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On the use of Ti centres for estimating burial ages of Pleistocene sedimentary quartz: Multiple-grain data from Australia
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Quaternary Geochronology 1 (2006) 151–158 www.elsevier.com/locate/quageo
Research paper
On the use of Ti centres for estimating burial ages of Pleistocene sedimentary quartz: Multiple-grain data from Australia Koen Beertena,, Johanna Lomaxa, Katrijn Cle´merb, Andre Stesmansb, Ulrich Radtkea a
Geographisches Institut der Universita¨t zu Ko¨ln, Albertus-Magnus-Platz, 50923 Ko¨ln, Germany Departement Natuurkunde en Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
b
Received 12 October 2005; received in revised form 23 May 2006; accepted 26 May 2006 Available online 25 July 2006
Abstract Ti-related paramagnetic centres in quartz are promising dosimeters to estimate burial ages of sedimentary deposits with electron spin resonance (ESR). In general, two different subspecies can be found, known as Ti–Li and Ti–H centres. Recent single- and multiple-grain ESR dating experiments have shown that apparent burial doses determined from these two centres seldom converge to a single value. In an attempt to further investigate this problem, optically stimulated luminescence (OSL) and ESR equivalent doses from Australian aeolian quartz are compared in this study. In general, the results confirm earlier findings that the Ti–H centre systematically yields lower burial doses compared to the Ti–Li centre. For ‘younger’ samples, the Ti–H-based data are consistent with the OSL data. Nevertheless, both Ti centres seem to bracket the OSL data over the investigated dose range from 0 to 350 Gy. At present, it appears that Ti centres can be used to define a general sedimentation age window for Pleistocene aeolian deposits. r 2006 Elsevier Ltd. All rights reserved. Keywords: ESR dating; OSL dating; Ti–Li centres; Ti–H centres; Aeolian deposits
1. Introduction Electron spin resonance (ESR) is one of the methods to estimate the last exposure to sunlight of quartz grains. Similar to the commonly used luminescence methods, apparent sedimentation ages can be determined with ESR. The thus obtained model age is equal to the true age if several criteria are met, one of these being sufficient resetting of the geological clock by sunlight exposure prior to burial and the geological stability of the measured signals. Several ESR centres in quartz are known to be light sensitive, and their relatively ‘slow’ response to ionising radiation may permit to date the entire Quaternary. The two most frequently studied light-sensitive ESR centres in quartz are the Al- and Ti-related impurity centres or defects. In this study, we will focus on the Ti centres, following promising results from single-grain ESR experiments (Beerten and Stesmans, in press). The main Corresponding author. Tel.: +49 0 221 470 2547; fax: +49 0 221 470 5124. E-mail address: [email protected] (K. Beerten).
1871-1014/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2006.05.037
advantage of Ti centres over Al centres is their bleaching behaviour, indicating that complete bleaching can be commonly achieved (Brumby and Yoshida, 1994; Tanaka et al., 1997; Toyoda et al., 2000; Tissoux et al., in press). However, recent studies have shown that in several cases a very small but detectable residual Ti signal could remain in modern fluvial quartz (Falgue`res, pers. comm.). The relevant family of Ti-related centres consists of three subcentres, denoted Ti–Li, Ti–H and Ti–Na, according to the charge-compensating cation. Here we will focus on the Ti–Li and Ti–H centres, because the Ti–Na centre is rarely observed. Recently, the potential of Ti centres in dating studies has been reinvestigated by Tissoux et al. (in press) for fluvial multiple-grain samples of quartz, and Beerten and Stesmans (in press) for fluvial and aeolian single grains of quartz. Notwithstanding the promising results, the main difficulty is that the Li- and H-compensated species rarely converge to a single equivalent dose value, the Ti–H-based estimate always being lower than the Ti–Li-based estimate. In single grains, it has even been observed that Ti–Li centres show overestimates in the presence of Ti–H centres, relative to grains where only Ti–Li centres could be
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encountered (Beerten and Stesmans, 2006a). In the latter study, as in Tissoux et al. (in press), it was argued that the Ti–H centre may not be stable enough to cover the entire Middle Pleistocene. Conversely, Ti–Li overestimates may be interpreted in terms of insufficient bleaching, as these centres have a smaller bleaching potential than Ti–H centres (Toyoda et al., 2000; Beerten and Stesmans, 2006b; Tissoux et al., in press). In an attempt to further investigate the problems related to the use of Ti centres in ESR dating studies of sedimentary quartz, a direct comparison between optically stimulated luminescence (OSL) equivalent doses and multiple-grain Ti-related ESR equivalent doses in Australian aeolian quartz is aimed at in this study. On the one hand, the choice for analysing multiple grain aliquots with ESR instead of single grains is based on recently presented evidence from single grains, showing Ti centres to be completely bleached in desert environments (Beerten and Stesmans, 2005). Furthermore, the surplus value of multiple-grain ESR dating is that it experimentally leads to results much faster than the single-grain approach. On the other hand, OSL has proven to be a robust dating tool for aeolian deposits (Radtke et al., 2001; Murray and Olley, 2002), and would therefore be a suitable reference to test the ESR results. OSL signals are thermally stable and extremely light sensitive—two problems that ESR centres may be confronted with. Nevertheless, the correct use of the Ti-related centres for dating applications is extremely important, given the potentially large application field in the Middle and Early Pleistocene. In this context, several data will be presented which may provide new perspectives on the use of Ti-related ESR centres for dating. 2. Samples A research area was chosen that allowed sampling to meet several important criteria: (1) the samples should be
arranged in a profile, (2) they should be ‘datable’ by luminescence methods and (3) they should be aeolian in origin to ensure extended exposure to sunlight. Accordingly, the northern part of the Australian Western Murray Basin was selected as appropriate area (Fig. 1). Sampling was carried out on clean faces exposed by backhoe trenching in the core of one of the closely spaced west–east trending longitudinal dunes. Eleven samples were taken up to a depth of almost 8 m from which five will be discussed in this study (Table 1): MS0, MS1, MS2, MS3 and MS9. The entire profile is aeolian in origin and consists of red–brown sand with various amounts of carbonate, either in pellets or as a matrix. Sample MS1 was taken from a calcrete that contains abundant red–brown sand grains. 3. Methodology 3.1. Sample preparation The 100–200 mm size quartz fraction of the bulk sample was extracted by sieving and then treated with HCl and H2O2 to remove carbonates and organic material. Sodium polytungstate was used to extract the quartz-rich fraction. The remaining grains were immersed in HF (40%) for 40 min for purification. Finally, the etched quartz grains were re-sieved and one part was mounted on stainless-steel discs for luminescence measurements (covering the central 1 mm of the disc with 50725 grains) while another was divided into several aliquots (300 mg) for ESR measurements. 3.2. ESR measurements The ESR spectra were recorded with a Bruker X-band (ESP 300) operating at a microwave frequency of 9.58 GHz and a modulation frequency of 100 kHz. Measurements were made at cryogenic temperatures using
Fig. 1. (a) General location of the investigated profile. (b) Synthetic cross-section of the studied profile. Samples MS0, MS1, MS2, MS3 and MS9 are discussed in this study.
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Table 1 ESR and OSL equivalent doses for several samples from the investigated profile Sample
Depth (m)
OSL na
MS9 MS3 MS2 MS1 MS0
1.5 4.3 4.9 5.6 7.5
11 49 42 44 41
ESR RSDb
9.59 48.4 27.0 40.1 53.2
Skewness
1.83 2.32 0.41 1.91 2.03
Equivalent dose (Gy)
Equivalent dose (Gy)
Weighted mean
Median
Arithmetic mean
Ti–H
Total Ti
Ti–Li
0.0970.01 39.672.0 75.570.2 18471 27571
0.0870.01 39.474.0 77.074.3 218717 346736
0.0970.01 46.773.2 81.073.4 247715 405734
071 3676 7576 169726 207725
1472 68710 10477 27573 387727
5275 132710 15779 363716 671763
Uncertainties are quoted as standard errors (1s). a Number of OSL aliquots. b Relative standard deviation.
a quartz dewar in the cavity. The lowest possible temperature with optimal background stability was found to be 115 K. The modulation amplitude was set to 5 G to increase the S/N ratio, allowing one to observe hyperfine splitting (I ¼ 1/2) due to the presence of Ti–H centres. The centre field was set to 3450 G (exploratory measurements) or 3520 G (routine measurements), and the sweep width to 300 G (exploratory measurements) or 160 G (routine measurements). The conversion time and time constant were set to 20.48 ms, a value which was found to produce a fair balance between signal resolution and scan time. The Ti-related absorption signals were found not to be in saturation using a microwave power of 20 mW at 115 K. The quartz aliquots were measured in quartz tubes and sealed off from light during the measurement process. Short exposure to laboratory light occurred while mounting and unmounting the sample in the cavity, but this does not seem to affect the ESR signals (Beerten and Stesmans, 2006a). Indeed, the intensity and spectrum of solar light is completely different from laboratory light. Several measurements were taken to calibrate the corresponding gvalues of Ti-related ESR absorption lines against a Si:P marker sample with known g-value of 1.999. The large intensity and small line width of the marker signal makes it an excellent standard, even if it is superimposed on other ESR signals (e.g., the Al signal; cf. infra). Simulated spectra of Ti-related impurity centres were used to fit the measured spectra in WINEPR SimFonia after baseline correction (with either a linear or quadratic function). The intensity of the simulated spectra was adjusted until the best fit—visually—was obtained. The corresponding g-values are taken from Okada et al. (1971), while the hyperfine splitting matrix A elements were determined using the data from Isoya et al. (1988) and Rinneberg and Weil (1972) for the Ti–Li and Ti–H centres, respectively (see Appendix A.). Equivalent doses were estimated using a multiple aliquot regenerative dose method. The aliquots were heated for 2 h at 300 1C in order to deactivate all Ti-related ESR centres. Three aliquots were used to determine the equivalent dose of each sample. The artificial doses (g-rays from a 60Co
with a dose rate of 2–3 Gy/min for quartz) were given such that the three dose points would encompass the expected dose. Subsequently, the non-zero dose points were fitted with a linear function, and the equivalent dose was determined by interpolation of the natural intensity. Most dose curves indeed show a linear trend, and even if there would be a slight tendency towards saturation this would not affect the equivalent dose significantly because of the weak curvature. In this context, the surplus value of linear fitting is that it produces much smaller errors. Previously, this procedure was found to produce promising results in single-grain ESR dating (Beerten and Stesmans, 2006a; Beerten and Stesmans, in press). Sensitivity change tests were performed on a sample with a natural near-zero OSL dose (sample MS9; cf. infra). Several aliquots of this sample were given an artificial gamma dose of 300 Gy after thermal treatment (in the range of 20 1C to 400 1C) for 2 h. 3.3. OSL measurements OSL measurements were carried out on an automated Risø Minisys TL/OSL reader-type TL-DA-15 equipped with blue light emitting diodes (470730 nm) for illumination and a 90Sr/90Y beta source for irradiation (BøtterJensen et al., 1999). After passing through an U340 optical filter, OSL signals were measured for 50 s at 125 1C with an EMI 9235 photomultiplier tube. The equivalent dose was estimated following the Single Aliquot Regenerative (SAR) Dose protocol (Murray and Wintle, 2000). The appropriate preheat temperature was tested on samples that were artificially bleached (100 s at 125 1C, blue LED) and irradiated with a known beta dose. To recover the given dose, the SAR protocol was applied with preheat temperatures ranging from 180 to 280 1C in 20 1C steps and a fixed cut heat of 160 1C. From 200 to 280 1C, the recovered dose matches the given dose, indicating no influence of thermal transfer in this temperature range. On this basis, preheat temperatures in the palaeodose measurements of the naturally dosed samples were set to 240 1C.
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4. Results 4.1. ESR spectra A typical ESR spectrum is shown in Fig. 2a for sample MS0 (regenerated ESR spectrum following thermal annealing at 300 1C for 2 h). The large narrow absorption line at 3424 G originates from the Si:P marker and is superimposed on the Al signal. In this study, however, we will focus on the absorption lines towards higher magnetic field values that are due to Ti-related impurity defects. Two different centres have been observed which are ascribed to the paramagnetic species, denoted Ti–Li and Ti–H, according to the charge-compensating cation. The measured g-values for the different Ti centres are indicated. An interesting feature to observe is the prominent co-presence of Ti–H centres (Fig. 2a). This is most clearly seen in the doublet-like absorption line at 3545 G, which arises from hyperfine splitting of the nearby proton with nuclear spin I ¼ 1/2. Note that the absorption lines of Ti–H and Ti–Li centres are coinciding at this g-value. Unfortunately, the absorption lines corresponding to g1 and g3 values of the Ti–H centres are less clear, due to interference with the marker and the g3 absorption peak of the Ti–Li defect, respectively. Hyperfine splitting of the latter due to the Li cation (I ¼ 2/3) is not resolved. The ESR spectra were simulated in order to determine the ESR intensity of the various Ti centres (see Appendix A for more details on the hyperfine matrix). The simulation process proceeded as follows. First, the Ti–Li spectrum was simulated and fitted to the measured spectrum taking the absorption line at g1 (Fig. 2a) for this centre as a reference (Fig. 2b; upper spectrum). Subsequently, the simulated spectrum was subtracted from the measured one, and the
(a)
resulting spectrum was simulated using the parameters for the Ti–H centre (Fig. 2b; middle spectrum). Again, the simulated spectrum was subtracted, and the resulting spectrum from this operation did not contain any significant residual ESR signals (Fig. 2b; lower spectrum), indicating that satisfactory fittings have been achieved. Finally, the simulated spectra were numerically doubleintegrated to obtain separate relative defect intensities for the Ti–Li and Ti–H centres, and from these data the relative total Ti defect concentration was determined. 4.2. Sensitivity change tests of Ti-related ESR centres As will be shown later, ESR equivalent doses were estimated with the regenerative approach using thermal annealing. In order to justify this approach, and to find the proper annealing temperature range, several aliquots of sample MS9 were given a gamma dose after various temperature treatments. The observed intensities plotted in Fig. 3 are as measured, except for the aliquot at room temperature. For the latter, the weak ESR spectrum was subtracted from the spectrum observed following a 300 Gy gamma dose. As can be seen from Fig. 3, sensitivity changes for both Ti–Li and Ti–H centres appear to be weak, if any at all, over the investigated temperature range. Furthermore, any influence due to sensitivity changes will automatically be reduced because only one regeneration cycle (one thermal resetting) is involved here. 4.3. Comparison of ESR and OSL equivalent doses A comparison of OSL and ESR dose curves for sample MS0 is presented in Fig. 4. Data on the OSL dose
(b)
Fig. 2. (a) X-band ESR spectrum from sample MS0 (450 Gy regeneration dose following thermal annealing). The very strong absorption line around 3425 G stems from the Si:P marker. Absorption lines from Ti–H and Ti–Li centres can be seen at higher magnetic field values. The g-values are calibrated against the marker and are consistent with those cited in Okada et al., (1971). However, a systematic deviation of 0.001 can be observed, possibly caused by distortion of the marker signal due to the presence of the Al centre. (b) Demonstration of the three-step procedure followed to determine ESR intensities from Ti-related ESR spectra: (1) comparison between the measured spectrum and the simulated Ti–Li spectrum—the absorption line at g1 is used to fit the spectrum (cf. Fig. 2a); (2) comparison between the resulting difference in spectrum from the previous operation and a simulated T–H spectrum; and (3) resultant spectrum left from the previous subtraction.
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mean of 5 values
Ti-Li
between 200-400 °C
Ti-H Ti
relative ESR intensity
0.6
0.4
0.2
0.0 0
100
200 300 annealing temperature (°C)
400
Fig. 3. Sensitivity change tests after thermal annealing of several aliquots from sample MS9. After a temperature treatment of 2 h (at temperatures between 20 and 400 1C), several aliquots were given a gamma dose of 300 Gy. The resulting ESR intensities are plotted as a function of the temperature. The ESR spectrum of the as-sampled aliquot (natural spectrum) was subtracted from the spectrum obtained after room temperature annealing and artificial dosing. Note the good consistency among the data, indicating that sensitivity changes of the various Ti centres are weak or non-existing over the investigated temperature range. The slightly higher intensities for the aliquot at 200 1C may indicate incomplete thermal resetting.
normalised OSL (Lx/Tx) and ESR (arb. uits) intensity
K. Beerten et al. / Quaternary Geochronology 1 (2006) 151–158
155
OSL MS0-09 OSL MS0-14 ESR Ti-Li ESR Ti-H ESR total Ti exp. growth exp. + linear growth
2
1
0 0
200
400
600
dose (Gy)
Fig. 4. Comparison of normalised (natural intensity ¼ 1) OSL and Ti-related ESR dose curves in quartz aliquots from sample MS0. Whereas some aliquots already show a strong tendency towards OSL signal saturation at doses beyond 200 Gy, the ESR signals tend to rise monotonically up to doses of at least 450 Gy. This behaviour of ESR centres reflects the most important advantage of ESR dating over OSL dating.
10
frequency
MS1
distributions and equivalent doses are given in Table 1 and Figs. 5 and 6. The OSL distributions show a relatively large standard deviation and positive skewness as illustrated in Fig. 5 for sample MS1. Regarding the OSL equivalent doses (cf. Table 1), considerable differences can be observed between the weighted mean, arithmetic mean and the median of the obtained dose distributions, especially for the two oldest samples. It is beyond the scope of the present study to indicate which one would most reliably reflect the true average burial dose of the samples. However, we advocate that the median would be a good compromise to compare the ESR data with (cf. infra). The ESR doses are determined from multiple aliquot experiments, in which case a single value for each sample is obtained instead of a dose distribution. Apparently, the data show a clear trend, the Ti–H equivalent doses being smaller than the Ti–Li doses (Table 1 and Fig. 6). The data based on the total Ti defect concentration are situated in between (Table 1 and Fig. 6). For the youngest samples (MS9, MS3 and MS2), it can be observed that the Ti–H data are in fair agreement with the OSL doses. However, an apparent underestimate seems to result from the Ti–H data relative to the OSL data for samples MS1 and MS0. The opposite is true for the Ti–Li data, showing systematically larger equivalent doses compared to the OSL data. The dose difference between both even seems to increase with sampling depth. The existence of a significant nonzero Ti–Li equivalent dose for the youngest sample (MS9)
5
0 0
100
200
300 400 500 equivalent dose (Gy)
600
700
Fig. 5. Inferred OSL dose distribution of sample MS1. The histogram shows a clear positive skewness, probably related to dose rate inhomogeneities and/or OSL signal saturation of the aliquots towards higher doses.
strongly indicates the presence of an unbleached or unbleachable residual dose in this sample. 5. Discussion 5.1. Reliability of the OSL data The currently addressed equivalent doses are relatively high in terms of OSL dating, thereby decreasing the reliability of the OSL data. Not much is known about the
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1
depth (m)
2
MS9
3
1 2 depth (m)
OSL median ESR Ti-Li ESR Ti-H ESR total Ti
4
MS3 MS2
5
MS3 MS2
5 6
4
MS9
3
0
50 100 150 equivalent dose (Gy)
MS1 6 7 MS0 8 0
200 400 600 equivalent dose (Gy)
800
Fig. 6. Comparison of OSL and ESR equivalent doses. Whereas the OSL and Ti–H ESR data are very consistent for the upper three samples in the profile, this appears not to be the case for the lower two samples. Here, the Ti–H data show a clear underestimate with regard to the OSL data. The Ti–Li-based data show a systematic overestimation relative to the OSL doses. In any case, in all samples, the individual Ti centres seem to bracket the OSL data. Furthermore, the equivalent doses based on the total Ti defect concentration seem to be consistent with the OSL data over the entire profile, except for a slight offset over a few tens of Gy.
validity of the SAR protocol for determining high environmental doses. Nevertheless, in earlier studies, promising results for SAR-OSL doses of up to 300 Gy have been obtained (Banerjee et al., 2003). The uncertainty on the OSL data seems to be reflected in the large differences between the various average equivalent dose estimates, especially for the older samples. Nevertheless, we propose that the median would be a good compromise. Previously, it has been suggested that non-linear growth may produce positively skewed dose distributions, in which case the median would be a better estimate of the equivalent dose (Murray and Funder, 2003). Indeed, the measured doses of the oldest two samples in the profile were situated in the non-linear part of the growth curves, and a strongly positively skewed distribution was observed. Other possible factors influencing the OSL dose distribution could be bioturbation, insufficient bleaching and/or an inhomogeneous radiation field. Field evidence and the nature of the deposits (i.e., aeolian) seem to exclude the first two possibilities. Single-grain OSL experiments are underway to investigate the contribution of an inhomogeneous dose rate. 5.2. Interpretation of the ESR results In general, the present study confirms earlier results on the use of Ti centres in both single- (Beerten and Stesmans, 2006a) and multiple-grain (Yoshida, 1996; Tissoux et al., in press) samples. Clearly, the Ti–H equivalent doses are in fair agreement with the OSL equivalent doses for the upper three samples (Table 1), whereas the Ti–Li equivalent doses
are systematically higher. Accordingly, the difference in this part of the profile could simply be interpreted in terms of the bleaching difference between Ti–H centres (faster bleaching behaviour and thus completely bleached) and Ti–Li centres (slower bleaching behaviour and thus incompletely bleached). This raises the question whether this difference is really due to insufficient bleaching or to the presence of unbleachable residuals. We are currently performing bleaching experiments to elucidate this problem. The preliminary results indicate that, at least for sample MS9, a residual dose of 70% of the natural Ti–Li intensity remains after 120 h bleaching in a SOL-2 solar simulator. In the same context, it is interesting to note that preliminary observations indicate similar results for the (asfound) Al centre and Ti–Li centre equivalent doses for this sample. Towards the lower part of the profile, the Ti–H equivalent doses tend to underestimate the OSL results, whether or not the median is used as reference OSL dose. Perhaps, this may indicate that the Ti–H centre, as defect site, is not stable enough to be used in ESR dating of older samples (i.e., Middle Pleistocene and beyond), as has been suggested by Tissoux et al. (in press). At the same time, the absolute dose difference between the OSL and Ti–Li data seems to increase with depth. This is not true for the relative difference, but we believe that residual doses should be evaluated using the absolute difference. On the one hand, assuming that the residual Ti–Li dose is due to incomplete bleaching, it is difficult to believe that the older samples would be systematically less bleached than the younger ones, given the similar depositional environment. On the other hand, if the difference is due to the presence of unbleachable doses, there is no reason why the older samples would have more unbleachable Ti centres than the younger ones at the time of deposition. In this respect, we note again that the increasing difference between the Ti–Li data and the OSL data appears to be linked with an increasing underestimate based on the Ti–H centre. This may suggest that there is a link between both centres, in which case the separate use of these centres may not be appropriate. In previous works we have already stressed this point, and recent single-grain work seems to indicate that the total Ti defect concentration appears to be a relatively good dosimeter for the true burial dose (Beerten and Stesmans, in press). Do Ti-related ESR centres contain any useful information for sediment dating at all? Apparently, the ESR data always bracket the OSL data. If we now assume that Ti–H centres give a minimum estimate and Ti–Li centres a maximum estimate, it can be seen that there is an apparent hiatus between samples MS1 and MS2 (see Appendix B): the maximum age—based on Ti–Li centres—for the upper sample (MS2) is lower than the minimum age—based on Ti–H centers—of the lower sample (MS1). We then would be able to divide the profile in Late Pleistocene deposits (upper three samples) and Middle Pleistocene deposits (lower two samples). Moreover, the maximum estimate for
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sample MS0 would indicate that the entire profile is younger than the Early Pleistocene. In the present study, the OSL data could be used as some kind of reference for the older deposits. However, in many cases, the saturation behaviour of OSL signals does not permit dating beyond the Late Pleistocene. Ti-related ESR centres could be very useful then, even with all the remaining problems discussed above. But, as indicated earlier, there are reasons to believe that more accurate dating is possible if these problems could be resolved. The solution could lie in the determination of unbleachable residual doses or in the use of the total Ti defect concentration rather than the individual Ti centres. 6. Conclusion A comparison of equivalent doses from ESR active Ti defect centres in quartz and OSL data has revealed several important trends with regard to the feasibility of ESR dating of aeolian quartz. Earlier evidence that the individual Ti–H and Ti–Li sub-centres yield inconsistent results has been confirmed in this study. Nevertheless, the combination of both centres may allow identification of a sedimentation age window, which, in the absence of OSL data, could be used to establish a global geochronological framework. However, it should be clearly stated that— before generalising these results—more consolidating comparisons between ESR and OSL data and independent age control will be needed. Much older deposits should be investigated, and age control other than luminescence should be looked for as well. We believe that these results encourage further research on Ti-related centres as it may open new possibilities for ESR dating of sedimentary quartz.
157
and Weil (1972) for the Ti–Li and Ti–H centres, respectively. The resulting hyperfine splitting matrices are given below: 1:275 ATiLi ðGÞ ¼ 0:049 0:000
0:049 1:116
0:000 0:000
0:000
2:691
4:7413
0:0014
0:9260
ATiH ðGÞ ¼ 0:0007 0:9220
4:3471
0:1405 .
0:1425
9:1815
and
The g-values used to simulate the Ti-related ESR spectra in quartz are taken from Okada et al. (1971). For the Ti–Li centre, these are g1 ¼ 1.979, g2 ¼ 1.931 and g3 ¼ 1.912. For the Ti–H centre, the g-values are g1 ¼ 1.986, g2 ¼ 1.931 and g3 ¼ 1.915. Appendix B. Dose rate data and preliminary ESR ages The dose rates are based on U, Th and K concentrations derived from high-resolution gamma spectroscopy, plus a small cosmic dose (see Lomax et al. (2003) for the methodology). The Ti–Li age for sample MS2 is based on a dose rate of 1.3470.04 Gy/ka, yielding a maximum age of 11778 ka. The Ti–H age for sample MS1 is based on a dose rate of 1.0670.04 Gy/ka, yielding a minimum age of 159725 ka. These data suggest that the Middle to Late Pleistocene transition is situated in between both samples. Finally, the Ti–Li age for sample MS0 is based on a dose rate of 0.9770.04 Gy/ka, yielding a maximum age of 691770 ka for this sample. Consequently, all samples are believed to be younger than the Early Pleistocene.
Acknowledgements
References
We kindly thank R. Schoonheydt (Centrum voor Oppervlaktechemie en katalyse, K.U. Leuven) for access to the ESP300 Bruker X-band spectrometer. Thanks are due to N. Bal and K. Feyen (Sint-Maartenziekenhuis, Duffel) for help with artificial irradiation.
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Appendix A. Simulation parameters The hyperfine splitting parameters needed for simulating the ESR spectra of the Ti–Li and Ti–H centres were determined as follows. Due to the symmetry of the defect system, the principal axes of the g-matrix and hyperfine matrix A do not coincide. Therefore they cannot be simultaneously diagonalised. The choice to describe the system in the principal axis coordinate system of the g-matrix thus results in three principal g-values and a symmetric non-diagonal A matrix. The principal values of the two parameter matrices and the polar and azimuthal angles that link each principal axis to the crystal coordinate system were taken from Isoya et al. (1988) and Rinneberg
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Lomax, J., Hilgers, A., Wopfner, H., Gru¨n, R., Twidale, C.R., Radtke, U., 2003. The onset of dune formation in the Strzelecki Desert, South Australia. Quaternary Science Reviews 22, 1067–1076. Murray, A.S., Funder, S., 2003. Optically stimulated luminescence dating of a Danish Eemian coastal marine deposit: a test of accuracy. Quaternary Science Reviews 22, 1177–1183. Murray, A.S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometria 21, 1–16. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73. Okada, M., Rinnenberg, J., Weil, J.A., Wright, P.M., 1971. EPR of Ti3+ centers in alfa quartz. Chemical Physics Letters 11, 275–276. Radtke, U., Janotta, A., Hilgers, A., Murray, A.S., 2001. The potential of OSL and TL for dating Lateglacial and Holocene dune sands tested with independent age control of the Laacher See tephra (12 880 a) at the Section Mainz-Gonsenheim. Quaternary Science Reviews 20, 719–724.
Rinneberg, J., Weil, J.A., 1972. EPR studies of Ti3+–H+ centers in X-irradiated alpha quartz. Journal of Chemical Physics 56, 2019–2028. Tanaka, K., Hataya, R., Spooner, N.A., Questiaux, D.G., Saito, Y., Hashimoto, T., 1997. Dating of marine terrace sediments by ESR, TL and OSL methods and their applicabilities. Quaternary Science Reviews 16, 257–264. Tissoux, H., Falgue`res, C., Voinchet, P., Toyoda, S., Bahain, J.J., Desprie´e, J., in press. Potential use of Ti-center in ESR dating of fluvial sediment. Quaternary Geochronology, doi:10.1016/j.quageo. 2006.04.006. Toyoda, S., Voinchet, P., Falgue`res, C., Dolo, J.M., Laurent, M., 2000. Bleaching of ESR signals by the sunlight: a laboratory experiment for establishing the ESR dating of sediments. Applied Radiation and Isotopes 52, 1357–1362. Yoshida, H., 1996. Quaternary dating studies using ESR signals, with emphasis on shell, coral, tooth enamel and quartz. Ph.D. Thesis, Australian National University, Canberra, Australia.
Quaternary Geochronology 1 (2006) 151–158 www.elsevier.com/locate/quageo
Research paper
On the use of Ti centres for estimating burial ages of Pleistocene sedimentary quartz: Multiple-grain data from Australia Koen Beertena,, Johanna Lomaxa, Katrijn Cle´merb, Andre Stesmansb, Ulrich Radtkea a
Geographisches Institut der Universita¨t zu Ko¨ln, Albertus-Magnus-Platz, 50923 Ko¨ln, Germany Departement Natuurkunde en Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
b
Received 12 October 2005; received in revised form 23 May 2006; accepted 26 May 2006 Available online 25 July 2006
Abstract Ti-related paramagnetic centres in quartz are promising dosimeters to estimate burial ages of sedimentary deposits with electron spin resonance (ESR). In general, two different subspecies can be found, known as Ti–Li and Ti–H centres. Recent single- and multiple-grain ESR dating experiments have shown that apparent burial doses determined from these two centres seldom converge to a single value. In an attempt to further investigate this problem, optically stimulated luminescence (OSL) and ESR equivalent doses from Australian aeolian quartz are compared in this study. In general, the results confirm earlier findings that the Ti–H centre systematically yields lower burial doses compared to the Ti–Li centre. For ‘younger’ samples, the Ti–H-based data are consistent with the OSL data. Nevertheless, both Ti centres seem to bracket the OSL data over the investigated dose range from 0 to 350 Gy. At present, it appears that Ti centres can be used to define a general sedimentation age window for Pleistocene aeolian deposits. r 2006 Elsevier Ltd. All rights reserved. Keywords: ESR dating; OSL dating; Ti–Li centres; Ti–H centres; Aeolian deposits
1. Introduction Electron spin resonance (ESR) is one of the methods to estimate the last exposure to sunlight of quartz grains. Similar to the commonly used luminescence methods, apparent sedimentation ages can be determined with ESR. The thus obtained model age is equal to the true age if several criteria are met, one of these being sufficient resetting of the geological clock by sunlight exposure prior to burial and the geological stability of the measured signals. Several ESR centres in quartz are known to be light sensitive, and their relatively ‘slow’ response to ionising radiation may permit to date the entire Quaternary. The two most frequently studied light-sensitive ESR centres in quartz are the Al- and Ti-related impurity centres or defects. In this study, we will focus on the Ti centres, following promising results from single-grain ESR experiments (Beerten and Stesmans, in press). The main Corresponding author. Tel.: +49 0 221 470 2547; fax: +49 0 221 470 5124. E-mail address: [email protected] (K. Beerten).
1871-1014/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2006.05.037
advantage of Ti centres over Al centres is their bleaching behaviour, indicating that complete bleaching can be commonly achieved (Brumby and Yoshida, 1994; Tanaka et al., 1997; Toyoda et al., 2000; Tissoux et al., in press). However, recent studies have shown that in several cases a very small but detectable residual Ti signal could remain in modern fluvial quartz (Falgue`res, pers. comm.). The relevant family of Ti-related centres consists of three subcentres, denoted Ti–Li, Ti–H and Ti–Na, according to the charge-compensating cation. Here we will focus on the Ti–Li and Ti–H centres, because the Ti–Na centre is rarely observed. Recently, the potential of Ti centres in dating studies has been reinvestigated by Tissoux et al. (in press) for fluvial multiple-grain samples of quartz, and Beerten and Stesmans (in press) for fluvial and aeolian single grains of quartz. Notwithstanding the promising results, the main difficulty is that the Li- and H-compensated species rarely converge to a single equivalent dose value, the Ti–H-based estimate always being lower than the Ti–Li-based estimate. In single grains, it has even been observed that Ti–Li centres show overestimates in the presence of Ti–H centres, relative to grains where only Ti–Li centres could be
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encountered (Beerten and Stesmans, 2006a). In the latter study, as in Tissoux et al. (in press), it was argued that the Ti–H centre may not be stable enough to cover the entire Middle Pleistocene. Conversely, Ti–Li overestimates may be interpreted in terms of insufficient bleaching, as these centres have a smaller bleaching potential than Ti–H centres (Toyoda et al., 2000; Beerten and Stesmans, 2006b; Tissoux et al., in press). In an attempt to further investigate the problems related to the use of Ti centres in ESR dating studies of sedimentary quartz, a direct comparison between optically stimulated luminescence (OSL) equivalent doses and multiple-grain Ti-related ESR equivalent doses in Australian aeolian quartz is aimed at in this study. On the one hand, the choice for analysing multiple grain aliquots with ESR instead of single grains is based on recently presented evidence from single grains, showing Ti centres to be completely bleached in desert environments (Beerten and Stesmans, 2005). Furthermore, the surplus value of multiple-grain ESR dating is that it experimentally leads to results much faster than the single-grain approach. On the other hand, OSL has proven to be a robust dating tool for aeolian deposits (Radtke et al., 2001; Murray and Olley, 2002), and would therefore be a suitable reference to test the ESR results. OSL signals are thermally stable and extremely light sensitive—two problems that ESR centres may be confronted with. Nevertheless, the correct use of the Ti-related centres for dating applications is extremely important, given the potentially large application field in the Middle and Early Pleistocene. In this context, several data will be presented which may provide new perspectives on the use of Ti-related ESR centres for dating. 2. Samples A research area was chosen that allowed sampling to meet several important criteria: (1) the samples should be
arranged in a profile, (2) they should be ‘datable’ by luminescence methods and (3) they should be aeolian in origin to ensure extended exposure to sunlight. Accordingly, the northern part of the Australian Western Murray Basin was selected as appropriate area (Fig. 1). Sampling was carried out on clean faces exposed by backhoe trenching in the core of one of the closely spaced west–east trending longitudinal dunes. Eleven samples were taken up to a depth of almost 8 m from which five will be discussed in this study (Table 1): MS0, MS1, MS2, MS3 and MS9. The entire profile is aeolian in origin and consists of red–brown sand with various amounts of carbonate, either in pellets or as a matrix. Sample MS1 was taken from a calcrete that contains abundant red–brown sand grains. 3. Methodology 3.1. Sample preparation The 100–200 mm size quartz fraction of the bulk sample was extracted by sieving and then treated with HCl and H2O2 to remove carbonates and organic material. Sodium polytungstate was used to extract the quartz-rich fraction. The remaining grains were immersed in HF (40%) for 40 min for purification. Finally, the etched quartz grains were re-sieved and one part was mounted on stainless-steel discs for luminescence measurements (covering the central 1 mm of the disc with 50725 grains) while another was divided into several aliquots (300 mg) for ESR measurements. 3.2. ESR measurements The ESR spectra were recorded with a Bruker X-band (ESP 300) operating at a microwave frequency of 9.58 GHz and a modulation frequency of 100 kHz. Measurements were made at cryogenic temperatures using
Fig. 1. (a) General location of the investigated profile. (b) Synthetic cross-section of the studied profile. Samples MS0, MS1, MS2, MS3 and MS9 are discussed in this study.
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Table 1 ESR and OSL equivalent doses for several samples from the investigated profile Sample
Depth (m)
OSL na
MS9 MS3 MS2 MS1 MS0
1.5 4.3 4.9 5.6 7.5
11 49 42 44 41
ESR RSDb
9.59 48.4 27.0 40.1 53.2
Skewness
1.83 2.32 0.41 1.91 2.03
Equivalent dose (Gy)
Equivalent dose (Gy)
Weighted mean
Median
Arithmetic mean
Ti–H
Total Ti
Ti–Li
0.0970.01 39.672.0 75.570.2 18471 27571
0.0870.01 39.474.0 77.074.3 218717 346736
0.0970.01 46.773.2 81.073.4 247715 405734
071 3676 7576 169726 207725
1472 68710 10477 27573 387727
5275 132710 15779 363716 671763
Uncertainties are quoted as standard errors (1s). a Number of OSL aliquots. b Relative standard deviation.
a quartz dewar in the cavity. The lowest possible temperature with optimal background stability was found to be 115 K. The modulation amplitude was set to 5 G to increase the S/N ratio, allowing one to observe hyperfine splitting (I ¼ 1/2) due to the presence of Ti–H centres. The centre field was set to 3450 G (exploratory measurements) or 3520 G (routine measurements), and the sweep width to 300 G (exploratory measurements) or 160 G (routine measurements). The conversion time and time constant were set to 20.48 ms, a value which was found to produce a fair balance between signal resolution and scan time. The Ti-related absorption signals were found not to be in saturation using a microwave power of 20 mW at 115 K. The quartz aliquots were measured in quartz tubes and sealed off from light during the measurement process. Short exposure to laboratory light occurred while mounting and unmounting the sample in the cavity, but this does not seem to affect the ESR signals (Beerten and Stesmans, 2006a). Indeed, the intensity and spectrum of solar light is completely different from laboratory light. Several measurements were taken to calibrate the corresponding gvalues of Ti-related ESR absorption lines against a Si:P marker sample with known g-value of 1.999. The large intensity and small line width of the marker signal makes it an excellent standard, even if it is superimposed on other ESR signals (e.g., the Al signal; cf. infra). Simulated spectra of Ti-related impurity centres were used to fit the measured spectra in WINEPR SimFonia after baseline correction (with either a linear or quadratic function). The intensity of the simulated spectra was adjusted until the best fit—visually—was obtained. The corresponding g-values are taken from Okada et al. (1971), while the hyperfine splitting matrix A elements were determined using the data from Isoya et al. (1988) and Rinneberg and Weil (1972) for the Ti–Li and Ti–H centres, respectively (see Appendix A.). Equivalent doses were estimated using a multiple aliquot regenerative dose method. The aliquots were heated for 2 h at 300 1C in order to deactivate all Ti-related ESR centres. Three aliquots were used to determine the equivalent dose of each sample. The artificial doses (g-rays from a 60Co
with a dose rate of 2–3 Gy/min for quartz) were given such that the three dose points would encompass the expected dose. Subsequently, the non-zero dose points were fitted with a linear function, and the equivalent dose was determined by interpolation of the natural intensity. Most dose curves indeed show a linear trend, and even if there would be a slight tendency towards saturation this would not affect the equivalent dose significantly because of the weak curvature. In this context, the surplus value of linear fitting is that it produces much smaller errors. Previously, this procedure was found to produce promising results in single-grain ESR dating (Beerten and Stesmans, 2006a; Beerten and Stesmans, in press). Sensitivity change tests were performed on a sample with a natural near-zero OSL dose (sample MS9; cf. infra). Several aliquots of this sample were given an artificial gamma dose of 300 Gy after thermal treatment (in the range of 20 1C to 400 1C) for 2 h. 3.3. OSL measurements OSL measurements were carried out on an automated Risø Minisys TL/OSL reader-type TL-DA-15 equipped with blue light emitting diodes (470730 nm) for illumination and a 90Sr/90Y beta source for irradiation (BøtterJensen et al., 1999). After passing through an U340 optical filter, OSL signals were measured for 50 s at 125 1C with an EMI 9235 photomultiplier tube. The equivalent dose was estimated following the Single Aliquot Regenerative (SAR) Dose protocol (Murray and Wintle, 2000). The appropriate preheat temperature was tested on samples that were artificially bleached (100 s at 125 1C, blue LED) and irradiated with a known beta dose. To recover the given dose, the SAR protocol was applied with preheat temperatures ranging from 180 to 280 1C in 20 1C steps and a fixed cut heat of 160 1C. From 200 to 280 1C, the recovered dose matches the given dose, indicating no influence of thermal transfer in this temperature range. On this basis, preheat temperatures in the palaeodose measurements of the naturally dosed samples were set to 240 1C.
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4. Results 4.1. ESR spectra A typical ESR spectrum is shown in Fig. 2a for sample MS0 (regenerated ESR spectrum following thermal annealing at 300 1C for 2 h). The large narrow absorption line at 3424 G originates from the Si:P marker and is superimposed on the Al signal. In this study, however, we will focus on the absorption lines towards higher magnetic field values that are due to Ti-related impurity defects. Two different centres have been observed which are ascribed to the paramagnetic species, denoted Ti–Li and Ti–H, according to the charge-compensating cation. The measured g-values for the different Ti centres are indicated. An interesting feature to observe is the prominent co-presence of Ti–H centres (Fig. 2a). This is most clearly seen in the doublet-like absorption line at 3545 G, which arises from hyperfine splitting of the nearby proton with nuclear spin I ¼ 1/2. Note that the absorption lines of Ti–H and Ti–Li centres are coinciding at this g-value. Unfortunately, the absorption lines corresponding to g1 and g3 values of the Ti–H centres are less clear, due to interference with the marker and the g3 absorption peak of the Ti–Li defect, respectively. Hyperfine splitting of the latter due to the Li cation (I ¼ 2/3) is not resolved. The ESR spectra were simulated in order to determine the ESR intensity of the various Ti centres (see Appendix A for more details on the hyperfine matrix). The simulation process proceeded as follows. First, the Ti–Li spectrum was simulated and fitted to the measured spectrum taking the absorption line at g1 (Fig. 2a) for this centre as a reference (Fig. 2b; upper spectrum). Subsequently, the simulated spectrum was subtracted from the measured one, and the
(a)
resulting spectrum was simulated using the parameters for the Ti–H centre (Fig. 2b; middle spectrum). Again, the simulated spectrum was subtracted, and the resulting spectrum from this operation did not contain any significant residual ESR signals (Fig. 2b; lower spectrum), indicating that satisfactory fittings have been achieved. Finally, the simulated spectra were numerically doubleintegrated to obtain separate relative defect intensities for the Ti–Li and Ti–H centres, and from these data the relative total Ti defect concentration was determined. 4.2. Sensitivity change tests of Ti-related ESR centres As will be shown later, ESR equivalent doses were estimated with the regenerative approach using thermal annealing. In order to justify this approach, and to find the proper annealing temperature range, several aliquots of sample MS9 were given a gamma dose after various temperature treatments. The observed intensities plotted in Fig. 3 are as measured, except for the aliquot at room temperature. For the latter, the weak ESR spectrum was subtracted from the spectrum observed following a 300 Gy gamma dose. As can be seen from Fig. 3, sensitivity changes for both Ti–Li and Ti–H centres appear to be weak, if any at all, over the investigated temperature range. Furthermore, any influence due to sensitivity changes will automatically be reduced because only one regeneration cycle (one thermal resetting) is involved here. 4.3. Comparison of ESR and OSL equivalent doses A comparison of OSL and ESR dose curves for sample MS0 is presented in Fig. 4. Data on the OSL dose
(b)
Fig. 2. (a) X-band ESR spectrum from sample MS0 (450 Gy regeneration dose following thermal annealing). The very strong absorption line around 3425 G stems from the Si:P marker. Absorption lines from Ti–H and Ti–Li centres can be seen at higher magnetic field values. The g-values are calibrated against the marker and are consistent with those cited in Okada et al., (1971). However, a systematic deviation of 0.001 can be observed, possibly caused by distortion of the marker signal due to the presence of the Al centre. (b) Demonstration of the three-step procedure followed to determine ESR intensities from Ti-related ESR spectra: (1) comparison between the measured spectrum and the simulated Ti–Li spectrum—the absorption line at g1 is used to fit the spectrum (cf. Fig. 2a); (2) comparison between the resulting difference in spectrum from the previous operation and a simulated T–H spectrum; and (3) resultant spectrum left from the previous subtraction.
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mean of 5 values
Ti-Li
between 200-400 °C
Ti-H Ti
relative ESR intensity
0.6
0.4
0.2
0.0 0
100
200 300 annealing temperature (°C)
400
Fig. 3. Sensitivity change tests after thermal annealing of several aliquots from sample MS9. After a temperature treatment of 2 h (at temperatures between 20 and 400 1C), several aliquots were given a gamma dose of 300 Gy. The resulting ESR intensities are plotted as a function of the temperature. The ESR spectrum of the as-sampled aliquot (natural spectrum) was subtracted from the spectrum obtained after room temperature annealing and artificial dosing. Note the good consistency among the data, indicating that sensitivity changes of the various Ti centres are weak or non-existing over the investigated temperature range. The slightly higher intensities for the aliquot at 200 1C may indicate incomplete thermal resetting.
normalised OSL (Lx/Tx) and ESR (arb. uits) intensity
K. Beerten et al. / Quaternary Geochronology 1 (2006) 151–158
155
OSL MS0-09 OSL MS0-14 ESR Ti-Li ESR Ti-H ESR total Ti exp. growth exp. + linear growth
2
1
0 0
200
400
600
dose (Gy)
Fig. 4. Comparison of normalised (natural intensity ¼ 1) OSL and Ti-related ESR dose curves in quartz aliquots from sample MS0. Whereas some aliquots already show a strong tendency towards OSL signal saturation at doses beyond 200 Gy, the ESR signals tend to rise monotonically up to doses of at least 450 Gy. This behaviour of ESR centres reflects the most important advantage of ESR dating over OSL dating.
10
frequency
MS1
distributions and equivalent doses are given in Table 1 and Figs. 5 and 6. The OSL distributions show a relatively large standard deviation and positive skewness as illustrated in Fig. 5 for sample MS1. Regarding the OSL equivalent doses (cf. Table 1), considerable differences can be observed between the weighted mean, arithmetic mean and the median of the obtained dose distributions, especially for the two oldest samples. It is beyond the scope of the present study to indicate which one would most reliably reflect the true average burial dose of the samples. However, we advocate that the median would be a good compromise to compare the ESR data with (cf. infra). The ESR doses are determined from multiple aliquot experiments, in which case a single value for each sample is obtained instead of a dose distribution. Apparently, the data show a clear trend, the Ti–H equivalent doses being smaller than the Ti–Li doses (Table 1 and Fig. 6). The data based on the total Ti defect concentration are situated in between (Table 1 and Fig. 6). For the youngest samples (MS9, MS3 and MS2), it can be observed that the Ti–H data are in fair agreement with the OSL doses. However, an apparent underestimate seems to result from the Ti–H data relative to the OSL data for samples MS1 and MS0. The opposite is true for the Ti–Li data, showing systematically larger equivalent doses compared to the OSL data. The dose difference between both even seems to increase with sampling depth. The existence of a significant nonzero Ti–Li equivalent dose for the youngest sample (MS9)
5
0 0
100
200
300 400 500 equivalent dose (Gy)
600
700
Fig. 5. Inferred OSL dose distribution of sample MS1. The histogram shows a clear positive skewness, probably related to dose rate inhomogeneities and/or OSL signal saturation of the aliquots towards higher doses.
strongly indicates the presence of an unbleached or unbleachable residual dose in this sample. 5. Discussion 5.1. Reliability of the OSL data The currently addressed equivalent doses are relatively high in terms of OSL dating, thereby decreasing the reliability of the OSL data. Not much is known about the
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1
depth (m)
2
MS9
3
1 2 depth (m)
OSL median ESR Ti-Li ESR Ti-H ESR total Ti
4
MS3 MS2
5
MS3 MS2
5 6
4
MS9
3
0
50 100 150 equivalent dose (Gy)
MS1 6 7 MS0 8 0
200 400 600 equivalent dose (Gy)
800
Fig. 6. Comparison of OSL and ESR equivalent doses. Whereas the OSL and Ti–H ESR data are very consistent for the upper three samples in the profile, this appears not to be the case for the lower two samples. Here, the Ti–H data show a clear underestimate with regard to the OSL data. The Ti–Li-based data show a systematic overestimation relative to the OSL doses. In any case, in all samples, the individual Ti centres seem to bracket the OSL data. Furthermore, the equivalent doses based on the total Ti defect concentration seem to be consistent with the OSL data over the entire profile, except for a slight offset over a few tens of Gy.
validity of the SAR protocol for determining high environmental doses. Nevertheless, in earlier studies, promising results for SAR-OSL doses of up to 300 Gy have been obtained (Banerjee et al., 2003). The uncertainty on the OSL data seems to be reflected in the large differences between the various average equivalent dose estimates, especially for the older samples. Nevertheless, we propose that the median would be a good compromise. Previously, it has been suggested that non-linear growth may produce positively skewed dose distributions, in which case the median would be a better estimate of the equivalent dose (Murray and Funder, 2003). Indeed, the measured doses of the oldest two samples in the profile were situated in the non-linear part of the growth curves, and a strongly positively skewed distribution was observed. Other possible factors influencing the OSL dose distribution could be bioturbation, insufficient bleaching and/or an inhomogeneous radiation field. Field evidence and the nature of the deposits (i.e., aeolian) seem to exclude the first two possibilities. Single-grain OSL experiments are underway to investigate the contribution of an inhomogeneous dose rate. 5.2. Interpretation of the ESR results In general, the present study confirms earlier results on the use of Ti centres in both single- (Beerten and Stesmans, 2006a) and multiple-grain (Yoshida, 1996; Tissoux et al., in press) samples. Clearly, the Ti–H equivalent doses are in fair agreement with the OSL equivalent doses for the upper three samples (Table 1), whereas the Ti–Li equivalent doses
are systematically higher. Accordingly, the difference in this part of the profile could simply be interpreted in terms of the bleaching difference between Ti–H centres (faster bleaching behaviour and thus completely bleached) and Ti–Li centres (slower bleaching behaviour and thus incompletely bleached). This raises the question whether this difference is really due to insufficient bleaching or to the presence of unbleachable residuals. We are currently performing bleaching experiments to elucidate this problem. The preliminary results indicate that, at least for sample MS9, a residual dose of 70% of the natural Ti–Li intensity remains after 120 h bleaching in a SOL-2 solar simulator. In the same context, it is interesting to note that preliminary observations indicate similar results for the (asfound) Al centre and Ti–Li centre equivalent doses for this sample. Towards the lower part of the profile, the Ti–H equivalent doses tend to underestimate the OSL results, whether or not the median is used as reference OSL dose. Perhaps, this may indicate that the Ti–H centre, as defect site, is not stable enough to be used in ESR dating of older samples (i.e., Middle Pleistocene and beyond), as has been suggested by Tissoux et al. (in press). At the same time, the absolute dose difference between the OSL and Ti–Li data seems to increase with depth. This is not true for the relative difference, but we believe that residual doses should be evaluated using the absolute difference. On the one hand, assuming that the residual Ti–Li dose is due to incomplete bleaching, it is difficult to believe that the older samples would be systematically less bleached than the younger ones, given the similar depositional environment. On the other hand, if the difference is due to the presence of unbleachable doses, there is no reason why the older samples would have more unbleachable Ti centres than the younger ones at the time of deposition. In this respect, we note again that the increasing difference between the Ti–Li data and the OSL data appears to be linked with an increasing underestimate based on the Ti–H centre. This may suggest that there is a link between both centres, in which case the separate use of these centres may not be appropriate. In previous works we have already stressed this point, and recent single-grain work seems to indicate that the total Ti defect concentration appears to be a relatively good dosimeter for the true burial dose (Beerten and Stesmans, in press). Do Ti-related ESR centres contain any useful information for sediment dating at all? Apparently, the ESR data always bracket the OSL data. If we now assume that Ti–H centres give a minimum estimate and Ti–Li centres a maximum estimate, it can be seen that there is an apparent hiatus between samples MS1 and MS2 (see Appendix B): the maximum age—based on Ti–Li centres—for the upper sample (MS2) is lower than the minimum age—based on Ti–H centers—of the lower sample (MS1). We then would be able to divide the profile in Late Pleistocene deposits (upper three samples) and Middle Pleistocene deposits (lower two samples). Moreover, the maximum estimate for
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sample MS0 would indicate that the entire profile is younger than the Early Pleistocene. In the present study, the OSL data could be used as some kind of reference for the older deposits. However, in many cases, the saturation behaviour of OSL signals does not permit dating beyond the Late Pleistocene. Ti-related ESR centres could be very useful then, even with all the remaining problems discussed above. But, as indicated earlier, there are reasons to believe that more accurate dating is possible if these problems could be resolved. The solution could lie in the determination of unbleachable residual doses or in the use of the total Ti defect concentration rather than the individual Ti centres. 6. Conclusion A comparison of equivalent doses from ESR active Ti defect centres in quartz and OSL data has revealed several important trends with regard to the feasibility of ESR dating of aeolian quartz. Earlier evidence that the individual Ti–H and Ti–Li sub-centres yield inconsistent results has been confirmed in this study. Nevertheless, the combination of both centres may allow identification of a sedimentation age window, which, in the absence of OSL data, could be used to establish a global geochronological framework. However, it should be clearly stated that— before generalising these results—more consolidating comparisons between ESR and OSL data and independent age control will be needed. Much older deposits should be investigated, and age control other than luminescence should be looked for as well. We believe that these results encourage further research on Ti-related centres as it may open new possibilities for ESR dating of sedimentary quartz.
157
and Weil (1972) for the Ti–Li and Ti–H centres, respectively. The resulting hyperfine splitting matrices are given below: 1:275 ATiLi ðGÞ ¼ 0:049 0:000
0:049 1:116
0:000 0:000
0:000
2:691
4:7413
0:0014
0:9260
ATiH ðGÞ ¼ 0:0007 0:9220
4:3471
0:1405 .
0:1425
9:1815
and
The g-values used to simulate the Ti-related ESR spectra in quartz are taken from Okada et al. (1971). For the Ti–Li centre, these are g1 ¼ 1.979, g2 ¼ 1.931 and g3 ¼ 1.912. For the Ti–H centre, the g-values are g1 ¼ 1.986, g2 ¼ 1.931 and g3 ¼ 1.915. Appendix B. Dose rate data and preliminary ESR ages The dose rates are based on U, Th and K concentrations derived from high-resolution gamma spectroscopy, plus a small cosmic dose (see Lomax et al. (2003) for the methodology). The Ti–Li age for sample MS2 is based on a dose rate of 1.3470.04 Gy/ka, yielding a maximum age of 11778 ka. The Ti–H age for sample MS1 is based on a dose rate of 1.0670.04 Gy/ka, yielding a minimum age of 159725 ka. These data suggest that the Middle to Late Pleistocene transition is situated in between both samples. Finally, the Ti–Li age for sample MS0 is based on a dose rate of 0.9770.04 Gy/ka, yielding a maximum age of 691770 ka for this sample. Consequently, all samples are believed to be younger than the Early Pleistocene.
Acknowledgements
References
We kindly thank R. Schoonheydt (Centrum voor Oppervlaktechemie en katalyse, K.U. Leuven) for access to the ESP300 Bruker X-band spectrometer. Thanks are due to N. Bal and K. Feyen (Sint-Maartenziekenhuis, Duffel) for help with artificial irradiation.
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Appendix A. Simulation parameters The hyperfine splitting parameters needed for simulating the ESR spectra of the Ti–Li and Ti–H centres were determined as follows. Due to the symmetry of the defect system, the principal axes of the g-matrix and hyperfine matrix A do not coincide. Therefore they cannot be simultaneously diagonalised. The choice to describe the system in the principal axis coordinate system of the g-matrix thus results in three principal g-values and a symmetric non-diagonal A matrix. The principal values of the two parameter matrices and the polar and azimuthal angles that link each principal axis to the crystal coordinate system were taken from Isoya et al. (1988) and Rinneberg
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