OSL
Luochuan (2 old samples)
(a)
Std. error (%)
(b)
Normalised OSL
Fig. 3. Growth curves of samples from different sections. (a) For all 12 samples from the four sections; (b) for 7 young samples (< 270 ka); (c) for 5 old samples (> 650 ka).
Std. error (%)
causes of the slight difference in the form of the growth curves for the two groups are not known yet.
Normalised OSL
4. De determination using SGC
Luochuan (2 young samples)
For dating purposes, 31 samples from three sites (sites CH02/2, CH02/3 and CH02/4) in Yuanbao were measured for De determination using the same SAR conditions (preheat at 260 ◦ C for 10 s, cut-heat at 220 ◦ C for 10 s, test dose
Luochuan section (4 samples)
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
15
Table 4 The average normalised OSL (Lx /Tx ) obtained by SAR for all four sections Regeneration dose (Gy)
69 161 380 575 1036 1498 2074 00 161
All 12 samples
All 7 young samples
All 5 old samples
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
0.35 ± 0.01 0.72 ± 0.03 1.4 ± 0.05 1.87 ± 0.06 2.66 ± 0.05 3.22 ± 0.03 3.76 ± 0.02 0.03 ± 0.003 0.71 ± 0.03
3.8 4.3 3.7 3.0 1.8 1.0 0.5 10.6 4.1
0.38 ± 0.01 0.78 ± 0.01 1.5 ± 0.01 1.98 ± 0.01 2.76 ± 0.01 3.28 ± 0.02 3.75 ± 0.03 0.02 ± 0.001 0.77 ± 0.004
1.4 0.7 0.6 0.6 0.5 0.6 0.8 2.3 0.6
0.3 ± 0.01 0.6 ± 0.02 1.19 ± 0.03 1.65 ± 0.03 2.47 ± 0.03 3.11 ± 0.02 3.78 ± 0.020 0.04 ± 0.003 0.6 ± 0.02
3.0 2.8 2.5 1.9 1.3 0.7 0.6 6.8 2.8
250
100
200
80
SGC De (Gy)
Standardised OSL ((Lx / Tx )*TD)
120
60 40 20
150 100 50
0 0
50
100 150 Dose (Gy)
200
250
0
Fig. 4. SGC constructed using 7 samples from Yuanbao. The data are fitted using an equation containing an exponential and a linear component.
28.8 Gy). For each sample 8–12 aliquots were measured. In order to construct a SGC for these 31 samples, the data for the oldest and youngest samples at each site were used. In site CH02/3, an extra sample in the middle was also used because of the thickness of the site (18 m). The 7 samples so selected were CH02/2/7, CH02/2/12, CH02/3/2, CH02/3/15, CH02/3/31, CH02/4/20 and CH02/4/39. For the construction of SGC, the standardised OSL ((LX /TX )TD ) of a specific regeneration dose point is the average of all aliquots having the same regeneration dose. The data set is then fitted using an exponential plus linear equation as shown below: Standardised OSL (I )= Imax [1 − exp(−X/D0 )] + aX,
0
(1)
where X is the regeneration dose, Imax (80.1 a.u.) is the normalised OSL intensity at the saturation level, D0 (89.5 Gy) is the characteristic dose at which point the slope of the growth curve is equal to 1/e of the initial value, and a (0.14) is a constant defining the slope of the linear fitting. Fig. 4 shows all the dose points and the SGC thus constructed. For De determination using SGC on an aliquot, the standardised natural signal level ((LN /TN )TD ) of the aliquot is
50
100
150
200
250
SAR De (Gy) Fig. 5. Comparison of De value obtained by SGC with that by conventional SAR for 24 loess samples, with the 1:1 ratio being given by the dotted line.
fitted into the SGC. The average of all these De s are treated as the De value for this sample. The comparison of the SGC and conventional SAR De values for 24 samples (out of 31 samples mentioned above, as 7 samples used for SGC construction were excluded) is shown in Fig. 5. For De s up to 202 Gy, the SAR De and the SGC De are in agreement. As a result, SGC is an alternative for De determination for loess samples, which allows dramatic reduction of measurement time. For example, the measurement of a SAR De for an aliquot with a De of about 100 Gy needs about 2.5 h, assuming a beta source strength of about 7 Gy/min, while the measurement of natural OSL and OSL response to a test dose needs only about 10 min (0.17 h), with a reduction of about 93% of measurement time.
5. Conclusions A common growth curve exists for samples collected from four different sections in the Chinese Loess Plateau. For
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Z. Lai / Radiation Measurements 41 (2006) 9 – 16
those samples younger than about 270 ka, the scatter of normalised OSL of natural doses in a group of aliquots is higher than that of any regeneration dose (except the zero dose), implying that the scatter in De s for each sample is mainly from the scatter of the natural signal, not from that of the growth curve. There is a slight difference in the shape of the growth curve for the young samples (< 270 ka) and that of the old samples (c. 0.65–2.5 Ma). It may be due to the difference in ages (total natural irradiation dose) or dust sources (origins of quartz). This requires further study. For 24 samples, up to a De of c. 200 Gy, the De determined by SGC is in agreement with that by SAR. As a result, SGC is an alternative method for De determination for loess samples, which allows dramatic reduction of measurement time. When samples are well bleached before deposition, as is the case with loess, this offers substantial procedural advantages for situations where many samples from the same section have to be dated for the establishment of high-resolution chronology.
Acknowledgements Thanks are due to Dr Richard Bailey for his spreadsheet for De calculation, and Prof. David Thomas for proofreading the manuscript. The author is grateful to two anonymous referees for their comments that have significantly improved the manuscript. The author also thanks Oxford University for a Clarendon Scholarship of full support of his D.Phil. study, and Jesus College, Oxford University, for grants for field work.
References An, Z.S., Liu, T.S., Liu, Y.C., Porter, S.C., Kukla, G., Wu, X.H., Hua, Y.M., 1990. The long-term paleomonsoon variation recorded by the loess paleosol sequence in central China. Quat. Int. 118, 91–95. Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiat. Meas. 27, 123–136. Balsam, W., Ji, J.F., Chen, J., 2004. Climatic interpretation of the Luochuan and Lingtai loess sections, China, based on changing iron oxide mineralogy and magnetic susceptibility. Earth Planetary Sci. Lett. 223, 335–348. Berger, G.W., Mulhern, P.J., Huntley, D.J., 1980. Isolation of siltsized quartz from sediments. Ancient TL 11, 8–9. BZtter-Jensen, L., 1997. Luminescence techniques: instrumentation and methods. Radiat. Meas. 27, 749–768.
Chen, F.H., Qiang, M.R., Feng, Z.D., Wang, H.B., Bloemendal, J., 2003. Stable East Asian monsoon climate during the last interglacial (Eemian) indicated by paleosol S1 in the western part of the Chinese Loess plateau. Global Planetary Change 36, 171–179. Duller, G.A.T., BZtter-Jensen, L., Murray, A.S., 2000. Optical dating of single sand-sized grains of quartz: sources of variability. Radiat. Meas. 32, 453–457. Jain, M., BZtter-Jensen, L., Murray, A.S., Jungner, H., 2002. Retrospective dosimetry: dose evaluation using unheated and heated quartz from a radioactive waste storage building. Radiat. Protect. Dosimetry 101, 525–530. Li, S.H., Wintle, A.G., 1992. Luminescence sensitivity change due to bleaching of sediments. Nucl. Tracks Radiat. Meas. 20, 567–573. Liu, T.S., 1985. Loess and the Environment. China Ocean Press, Beijing, 251pp. McCoy, D.G., Prescott, J.R., Nation, R.J., 2000. Some aspects of single-grain luminescence dating. Radiat. Meas. 32, 859–864. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57–73. Porat, N., Schwarcz, H.P., 1994. ESR dating of tooth enamel: a universal growth curve. In: Corrucini, R., Ciochon, R. (Eds.), Integrative Paths to the Past: Paleoanthropological Advances in Honor of F.C. Howell. Prentice-Hall, Englewood Cliffs, NJ, pp. 521–530. Rink, W.J., Schwarcz, H.P., 1994. Dose response of ESR signals in tooth enamel. Radiat. Meas. 23, 481–484. Roberts, H.M., Duller, G.A.T., 2004. Standardised growth curves for optical dating of sediment using multiple-grain aliquots. Radiat. Meas. 38, 241–252. Shlukov, A.I., Shakhovets, S.A., 1987. Kinetic studies of quartz thermoluminescence as applied to sediment dating. Ancient TL 5, 11–15. Shlukov, A.I., Shakovets, S.A., Voskovskaya, L.T., Lyaschenko, M.G., 1993. A criticism of standard TL-dating technology. Nucl. Instrum. Methods Phys. Res. 73, 373–381. Shlukov, A.I., Usova, M.G., Voskovskaya, L.T., Shakovets, S.A., 2001. New dating techniques for Quaternary sediments and their application on the Russian Plain. Quat. Sci. Rev. 20, 875–878. Smith, B.W., 1983. New applications of thermoluminescence and comparisons with other methods. Ph.D. Thesis, University of Adelaide. Thomsen, K.J., BZtter-Jensen, L., Murray, A.S., Solongo, S., 2002. Retrospective dosimetry using unheated quartz: a feasibility study. Radiat. Protect. Dosimetry 101, 345–348. Wintle, A.G., Murray, A.S., 1999. Luminescence sensitivity changes in quartz. Radiat. Meas. 30, 107–118. Yoshida, H., Roberts, R.G., Olley, J.M., Laslett, G.M., Galbraith, R.F., 2000. Extending the age range of optical dating using single ‘supergrains’ of quartz. Radiat. Meas. 32, 439–446. Zhou, L.P., Wintle, A.G., 1994. Sensitivity change of thermoluminescence signals after laboratory optical bleaching: experiments with loess fine grains. Quat. Sci. Rev. 13, 457–463.
Testing the use of an OSL standardised growth curve (SGC) for De determination on quartz from the Chinese Loess Plateau ZhongPing Lai∗ Centre for the Environment, University of Oxford, Mansfield Road, Oxford OX1 3TB, United Kingdom Received 19 November 2004; received in revised form 4 June 2005; accepted 13 June 2005
Abstract In the Standardised Growth Curve (SGC) method of De determination, once a SGC has been constructed, a De can be determined by measuring the natural OSL and the OSL response to a test dose only, instead of constructing a growth curve for each aliquot. This is a major advantage when a large number of samples from the same section, or the same geographical area, have to be dated, and the samples are well bleached before deposition. In the present study, the use of a SGC for De determination is tested using quartz extracted from loess samples from the Chinese Loess Plateau. It is demonstrated that: (a) A common growth curve exists for samples collected from four different sections in the Chinese Loess Plateau; (b) The scatter in De for samples younger than about 270 ka is dominated by the scatter of the natural signal levels, not that of the growth curve; (c) There is a slight difference between the shape of the growth curves of young samples (< 270 ka) and those from old samples (c. 0.65–2.5 Ma). The cause is not known, but may be due to the difference in ages (total natural irradiation dose) or in the dust sources (origins of quartz); (d) Up to a De of c. 200 Gy, the single aliquot regenerative-dose (SAR) De and the SGC De are in agreement. The ratio of the sum of SAR De s to the sum of SGC De s is 1.01, close to unity. It is therefore concluded that SGC is an alternative procedure for De determination for loess samples from the Chinese Loess Plateau. © 2005 Elsevier Ltd. All rights reserved. Keywords: Quartz OSL; Standardised growth curve; Luminescence; Chinese Loess
1. Introduction A standardised growth curve (SGC) in optically stimulated luminescence (OSL) of quartz has been reported by Roberts and Duller (2004) when using the single aliquot regenerative-dose (SAR) protocol for equivalent dose (De ) determination (Murray and Wintle, 2000). In SAR procedures, the OSL signal from the test dose (Tx ) is employed to correct for sensitivity changes over a series of regeneration measurements (Lx ) (Murray and Wintle, 2000). Roberts and Duller (2004) argued that such a test dose response can ∗ Fax: +44 1865 271929.
E-mail addresses: [email protected], [email protected] (Z. Lai). 1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.06.031
potentially be used as an inter-aliquot normalisation step, and that normalising the luminescence signal (Lx ) with the test dose response (Tx ) not only corrects for changes in luminescence sensitivity, but should also compensate for differences in natural signal intensity (LN ). When the normalised luminescence signal (Lx /Tx ) is multiplied by the size of the test dose (TD ), i.e. ((Lx /Tx )TD ) which they termed ‘standardised luminescence signal’, they found that a SGC exists for different samples from different continents. Attempts to construct a common growth curve for luminescence dating date back to the 1980s (Smith, 1983), but in general such efforts were not successful before. Sensitivity change in both feldspar and quartz after laboratory treatments (e.g. bleaching, irradiation, or preheat treatment) are commonplace (e.g., Li and Wintle, 1992; Zhou and Wintle,
10
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
1994; Wintle and Murray, 1999). This complicates the construction of a universal growth curve unless the sensitivity changes can be corrected for. To avoid sensitivity changes due to laboratory bleaching in the regeneration method, Smith (1983) attempted to provide a general growth curve for thermoluminescence (TL) dating using an additive approach on coarse-grained sedimentary quartz. He firstly constructed an additive growth curve in the laboratory using the youngest sample from a sedimentary sequence, and then obtained the De s for older samples by matching their normalised natural TL signal levels to this growth curve. The validity of this method assumes that for all the samples dated: (a) the TL residual level is similar; (b) the quartz TL has the same growth curve during burial. This was the first attempt to use a general growth; such an approach has difficulties in practice, as the assumptions are not universally valid. Shlukov and Shakhovets (1987) also proposed a general growth curve for samples from the Russian Plain. The studies by Shlukov and Shakhovets (1987) and Shlukov et al. (1993) found that laboratory irradiation can cause quartz TL signal loss. They showed that such “irradiation-induced fading” was even greater than the thermal fading, and proposed that the use of laboratory irradiation to reconstruct a growth curve should be abandoned. To obtain a growth curve without the use of laboratory irradiation, Shlukov and Shakhovets (1987) and Shlukov et al. (2001) suggested a method for quartz TL dating based on the assumption that all quartz in a specific geographical area has the same growth curve. The parameters that define the growth curve can be measured on a sample with known age and another saturated sample. To obtain an age for a sample, only the natural signal and the residual signal after laboratory bleaching are measured, in addition to the dose rate. However, it is difficult to accurately define the parameters for a growth curve in this way, and this method has not been widely adopted. A universal growth curve has been constructed by Porat and Schwarcz (1994) in ESR dating. They showed that the radiation sensitivity of all tooth enamel is quite similar, and have formed a universal growth curve for equivalent dose estimation. In a further test, using a saturating exponential equation of the form I = Imax (1 − exp −D/D0 ) to fit the data, Rink and Schwarcz (1994) found that both the fitting parameters (Imax and D0 ) and the sensitivity were similar for modern and fossil enamel from various sites. As a result, they concluded that the universal growth curve is a good predictor of equivalent dose. The development of a SAR protocol for quartz OSL (Murray and Wintle, 2000) made it easy to compare luminescence sensitivity for different aliquots or samples. The crucial step incorporated in SAR is the use of the OSL response to a test dose for correction of sensitivity changes. The implication of this is that the test-dose-normalised growth curve can be compared for many different samples (Roberts and Duller, 2004). Consequently, the SGC can be tested and constructed.
Some results suggest that the existence of a SGC is unlikely as different quartz grains have different luminescence characteristics. Based on single grain measurements, it has been shown that for quartz it is common for a small proportion of the brightest grains to be responsible for a large proportion of the total OSL signal (Duller et al., 2000; Mc Coy et al., 2000; Thomsen et al., 2002; Jain et al., 2002). In two aeolian samples from coastal dunes in southern Africa, Duller et al. (2000) found that 95% of the total OSL light sum of all grains was dominated by the signal from approximately 5% of the grains, and that there were large variations in the dose saturation characteristics of individual grains within a sample. Yoshida et al. (2000) also showed that different grains of sedimentary quartz had different D0 values characterising the saturating exponential function used to fit the data. Based on single aliquot measurements, Murray and Wintle (2000) also demonstrated that quartz from different samples had different saturation doses. However, in multiple-grain (coarse-grained) aliquots, in which there are about 1000 grains in an aliquot (Roberts and Duller, 2004) and the OSL measurements (e.g., intensity, growth curve) are the sum of all grains which may have a variety of different luminescence characteristics, Roberts and Duller (2004) found that the ratio of the SAR and SGC De values is consistent with unity for 22 samples from different continents. They have also obtained similar results for the quartz dominated post-IR OSL from fine-grains of 14 loess samples from three different sites along an east–west transect spanning 700 km in the Chinese Loess Plateau. They concluded that the SGC was applicable, and provided a rapid and accurate method for De determination for samples from the same section, or from a geographical area where the lithology and geological history are the same. Their finding is important as it opens a new door for De determination for quartz OSL. Loess from the Chinese Loess Plateau (CLP) is largely homogeneous due to the effect of long distance transport by wind on grain size sorting, and thus this sediment is ideal for further testing of the SGC. As most of the samples in Roberts and Duller (2004) were young and within linear fitting range, belonging to the Holocene epoch (out of their 36 quartz OSL samples, three samples had a De > 30 Gy, and one sample had a De > 40 Gy), the present work is a further test of the SGC in the non-linear dose range up to a De of c. 200 Gy. 2. Samples and measurement techniques Samples with ages spanning c. 10 ka to c. 2.5 Ma were collected from four different sections in the CLP; the Luochuan section (109◦ 25 E, 35◦ 45 N) from Shaanxi Province; the Lingtai section (107◦ 34 E, 34◦ 59 N) in Gansu Province in the central part of CLP; the Jiuzhoutai section near Lanzhou city in Gansu Province in the western part of CLP; and the Yuanbao Section (103◦ 08.8 E, 35◦ 38.4 N) in Gansu Province on the western edge of CLP. These sections have
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
given regeneration dose showing a standard error of within 1.7% (except for the zero dose). This pattern is seen for all 12 samples with a standard error of within 4.3%. It is worth noting that for those younger samples (LC/1 with an De of c. 27 Gy, LC/14 with an De of c. 140 Gy, LT/0.8 with an De of c. 60 Gy and LT/6.6 with an De of c. 228 Gy) the scatter in the signal observed from the natural dose is higher than that of any regeneration dose (except the zero dose), implying that the scatter in De s for each sample is mainly from the scatter in the natural signal, not from that of the growth curve.
5 Normalised OSL (L x / Tx )
11
4 3 2 1 0 0
50
100 150 200 Regeneration Dose (Gy)
250
Fig. 1. Growth curves of six aliquots from sample LC/1. The growth curves are identical for all 6 aliquots.
been well studied for their palaeoclimatic records (e.g., Liu, 1985; An et al., 1990; Balsam et al., 2004; Chen et al., 2003). In the laboratory, samples were treated with HCl and H2 O2 to remove carbonates and organics. The grain size from 45 to 63 m or 38 to 63 m was separated by dry sieving and treated with 35% fluorosilicic acid for about two weeks (Berger et al., 1980) to isolate pure quartz whose purity was tested by IR stimulation, and there is no obvious contamination of feldspar for all samples. The quartz grains were then deposited on stainless steel discs using silicone oil to make a multiple-grain aliquot. Experiments were carried out using a RisZ DA-15a automated TL/OSL system incorporating a Strontium-90 beta source (BZtter-Jensen, 1997). The OSL was detected using two U-340 filters. The SAR protocol (Murray and Wintle, 2000) was employed for De determination, in which the OSL of the first 0.4 s stimulation (background subtracted) was integrated to isolate the fast component (Bailey et al., 1997).
3.2. The comparison of growth curves of individual samples from the same section For each of the 12 samples, six aliquots were used to construct growth curves using the same SAR conditions. The growth curve for each sample is the average of the data from the six aliquots. Fig. 2a shows the growth curve of each of the three samples from the Lingtai section. The normalised signal levels for a given regeneration dose are identical with a standard error of 2% (except for the zero dose) (see Table 3). Similar results were observed in sections from Jiuzhoutai (three samples) (Fig. 2b), and Yuanbao (two samples) (Fig. 2c). In the Luochuan section (four samples) the scatter is larger with the normalised signal level for a given regeneration dose showing a standard error of 7% (except for the zero dose) (Fig. 2d). This may be related to the large difference in the ages of the samples. When the four samples were divided into two groups, with the two young samples LC/1 and LC/14 in one group, and the old samples L13/UP and CH02/20/1 in another group, the two growth curves in each group are very similar and over-lap (Fig. 2e and f). This will be discussed further in the following section. 3.3. Growth curves of samples from four sections in the Chinese Loess Plateau
3. The possibility of a common growth curve For this experiment, twelve samples from the four sections were selected (Table 1). Six aliquots were used for each sample, and the same SAR conditions (preheat at 260 ◦ C for 10 s, cut-heat at 220 ◦ C for 10 s, test dose 25.9 Gy) were applied to all aliquots. Seven regeneration doses (from 6.9 Gy to 207.4 Gy) were used. 3.1. The comparison of growth curves of individual aliquots from the same sample Table 2 shows the average normalised OSL signal level (Lx /Tx ) of each dose point from six aliquots for each of the 12 samples. Fig. 1 shows the 6 growth curves of six aliquots from sample LC/1. The growth curves of all 6 aliquots are very similar, with the test-dose normalised signal level for a
Fig. 3a shows the growth curves of all 12 samples. The normalised signal levels for a given regeneration dose have a standard error of within 5% (except for the zero dose) (Table 3). When the data are divided into two groups, 7 young samples with ages < 270 ka (LT/0.8, LT/6.6, LT/19.2, LC/1, LC/14, CH02/4/5 and CH02/4/10) and 5 old samples with ages from c. 0.65 to c. 2.5 Ma (CH02/5/1, CH02/5/2, CH02/5/3, L15/UP and CH02/20/1), within each group the growth curves are very similar with the normalised signal level for a given regeneration dose showing a standard error of within 3% (except for the zero dose) (Fig. 3b, c and Table 4). For practical reasons, the group that is of interest for De determination is that containing the younger samples (< 270 ka), where the charge trapping centres have not become saturated during the period of burial. For these samples the forms of the growth curves are very similar. The
12
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
Table 1 Sample information Sample ID
Section name
Collected from loess/soil layer
Expected agea (approximate) (ka)
LC/1 LC/14 L15/UP CH02/20/1 LT/0.8 LT/6.6 LT/19.2 CH02/5/3 CH02/5/2 CH02/5/1 CH02/4/10 CH02/4/5
Luochuan Luochuan Luochuan Luochuan Lingtai Lingtai Lingtai Jiuzhoutai Jiuzhoutai Jiuzhoutai Yuanbao Yuanbao
S0 L1 middle L15 bottom of Wucheng Loess L1 top L1 bottom L3 top L7 L8 L8 S1 middle S1 middle
8 40 1000 2500 15 70 260 650 750 750 110 120
a For those younger than 70 ka, the expected age is based on luminescence dating, and for those older than 70 ka it is based on the loess stratigraphy (Liu, 1985).
Normalised OSL (L x / Tx )
(a)
5
5
4
4
3
3
2
2
1
1
0 0
50
100
150
200
250
5
5
4
4
3
3
2
2
1
1
0
50
100
150
200
250
50
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200
250
50 100 150 200 Regeneration Dose (Gy)
250
0 0
(c)
50
100
150
200
250
(d)
5
5
4
4
3
3
2
2
1
1
0
0
0 0 (e)
0 (b) 0
50 100 150 200 Regeneration Dose (Gy)
0
250 (f)
Fig. 2. Growth curves for samples from the same section. (a) Three samples from Lingtai section. (b) Three samples from Jiuzhoutai. (c) Two samples from Yuanbao section. (d) Four samples from Luochuan section. (e) The two young samples (LC/1 and LC/14) from Luochuan section. (f) The two old samples (L15/UP and CH02/20/1) from Luochuan section.
Table 2 The average normalised OSL (Lx /Tx ) obtained by SAR (6 aliquots each) Normalised OSL
Std. error (%)
Normalised OSL
N 6.9 16.1 38 57.5 103.6 149.8 207.4 0 16.1
LC/1 1.22 ± 0.04 0.35 ± 0.01 0.78 ± 0.01 1.53 ± 0.01 2.03 ± 0.01 2.81 ± 0.02 3.34 ± 0.03 3.80 ± 0.05 0.02 ± 0.001 0.78 ± 0.01
3.3 1.7 1.7 0.9 0.7 0.8 0.9 1.2 4.3 1.0
LC/14 3.06 ± 0.07 0.38 ± 0.003 0.79 ± 0.002 1.51 ± 0.01 1.98 ± 0.02 2.74 ± 0.03 3.24 ± 0.03 3.69 ± 0.06 0.02 ± 0.001 0.77 ± 0.004
N 6.9 16.1 38 57.5 103.6 149.8 207.4 0 16.1
LT/19.2 4.83 ± 0.17 0.38 ± 0.004 0.76 ± 0.01 1.49 ± 0.03 1.94 ± 0.06 2.71 ± 0.11 3.22 ± 0.14 3.67 ± 0.16 0.02 ± 0.001 0.76 ± 0.01
3.6 1.3 1.6 2.3 2.9 3.9 4.3 4.3 5.8 1.5
CH02/5/1 5.46 ± 0.03 0.28 ± 0.01 0.56 ± 0.01 1.14 ± 0.02 1.56 ± 0.03 2.38 ± 0.05 3.04 ± 0.05 3.74 ± 0.07 0.04 ± 0.004 0.56 ± 0.01
Std. error (%)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
2.4 0.9 0.3 0.5 0.9 1.0 1.0 1.5 2.9 0.6
L15/UP 5.15 ± 0.11 0.32 ± 0.004 0.63 ± 0.01 1.24 ± 0.03 1.69 ± 0.04 2.50 ± 0.05 3.31 ± 0.07 3.13 ± 0.10 0.04 ± 0.003 0.63 ± 0.01
2.1 1.5 1.9 2.2 2.7 2.0 2.4 2.7 6.1 1.7
CH02/20/1 5.50 ± 0.17 0.33 ± 0.01 0.64 ± 0.01 1.28 ± 0.02 1.74 ± 0.01 2.58 ± 0.03 3.16 ± 0.04 3.85 ± 0.06 0.03 ± 0.003 0.64 ± 0.01
3.1 2.1 1.5 1.3 0.8 1.2 1.2 1.5 5.2 1.1
LT/0.8 1.84 ± 0.06 0.37 ± 0.01 0.78 ± 0.01 1.51 ± 0.01 1.99 ± 0.02 2.76 ± 0.05 3.23 ± 0.07 3.66 ± 0.10 0.02 ± 0.003 0.78 ± 0.01
3.4 1.5 1.0 0.8 0.9 1.8 2.3 2.7 6.1 1.3
LT/6.6 3.94 ± 0.12 0.39 ± 0.01 0.80 ± 0.01 1.52 ± 0.01 2.01 ± 0.02 2.80 ± 0.05 3.36 ± 0.08 3.85 ± 0.10 0.02 ± 0.004 0.78 ± 0.01
3.1 1.5 1.5 0.7 0.8 1.8 2.4 2.0 2.0 1.1
0.6 3.4 2.1 2.1 1.9 2.2 1.6 1.9 13.0 2.1
CH02/5/2 5.2 ± 0.11 0.3 ± 0.004 0.57 ± 0.01 1.17 ± 0.01 1.64 ± 0.02 2.45 ± 0.05 3.15 ± 0.04 3.80 ± 0.06 0.04 ± 0.003 0.57 ± 0.01
2.2 1.5 1.9 0.9 0.9 2.0 1.4 1.5 7.1 1.2
CH02/5/3 5.54 ± 0.13 0.28 ± 0.01 0.58 ± 0.01 1.14 ± 0.04 1.61 ± 0.05 2.47 ± 0.07 3.08 ± 0.07 3.79 ± 0.08 0.05 ± 0.004 0.57 ± 0.02
2.3 2.1 1.5 3.1 2.9 2.8 2.2 2.2 8.2 2.9
CH02/4/5 4.47 ± 0.18 0.39 ± 0.004 0.78 ± 0.01 1.49 ± 0.01 1.98 ± 0.01 2.76 ± 0.02 3.29 ± 0.05 3.79 ± 0.07 0.02 ± 0.001 0.76 ± 0.01
4.1 1.0 1.2 0.8 0.7 0.9 1.4 1.8 5.5 1.0
CH02/4/10 4.33 ± 0.20 0.38 ± 0.01 0.76 ± 0.01 1.46 ± 0.01 1.95 ± 0.02 2.73 ± 0.04 3.29 ± 0.07 3.79 ± 0.10 0.02 ± 0.001 0.75 ± 0.01
4.5 1.7 1.2 0.9 1.0 1.5 2.2 2.7 3.2 1.7
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
Dose (Gy)
13
14
Table 3 The average normalised OSL (Lx /Tx ) obtained by SAR for all samples from the same section Regeneration dose (Gy)
Lingtai section (3 samples)
Jiuzhoutai section (3 samples)
Yuanbao section (2 samples)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
0.36 ± 0.01 0.79 ± 0.01 1.52 ± 0.01 2 ± 0.03 2.78 ± 0.03 3.29 ± 0.05 3.25 ± 0.05 0.02 ± 0.001 0.78 ± 0.004
3.9 0.7 0.6 1.3 1.2 1.5 1.4 4.9 0.5
0.32 ± 0.01 0.64 ± 0.01 1.26 ± 0.02 1.71 ± 0.02 2.54 ± 0.04 3.15 ± 0.02 3.78 ± 0.07 0.03 ± 0.003 0.64 ± 0.01
1.7 1.2 1.5 1.4 1.5 0.5 1.9 10.8 1.0
2.0 1.1 0.9 1.6 1.1 1.0 0.5 6.8 0.5
0.38 ± 0.004 0.77 ± 0.01 1.48 ± 0.01 1.97 ± 0.01 2.75 ± 0.02 3.29 ± 0.003 3.79 ± 0.002 0.02 ± 0.001 0.76 ± 0.01
1.2 1.3 1.0 0.7 0.6 0.1 0.04 2.9 0.9
0.34 ± 0.01 0.71 ± 0.04 1.39 ± 0.07 1.86 ± 0.09 2.66 ± 0.07 3.22 ± 0.05 3.76 ± 0.04 0.03 ± 0.004 0.71 ± 0.04
3.9 6.1 5.4 4.6 2.7 1.4 1.0 14.5 5.7
(c)
Normalised OSL (L x / Tx )
5
4
3
2
1
0
5
4
3
2
1
0
0
0
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50
50
100
100
100
150
150
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200
200
250
250
250
Regeneration Dose (Gy)
0
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
0.29 ± 0.01 0.57 ± 0.01 1.15 ± 0.01 1.6 ± 0.03 2.43 ± 0.03 3.09 ± 0.03 3.78 ± 0.02 0.04 ± 0.003 0.57 ± 0.003
5
1.9 1.3 0.6 1.0 0.9 1.3 1.7 4.4 1.0
4
0.38 ± 0.01 0.78 ± 0.01 1.50 ± 0.01 1.98 ± 0.02 2.76 ± 0.03 3.27 ± 0.04 3.72 ± 0.06 0.02 ± 0.001 0.77 ± 0.01
3
69 161 380 575 1036 1498 2074 00 161
Std. error (%)
2
Normalised OSL
1
Std. error (%)
0
Normalised OSL
Luochuan (2 old samples)
(a)
Std. error (%)
(b)
Normalised OSL
Fig. 3. Growth curves of samples from different sections. (a) For all 12 samples from the four sections; (b) for 7 young samples (< 270 ka); (c) for 5 old samples (> 650 ka).
Std. error (%)
causes of the slight difference in the form of the growth curves for the two groups are not known yet.
Normalised OSL
4. De determination using SGC
Luochuan (2 young samples)
For dating purposes, 31 samples from three sites (sites CH02/2, CH02/3 and CH02/4) in Yuanbao were measured for De determination using the same SAR conditions (preheat at 260 ◦ C for 10 s, cut-heat at 220 ◦ C for 10 s, test dose
Luochuan section (4 samples)
Z. Lai / Radiation Measurements 41 (2006) 9 – 16
15
Table 4 The average normalised OSL (Lx /Tx ) obtained by SAR for all four sections Regeneration dose (Gy)
69 161 380 575 1036 1498 2074 00 161
All 12 samples
All 7 young samples
All 5 old samples
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
Normalised OSL
Std. error (%)
0.35 ± 0.01 0.72 ± 0.03 1.4 ± 0.05 1.87 ± 0.06 2.66 ± 0.05 3.22 ± 0.03 3.76 ± 0.02 0.03 ± 0.003 0.71 ± 0.03
3.8 4.3 3.7 3.0 1.8 1.0 0.5 10.6 4.1
0.38 ± 0.01 0.78 ± 0.01 1.5 ± 0.01 1.98 ± 0.01 2.76 ± 0.01 3.28 ± 0.02 3.75 ± 0.03 0.02 ± 0.001 0.77 ± 0.004
1.4 0.7 0.6 0.6 0.5 0.6 0.8 2.3 0.6
0.3 ± 0.01 0.6 ± 0.02 1.19 ± 0.03 1.65 ± 0.03 2.47 ± 0.03 3.11 ± 0.02 3.78 ± 0.020 0.04 ± 0.003 0.6 ± 0.02
3.0 2.8 2.5 1.9 1.3 0.7 0.6 6.8 2.8
250
100
200
80
SGC De (Gy)
Standardised OSL ((Lx / Tx )*TD)
120
60 40 20
150 100 50
0 0
50
100 150 Dose (Gy)
200
250
0
Fig. 4. SGC constructed using 7 samples from Yuanbao. The data are fitted using an equation containing an exponential and a linear component.
28.8 Gy). For each sample 8–12 aliquots were measured. In order to construct a SGC for these 31 samples, the data for the oldest and youngest samples at each site were used. In site CH02/3, an extra sample in the middle was also used because of the thickness of the site (18 m). The 7 samples so selected were CH02/2/7, CH02/2/12, CH02/3/2, CH02/3/15, CH02/3/31, CH02/4/20 and CH02/4/39. For the construction of SGC, the standardised OSL ((LX /TX )TD ) of a specific regeneration dose point is the average of all aliquots having the same regeneration dose. The data set is then fitted using an exponential plus linear equation as shown below: Standardised OSL (I )= Imax [1 − exp(−X/D0 )] + aX,
0
(1)
where X is the regeneration dose, Imax (80.1 a.u.) is the normalised OSL intensity at the saturation level, D0 (89.5 Gy) is the characteristic dose at which point the slope of the growth curve is equal to 1/e of the initial value, and a (0.14) is a constant defining the slope of the linear fitting. Fig. 4 shows all the dose points and the SGC thus constructed. For De determination using SGC on an aliquot, the standardised natural signal level ((LN /TN )TD ) of the aliquot is
50
100
150
200
250
SAR De (Gy) Fig. 5. Comparison of De value obtained by SGC with that by conventional SAR for 24 loess samples, with the 1:1 ratio being given by the dotted line.
fitted into the SGC. The average of all these De s are treated as the De value for this sample. The comparison of the SGC and conventional SAR De values for 24 samples (out of 31 samples mentioned above, as 7 samples used for SGC construction were excluded) is shown in Fig. 5. For De s up to 202 Gy, the SAR De and the SGC De are in agreement. As a result, SGC is an alternative for De determination for loess samples, which allows dramatic reduction of measurement time. For example, the measurement of a SAR De for an aliquot with a De of about 100 Gy needs about 2.5 h, assuming a beta source strength of about 7 Gy/min, while the measurement of natural OSL and OSL response to a test dose needs only about 10 min (0.17 h), with a reduction of about 93% of measurement time.
5. Conclusions A common growth curve exists for samples collected from four different sections in the Chinese Loess Plateau. For
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Z. Lai / Radiation Measurements 41 (2006) 9 – 16
those samples younger than about 270 ka, the scatter of normalised OSL of natural doses in a group of aliquots is higher than that of any regeneration dose (except the zero dose), implying that the scatter in De s for each sample is mainly from the scatter of the natural signal, not from that of the growth curve. There is a slight difference in the shape of the growth curve for the young samples (< 270 ka) and that of the old samples (c. 0.65–2.5 Ma). It may be due to the difference in ages (total natural irradiation dose) or dust sources (origins of quartz). This requires further study. For 24 samples, up to a De of c. 200 Gy, the De determined by SGC is in agreement with that by SAR. As a result, SGC is an alternative method for De determination for loess samples, which allows dramatic reduction of measurement time. When samples are well bleached before deposition, as is the case with loess, this offers substantial procedural advantages for situations where many samples from the same section have to be dated for the establishment of high-resolution chronology.
Acknowledgements Thanks are due to Dr Richard Bailey for his spreadsheet for De calculation, and Prof. David Thomas for proofreading the manuscript. The author is grateful to two anonymous referees for their comments that have significantly improved the manuscript. The author also thanks Oxford University for a Clarendon Scholarship of full support of his D.Phil. study, and Jesus College, Oxford University, for grants for field work.
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