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Assessment of bleaching of K-feldspar grains
Radiation Measurements 33 (2001) 103±108
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Assessment of bleaching of K-feldspar grains Jia-Fu Zhang, Shenghua Li*, M-Y.W. Tso Radioisotope Unit, The University of Hong Kong, Pokfulam Road, Hong Kong Received 18 October 1999; received in revised form 31 March 2000; accepted 5 May 2000
Abstract Based on the dierence in the bleaching rate between IRSL and TL signals, a new method of assessing the bleaching of sedimentary feldspar is proposed. All measurements for De and the bleaching factor (normalized ratio of IRSL to TL) are carried out on the same grain. K-feldspar grains from two sediments, a marine and an alluvium, from Hong Kong were tested. Their bleaching factors indicate that bleaching of the marine sediment was not uniform, whereas the latter was relatively homogeneous. Furthermore, the relatively well-bleached grains in each sample were identi®ed. 7 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Insucient bleaching of feldspar grains at the time of deposition is one of the major factors causing scatter of their palaeodoses (De ). Many procedures have been developed to assess the bleaching degree, or homogeneity, of sediments at deposition. These include the age comparisons between polymineral ®ne grains and coarse grains of K-feldspar (Li and Wintle, 1992) and between feldspar and quartz (Hansen et al., 1999), and the De comparisons between IRSL and TL (Li, 1992; Duller, 1994). Besides these comparisons, data dispersion was also used to identify incomplete bleaching, such as the pattern of the distribution of single aliquot palaeodoses versus natural IRSL intensities (Li, 1994). In single-grain measurements, the statistical distribution of the ratios of natural-plus-beta-induced IRSL to the natural IRSL from individual grains was used as an indicator of homogeneity of bleaching (Lamothe and Auclair, 1997). Generally, the above assessment methods require at least two aliquots or grains. In this paper, the dierence in the bleaching rate between
* Corresponding author.
IRSL and TL from an individual grain was used to assess the degree of bleaching at deposition. This assessment could be carried out following the De measurement. 2. Experimental details All the experiments reported here were carried out using an automated Risù TL/OSL reader system (TLDA-15) (Markey et al., 1997). The IR stimulation is at 880D80 nm and the IR power at the sample is about 40 mW/cm2. The IRSL signal was detected by an EMI 9635Q photomultiplier tube with a Schott BG-39 ®lter for IR rejection and a Corning 5-58 ®lter to isolate the blue emission. For the bleaching experiments, the bleaching light wavelength is from 420 to approximately 550 nm given by a ®ltered light source in the reader. The reader is also equipped with a 90Sr/90Y beta source delivering 0.09 Gy/s to the sample. The IRSL signal measurements were made for 0.3 s at a constant sample temperature of 508C. K-feldspar grains, extracted from two sedimentary samples, were used. Sample B4, a marine sediment, was collected from an oshore core from Borehole A5/
1350-4487/01/$ - see front matter 7 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 1 1 3 - X
104
J.-F. Zhang et al. / Radiation Measurements 33 (2001) 103±108
2 in the West Lamma Channel, Hong Kong. Sample SHT3, an alluvium sediment, was taken from Yuen Long, Hong Kong. Feldspar grains were separated from the samples using conventional methods. Grain size selection was achieved by dry sieving before and after etching. Individual grains of 425±500 mm were mounted on 10 mm diameter aluminum discs, being attached to the discs with Silicone spray.
bleached grain is greater than one for a well-bleached grain, but less than that for an unbleached grain. Based on the residual signals and the dierence in the bleaching rate between IRSL and TL, the ratio Rresidual of the residual signals for dierent degrees of bleaching has the following relation: Rresidual unbleached > Rresidual poorly-bleached > Rresidual well-bleached
3. Identi®cation of bleaching 3.1. Ratio of IRSL and TL The relationships between IRSL and TL signals from three grains that have been given beta doses are illustrated in Fig. 1. It shows that IRSL intensity of a grain is proportional to its TL intensity. The ratio (R ) of IRSL to TL from a grain is constant if there is no bleaching. The ®gure also demonstrates that the ratios for dierent grains are not identical. Godfrey-Smith et al. (1988) ®rst reported that the bleaching of OSL is much more rapid and eective than that of TL for quartz and feldspar. R, like the IRSL signal, reduces rapidly as the bleaching time increases (Fig. 2). The implication is that the value of R can indicate the degree of bleaching of a grain. 3.2. Bleaching degree The natural IRSL In or TL Tn signals from a sedimentary feldspar grain consists of the residuals Iresidual and Tresidual and the signals Ipost and Tpost induced after deposition. For most sediment types, the degree of bleaching can be classi®ed into well-bleached, poorly-bleached and unbleached. Iresidual from a poorly-
Fig. 1. The beta-induced IRSL (0.3 s) and TL (250±4008C) signals from individual grains from sample B4.
1
This is consistent with the curve for R in Fig. 2. However, the residuals cannot be obtained unless the luminescence signals are measured immediately after they are bleached, i.e., at deposition. This is because Iresidual and Ipost , or Tresidual and Tpost cannot be measured separately. Hence, for the natural signals, the ratio, Rn , is in the following form: Rn
Iresidual Ipost In Tn Tresidual Tpost
2
Based on relation (1), the following relation is also tenable. Rn unbleached > Rn poorly-bleached > Rn well-bleached
3
Relation (3) suggests that the bleaching degree of a sedimentary feldspar grain can also be indicated by its Rn : If the value of Rn is relatively small, then the bleaching was relatively complete. If a grain is well bleached, Iresidual tends to be zero, and Rn well-bleached is the smallest value. 3.3. Identi®cation of bleaching There is only one value of Rn for a grain. Further-
Fig. 2. The IRSL and TL signals from feldspar of sample B4 are reduced by exposure to light. The ratio (R ) of IRSL to TL (shown as triangles) decreases rapidly as the bleaching time increases.
J.-F. Zhang et al. / Radiation Measurements 33 (2001) 103±108
105
Table 1 Procedure used to test Eq. (4) Step
Treatment
Measure
Observe
Bleaching factor
1 2 3 4 5 6 7
5008C heat 270 Gy beta dose 2208C preheat for 60 s 0.3 s IRSL Bleachinga 0.3 s IRSL 5008C TL
IRSLbleached TLbleached
(2) Rbleached
8 9 10 11
270 Gy beta dose 2208C preheat for 60 s 0.3 s IRSL 5008C TL
IRSLunbleached TLunbleached
Runbleached
(1) Remaining % of IRSL due to bleaching
IRSLbleached IRSLunbleached
100
IRSLunbleached IRSLbleached TLbleached
R0
Rbleached Runbleached
IRSLunbleached TLunbleached
a In experiment 1, the bleaching time for dierent grains is dierent, from 0.1 to 1000 s; in experiment 2, the bleaching time for all grains is the same, 10 s.
more, even if they have not been bleached, R values of dierent grains are not completely identical (Fig. 1). Hence, there is a need for Rn to be normalized between grains. The normalization is achieved by dividing Rn by Rn unbleached : Rn unbleached can be represented by the ratio Rb of laboratory beta-induced IRSL to TL signals from the same grain. The normalized Rn is de®ned as the bleaching factor (R '). R0
Rn Rb
4
If a grain has not been bleached, the value of R ' should be 1. If it is poorly bleached, R' is less than 1. The smaller R', the more complete the bleaching is. Two experiments were carried out to test function (4). The procedures are listed in Table 1. The results are illustrated in Figs. 3 and 4, respectively. Fig. 3 shows that not all of the values of Rbleached reduce as the bleaching time increases. After normalization, all the R' values decrease as the bleaching time increases and the curve is very similar to the IRSL bleaching curve (inset). Fig. 4 shows that, although all the grains were bleached for the same time, their bleaching degrees are not identical. The remaining percentage of the initial IRSL signal varies from 15% to 77%. Correspondingly, R' ranges from 0.29 to 0.80. The R' indicates the degree of bleaching of these grains. 4. Relation between De and bleaching factor 4.1. De of single grains The three-stage single-aliquot additive-dose method was used to determine De (Zhang et al., 2001). The procedure is divided into three stages. In the ®rst
stage, the preheat/measurement cycle is repeated several times to construct a pulsed decay curve for the natural IRSL signal. During the second stage, the grain is beta irradiated, then preheated before the measurement of the IRSL signal. This dose/preheat/ measurement cycle is repeated for as many doses as required (m times) to generate a growth curve. In the third stage, the preheat/measurement cycle is repeated in the same manner as in the ®rst stage to construct a pulsed decay curve for the N mb IRSL signal. The loss of IRSL signal due to repeated preheats and measurements is corrected for using the luminescence correction method. If the natural and each beta induced IRSL signal behave identically, then, after cor-
Fig. 3. The relationship between the ratios Rbleached from Table 1 of bleached IRSL to TL signals from nine single grains of sample B4 (left Y axis), or bleaching factor R ' from Table 1 (right Y axis), and bleaching time (on logarithmic axis). Each grain was bleached for dierent periods of time. The inset is the remaining percentage of the initial IRSL signals after bleaching IRSLbleached =IRSLunbleached from Table 1).
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J.-F. Zhang et al. / Radiation Measurements 33 (2001) 103±108
rection, the data points from the ®rst and the third stages should be constant within each stage. In this case, the preheating is successful. In contrast, if the corrected data points from the third stage are not constant, it suggests that the natural and the beta induced IRSL signals decay dierently. In this case, the preheat condition needs to be adjusted until the decays of all signals are identical. De is obtained by ®tting the corrected growth curve at the second stage. The most appropriate preheats for the single grains of 425±500 mm from samples B4 and SHT3 are 2208C for 10 min and 2308C for 1 min, respectively. Under these preheating conditions, palaeodoses of 26 grains of B4 and 24 grains for SHT3 were measured, respectively. They range from 104 to 385 Gy for B4 and from 63 to 174 Gy for SHT3, respectively. This scatter in De is so large that their bleaching at the time of deposition must be examined.
ment. Thus, Rb is obtained. Finally, R' is calculated as Rn =Rb : The feldspar grains from samples B4 and SHT3 were tested. In the three-stage single-aliquot additivedose method discussed above, stage 3 was replaced by measuring IRSL and TL for calculating R ', after the most appropriate preheat for a sample was found. The subsequent beta dose was 216 Gy. The results are illustrated by plots of De versus R ' (Fig. 5). For sample B4, R' values ranging from 0.23 to 1.00 suggest that the bleaching of the sample is heterogeneous. The grains can be classi®ed into well-bleached, poorlybleached and unbleached (Fig. 5). Their bleaching factors R' are 0.24±0.40, 0.42±0.69 and 0.95±1.0, respectively; and their De values are 104±147, 151±276 and 282±385 Gy, respectively. The values of De for the unbleached grains are almost three times larger than
4.2. Bleaching factor of single grains The value of R ' of a grain can be obtained after its De measurement with the additive dose method. Firstly, N b TL Tnb is measured immediately after the De measurement. Although some of the natural TL signal Tn is removed due to the successive preheats and TL Tb is induced by each beta irradiation used to construct the additive dose growth curve, the removal of Tn and the accumulation of Tb are proportional to Tn : So, TNb is proportional to Tn : Tn in function (2) can be replaced by Tnb : Thus, Rn is obtained. Secondly, the grain is given a subsequent dose and then its IRSL and TL signals are measured following the identical preheat as in the De measure-
Fig. 4. The relationship between R' and the percentage of the initial IRSL signals from individual grains from sample B4 remaining after the bleaching of 10 s.
Fig. 5. The relationship between the palaeodoses (De ) and the bleaching factors (R') for grains from samples B4 and SHT3.
J.-F. Zhang et al. / Radiation Measurements 33 (2001) 103±108
for the well-bleached grains. For sample SHT3, R ' ranges from 0.18 to 0.44, which suggests that the bleaching of this sample is relatively homogeneous. The grains can be further classi®ed into well-bleached and poorly-bleached, based on their bleaching factors (R') of 0.18±0.23 and 0.26±0.44, respectively; and their De are 63±132 and 113±174 Gy, respectively. The bleaching factors show that the bleaching of SHT3 is more complete and homogenous than B4. The scatter of De for SHT3 is much less than for B4.
107
6. Conclusion The measured bleaching factor R ' is an indicator of the degree of bleaching of a grain. This factor can be measured for each grain. All measurements on De and R ' can be carried out on the same grain. The dierences in the properties of mineral and luminescence among grains are overcome. Based on the value of R ', not only is the homogeneity of bleaching of a sample identi®ed, but also the relatively well-bleached grains are identi®ed and their De values are given.
5. Discussion In luminescence dating it is customary to assume that all the grains in a sediment unit are bleached to the same degree (Aitken, 1998). In practice, the bleaching is aected by many factors, such as variation in bleachability, variation in light-absorbent coatings, and variation in duration of bleaching. The bleaching also depends on the weather and the depositional conditions at the time of deposition. Even under the same bleaching conditions in the laboratory, the bleaching rates of dierent gains are not identical (Fig. 4). So, it is almost impossible that all the grains extracted from a bulk sediment sample were bleached to the same degree. The grain-to-grain variation in bleaching suggests that the assessment of bleaching of a sample should be based on individual grains. The relatively well-bleached grains can be identi®ed by comparing values of R' from grain-to-grain. The constant ratio of IRSL to TL from an unbleached grain is its inherent property. The implication is that the traps that give rise to IRSL are proportional to the traps responsible for TL in a grain regardless of whether any common charge is being measured, or whether the two signals are wholly independent. The similarity in anomalous fading of IRSL and TL (Spooner, 1992; Balescu et al., 1997) implies that anomalous fading does not aect the ratio. It is mainly aected by bleaching. If samples approach saturation, their bleaching degrees at the time of deposition cannot be identi®ed because of the same R ' values for all grains. In general, the younger the sample, the easier is the identi®cation of its degree of bleaching. Fig. 5 suggests that the large scatter of palaeodoses for individual grains is mainly caused by the inhomogeneity of bleaching. The grains can be grouped by their bleaching factors. In each group, the scatter is relatively small. De in the well-bleached group is much smaller than that in the unbleached group. Thus, the best estimate of the true palaeodose is given by the lowest values obtained. This is consistent with the results reported by Li (1994), Murray et al. (1995) and Olley et al. (1998).
Acknowledgements This work was fully supported by a grant from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project No. 7105/97P).
References Aitken, M.J., 1998. An Introduction to Optical Dating: The Dating of Quaternary Sediments by the Use of PhotonStimulated Luminescence. Oxford University Press, Oxford. Balescu, S., Lamothe, M., Lautridou, J.-P., 1997. Luminescence evidence for two Middle Pleistocene interglacial events at Tourville, northwestern France. Boreas 26, 61±72. Duller, G.A.T., 1994. Luminescence dating of poorly bleached sediments from Scotland. Quaternary Science Reviews 13, 521±524. Godfrey-Smith, D.I., Huntley, D.J., Chen, W.-H., 1998. Optical dating studies of quartz and feldspar sediment extracts. Quaternary Science Reviews 7, 379±385. Hansen, L., Funder, S., Murray, A.S., Mejdahl, V., 1999. Luminescence dating of the last Weichselian Glacier advance in East Greenland. Quaternary Science Reviews 18, 179±190. Lamothe, M., Auclair, M., 1997. Assessing the datability of young sediments by IRSL using an intrinsic laboratory protocol. Radiation Measurements 27, 107±118. Li, S.-H., (1992). Development and application of stimulated luminescence dating methods for sediments. Unpublished Ph.D. thesis. University College of Wales, Aberystwyth. Li, S.-H., 1994. Optical dating: insuciently bleached sediments. Radiation Measurements 23, 563±568. Li, S.-H., Wintle, A.G., 1992. A global view of the stability of luminescence signals from loess. Quaternary Science Reviews 11, 133±137. Markey, B.G., Bùtter-Jensen, L., Duller, G.A.T., 1997. A new ¯exible system for measuring thermally and optically stimulated luminescence. Radiation Measurements 27, 83± 89. Murray, A.S., Olley, J.M., Caitcheon, G.G., 1995. Measurements of equivalent doses in quartz from contem-
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porary water-lain sediments using optically stimulated luminescence. Quaternary Science Reviews 14, 365±371. Olley, J.M., Caitcheon, G., Murray, A.S., 1998. The distribution of apparent dose as determined by optically stimulated luminescence in small aliquots of ¯uvial quartz: implications for dating young sediments. Quaternary Science Reviews 17, 1033±1040.
Spooner, N.A., 1992. Optical dating: Preliminary results on the anomalous fading of luminescence from feldspars. Quaternary Science Reviews 11, 139±145. Zhang, J.-F., Li, S.-H., Tso, M-Y.W., 2001. Improvement of the equivalent dose determination using single aliquots of potassium feldspar. Radiation Measurements 33, 65± 71.
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Assessment of bleaching of K-feldspar grains Jia-Fu Zhang, Shenghua Li*, M-Y.W. Tso Radioisotope Unit, The University of Hong Kong, Pokfulam Road, Hong Kong Received 18 October 1999; received in revised form 31 March 2000; accepted 5 May 2000
Abstract Based on the dierence in the bleaching rate between IRSL and TL signals, a new method of assessing the bleaching of sedimentary feldspar is proposed. All measurements for De and the bleaching factor (normalized ratio of IRSL to TL) are carried out on the same grain. K-feldspar grains from two sediments, a marine and an alluvium, from Hong Kong were tested. Their bleaching factors indicate that bleaching of the marine sediment was not uniform, whereas the latter was relatively homogeneous. Furthermore, the relatively well-bleached grains in each sample were identi®ed. 7 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Insucient bleaching of feldspar grains at the time of deposition is one of the major factors causing scatter of their palaeodoses (De ). Many procedures have been developed to assess the bleaching degree, or homogeneity, of sediments at deposition. These include the age comparisons between polymineral ®ne grains and coarse grains of K-feldspar (Li and Wintle, 1992) and between feldspar and quartz (Hansen et al., 1999), and the De comparisons between IRSL and TL (Li, 1992; Duller, 1994). Besides these comparisons, data dispersion was also used to identify incomplete bleaching, such as the pattern of the distribution of single aliquot palaeodoses versus natural IRSL intensities (Li, 1994). In single-grain measurements, the statistical distribution of the ratios of natural-plus-beta-induced IRSL to the natural IRSL from individual grains was used as an indicator of homogeneity of bleaching (Lamothe and Auclair, 1997). Generally, the above assessment methods require at least two aliquots or grains. In this paper, the dierence in the bleaching rate between
* Corresponding author.
IRSL and TL from an individual grain was used to assess the degree of bleaching at deposition. This assessment could be carried out following the De measurement. 2. Experimental details All the experiments reported here were carried out using an automated Risù TL/OSL reader system (TLDA-15) (Markey et al., 1997). The IR stimulation is at 880D80 nm and the IR power at the sample is about 40 mW/cm2. The IRSL signal was detected by an EMI 9635Q photomultiplier tube with a Schott BG-39 ®lter for IR rejection and a Corning 5-58 ®lter to isolate the blue emission. For the bleaching experiments, the bleaching light wavelength is from 420 to approximately 550 nm given by a ®ltered light source in the reader. The reader is also equipped with a 90Sr/90Y beta source delivering 0.09 Gy/s to the sample. The IRSL signal measurements were made for 0.3 s at a constant sample temperature of 508C. K-feldspar grains, extracted from two sedimentary samples, were used. Sample B4, a marine sediment, was collected from an oshore core from Borehole A5/
1350-4487/01/$ - see front matter 7 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 1 1 3 - X
104
J.-F. Zhang et al. / Radiation Measurements 33 (2001) 103±108
2 in the West Lamma Channel, Hong Kong. Sample SHT3, an alluvium sediment, was taken from Yuen Long, Hong Kong. Feldspar grains were separated from the samples using conventional methods. Grain size selection was achieved by dry sieving before and after etching. Individual grains of 425±500 mm were mounted on 10 mm diameter aluminum discs, being attached to the discs with Silicone spray.
bleached grain is greater than one for a well-bleached grain, but less than that for an unbleached grain. Based on the residual signals and the dierence in the bleaching rate between IRSL and TL, the ratio Rresidual of the residual signals for dierent degrees of bleaching has the following relation: Rresidual unbleached > Rresidual poorly-bleached > Rresidual well-bleached
3. Identi®cation of bleaching 3.1. Ratio of IRSL and TL The relationships between IRSL and TL signals from three grains that have been given beta doses are illustrated in Fig. 1. It shows that IRSL intensity of a grain is proportional to its TL intensity. The ratio (R ) of IRSL to TL from a grain is constant if there is no bleaching. The ®gure also demonstrates that the ratios for dierent grains are not identical. Godfrey-Smith et al. (1988) ®rst reported that the bleaching of OSL is much more rapid and eective than that of TL for quartz and feldspar. R, like the IRSL signal, reduces rapidly as the bleaching time increases (Fig. 2). The implication is that the value of R can indicate the degree of bleaching of a grain. 3.2. Bleaching degree The natural IRSL In or TL Tn signals from a sedimentary feldspar grain consists of the residuals Iresidual and Tresidual and the signals Ipost and Tpost induced after deposition. For most sediment types, the degree of bleaching can be classi®ed into well-bleached, poorly-bleached and unbleached. Iresidual from a poorly-
Fig. 1. The beta-induced IRSL (0.3 s) and TL (250±4008C) signals from individual grains from sample B4.
1
This is consistent with the curve for R in Fig. 2. However, the residuals cannot be obtained unless the luminescence signals are measured immediately after they are bleached, i.e., at deposition. This is because Iresidual and Ipost , or Tresidual and Tpost cannot be measured separately. Hence, for the natural signals, the ratio, Rn , is in the following form: Rn
Iresidual Ipost In Tn Tresidual Tpost
2
Based on relation (1), the following relation is also tenable. Rn unbleached > Rn poorly-bleached > Rn well-bleached
3
Relation (3) suggests that the bleaching degree of a sedimentary feldspar grain can also be indicated by its Rn : If the value of Rn is relatively small, then the bleaching was relatively complete. If a grain is well bleached, Iresidual tends to be zero, and Rn well-bleached is the smallest value. 3.3. Identi®cation of bleaching There is only one value of Rn for a grain. Further-
Fig. 2. The IRSL and TL signals from feldspar of sample B4 are reduced by exposure to light. The ratio (R ) of IRSL to TL (shown as triangles) decreases rapidly as the bleaching time increases.
J.-F. Zhang et al. / Radiation Measurements 33 (2001) 103±108
105
Table 1 Procedure used to test Eq. (4) Step
Treatment
Measure
Observe
Bleaching factor
1 2 3 4 5 6 7
5008C heat 270 Gy beta dose 2208C preheat for 60 s 0.3 s IRSL Bleachinga 0.3 s IRSL 5008C TL
IRSLbleached TLbleached
(2) Rbleached
8 9 10 11
270 Gy beta dose 2208C preheat for 60 s 0.3 s IRSL 5008C TL
IRSLunbleached TLunbleached
Runbleached
(1) Remaining % of IRSL due to bleaching
IRSLbleached IRSLunbleached
100
IRSLunbleached IRSLbleached TLbleached
R0
Rbleached Runbleached
IRSLunbleached TLunbleached
a In experiment 1, the bleaching time for dierent grains is dierent, from 0.1 to 1000 s; in experiment 2, the bleaching time for all grains is the same, 10 s.
more, even if they have not been bleached, R values of dierent grains are not completely identical (Fig. 1). Hence, there is a need for Rn to be normalized between grains. The normalization is achieved by dividing Rn by Rn unbleached : Rn unbleached can be represented by the ratio Rb of laboratory beta-induced IRSL to TL signals from the same grain. The normalized Rn is de®ned as the bleaching factor (R '). R0
Rn Rb
4
If a grain has not been bleached, the value of R ' should be 1. If it is poorly bleached, R' is less than 1. The smaller R', the more complete the bleaching is. Two experiments were carried out to test function (4). The procedures are listed in Table 1. The results are illustrated in Figs. 3 and 4, respectively. Fig. 3 shows that not all of the values of Rbleached reduce as the bleaching time increases. After normalization, all the R' values decrease as the bleaching time increases and the curve is very similar to the IRSL bleaching curve (inset). Fig. 4 shows that, although all the grains were bleached for the same time, their bleaching degrees are not identical. The remaining percentage of the initial IRSL signal varies from 15% to 77%. Correspondingly, R' ranges from 0.29 to 0.80. The R' indicates the degree of bleaching of these grains. 4. Relation between De and bleaching factor 4.1. De of single grains The three-stage single-aliquot additive-dose method was used to determine De (Zhang et al., 2001). The procedure is divided into three stages. In the ®rst
stage, the preheat/measurement cycle is repeated several times to construct a pulsed decay curve for the natural IRSL signal. During the second stage, the grain is beta irradiated, then preheated before the measurement of the IRSL signal. This dose/preheat/ measurement cycle is repeated for as many doses as required (m times) to generate a growth curve. In the third stage, the preheat/measurement cycle is repeated in the same manner as in the ®rst stage to construct a pulsed decay curve for the N mb IRSL signal. The loss of IRSL signal due to repeated preheats and measurements is corrected for using the luminescence correction method. If the natural and each beta induced IRSL signal behave identically, then, after cor-
Fig. 3. The relationship between the ratios Rbleached from Table 1 of bleached IRSL to TL signals from nine single grains of sample B4 (left Y axis), or bleaching factor R ' from Table 1 (right Y axis), and bleaching time (on logarithmic axis). Each grain was bleached for dierent periods of time. The inset is the remaining percentage of the initial IRSL signals after bleaching IRSLbleached =IRSLunbleached from Table 1).
106
J.-F. Zhang et al. / Radiation Measurements 33 (2001) 103±108
rection, the data points from the ®rst and the third stages should be constant within each stage. In this case, the preheating is successful. In contrast, if the corrected data points from the third stage are not constant, it suggests that the natural and the beta induced IRSL signals decay dierently. In this case, the preheat condition needs to be adjusted until the decays of all signals are identical. De is obtained by ®tting the corrected growth curve at the second stage. The most appropriate preheats for the single grains of 425±500 mm from samples B4 and SHT3 are 2208C for 10 min and 2308C for 1 min, respectively. Under these preheating conditions, palaeodoses of 26 grains of B4 and 24 grains for SHT3 were measured, respectively. They range from 104 to 385 Gy for B4 and from 63 to 174 Gy for SHT3, respectively. This scatter in De is so large that their bleaching at the time of deposition must be examined.
ment. Thus, Rb is obtained. Finally, R' is calculated as Rn =Rb : The feldspar grains from samples B4 and SHT3 were tested. In the three-stage single-aliquot additivedose method discussed above, stage 3 was replaced by measuring IRSL and TL for calculating R ', after the most appropriate preheat for a sample was found. The subsequent beta dose was 216 Gy. The results are illustrated by plots of De versus R ' (Fig. 5). For sample B4, R' values ranging from 0.23 to 1.00 suggest that the bleaching of the sample is heterogeneous. The grains can be classi®ed into well-bleached, poorlybleached and unbleached (Fig. 5). Their bleaching factors R' are 0.24±0.40, 0.42±0.69 and 0.95±1.0, respectively; and their De values are 104±147, 151±276 and 282±385 Gy, respectively. The values of De for the unbleached grains are almost three times larger than
4.2. Bleaching factor of single grains The value of R ' of a grain can be obtained after its De measurement with the additive dose method. Firstly, N b TL Tnb is measured immediately after the De measurement. Although some of the natural TL signal Tn is removed due to the successive preheats and TL Tb is induced by each beta irradiation used to construct the additive dose growth curve, the removal of Tn and the accumulation of Tb are proportional to Tn : So, TNb is proportional to Tn : Tn in function (2) can be replaced by Tnb : Thus, Rn is obtained. Secondly, the grain is given a subsequent dose and then its IRSL and TL signals are measured following the identical preheat as in the De measure-
Fig. 4. The relationship between R' and the percentage of the initial IRSL signals from individual grains from sample B4 remaining after the bleaching of 10 s.
Fig. 5. The relationship between the palaeodoses (De ) and the bleaching factors (R') for grains from samples B4 and SHT3.
J.-F. Zhang et al. / Radiation Measurements 33 (2001) 103±108
for the well-bleached grains. For sample SHT3, R ' ranges from 0.18 to 0.44, which suggests that the bleaching of this sample is relatively homogeneous. The grains can be further classi®ed into well-bleached and poorly-bleached, based on their bleaching factors (R') of 0.18±0.23 and 0.26±0.44, respectively; and their De are 63±132 and 113±174 Gy, respectively. The bleaching factors show that the bleaching of SHT3 is more complete and homogenous than B4. The scatter of De for SHT3 is much less than for B4.
107
6. Conclusion The measured bleaching factor R ' is an indicator of the degree of bleaching of a grain. This factor can be measured for each grain. All measurements on De and R ' can be carried out on the same grain. The dierences in the properties of mineral and luminescence among grains are overcome. Based on the value of R ', not only is the homogeneity of bleaching of a sample identi®ed, but also the relatively well-bleached grains are identi®ed and their De values are given.
5. Discussion In luminescence dating it is customary to assume that all the grains in a sediment unit are bleached to the same degree (Aitken, 1998). In practice, the bleaching is aected by many factors, such as variation in bleachability, variation in light-absorbent coatings, and variation in duration of bleaching. The bleaching also depends on the weather and the depositional conditions at the time of deposition. Even under the same bleaching conditions in the laboratory, the bleaching rates of dierent gains are not identical (Fig. 4). So, it is almost impossible that all the grains extracted from a bulk sediment sample were bleached to the same degree. The grain-to-grain variation in bleaching suggests that the assessment of bleaching of a sample should be based on individual grains. The relatively well-bleached grains can be identi®ed by comparing values of R' from grain-to-grain. The constant ratio of IRSL to TL from an unbleached grain is its inherent property. The implication is that the traps that give rise to IRSL are proportional to the traps responsible for TL in a grain regardless of whether any common charge is being measured, or whether the two signals are wholly independent. The similarity in anomalous fading of IRSL and TL (Spooner, 1992; Balescu et al., 1997) implies that anomalous fading does not aect the ratio. It is mainly aected by bleaching. If samples approach saturation, their bleaching degrees at the time of deposition cannot be identi®ed because of the same R ' values for all grains. In general, the younger the sample, the easier is the identi®cation of its degree of bleaching. Fig. 5 suggests that the large scatter of palaeodoses for individual grains is mainly caused by the inhomogeneity of bleaching. The grains can be grouped by their bleaching factors. In each group, the scatter is relatively small. De in the well-bleached group is much smaller than that in the unbleached group. Thus, the best estimate of the true palaeodose is given by the lowest values obtained. This is consistent with the results reported by Li (1994), Murray et al. (1995) and Olley et al. (1998).
Acknowledgements This work was fully supported by a grant from the Research Grant Council of the Hong Kong Special Administrative Region, China (Project No. 7105/97P).
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