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The use of Ti centers for estimating burial doses of single quartz grains: A case study from an aeolian deposit ∼2 Ma old
Radiation Measurements 41 (2006) 418 – 424 www.elsevier.com/locate/radmeas
The use of Ti centers for estimating burial doses of single quartz grains: A case study from an aeolian deposit ∼2 Ma old Koen Beertena,∗,1 , Andre Stesmansb a Laboratorium voor Stratigrafie, Afdeling Historische Geologie, Departement Geografie-Geologie, Katholieke Universiteit Leuven,
Redingenstraat 16, 3000 Leuven, Belgium b Afdeling Halfgeleiderfysica, Departement Natuurkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
Received 17 June 2005; received in revised form 6 October 2005; accepted 12 October 2005
Abstract Several single quartz grains from an aeolian deposit (Beerse Member, N. Belgium, ∼ 2 Ma old) are investigated using Q-band electron spin resonance (ESR) spectroscopy. Palaeodose estimates based on Ti centers are presented both for grains containing a mixture of Ti–Li and Ti–H centers (group 1), and grains containing only the Li compensated type of Ti centers (group 2). The results are obtained using the regenerative dose method after thermal zeroing. The separate Ti–Li and Ti–H palaeodoses from group 1 show severe over- and underestimates of the expected burial dose, respectively. In contrast, palaeodoses based on the total Ti defect concentration are in fair agreement with the expected dose, as are the Ti–Li palaeodoses from grains of group 2. As to the origin of this, several possible mechanisms are proposed, but none appears adequate to provide a satisfactory explanation. Nevertheless, this study shows that reasonable palaeodose estimates can be obtained if they are based on the use of either all Ti absorption lines or only the Ti–Li ones, depending on which centers are present in the quartz grains. These results may provide an important step forward in the development of a solid ESR dating protocol for sedimentary deposits up to 2 Ma old. © 2005 Elsevier Ltd. All rights reserved.
1. Introduction Ti-related impurity defects in natural quartz are known to be useful dosimeters for estimating accumulated doses over geological timescales (review in Rink, 1997). In general, three different Ti centers exist, labelled Ti–Li, Ti–Na or Ti–H, according to the charge compensating cation (Okada et al., 1971). Laboratory experiments have shown that they are completely bleachable by sunlight and/or artificial light, underlining their potential for dating sedimentary deposits ∗ Corresponding author. Tel.: +49 221 470 2547; +49 221 470 4917. E-mail address: [email protected] (K. Beerten). 1 Present address: Geographisches Institut der Universität zu Köln, Albertus-Magnus-Platz, 50923 Köln, Germany.
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(Brumby and Yoshida, 1994; Yoshida, 1996; Toyoda et al., 2000). Complete resetting may take days to weeks and there appears to be no unbleachable residual (as opposed to the Al center; Voinchet et al., 2003). However, the rare attempts reported so far show problems related to complete solar resetting of the dating signal before burial in some real sedimentary environments. Age overestimates using the Ti–Li center have been reported by Yoshida (1996) and Tanaka et al. (1997) for wind-blown cave deposits and marine terrace deposits. Better results have been obtained by Yoshida (1996) using so-called ‘new light-sensitive centers’ in quartz—presumably Ti–H centers. Bleaching experiments indeed indicate that these centers are more easily bleached than Ti–Li centers (Yoshida, 1996; Toyoda et al., 2000; Beerten, 2005). Recently, Q-band ESR analysis of quartz sediments at the single grain level revealed excellent
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bleaching of Ti–H and Ti–Li centers in most grains of a modern semi-arid desert surface deposit (Beerten and Stesmans, 2005a). In contrast, recent fluvial deposits appear to contain a mixture of grains with large residual doses (Beerten and Stesmans, 2005b) and sufficiently bleached grains (Beerten et al., 2003). However, this could only be confirmed for Ti–Li centers because Ti–H defects were lacking in these deposits. Besides bleaching, another problem is of concern, namely, the stability of activated Ti centers over geological timescales. Up to the present, little is known about the mean lifetime of Ti centers. Toyoda and Ikeya (1991) and Miallier et al. (1994) estimated the mean lifetime of the Ti–Li center to be ca. 10 and 8 Ma, respectively, at average burial temperatures. Grün et al. (1999) determined a mean lifetime of 1 Ma at elevated temperatures (50 ◦ C). Further evidence suggests that the Al center in quartz may be totally reset (to zero ESR activity) at high ambient temperatures in geothermal areas (Shimokawa and Imai, 1987), which may be the case for Ti centers as well. Actual ESR dating results of volcanic rocks and heated quartz sediments based on Ti centers show a tendency of underestimation for ages above 60–100 ka (review in Rink, 1997). The latter author argues that this would be the upper limit for ESR dating of xenolithic quartz due to the inferred limited thermal stability. However, it has been pointed out that the thermal stability of ESR centers may be very much sample dependent (Grün et al., 1999; Woda et al., 2001). In any case, mean lifetimes appear to be lower in geothermally active areas but may be significantly larger at average burial temperatures. Usually, multiple grains are used in ESR dating studies of sediments, allowing powder spectra to be recorded. In contrast, in the single grain approach, single crystal spectra are acquired. Due to the anisotropic nature of the g tensor of Ti-related impurity defects in quartz, the position of absorption lines in such spectra is dependent on the relative angle between the crystallographic axes of the grain and the direction of the spectrometer’s applied magnetic field. ESR spectra will generally differ for the same grain after removal and repositioning in the cavity because any control on the position of the crystallographic axes is lacking. Nevertheless, even in very complex single grain spectra it is possible to identify different Ti related species (Beerten et al., 2003; Beerten and Stesmans, 2005a), enabling one, in principle, to carry out independent dating experiments based on each occurring Ti center separately. Evidently, an important drawback of the single grain approach is the difficulty to perform intensity measurements on individual Ti–H and Ti–Li centers due to signal overlap. Only if grains show very sharp absorption lines and measurement time is not restricted can both centers be used to calculate accumulated doses for Ti–Li and Ti–H centers separately. Such grains have been encountered in a particular Plio–Pleistocene deposit from northern Belgium and will therefore be the subject of this paper.
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The aim of the present study is twofold. First, it intends to explore the potential of extending the age range of ESR dating using different Ti centers (Ti–Li and Ti–H) in quartz. For this purpose, grains from a near 2 Ma old aeolian deposit were investigated. Such an old deposit is an excellent test case because of the high doses involved, and because the age may be near the upper limit in terms of thermal stability of ESR active centers. In this respect, we mention that younger samples have also been investigated with promising results. This, however, will be the subject of another paper (Beerten and Stesmans, 2005b). Second, the present case study is a further test for the single grain approach in ESR dating of sediments. The importance of this method has been outlined before (Beerten et al., 2003), especially if poor bleaching may be concerned. It is generally thought that analysing single grains may be a useful tool in the recognition of poor bleaching, as has been shown in optically stimulated luminescence (OSL) dating (Olley et al., 2004; Duller, 2004). The results of this study will allow further development towards a general methodology for ESR dating of (aeolian) sediments. Furthermore, the results show that a solid ESR protocol certainly merits its place among the conceivable methods for estimating ages of Quaternary deposits.
2. Materials and methods Several single grains were extracted from the quartz-rich Plio–Pleistocene Beerse Member (Dricot, 1961). For a general review of the litho- and chronostratigraphy of this member we refer to Gullentops et al. (2001) and Kasse and Bohncke (2001). The Beerse Member is generally regarded as the first cold climate deposit in northern Belgium and it is correlated with part of the Tiglian pollen stage on palaeobotanical grounds. The normal palaeomagnetic polarity of the overlying clayey Turnhout Member is attributed to the Olduvai subchron. Hence, a tentative age of 1.8–2 Ma is inferred for the Beerse Member. At the sampling location, a clear succession of the Beerse Member (ca. 2 m thick), the Turnhout Member (ca. 1.5 m) and a thin coversand unit (ca. 1 m) could be observed from bottom to top. The ESR sample was taken from a ca. 5 cm thick coarse sand layer within the otherwise fine and very homogeneous aeolian sand deposits of the Beerse Member. It consists of ca. 100% pure quartz and is interpreted as a fluvially reworked layer because of the slightly increased grain size in these otherwise fine sands. The depth of the sample is ca. 3.1 m below the surface. The exact location for sampling was chosen to enable retrieval of sufficiently large grains and to provide a homogeneous gamma radiation field. Sample preparation was carried out in a dark room with dimmed red light. After washing and treatment with HCl the samples were dried and mechanically sieved. The coarse fraction (0.7–1 mm) was retained for further sample processing. Transparent quartz grains were picked under the microscope using strongly subdued light from a conventional
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light source. The exposure time was in the range of 10 s to 60 s (room temperature). A final selection of a few tens of grains was treated with concentrated HF (49% by volume) for 30 min in order to remove surface area defects induced by external alpha radiation. Finally, the grains were washed in purified water and dried in an oven (T ∼ 30 ◦ C). ESR measurements were done with an EMX Bruker Qband ESR spectrometer at a frequency of ca. 34 GHz, with an incident microwave power of ca. 1.6 mW. The sweep width was selected as 700 G (center field at 12450 G). The field modulation amplitude (100 kHz) was set in the range 0.5–1 G. Scan times were 42 s. Grains were measured at 3–4 different field angles (B ) averaging over 10–30 scans for each angle, depending on spectrometer stability and temperature stability. Average intensities were calculated using the mean intensity of several measurements, and error bars indicate standard deviation of the mean. The relative concentration of defects in each grain was calculated by double integration of absorption-derivative peaks relative to the co-mounted Si:P marker. The overall density of a particular defect (ESR intensity) as determined by integration of the total spectrum should be the same at any angle. Details on the interpretation of single quartz grain ESR spectra are given in Beerten et al. (2003). Different Ti centers can be identified on the basis of their g value range, specific hyperfine properties and the number of absorption lines. A total number of nine grains was used for the current experiment. Five grains were used for De determination (arbitrarily labelled grains 1–5), two grains for exploratory measurements (test grains A and B) and two more grains for stability tests (test grains C and D). Equivalent doses were calculated using the regenerative dose method. Using the same method, first and second regenerative dose curves were constructed after heating the grains up to 300 ◦ C for 2 h. This treatment had been checked
to result in complete zeroing. Individual grains were irradiated using a 60 Co gamma-ray source with dose rates of ∼ 2 Gy/min. The quartz grains were put in an aluminium sample holder designed with small holes (∼ 1 mm deep and ∼ 5 mm spacing) which is overlain by a 2 mm thick aluminium plate to allow for build up of secondary electron equilibrium. The resulting equivalent doses are compared with the expected dose range for this particular sample. The range is calculated from the inferred age of the deposit (1.8–2 Ma) and a preliminary dose rate. The latter is derived from radionuclide concentrations based on ICP–MS analyses of the bulk sample (gamma rays) and smaller subsamples (beta and alpha rays) taken from the 5 cm thick layer from which the ESRdated grains were extracted. Due to the extremely low radionuclide concentration, the internal alpha and cosmic dose rates turned out to be the major contributors. The k-value for alpha rays was not determined directly. Nevertheless, two different values were used (0.05 and 0.1) in order to encompass uncertainties on the estimated k-value of 0.07 which is cited in Buhay et al. (1992) and subsequently used in this study. Furthermore, two different burial scenarios are taken into account with and without excess overburden (10 m) respectively. These considerations regarding the alpha and cosmic dose rate result in two extreme dose rate scenarios (0.21 and 0.29 Gy/ka) and the expected dose range of 380–580 Gy.
3. Results 3.1. Single grain ESR spectra of Ti centers In general, two types of different single quartz grain ESR spectra have been encountered in grains from the Beerse deposit. Some grains show the Ti–Li center to be the only Tirelated impurity defect (Fig. 1a). In other grains a mixture of
g value range Ti-Li
g value range Ti-Li
g value range Ti-H
12200 12400 12600 magnetic field (gauss) (a)
12800
12200 12400 12600 magnetic field (gauss) (b)
12800
Fig. 1. Single quartz grain Q-band ESR spectra from sample Beerse. (a) Test grain A: the three single absorption lines observed indicate the unique presence of the Ti–Li center in this grain; the large narrow absorption line to the left originates from the enclosed Si:P marker. (b) Test grain B: the largest absorption line around 12150 G originates from the marker and separates absorption lines from Al centers (left) and Ti centers (right); the g value range is slightly different for Ti–Li and Ti–H centers (as indicated by the horizontal line) and can thus be used as a criterion to identify both centers; six doublet absorption lines (indicated by arrows) show the presence of Ti–H centers whereas the three single absorption lines originate from Ti–Li centers.
K. Beerten, A. Stesmans / Radiation Measurements 41 (2006) 418 – 424 Ti-Li, grain 1 regenerative dose points
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relative ESR intensity
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total Ti defect concentration, grain 1 regenerative dose points
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1.0 relative ESR intensity
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relative ESR intensity
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Ti-Li, grain 1 additive regenerative
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Ti-H, grain 1 additive regenerative
1.5 relative ESR intensity
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total Ti defect concentration, grain 1 additive regenerative
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Ti-H, grain 1 regenerative dose points
relative ESR intensity
relative ESR intensity
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200 400 600 800 1000 (f) dose (Gy)
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Fig. 2. Regenerative dose curves inferred from different Ti centers in grain 1. (a–c) The data are fitted with a linear function and palaeodoses are estimated by interpolation of the natural intensity (horizontal lines); note the difference in vertical scale over the various diagrams. (d–f) Comparison of additive and regenerative dose method; within error limits, both data sets appear to follow similar trends; it should be stressed that these data alone do not provide conclusive evidence for a linear trend at higher and/or lower doses, except perhaps for data of the Ti–H center (at lower doses).
Ti–Li and Ti–H centers can be observed (Fig. 1b). Due to the very sharp absorption lines, detailed tuning of the different spectroscopic measurement parameters and careful rotation of the grain in the cavity, the separate absorption lines of Ti–Li and Ti–H centers could be well separated and hence integrated (double integral), thus enabling one to calculate (relative) separate defect intensities. For clarity, it should be noted that the Ti–Na center could not be observed in any of the investigated grains.
3.2. Palaeodoses based on different Ti centers in single grains containing Ti–Li and Ti–H centers Two grains were used to calculate individual palaeodoses based on Ti–Li and Ti–H centers. The regenerative dose curves of one such grain are shown in Figs. 2a–c. The dose points show an apparent linear trend and were fitted with a simple linear function. Fig. 3 shows the palaeodose results. Individual estimates for the Ti–Li and Ti–H centers are,
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Ti-Li Ti-H Ti (Ti-Li + Ti-H) Ti-Li (1st regen) Ti-Li (2nd regen)
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palaeodose (Gy)
1000 800 600 400 200
12 10 relative ESR intensity
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Ti-Li, grain 3 1st regeneration 2nd regeneration
8 6 4 2 0
0 1
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3 grain no.
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Fig. 3. Palaeodose estimates based on different Ti components in single grains from sample Beerse, plotted against the expected dose (hatched area). Data for the Ti–Li center in grains 1 and 2 are far above the expected dose whereas the opposite can be observed for the Ti–H center. In contrast, data inferred from the total Ti defect concentration are in agreement with the estimated dose. In grains where the Li compensated one is the only Ti related center (grains 3–5) the Ti–Li based palaeodoses are more or less within the expected dose range. The estimated dose range is based on the age of the sample (1.8–2 Ma) and preliminary dose rate calculations (see text).
respectively, much higher and lower than the expected dose range. Palaeodoses based on the total Ti defect concentration (i.e. dose points calculated using all absorption lines from Ti centers) seem to fall within the expected range. A few doses were added to the natural intensity prior to thermal annealing in order to check for severe sensitivity changes (Figs. 2d–f). In spite of the limited number of dose points used, it can be seen that in general the dose curves follow similar trends. However, it should be stressed that detailed information on sensitivity change cannot be deduced from such an experiment. 3.3. Palaeodoses based on Ti centers in single grains containing only Ti–Li centers Regenerative dose curves of a grain containing only Li compensated Ti centers are shown in Fig. 4. Apparently, dose points from the 1st and 2nd regeneration cycles coincide closely, suggesting limited or even non-existing sensitivity change in between both cycles. The resulting palaeodoses are plotted in Fig. 3. Within error limits, the data are in fair agreement with the expected dose.
0
200 400 600 regenerative dose (Gy)
800
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Fig. 4. Comparison of two consecutive regenerative dose curves for the Ti–Li center observed in grain 3. Within error limits, both data sets appear to follow similar trends, indicating that sensitivity change is minimal.
estimates, respectively) if both centers are present simultaneously (in ESR detectable amount) in a single quartz grain. It appears that Ti–Li centers lead to strong overestimates in the presence of Ti–H centers, whereas grains containing only the Ti–Li center produce relatively good results. On top of this, results for the total Ti defect concentration are in fair agreement with the expected dose, irrespective of the results of the individual Li and H compensated components. In search of an adequate model, we consider various possible origins for this remarkable conduct. 4.1. Sensitivity change Notwithstanding the limited number of data points, it can be argued that sensitivity changes, if any at all, cannot account for the observed difference in results based on Ti–Li and Ti–H centers. Extended additive dose curves could have resolved this problem, but in general the additive and regenerative dose behavior of the different Ti centers appears to be similar. We have inferred an apparent linear trend from the few added dose points. However, it is noted that linear fitting of additive dose points may be very risky (Grün, 1996), especially if high doses are to be determined. In any case, our observations on sensitivity change are in general agreement with earlier work in which enhanced or reduced sensitivity of Ti centers was found only after annealing above 400 ◦ C (Imai et al., 1992; Poolton et al., 2000). 4.2. Differences in light-sensitivity
4. Discussion Clearly, the main challenge is to explain why palaeodose inferences based on Ti–Li centers and Ti–H centers show strong deviations from the expected dose (over- and under-
Sample handling. As the Ti–H center is slightly more sensitive to optical bleaching than the Ti–Li center, over-exposure to light during sample handling might have caused erroneously low palaeodose estimates based on the
K. Beerten, A. Stesmans / Radiation Measurements 41 (2006) 418 – 424
Ti-Li Ti-H normailzed ESR intensity
Ti–H center. However, results from a bleaching experiment (Beerten, 2005) show that no significant difference in relative intensity of both centers could be found after ca. 10 min of direct exposure to the light source used for grain selection. These results indicate that sample handling is not responsible for the encountered difference in inferred palaeodoses. Incomplete bleaching of Ti–Li centers. For the same reason (lower light-sensitivity of Ti–H centers than Ti–Li centers), over-estimates for the Ti–Li center may have resulted from incomplete bleaching prior to deposition. Remarkable, however, is that only if absorption lines from Ti–H and Ti–Li centers are co-observed do the latter centers lead to palaeodose overestimates. Assuming that this does not concern just coincidence, it suggests that the mere presence of Ti–H may induce changes in the bleaching behavior of Ti–Li centers, i.e. making them more light-insensitive. We did not perform any bleaching experiments to check this hypothesis, but from previous studies we know that Ti–Li centers from presentday semi-arid aeolian environments appear to be completely bleached in the presence of Ti–H centers (Beerten and Stesmans, 2005a).
423
100
50
0 0
100
200 temperature (°C)
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Fig. 5. Isochronal anneal (15 min) plot for Ti centers in a particular quartz grain from sample Beerse (test grain C). The data points are normalized to the natural as-sampled intensity. Individual data points exhibit a scattered appearance but nevertheless suggest that the relative stabilities of Ti–Li and Ti–H centers are similar, at least in this grain.
4.3. Thermal stability Another possibility for the age shortfall inferred from Ti–H centers may concern a limited thermal stability and hence short mean lifetimes. The relative stabilities of Ti–H and Ti–Li centers with regard to temperature increase were investigated with an isochronal annealing experiment. One particular grain was stepwise heated for 15 min to temperatures ranging from room temperature to 325 ◦ C. The results of this experiment are shown in Fig. 5. It is suggested that both centers appear to have similar stabilities with regard to temperature increase. This suggests that their mean lifetimes would be more or less equal at burial temperature and hence this explanation appears unlikely. 4.4. Fading of Ti–H centers Fading of Ti–H centers may also be considered as responsible for the observed palaeodose underestimates based on this center, similar to fading of luminescence from feldspars (Wintle, 1973). This was checked for another grain (test grain D), the results indicating that nine months after irradiation neither the total Ti defect concentration nor the relative proportion of Ti–Li and Ti–H centers had changed, even when the grain was stored at 100 ◦ C for 2 days (Beerten, 2005).
center populations do not change. It suggests that such a transfer does simply not occur, or, in another view, that the time span in between measurements was too short in comparison with the rate of this process in nature, even at slightly elevated temperature. 4.6. Difference in production efficiency of Ti–Li and Ti–H centers One could also speculate that different types of radiation (alpha, beta, gamma and cosmic rays) affect dissimilar defects with different efficiency. In theory, this is corrected for in the case of alpha rays (k-value). However, the k-value used here is only a rough estimate, and it may even be different for the various types of defects. 4.7. Dose rate effect Up to the present, we have no information regarding any influence of the dose rate on the creation of Ti–H and Ti–Li centers. If feasible, any explanation in this direction should take into account the favourable creation of Ti–H centers relative to Ti–Li centers due to high laboratory gamma dose rates. 5. Conclusion
4.5. Transfer of centers As another hypothesis, one may notice that any transfer of Ti–H into Ti–Li centers during burial may explain the data. If this would be the case, such transfer may occur after artificial irradiation as well. However, as indicated in (4.4), our preliminary storage tests indicate that the respective Ti
Titanium-related centers in single quartz grains from a 2 Ma old aeolian deposit were investigated using Q-band ESR spectroscopy. Exceptionally, we were able to perform intensity measurements on Ti–Li and Ti–H centers individually in several single grains. Palaeodoses based on these individual Ti centers show a strong offset from the expected
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dose: severe over- and underestimates were found for results based on the Ti–Li and Ti–H centers, respectively, using the regeneration method. Remarkably, if the total Ti defect concentration is used for dose calculation, the individual palaeodose results appear to be consistent with each other, regardless of the relative proportion of Ti–Li and Ti–H centers, and are in fair agreement with the expected dose. Several possible explanations for this unusual behavior have been critically evaluated, but none of these seems to be fully satisfactory. Notwithstanding the fact that some experimental problems remain, we think that ESR dating of single aeolian quartz grains using Ti centers may be a valuable tool for determining the last exposure to sunlight if large natural doses are to be recovered. But clearly, more data points are needed to increase the reliability of the method. This, however, may emerge as a limitation of single grain ESR dating because of the extremely large measurement times. Acknowledgements Thanks are extended to Natalie Bal (Sint-Maartenziekenhuis, Duffel) and Hans Ooms (SCK, Mol) for their help with artificial irradiation. Preliminary ICP-MS analyses were performed by Jan Hertogen (Fysico-chemische Geologie, KULeuven). Frans Gullentops (Historische Geologie, KULeuven) guided us to the sampling location. This work is financially supported by the IWT, Flanders, Belgium. References Beerten, K., 2005. The use of electron spin resonance for estimating sedimentation ages of single quartz grains. Ph.D. Thesis, Katholieke Universiteit Leuven. Belgium. Beerten, K., Pierreux, D., Stesmans, A., 2003. Towards single grain ESR dating of sedimentary quartz: first results. Quaternary Sci. Rev. 22, 1329–1334. Beerten, K., Stesmans, A., 2005a. Single quartz grain ESR dating of a contemporary desert surface deposit, Eastern Desert, Egypt. Quaternary Sci. Rev. 24, 223–231. Beerten, K., Stesmans, A., 2005b. Electron spin resonance (ESR) dating of sedimentary quartz: possibilities and limitations of the single grain approach. Book of Abstracts of the 11th International Conference on Luminescence and Electron Spin Resonance Dating, Cologne, Germany, p. 18. Brumby, S., Yoshida, H., 1994. An investigation of the effect of sunlight on the ESR spectra of quartz centers: implications for dating. Quaternary Sci. Rev. 13, 615–618. Buhay, W.M., Clifford, P.M., Schwarcz, H.P., 1992. ESR dating of the Rotoiti Breccia in the Taupo volcanic zone, New Zealand. Quaternary Sci. Rev. 11, 267–271. Dricot, E.M., 1961. Microstratigraphie des Argiles de Campine. Bull. Soc. Belge Géol. 70, 113–141. Duller, G.A.T., 2004. Luminescence dating of Quaternary sediments: recent advances. J. Quaternary Sci. 19, 183–192. Grün, R., 1996. Errors in dose assessment introduced by the use of the “linear part” of a saturating dose response curve. Radiat. Meas. 26, 297–302.
Grün, R., Tani, A., Gurbanov, A., Koschung, D., Williams, I., Braun, J., 1999. A new method for the estimation of cooling and denudation rates using paramagnetic centers in quartz: A case study on the Eldzhurtinskiy Granite, Caucasus. J. Geophys. Res. 104, 17531–17549. Gullentops, F., Bogemans, F., De Moor, G., Paulissen, E., Pissart A., 2001. Quaternary lithostratigraphic units (Belgium). In: Bultynck, Dejonghe (Eds.), Guide to a Revised Lithostratigraphic Scale of Belgium (Geologica Belgica 4, 153–164). Imai, N., Shimokawa, K., Sakaguchi, K., Takada, M., 1992. ESR dates and thermal behaviour of Al and Ti centers in quartz for the tephra and welded tuff in Japan. Quaternary Sci. Rev. 11, 257–265. Kasse, C., Bohncke, S., 2001. Early Pleistocene fluvial and estuarine records of climate change in southern Netherlands and northern Belgium. In: Maddy, D., Macklin, M.G., Woodward, J.C. (Eds.), River Basin Sediment Systems: Archives of Environmental Change. Balkema, Rotterdam, pp. 171–194. Miallier, D., Sanzelle, S., Falguères, C., Faı¨n, J., Montret, M., Pilleyre, Th., Soumana, S., Laurent, M., Camus, G., De Goër de Hervé, A., 1994. Intercomparisons of red TL and ESR signals from heated quartz grains. Radiat. Meas. 23, 143–153. Okada, M., Rinneberg, J., Weil, J.A., Wright, P.M., 1971. EPR of Ti3+ centers in alpha quartz. Chem. Phys. Lett. 11, 275–276. Olley, J.M., Pietsch, T., Roberts, R.G., 2004. Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Geomorphology 60, 337–358. Poolton, N.R.J., Smith, G.M., Riedi, P.C., Bulur, E., BZtter-Jensen, L., Murray, A.S., Adrian, M., 2000. Luminescence sensitivity changes in natural quartz induced by high temperature annealing: a high frequency EPR and OSL study. J. Phys. D Appl. Phys. 33, 1007–1017. Rink, W.J., 1997. Electron spin resonance (ESR) dating and ESR applications in Quaternary science and archaeometry. Radiat. Meas. 27, 975–1025. Shimokawa, K., Imai, N., 1987. Simultaneous determination of alteration and eruption ages of volcanic rocks by electron spin resonance. Geochim. Cosmochim. Ac. 51, 115–119. 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 Sci. Rev. 16, 257–264. Toyoda, S., Ikeya, M., 1991. Thermal stabilities of paramagnetic defects and impurity centers in quartz: basis for ESR dating of thermal history. Geochem. J. 25, 437–445. Toyoda, S., Voinchet, P., Falguères, C., Dolo, J.M., Laurent, M., 2000. Bleaching of ESR signals by the sunlight: a laboratory experiment for establishing the ESR dating of sediments. Appl. Radiat. Isotop. 52, 1357–1362. Voinchet, P., Falguères, C., Laurent, M., Toyoda, S., Bahain, J.J., Dolo, J.M., 2003. Artificial optical bleaching of the aluminium center in quartz implications to ESR dating of sediments. Quaternary Sci. Rev. 22, 1335–1338. Wintle, A.G., 1973. Anomalous fading of thermoluminescence in mineral samples. Nature 245, 143–144. Woda, C., Mangini, A., Wagner, G.A., 2001. ESR dating of xenolithic quartz in volcanic rocks. Quaternary Sci. Rev. 20, 993–998. 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.
The use of Ti centers for estimating burial doses of single quartz grains: A case study from an aeolian deposit ∼2 Ma old Koen Beertena,∗,1 , Andre Stesmansb a Laboratorium voor Stratigrafie, Afdeling Historische Geologie, Departement Geografie-Geologie, Katholieke Universiteit Leuven,
Redingenstraat 16, 3000 Leuven, Belgium b Afdeling Halfgeleiderfysica, Departement Natuurkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
Received 17 June 2005; received in revised form 6 October 2005; accepted 12 October 2005
Abstract Several single quartz grains from an aeolian deposit (Beerse Member, N. Belgium, ∼ 2 Ma old) are investigated using Q-band electron spin resonance (ESR) spectroscopy. Palaeodose estimates based on Ti centers are presented both for grains containing a mixture of Ti–Li and Ti–H centers (group 1), and grains containing only the Li compensated type of Ti centers (group 2). The results are obtained using the regenerative dose method after thermal zeroing. The separate Ti–Li and Ti–H palaeodoses from group 1 show severe over- and underestimates of the expected burial dose, respectively. In contrast, palaeodoses based on the total Ti defect concentration are in fair agreement with the expected dose, as are the Ti–Li palaeodoses from grains of group 2. As to the origin of this, several possible mechanisms are proposed, but none appears adequate to provide a satisfactory explanation. Nevertheless, this study shows that reasonable palaeodose estimates can be obtained if they are based on the use of either all Ti absorption lines or only the Ti–Li ones, depending on which centers are present in the quartz grains. These results may provide an important step forward in the development of a solid ESR dating protocol for sedimentary deposits up to 2 Ma old. © 2005 Elsevier Ltd. All rights reserved.
1. Introduction Ti-related impurity defects in natural quartz are known to be useful dosimeters for estimating accumulated doses over geological timescales (review in Rink, 1997). In general, three different Ti centers exist, labelled Ti–Li, Ti–Na or Ti–H, according to the charge compensating cation (Okada et al., 1971). Laboratory experiments have shown that they are completely bleachable by sunlight and/or artificial light, underlining their potential for dating sedimentary deposits ∗ Corresponding author. Tel.: +49 221 470 2547; +49 221 470 4917. E-mail address: [email protected] (K. Beerten). 1 Present address: Geographisches Institut der Universität zu Köln, Albertus-Magnus-Platz, 50923 Köln, Germany.
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.10.004
(Brumby and Yoshida, 1994; Yoshida, 1996; Toyoda et al., 2000). Complete resetting may take days to weeks and there appears to be no unbleachable residual (as opposed to the Al center; Voinchet et al., 2003). However, the rare attempts reported so far show problems related to complete solar resetting of the dating signal before burial in some real sedimentary environments. Age overestimates using the Ti–Li center have been reported by Yoshida (1996) and Tanaka et al. (1997) for wind-blown cave deposits and marine terrace deposits. Better results have been obtained by Yoshida (1996) using so-called ‘new light-sensitive centers’ in quartz—presumably Ti–H centers. Bleaching experiments indeed indicate that these centers are more easily bleached than Ti–Li centers (Yoshida, 1996; Toyoda et al., 2000; Beerten, 2005). Recently, Q-band ESR analysis of quartz sediments at the single grain level revealed excellent
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bleaching of Ti–H and Ti–Li centers in most grains of a modern semi-arid desert surface deposit (Beerten and Stesmans, 2005a). In contrast, recent fluvial deposits appear to contain a mixture of grains with large residual doses (Beerten and Stesmans, 2005b) and sufficiently bleached grains (Beerten et al., 2003). However, this could only be confirmed for Ti–Li centers because Ti–H defects were lacking in these deposits. Besides bleaching, another problem is of concern, namely, the stability of activated Ti centers over geological timescales. Up to the present, little is known about the mean lifetime of Ti centers. Toyoda and Ikeya (1991) and Miallier et al. (1994) estimated the mean lifetime of the Ti–Li center to be ca. 10 and 8 Ma, respectively, at average burial temperatures. Grün et al. (1999) determined a mean lifetime of 1 Ma at elevated temperatures (50 ◦ C). Further evidence suggests that the Al center in quartz may be totally reset (to zero ESR activity) at high ambient temperatures in geothermal areas (Shimokawa and Imai, 1987), which may be the case for Ti centers as well. Actual ESR dating results of volcanic rocks and heated quartz sediments based on Ti centers show a tendency of underestimation for ages above 60–100 ka (review in Rink, 1997). The latter author argues that this would be the upper limit for ESR dating of xenolithic quartz due to the inferred limited thermal stability. However, it has been pointed out that the thermal stability of ESR centers may be very much sample dependent (Grün et al., 1999; Woda et al., 2001). In any case, mean lifetimes appear to be lower in geothermally active areas but may be significantly larger at average burial temperatures. Usually, multiple grains are used in ESR dating studies of sediments, allowing powder spectra to be recorded. In contrast, in the single grain approach, single crystal spectra are acquired. Due to the anisotropic nature of the g tensor of Ti-related impurity defects in quartz, the position of absorption lines in such spectra is dependent on the relative angle between the crystallographic axes of the grain and the direction of the spectrometer’s applied magnetic field. ESR spectra will generally differ for the same grain after removal and repositioning in the cavity because any control on the position of the crystallographic axes is lacking. Nevertheless, even in very complex single grain spectra it is possible to identify different Ti related species (Beerten et al., 2003; Beerten and Stesmans, 2005a), enabling one, in principle, to carry out independent dating experiments based on each occurring Ti center separately. Evidently, an important drawback of the single grain approach is the difficulty to perform intensity measurements on individual Ti–H and Ti–Li centers due to signal overlap. Only if grains show very sharp absorption lines and measurement time is not restricted can both centers be used to calculate accumulated doses for Ti–Li and Ti–H centers separately. Such grains have been encountered in a particular Plio–Pleistocene deposit from northern Belgium and will therefore be the subject of this paper.
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The aim of the present study is twofold. First, it intends to explore the potential of extending the age range of ESR dating using different Ti centers (Ti–Li and Ti–H) in quartz. For this purpose, grains from a near 2 Ma old aeolian deposit were investigated. Such an old deposit is an excellent test case because of the high doses involved, and because the age may be near the upper limit in terms of thermal stability of ESR active centers. In this respect, we mention that younger samples have also been investigated with promising results. This, however, will be the subject of another paper (Beerten and Stesmans, 2005b). Second, the present case study is a further test for the single grain approach in ESR dating of sediments. The importance of this method has been outlined before (Beerten et al., 2003), especially if poor bleaching may be concerned. It is generally thought that analysing single grains may be a useful tool in the recognition of poor bleaching, as has been shown in optically stimulated luminescence (OSL) dating (Olley et al., 2004; Duller, 2004). The results of this study will allow further development towards a general methodology for ESR dating of (aeolian) sediments. Furthermore, the results show that a solid ESR protocol certainly merits its place among the conceivable methods for estimating ages of Quaternary deposits.
2. Materials and methods Several single grains were extracted from the quartz-rich Plio–Pleistocene Beerse Member (Dricot, 1961). For a general review of the litho- and chronostratigraphy of this member we refer to Gullentops et al. (2001) and Kasse and Bohncke (2001). The Beerse Member is generally regarded as the first cold climate deposit in northern Belgium and it is correlated with part of the Tiglian pollen stage on palaeobotanical grounds. The normal palaeomagnetic polarity of the overlying clayey Turnhout Member is attributed to the Olduvai subchron. Hence, a tentative age of 1.8–2 Ma is inferred for the Beerse Member. At the sampling location, a clear succession of the Beerse Member (ca. 2 m thick), the Turnhout Member (ca. 1.5 m) and a thin coversand unit (ca. 1 m) could be observed from bottom to top. The ESR sample was taken from a ca. 5 cm thick coarse sand layer within the otherwise fine and very homogeneous aeolian sand deposits of the Beerse Member. It consists of ca. 100% pure quartz and is interpreted as a fluvially reworked layer because of the slightly increased grain size in these otherwise fine sands. The depth of the sample is ca. 3.1 m below the surface. The exact location for sampling was chosen to enable retrieval of sufficiently large grains and to provide a homogeneous gamma radiation field. Sample preparation was carried out in a dark room with dimmed red light. After washing and treatment with HCl the samples were dried and mechanically sieved. The coarse fraction (0.7–1 mm) was retained for further sample processing. Transparent quartz grains were picked under the microscope using strongly subdued light from a conventional
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light source. The exposure time was in the range of 10 s to 60 s (room temperature). A final selection of a few tens of grains was treated with concentrated HF (49% by volume) for 30 min in order to remove surface area defects induced by external alpha radiation. Finally, the grains were washed in purified water and dried in an oven (T ∼ 30 ◦ C). ESR measurements were done with an EMX Bruker Qband ESR spectrometer at a frequency of ca. 34 GHz, with an incident microwave power of ca. 1.6 mW. The sweep width was selected as 700 G (center field at 12450 G). The field modulation amplitude (100 kHz) was set in the range 0.5–1 G. Scan times were 42 s. Grains were measured at 3–4 different field angles (B ) averaging over 10–30 scans for each angle, depending on spectrometer stability and temperature stability. Average intensities were calculated using the mean intensity of several measurements, and error bars indicate standard deviation of the mean. The relative concentration of defects in each grain was calculated by double integration of absorption-derivative peaks relative to the co-mounted Si:P marker. The overall density of a particular defect (ESR intensity) as determined by integration of the total spectrum should be the same at any angle. Details on the interpretation of single quartz grain ESR spectra are given in Beerten et al. (2003). Different Ti centers can be identified on the basis of their g value range, specific hyperfine properties and the number of absorption lines. A total number of nine grains was used for the current experiment. Five grains were used for De determination (arbitrarily labelled grains 1–5), two grains for exploratory measurements (test grains A and B) and two more grains for stability tests (test grains C and D). Equivalent doses were calculated using the regenerative dose method. Using the same method, first and second regenerative dose curves were constructed after heating the grains up to 300 ◦ C for 2 h. This treatment had been checked
to result in complete zeroing. Individual grains were irradiated using a 60 Co gamma-ray source with dose rates of ∼ 2 Gy/min. The quartz grains were put in an aluminium sample holder designed with small holes (∼ 1 mm deep and ∼ 5 mm spacing) which is overlain by a 2 mm thick aluminium plate to allow for build up of secondary electron equilibrium. The resulting equivalent doses are compared with the expected dose range for this particular sample. The range is calculated from the inferred age of the deposit (1.8–2 Ma) and a preliminary dose rate. The latter is derived from radionuclide concentrations based on ICP–MS analyses of the bulk sample (gamma rays) and smaller subsamples (beta and alpha rays) taken from the 5 cm thick layer from which the ESRdated grains were extracted. Due to the extremely low radionuclide concentration, the internal alpha and cosmic dose rates turned out to be the major contributors. The k-value for alpha rays was not determined directly. Nevertheless, two different values were used (0.05 and 0.1) in order to encompass uncertainties on the estimated k-value of 0.07 which is cited in Buhay et al. (1992) and subsequently used in this study. Furthermore, two different burial scenarios are taken into account with and without excess overburden (10 m) respectively. These considerations regarding the alpha and cosmic dose rate result in two extreme dose rate scenarios (0.21 and 0.29 Gy/ka) and the expected dose range of 380–580 Gy.
3. Results 3.1. Single grain ESR spectra of Ti centers In general, two types of different single quartz grain ESR spectra have been encountered in grains from the Beerse deposit. Some grains show the Ti–Li center to be the only Tirelated impurity defect (Fig. 1a). In other grains a mixture of
g value range Ti-Li
g value range Ti-Li
g value range Ti-H
12200 12400 12600 magnetic field (gauss) (a)
12800
12200 12400 12600 magnetic field (gauss) (b)
12800
Fig. 1. Single quartz grain Q-band ESR spectra from sample Beerse. (a) Test grain A: the three single absorption lines observed indicate the unique presence of the Ti–Li center in this grain; the large narrow absorption line to the left originates from the enclosed Si:P marker. (b) Test grain B: the largest absorption line around 12150 G originates from the marker and separates absorption lines from Al centers (left) and Ti centers (right); the g value range is slightly different for Ti–Li and Ti–H centers (as indicated by the horizontal line) and can thus be used as a criterion to identify both centers; six doublet absorption lines (indicated by arrows) show the presence of Ti–H centers whereas the three single absorption lines originate from Ti–Li centers.
K. Beerten, A. Stesmans / Radiation Measurements 41 (2006) 418 – 424 Ti-Li, grain 1 regenerative dose points
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relative ESR intensity
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total Ti defect concentration, grain 1 regenerative dose points
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relative ESR intensity
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Ti-Li, grain 1 additive regenerative
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Ti-H, grain 1 additive regenerative
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total Ti defect concentration, grain 1 additive regenerative
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Ti-H, grain 1 regenerative dose points
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relative ESR intensity
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200 400 600 800 1000 (f) dose (Gy)
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Fig. 2. Regenerative dose curves inferred from different Ti centers in grain 1. (a–c) The data are fitted with a linear function and palaeodoses are estimated by interpolation of the natural intensity (horizontal lines); note the difference in vertical scale over the various diagrams. (d–f) Comparison of additive and regenerative dose method; within error limits, both data sets appear to follow similar trends; it should be stressed that these data alone do not provide conclusive evidence for a linear trend at higher and/or lower doses, except perhaps for data of the Ti–H center (at lower doses).
Ti–Li and Ti–H centers can be observed (Fig. 1b). Due to the very sharp absorption lines, detailed tuning of the different spectroscopic measurement parameters and careful rotation of the grain in the cavity, the separate absorption lines of Ti–Li and Ti–H centers could be well separated and hence integrated (double integral), thus enabling one to calculate (relative) separate defect intensities. For clarity, it should be noted that the Ti–Na center could not be observed in any of the investigated grains.
3.2. Palaeodoses based on different Ti centers in single grains containing Ti–Li and Ti–H centers Two grains were used to calculate individual palaeodoses based on Ti–Li and Ti–H centers. The regenerative dose curves of one such grain are shown in Figs. 2a–c. The dose points show an apparent linear trend and were fitted with a simple linear function. Fig. 3 shows the palaeodose results. Individual estimates for the Ti–Li and Ti–H centers are,
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Ti-Li Ti-H Ti (Ti-Li + Ti-H) Ti-Li (1st regen) Ti-Li (2nd regen)
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1000 800 600 400 200
12 10 relative ESR intensity
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Ti-Li, grain 3 1st regeneration 2nd regeneration
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Fig. 3. Palaeodose estimates based on different Ti components in single grains from sample Beerse, plotted against the expected dose (hatched area). Data for the Ti–Li center in grains 1 and 2 are far above the expected dose whereas the opposite can be observed for the Ti–H center. In contrast, data inferred from the total Ti defect concentration are in agreement with the estimated dose. In grains where the Li compensated one is the only Ti related center (grains 3–5) the Ti–Li based palaeodoses are more or less within the expected dose range. The estimated dose range is based on the age of the sample (1.8–2 Ma) and preliminary dose rate calculations (see text).
respectively, much higher and lower than the expected dose range. Palaeodoses based on the total Ti defect concentration (i.e. dose points calculated using all absorption lines from Ti centers) seem to fall within the expected range. A few doses were added to the natural intensity prior to thermal annealing in order to check for severe sensitivity changes (Figs. 2d–f). In spite of the limited number of dose points used, it can be seen that in general the dose curves follow similar trends. However, it should be stressed that detailed information on sensitivity change cannot be deduced from such an experiment. 3.3. Palaeodoses based on Ti centers in single grains containing only Ti–Li centers Regenerative dose curves of a grain containing only Li compensated Ti centers are shown in Fig. 4. Apparently, dose points from the 1st and 2nd regeneration cycles coincide closely, suggesting limited or even non-existing sensitivity change in between both cycles. The resulting palaeodoses are plotted in Fig. 3. Within error limits, the data are in fair agreement with the expected dose.
0
200 400 600 regenerative dose (Gy)
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Fig. 4. Comparison of two consecutive regenerative dose curves for the Ti–Li center observed in grain 3. Within error limits, both data sets appear to follow similar trends, indicating that sensitivity change is minimal.
estimates, respectively) if both centers are present simultaneously (in ESR detectable amount) in a single quartz grain. It appears that Ti–Li centers lead to strong overestimates in the presence of Ti–H centers, whereas grains containing only the Ti–Li center produce relatively good results. On top of this, results for the total Ti defect concentration are in fair agreement with the expected dose, irrespective of the results of the individual Li and H compensated components. In search of an adequate model, we consider various possible origins for this remarkable conduct. 4.1. Sensitivity change Notwithstanding the limited number of data points, it can be argued that sensitivity changes, if any at all, cannot account for the observed difference in results based on Ti–Li and Ti–H centers. Extended additive dose curves could have resolved this problem, but in general the additive and regenerative dose behavior of the different Ti centers appears to be similar. We have inferred an apparent linear trend from the few added dose points. However, it is noted that linear fitting of additive dose points may be very risky (Grün, 1996), especially if high doses are to be determined. In any case, our observations on sensitivity change are in general agreement with earlier work in which enhanced or reduced sensitivity of Ti centers was found only after annealing above 400 ◦ C (Imai et al., 1992; Poolton et al., 2000). 4.2. Differences in light-sensitivity
4. Discussion Clearly, the main challenge is to explain why palaeodose inferences based on Ti–Li centers and Ti–H centers show strong deviations from the expected dose (over- and under-
Sample handling. As the Ti–H center is slightly more sensitive to optical bleaching than the Ti–Li center, over-exposure to light during sample handling might have caused erroneously low palaeodose estimates based on the
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Ti-Li Ti-H normailzed ESR intensity
Ti–H center. However, results from a bleaching experiment (Beerten, 2005) show that no significant difference in relative intensity of both centers could be found after ca. 10 min of direct exposure to the light source used for grain selection. These results indicate that sample handling is not responsible for the encountered difference in inferred palaeodoses. Incomplete bleaching of Ti–Li centers. For the same reason (lower light-sensitivity of Ti–H centers than Ti–Li centers), over-estimates for the Ti–Li center may have resulted from incomplete bleaching prior to deposition. Remarkable, however, is that only if absorption lines from Ti–H and Ti–Li centers are co-observed do the latter centers lead to palaeodose overestimates. Assuming that this does not concern just coincidence, it suggests that the mere presence of Ti–H may induce changes in the bleaching behavior of Ti–Li centers, i.e. making them more light-insensitive. We did not perform any bleaching experiments to check this hypothesis, but from previous studies we know that Ti–Li centers from presentday semi-arid aeolian environments appear to be completely bleached in the presence of Ti–H centers (Beerten and Stesmans, 2005a).
423
100
50
0 0
100
200 temperature (°C)
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Fig. 5. Isochronal anneal (15 min) plot for Ti centers in a particular quartz grain from sample Beerse (test grain C). The data points are normalized to the natural as-sampled intensity. Individual data points exhibit a scattered appearance but nevertheless suggest that the relative stabilities of Ti–Li and Ti–H centers are similar, at least in this grain.
4.3. Thermal stability Another possibility for the age shortfall inferred from Ti–H centers may concern a limited thermal stability and hence short mean lifetimes. The relative stabilities of Ti–H and Ti–Li centers with regard to temperature increase were investigated with an isochronal annealing experiment. One particular grain was stepwise heated for 15 min to temperatures ranging from room temperature to 325 ◦ C. The results of this experiment are shown in Fig. 5. It is suggested that both centers appear to have similar stabilities with regard to temperature increase. This suggests that their mean lifetimes would be more or less equal at burial temperature and hence this explanation appears unlikely. 4.4. Fading of Ti–H centers Fading of Ti–H centers may also be considered as responsible for the observed palaeodose underestimates based on this center, similar to fading of luminescence from feldspars (Wintle, 1973). This was checked for another grain (test grain D), the results indicating that nine months after irradiation neither the total Ti defect concentration nor the relative proportion of Ti–Li and Ti–H centers had changed, even when the grain was stored at 100 ◦ C for 2 days (Beerten, 2005).
center populations do not change. It suggests that such a transfer does simply not occur, or, in another view, that the time span in between measurements was too short in comparison with the rate of this process in nature, even at slightly elevated temperature. 4.6. Difference in production efficiency of Ti–Li and Ti–H centers One could also speculate that different types of radiation (alpha, beta, gamma and cosmic rays) affect dissimilar defects with different efficiency. In theory, this is corrected for in the case of alpha rays (k-value). However, the k-value used here is only a rough estimate, and it may even be different for the various types of defects. 4.7. Dose rate effect Up to the present, we have no information regarding any influence of the dose rate on the creation of Ti–H and Ti–Li centers. If feasible, any explanation in this direction should take into account the favourable creation of Ti–H centers relative to Ti–Li centers due to high laboratory gamma dose rates. 5. Conclusion
4.5. Transfer of centers As another hypothesis, one may notice that any transfer of Ti–H into Ti–Li centers during burial may explain the data. If this would be the case, such transfer may occur after artificial irradiation as well. However, as indicated in (4.4), our preliminary storage tests indicate that the respective Ti
Titanium-related centers in single quartz grains from a 2 Ma old aeolian deposit were investigated using Q-band ESR spectroscopy. Exceptionally, we were able to perform intensity measurements on Ti–Li and Ti–H centers individually in several single grains. Palaeodoses based on these individual Ti centers show a strong offset from the expected
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dose: severe over- and underestimates were found for results based on the Ti–Li and Ti–H centers, respectively, using the regeneration method. Remarkably, if the total Ti defect concentration is used for dose calculation, the individual palaeodose results appear to be consistent with each other, regardless of the relative proportion of Ti–Li and Ti–H centers, and are in fair agreement with the expected dose. Several possible explanations for this unusual behavior have been critically evaluated, but none of these seems to be fully satisfactory. Notwithstanding the fact that some experimental problems remain, we think that ESR dating of single aeolian quartz grains using Ti centers may be a valuable tool for determining the last exposure to sunlight if large natural doses are to be recovered. But clearly, more data points are needed to increase the reliability of the method. This, however, may emerge as a limitation of single grain ESR dating because of the extremely large measurement times. Acknowledgements Thanks are extended to Natalie Bal (Sint-Maartenziekenhuis, Duffel) and Hans Ooms (SCK, Mol) for their help with artificial irradiation. Preliminary ICP-MS analyses were performed by Jan Hertogen (Fysico-chemische Geologie, KULeuven). Frans Gullentops (Historische Geologie, KULeuven) guided us to the sampling location. This work is financially supported by the IWT, Flanders, Belgium. References Beerten, K., 2005. The use of electron spin resonance for estimating sedimentation ages of single quartz grains. Ph.D. Thesis, Katholieke Universiteit Leuven. Belgium. Beerten, K., Pierreux, D., Stesmans, A., 2003. Towards single grain ESR dating of sedimentary quartz: first results. Quaternary Sci. Rev. 22, 1329–1334. Beerten, K., Stesmans, A., 2005a. Single quartz grain ESR dating of a contemporary desert surface deposit, Eastern Desert, Egypt. Quaternary Sci. Rev. 24, 223–231. Beerten, K., Stesmans, A., 2005b. Electron spin resonance (ESR) dating of sedimentary quartz: possibilities and limitations of the single grain approach. Book of Abstracts of the 11th International Conference on Luminescence and Electron Spin Resonance Dating, Cologne, Germany, p. 18. Brumby, S., Yoshida, H., 1994. An investigation of the effect of sunlight on the ESR spectra of quartz centers: implications for dating. Quaternary Sci. Rev. 13, 615–618. Buhay, W.M., Clifford, P.M., Schwarcz, H.P., 1992. ESR dating of the Rotoiti Breccia in the Taupo volcanic zone, New Zealand. Quaternary Sci. Rev. 11, 267–271. Dricot, E.M., 1961. Microstratigraphie des Argiles de Campine. Bull. Soc. Belge Géol. 70, 113–141. Duller, G.A.T., 2004. Luminescence dating of Quaternary sediments: recent advances. J. Quaternary Sci. 19, 183–192. Grün, R., 1996. Errors in dose assessment introduced by the use of the “linear part” of a saturating dose response curve. Radiat. Meas. 26, 297–302.
Grün, R., Tani, A., Gurbanov, A., Koschung, D., Williams, I., Braun, J., 1999. A new method for the estimation of cooling and denudation rates using paramagnetic centers in quartz: A case study on the Eldzhurtinskiy Granite, Caucasus. J. Geophys. Res. 104, 17531–17549. Gullentops, F., Bogemans, F., De Moor, G., Paulissen, E., Pissart A., 2001. Quaternary lithostratigraphic units (Belgium). In: Bultynck, Dejonghe (Eds.), Guide to a Revised Lithostratigraphic Scale of Belgium (Geologica Belgica 4, 153–164). Imai, N., Shimokawa, K., Sakaguchi, K., Takada, M., 1992. ESR dates and thermal behaviour of Al and Ti centers in quartz for the tephra and welded tuff in Japan. Quaternary Sci. Rev. 11, 257–265. Kasse, C., Bohncke, S., 2001. Early Pleistocene fluvial and estuarine records of climate change in southern Netherlands and northern Belgium. In: Maddy, D., Macklin, M.G., Woodward, J.C. (Eds.), River Basin Sediment Systems: Archives of Environmental Change. Balkema, Rotterdam, pp. 171–194. Miallier, D., Sanzelle, S., Falguères, C., Faı¨n, J., Montret, M., Pilleyre, Th., Soumana, S., Laurent, M., Camus, G., De Goër de Hervé, A., 1994. Intercomparisons of red TL and ESR signals from heated quartz grains. Radiat. Meas. 23, 143–153. Okada, M., Rinneberg, J., Weil, J.A., Wright, P.M., 1971. EPR of Ti3+ centers in alpha quartz. Chem. Phys. Lett. 11, 275–276. Olley, J.M., Pietsch, T., Roberts, R.G., 2004. Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Geomorphology 60, 337–358. Poolton, N.R.J., Smith, G.M., Riedi, P.C., Bulur, E., BZtter-Jensen, L., Murray, A.S., Adrian, M., 2000. Luminescence sensitivity changes in natural quartz induced by high temperature annealing: a high frequency EPR and OSL study. J. Phys. D Appl. Phys. 33, 1007–1017. Rink, W.J., 1997. Electron spin resonance (ESR) dating and ESR applications in Quaternary science and archaeometry. Radiat. Meas. 27, 975–1025. Shimokawa, K., Imai, N., 1987. Simultaneous determination of alteration and eruption ages of volcanic rocks by electron spin resonance. Geochim. Cosmochim. Ac. 51, 115–119. 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 Sci. Rev. 16, 257–264. Toyoda, S., Ikeya, M., 1991. Thermal stabilities of paramagnetic defects and impurity centers in quartz: basis for ESR dating of thermal history. Geochem. J. 25, 437–445. Toyoda, S., Voinchet, P., Falguères, C., Dolo, J.M., Laurent, M., 2000. Bleaching of ESR signals by the sunlight: a laboratory experiment for establishing the ESR dating of sediments. Appl. Radiat. Isotop. 52, 1357–1362. Voinchet, P., Falguères, C., Laurent, M., Toyoda, S., Bahain, J.J., Dolo, J.M., 2003. Artificial optical bleaching of the aluminium center in quartz implications to ESR dating of sediments. Quaternary Sci. Rev. 22, 1335–1338. Wintle, A.G., 1973. Anomalous fading of thermoluminescence in mineral samples. Nature 245, 143–144. Woda, C., Mangini, A., Wagner, G.A., 2001. ESR dating of xenolithic quartz in volcanic rocks. Quaternary Sci. Rev. 20, 993–998. 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.