Single quartz grain electron spin resonance (ESR) dating of a contemporary desert surface deposit, Eastern Desert, Egypt

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Quaternary Science Reviews 24 (2005) 223–231

Single quartz grain electron spin resonance (ESR) dating of a contemporary desert surface deposit, Eastern Desert, Egypt Koen Beertena,, Andre´ Stesmansb a

Departement Geografie-Geologie, Laboratorium voor Stratigrafie, Afdeling Historische Geologie, Katholieke Universiteit Leuven, Redingenstraat 16, 3000 Leuven, Belgium b Departement Natuurkunde, Afdeling Halfgeleiderfysica, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium Received 18 February 2004; accepted 19 July 2004

Abstract Signal resetting by sunlight prior to burial is a crucial assumption in electron spin resonance (ESR) dating of sediments. This resetting process is expected to be completed to a greater extent in arid than in fluvial environments. The present paper investigates the natural and artificially irradiated signal intensity of Ti related centres in single quartz grains collected from the desert surface (Eastern Desert, Egypt) in order to test this hypothesis. The results suggest that in most grains both the Ti–Li and Ti–H signal are completely reset to zero. Additive dose curves based on the sum of both Ti centres show an anomaly in the low dose region. Possible causes for this behaviour are briefly discussed. Three fitting procedures are conducted and each of them shows a different palaeodose distribution with a rather large spread in DE values. It is concluded that similar fossil deposits would be datable by single grain ESR using Q-band measurements of the Ti–Li or Ti–H signals in quartz. r 2004 Elsevier Ltd. All rights reserved.

1. Introduction The acquisition of reliable age estimates of sediments based on electron spin resonance (ESR) dating, using ESR active (paramagnetic) defects rests on three important principles: (1) there must be a simple relationship between burial time and signal intensity of the ESR centres (i.e. ESR centre activation by environmental irradiation dose); (2) the centres should be stable over geological timescales and (3) they must be light sensitive, allowing the signal to be reset into the diamagnetic state by sunlight before burial. The latter crucial requirement in particular may meet difficulties as a number of studies have shown that relevant ESR centres in quartz have a very slow bleaching rate, as opposed to optically stimulated luminescence (OSL) and thermoluminescence (TL) signals used for the same Corresponding author. Tel.: +32-16-32-64-40; fax: +32-16-3264-01. E-mail address: [email protected] (K. Beerten).

0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.07.009

application. However, ESR offers the possibility to extend the time range of sediment dating over a few 100 ka, up to 1 Ma and beyond. This limit has been explored only recently in OSL (slow component), but the signals used are also much less rapidly bleached as the fast OSL components used earlier (Bailey, 2000; Singarayer et al., 2000). It is proposed that large age ranges can be addressed using the red emission of quartz thermoluminescence (red TL), and recently, this luminescence component was shown to be bleachable by polychromatic daylight (Stokes and Fattahi, 2003). In this context, ESR sediment dating would be a valuable alternative in establishing Early and Middle Pleistocene chronologies if the problem of slow bleaching, and thus insufficient resetting, could be properly handled. A first step in this direction is the single grain approach in ESR in which ages are estimated based on a set of individual grains, similar as in OSL single grain dating. This offers a great potential because insufficiently bleached grains can be identified and excluded from the analysis (Beerten et al., 2003).

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apparent anomaly in the lower dose region, regenerated dose curves (with and without preheating the grain prior to measurement) were composed in order to investigate the nature of this anomaly. So far, no firm conclusions could be made regarding the latter.

2. Materials and methods The sampling spot is located in the Eastern Desert of Egypt, along the course of Wadi Bili, ca. 30 km NW of Hurghada at an altitude of ca. 160 m above sea level (Fig. 1). Wadi Bili is a large wadi system that drains the Red Sea Mountains in a north-east to eastward direction to the Red Sea. Along the wadi a series of bedrock cut pediments can be found that form interfluvia for smaller wadi’s entering the main wadi (Fig. 2). These interfluvia are overlain by a sand cover consisting of rather coarse sand. During dry periods sand is blown from the exposed wadi channel and deposited on the interfluvia. Climatic conditions are hyperarid with a mean annual precipitation of 4 mm and high interannual variability (Moeyersons et al., 1999). Long almost complete dry periods are interrupted by rare high-magnitude rainfall 200 m

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In general there are three known different ESR active and light sensitive centres in natural quartz which can be used for dating sediments. In decreasing order of bleachability these are the Ge-, Ti- and the Al-centre. The first and the last were used in pioneering studies of ESR sediment dating (Tanaka et al., 1985; Yokoyama et al., 1985). The Ge-centre proved to bleach very fast with sunlight and artificial light (SOL 2 lamp; Walther and Zilles, 1994). However, difficulties in finding a natural Ge-signal suggests that it is not stable over geological time scales. The Al-centre, by contrast, was found to bleach very slowly with sunlight and artificial light from a variety of sources (weeks to months) and only to a non-bleachable residual level (Brumby and Yoshida, 1994; Walther and Zilles, 1994; Tanaka et al., 1995; Yoshida, 1996; Toyoda et al., 2000). On the other hand, investigations on zero age deposits indicated that the natural signal in fluvial sediments was down to the residual level (Laurent et al., 1998; Voinchet et al., 2003). Thus, acceptable age estimates for a range of deposits could be obtained using this centre by taking into account the residual level (Laurent et al., 1994, 1998; Falgue`res, 2003). Ti-related centres, in turn, are completely bleachable by light, but rates of bleaching vary dramatically. The natural Ti–Li signal is completely bleached after 15 min exposure to a UV lamp (200–290 nm; Tanaka et al., 1997). Yet, 132 days exposure to sunlight are needed to completely bleach the irradiated signal (Brumby and Yoshida, 1994). Dating results based on this centre overestimated the geological age estimates (Zilles, 1993; Yoshida, 1996; Tanaka et al., 1997). Recently, the bleaching behaviour of the Ti–H and Ti–Na centres has been investigated (Toyoda et al., 2000), reporting complete optical bleaching after about 40 h exposure to a halogen lamp. Although good dating results have been obtained in the past with the ESR method on bleached quartz, some doubts remain about signal resetting prior to burial of a sediment sample composed of a large number of grains. In a previous study we demonstrated that the Ti–Li signal from water-lain single grains exhibited a large spread in the apparent palaeodose (Beerten et al., 2003). We also demonstrated that a subrecent deposit contained at least a few fully bleached grains. In the current work, we investigate the possibility of dating sediments from an arid environment. Grains from a present day desert surface deposit in Eastern Egypt were sampled and subjected to an additive-dose single grain ESR dating protocol. Measurements were done on Ti-related centres only because these have shown to be completely bleachable by sunlight (cfr. supra). We determined the initial bleaching level of Ti-related signals (Ti–H and Ti–Li) from individual quartz grains before burial and it is shown that in most grains both signals are completely reset to zero. In addition, as most dose curves show an

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Fig. 1. (A) Location map of study area with north towards top of page. (B) Detail of map A. Course of Wadi Bili and main tributaries are dotted, smaller wadi’s are shown in thick line. Black dot marks the sampling spot. It is located on the north facing slope of an interfluvium.

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Fig. 2. Photograph of sampling spot. (A) Interfluvium bordered by a tributary wadi towards the reader and the flat floor of the main wadi in the background. Sample was taken in the small depression in the centre of the photograph. (B) Close-up of coarse sand cover at sampling location.

events. Flood recurrence intervals are estimated at 50 years based on recent fan deposits in the nearby and analogous Quseir area. Geomorphological studies indicate an alternation of dry and wet phases throughout the Early Holocene (Vermeersch, 2002) and modern conditions should prevail since 5000 BP (Moeyersons et al., 1999). Although the region is characterised by a severe drought it is to be expected that occasional rainfall would reorganise the internal fabric of the sand cover from time to time. The upper 5 cm of these sand deposits were sampled on the north facing slope of a small bedrock interfluvium (Fig. 2B). Quartz and orthoclase are the main minerals of the sand which is probably derived from local granites (Saı¨ d, 1993). The mean grain size is well above 0.5 mm but grains larger than 2 mm are rare. After dry sieving (at 0.35 and 1 mm), the grains were treated (ca. 20 min) with concentrated HF and dried in an oven for several hours at ca. 35 1C. Quartz grains of ca. 1 mm diameter were hand-picked under the microscope using a very short exposure of a halogen lamp (Beerten et al., 2003). ESR dating techniques have been described in detail by Gru¨n (1989), Ikeya (1993), Jonas (1997) and Rink (1997). The principle of ESR is based on the absorption

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of microwave energy by unpaired electrons (i.e. paramagnetic defects) immersed in an external magnetic field (B). Absorption will occur if the resonance condition for a particular defect is fulfilled, that is, when the B originated splitting between adjacent Zeeman levels, effectively governed by the defect’s g value for that B direction, exactly matches the energy of the incident microwave photon. The g value for a free electron is 2.0023. For an unpaired electron residing at a defect site, its g value will differ, mostly only slightly though, depending on the electron localization level, defect nature and atomic environment; it will even differ for nominally identical defects, but with different levels of distortion (strain). Consequently, different defects (i.e., trapped unpaired electrons at different types of defects) can be distinguished and identified by scanning the magnetic field over an appropriate range while keeping the incident microwave frequency constant. At resonance, absorption occurs and a characteristic absorption signal appears in the spectrum. Generally, the parameter g is a matrix, that is, an anisotropic physical quantity reflecting the defect’s symmetry. In the proper axes system, it reduces to three diagonal principle values, commonly referred to as g1, g2 and g3, and each defect is characterized by a unique g matrix, or, effectively g1, g2 and g3 values, by which the defect is identified. The g1, g2 and g3 values are generally anisotropic, which means that the actual g value differs according to the orientation of the defect’s crystallographic axes relative to the direction of the applied magnetic field. Consequently, the orientation of the axes must be known should one wish to identify a centre based on one ESR observation (one B orientation). On the other hand, in a powder, consisting of a virtually infinite number of tiny crystals, the effectively measured g value is an average, making the orientation of the sample with respect to B of no importance. As a result, an isotropic strongly modified, average ESR spectrum is observed, referred to as powder pattern (spectrum). But still, despite its blurred average shape, through the spectroscopic relationships, the spectrum has embedded the inherent typical spectroscopic properties (i.e., g matrix, symmetry) of the originating defect. And so, in that sense, the exhibited powder signal will also be, in principle, unique for the envisioned defect. However, due to its generally more blurred character as to the single crystal signals, more care is generally required when it comes to defect characterization (e.g., overlapping signals of different origin, broad signals, etc.). But, as said, since g factors are unique for a certain defect, most ESR measurements for dating are conveniently done on powders (isotropic ESR signal, signal enhancement through stacking (powder) grains, averaging (out) of differences of a targetted physical parameter over the various grains). In this approach, the powder pattern’s g values serve to identify a centre.

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However, when turning to the single grain approach, we again face single crystal properties: a particular defect system embedded in the single crystal will exhibit an anisotropic, multicomponent spectrum, and as the crystal orientation with respect to B is not known a priori other criteria must be used in order to identify the centre. These include the principle g values of the centre, number of absorption lines and relative intensities. Additionally, hyperfine splitting characteristics and temperature dependence of signals support identification (Beerten et al., 2003). Measurements were conducted at ca. 100 K using an EMX Bruker Q-band ESR spectrometer operating at a frequency of ca. 33 GHz, with an incident microwave power of ca. 1.65 mW (attenuation 20 dB). Generally, the chosen sweep width was 700 G (centre field nominally at 12 450 G), with the field modulation amplitude (100 kHz) set to 0.5 G and using scan times of 42 s. Individual grains were measured for 3–4 different orientations (different field angles, jB) of the grain with respect to the fixed B direction and performing signal averaging over 10 to 30 scans for each angle. The relative concentration of defects in each grain was calculated by integration of absorption peaks relative to a comounted Si:P marker. Irradiations were carried out using a 60Co gamma source (dose rate 1.29 Gy/min) and grains were put in an Al holder with an overlying 3 mm thick Al plate to allow for build-up of secondary electron equilibrium. After ESR measurement of the samples in as-sampled (initial) state (natural signal), each grain was exposed to a number of gamma-ray doses with ESR analysis between each irradiation step. In between irradiation and measurement, the grains were stored for a few hours. No preheating was used. Regenerated dose curves were composed after zeroing the sample with a xenon lamp (ORIEL 6253 150W Xe Arc lamp; ca. 25 kW/m2) after finishing the first dose curve. The experimental setup allowed light to pass through the quartz grain with minimal heating of the substrate. Temperature at the sample was estimated to raise only slightly above room temperature during bleaching. In one experiment, a preheat of 10 min at 200 1C was applied after irradiation and prior to ESR measurement. The signal was not zeroed in between successive irradiations.

3. Experimental results 3.1. Observed ESR centres A total number of 20 grains (labelled 1–20) was analysed. From these, only two showed a natural Ti signal. From the remaining 18 grains, nine showed a Ti signal after gamma-ray irradiation. Thus a set of 11

grains could be used for dating. Most grains show a rather complex spectrum in the g value range of Ti related centres, that is, between g1(Ti–H)=1.986 (Rinnenberg and Weil, 1972) and g3(Ti–Na)=1.899 (Bailey and Weil, 1992). Fig. 3A shows three spectra observed for different arbitrary directions of B, from grain 17 which contains only the Ti–Li signal (natural, after 105 Gy and after 315 Gy). No signals of Ti defects are observed in the natural grain. After irradiation, three absorption lines appear, however without any signs of hyperfine splitting due to H(I=1/2) nuclei, which means no evidence of the Ti–H centre is present. Spectra were taken for numerous field angles from where it appeared that the various absorption signals strongly shifted, yet confined to the range defined by the extreme g values of the Ti–Li signal, confirming the unique presence of the latter type of defect in this grain. The top and middle spectra in Fig. 3A were chosen to illustrate the extreme g values (g1(Ti–H)=1.986, Rinnenberg and Weil, 1972; g3(Ti–Li)=1.913, Isoya et al., 1988). However, the majority of the grains exhibit a mixed spectrum with variable concentrations of the Ti–Li and Ti–H defects. Fig. 3B, for instance, shows such a spectrum with a large Ti–Li signal and a minor Ti–H signal. The three spectra (different jB values) of grain 8 shown in Fig. 3C illustrate the presence of a dominant Ti–H signal intermixed with a smaller Ti–Li signal. Thus, the ratio of the Ti–Li/Ti–H signal intensity varied over the various grains studied. However, as to the kind of ESR active centres observed, we finally mention that no presence of the Ti–Na centre could be traced.

3.2. Additive dose curves and individual palaeodoses Dose curves for individual grains were established by measuring the ESR signal intensity after successive irradiation steps. The intensities of all observed signals in the range between the extreme g values g=1.986 and g=1.911 were summed up. Six sets of data are presented in Fig. 4. Dose curves of grains from which both the Ti–Li and Ti–H signals were used are shown in Fig. 4A–C (grains 2, 3 and 9, respectively). An important feature is the presence of a kink. It appears that, in some cases, the signal growth is enhanced after a slow initial intensity rise up to 200–300 Gy. In Fig. 4D the dose curve of a similar grain is shown (grain 7) for a total cumulative dose of up to ca. 1500 Gy. The intensity rise in the lower dose range is clearly visible: points (henceforth referred to as ‘low-dose points’) seem to be located off the general linear fit. Fig. 4E (grain 17) shows the dose curve plot for a grain which only contains the Ti–Li defect. A kink may be present at ca. 100 and 400 Gy. On the contrary, the dose curve of grain 5 (Fig. 4F) does not show a kink. This grain proved to contain only the Ti–H defect, and no observable Ti–Li defects.

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Fig. 3. Single grain Q-band ESR spectra observed at ca. 100 K; extreme g values for Ti–Li centre: g1=1.979, g3=1.911 (Isoya et al., 1988); extreme g values for Ti–H centre: g1=1.986, g3=1.913 (Rinnenberg and Weil, 1972). (A) Grain 17. Natural signal (lower curve), 105 Gy (middle curve) and 315 Gy (upper curve), taken at arbitrary angle of the grain’s crystallographic axes relative to direction of magnetic field. The intense, narrow absorption line at about 12 140 G stems from the Si:P marker. Typically three single absorption lines are observed, at g values within the extreme g value range of the Ti–Li centre, which indicate the presence of the latter. (B) Grain 20, irradiated to 315 Gy. Mixed spectrum with three Ti–Li (large singlet lines) and five Ti–H signals (doublets; one doublet is probably masked by the middle Ti–Li line). The g value of the doublet towards low magnetic field end crosses g1 of Ti–Li. (C) Grain 8, irradiated to 525 Gy. Three spectra at slightly different jB values. These illustrate the occurence of a double crystal case, the mixed spectra showing six absorption lines of Ti–Li (illustratively connected by dotted lines between spectra) and twelve doublets of Ti-H centres (full lines).

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Fig. 4. Dose response curves for several individual quartz grains. Linear fits with (full lines) and without (dashed lines) error weighting. For details about the observed centres in each grain see Table 1. (A) Grain 2; an apparent jump in the dose curve between 400 and 500 Gy is suggested. (B) Grain 3, showing a kink in the dose curve between 200 and 300 Gy. (C) Grain 9, similar as grain 3. (D) Grain 7; the dose points below 300 Gy exhibit an anomalously low intensity. (E) Grain 17, showing possible kinks at ca. 100 Gy and ca. 400 Gy; however, all data points may as well fit to one straight line within experimental error. (F) Grain 5, exhibiting a dose response without kinks.

All dose data points are fitted by a linear function y = a+bx, or more physically IESR = a+b*DE, with (full lines) and without (dashed lines) weighted errors (denoted as fitting procedures 1 and 2, respectively). We also fitted the curves using the lower three dose points only (at 0, 105 and 210 Gy), i.e., in the region of the low-intensity dose range (fitting procedure 3). They seem to occur below the inferred kinks and hence, palaeodoses calculated from these points may serve as a reference for palaeodoses based on the entire data plot. The fitting results are shown in Table 1. The different fittings produced a large range of palaeodoses for each individual grain. However, a few consistencies can be found within the inferred data set. First, grain 5 shows calculated palaeodoses which are very similar over all three fitting procedures and close to zero. For this grain, no low-intensity dose points could be observed. Second, as would be expected, grains that contained a natural signal (grain 2 and 8) consistently show a positive palaeodose. For all other grains, the

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Table 1 Inferred palaeodoses and associated error (Gy) for studied individual grains according to the three different fitting procedures applied (see text) Grain #

ESR centres

Natural signal

Kink in DRC

Fit 1 DE

2 3 5 7 8 9 13 16 17 19 20

Mix Mix Ti–H Mix Mix Mix Mix Mix Ti–Li Mix Mix

Yes No No No Yes No No No No No No

Yes Yes No Yes No Yes Yes Yes ? Yes Yes

Fit 2 Error

74 16 2 63 48 28 28 8 24 35 4

36 17 10 21 27 16 22 10 25 32 9

DE 28 34 5 25 33 48 17 32 34 33 38

Fit 3 DE

Error 35 29 8 27 39 36 15 26 29 25 51

50 8 6 10 8 2 17 2 9 4 17

Error 36 11 9 14 11 3 24 2 14 6 24

Additionally provided is information on observed type of ESR centres, signals observed in the natural state and any occurring kinks in the dose response curve.

Table 2 Linear and error weighted mean palaeodose values (DE, Gy) according to the three fitting procedures applied DE Fit 1 Linear mean

7

Error weighted mean

8

Fit 2 739a 720b 715c

11 7

Fit 3 731a 729b 720c

0 0

719a 714b 76c

Three types of errors are specified: (a) standard deviation, (b) linear mean of errors (see Table 1), (c) standard error of weighted mean.

calculations resulted in negative palaeodoses (except grain 20 for fitting procedure 2 and grain 16 for fitting procedure 3). Table 2 shows the mean palaeodoses calculated from the data set presented in Table 1. Fitting procedures 1 and 2 produce negative palaeodoses (positive values on the x-axis in the dose curve plots are regarded as negative palaeodoses) for both the linear and error weighted mean (ranging from 7 to 11 Gy). The standard deviation of the linear mean (39 and 31 Gy for procedures 1 and 2, respectively) is larger than the mean error calculated from individual palaeodose errors (20 and 29 Gy, respectively). The mean error of the weighted mean is significantly smaller (15 and 20 Gy, respectively). The mean palaeodose based on fitting procedure 3 is 0 Gy for both the linear and error weighted mean. For this procedure, the standard deviation of the linear mean (19 Gy) is larger than the mean error (14 Gy), whereas the calculated error of the error weighted mean is only 6 Gy. In any case, fitting procedure 3 produces smaller errors than procedures 1 and 2. But here we must realize this conclusion is based on three data points only.

3.3. Regenerated dose curves Subsequently, regeneration experiments were conducted to compare the natural and regenerated dose curve. Fig. 5A shows the results of this experiment for grain 2. Both sets of data show a clear trend of signal enhancement towards higher doses. Notwithstanding the relatively large scatter of the regenerated dose points, Fig. 5B shows a similar trend for grain 16. Alternatively, the signal intensities of the regenerated dose curve for grain 3 were measured after preheating the grain for 10 min at 200 1C (Fig. 5C). In the low-dose range up to 200–300 Gy, intensities of the regenerated dose points remain unaltered after preheat. At higher doses however, the data sets diverge strongly. This observation would suggest that Ti defects produced at higher artificial doses are much less stable to thermal treatment than the ones produced at lower doses. 3.4. Palaeodose distributions Palaeodose plots were constructed by summing up all (assumed Gaussian) distributions produced by

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Fig. 5. Comparison of natural (full circles) and regenerated (open squares) dose response curves. (A) Grain 2, no preheat; a similar trend of enhanced intensity rise towards higher doses is observed in both curves. (B) Grain 16, similar to grain 2. (C) Grain 3, preheated at 200 1C for 10 min prior to measurement of the regenerated dose curve points. A significant difference in signal intensity is observed at doses in excess of about 300 Gy.

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individual palaeodoses and their associated errors. The results are shown in Fig. 6. The distribution plot of the weighted error fitting procedure shows a clear asymmetry towards negative doses (Fig. 6A). The effect of two unbleached grains, demonstrated by the presence of a natural signal, seems almost totally cancelled by the overwhelming presence of negative palaeodoses. A similar plot is depicted in Fig. 6B, which shows the distribution plot of the unweighted error fitting procedure. The only difference with the previous plot is the slightly larger spread of the negative doses. Finally, the third plot contains the results of the fits where only the lower three dose points were used (Fig. 6C). It shows one large population, centred around 0 Gy. A slight asymmetry towards positive doses is visible. Most likely, it concerns an artefact resulting from the presence of two unbleached grains.

4. Discussion The main aim of the paper is to check if Ti related signals in as-picked grains from an aeolian environment are sufficiently bleached. From the 11 grains used in

this study, not less than nine were fully bleached as they did not exhibit any Ti related signal prior to irradiation. Surprisingly though, the mean palaeodoses based on the entire dose curves (Fig. 6) are found to be negative and show a large error. The corresponding palaeodose distribution plots show a very large distribution towards negative palaeodoses. On the other hand, if only the dose points at 0, 105 and 210 Gy are used, a mean palaeodose of 0 Gy is obtained, as well as a smaller estimated error compared to the other fitting procedures. The corresponding distribution plot shows one population of well bleached grains with a slight asymmetry which is attributed to unbleached grains. It is clear that the so-called kinks in the dose curves produce rather odd looking and highly scattered palaeodose distributions. Whatever the cause of this ‘anomaly’ in the dose curves might be, it is responsible for an offset from the real age and induces quite large errors. On the other hand, the systematic error that would originate from it seems relatively small compared to the random error for sufficiently extended dose curves (Fig. 4D). Since these anomalies in dose curves come as a systematic observation we may firmly reject the phenomenon to concern a

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random error. Moreover, our results suggest that the anomaly can be reproduced in the regenerated dose curve (Fig. 5). Possible explanations for this phenomenon will be discussed briefly. First, there might be the possibility of a systematic underestimation of small signals. Some of the low-dose points have relative ESR intensities of ca. 0.25 which is relatively close to the detection limit (ca. 0.1). However, as some of these data points have intensities of up to 1 and more, this hypothesis seems unlikely. One could suggest that small ESR signals are more prone to saturation and thus, for fixed incident Pm (microwave power), may be measured under (partial) saturation conditions. However, this was checked for the Ti–Li signal by varying Pm, and the results indicate that measurement conditions used in this study (ca. 1.6 mW at ca. 100 K) do not cause any observable saturation of the signal. Third, the kinks in the dose curves could mark real inflexion points. Indeed, such ‘kinked’ dose curves have been encountered before in mollusc shell (Katzenberger and Willems, 1988). Suggestions to overcome the problem of inflexion points in dose curves from mollusc shell and corals are presented by Schellmann and Radtke (1999 and 2001). In general, more data points than the number used in this study are needed to detect inflexion points and to apply the suggested approach of last mentioned authors. Finally, it cannot be ruled out that the anomaly represents supralinearity. This phenomenon has been observed in TL and OSL measurements of quartz grains as well (e.g. Aitken, 1985; Banerjee, 2001). Woda et al. (2002) showed TL dose curves where a signal rise enhancement starts at a dose of ca. 1000 Gy onwards. As most dose curves in the current study are based on the sum of Ti–Li and Ti–H centres, it is difficult to infer which type of defect would be mainly responsible for this behaviour and if there is a connection with TL measurements. Dose curves for grains which exhibit only one centre (grain 5 and 17) do not show a prominent signal rise enhancement. Instead, it seems that grains where both types of ESR active defects occur together are most sensitive to this phenomenon (except grain 8). However, at this stage of our research it is clear we cannot come to definite conclusions yet. More experiments are certainly needed to confirm the presence and to unravel the proper causal mechanism (if any) of these anomalous parts in the dose curves. Factors such as spatial distribution of centres (clustering) or the occurrence or absence of other electron or hole centres, either paramagnetic or not, could play an important role as well. Another question which remains unanswered is whether this phenomenon is due to artificial irradiation. In any case, our preliminary results suggest that the thermal stability of the Ti–Li centres formed at low doses is considerably higher than the ones formed at higher doses.

Besides optical bleaching, a crucial aspect concerns whether the thermal stability of the defects is sufficient to resist extreme desert temperatures. So far, only data for the Ti–Li centre are available. Gru¨n et al. (1999) estimated the mean lifetime of this centre at 106 1C to be only 1600 years. So, even though temperatures at the desert surface can reach 50 1C in summer (Kidron et al., 2000), to contribute to the ESR signal resetting long exposure times would be necessary (several 1000 years). Complete optical bleaching is much faster and is obtained after exposure times of weeks to months. At the moment it is still unclear whether the dose response of the sum of both signals can be used as a valid approach. Preliminary results from recent experiments suggest that it would be possible to infer intensities for each type of defect separately from Q-band ESR spectra. However, the single grain technique so far does not allow the isolation of different Ti signals properly for each grain. So in this work we used this type of dose response curves only to determine the bleaching history.

5. Conclusion Single grain ESR dating of a contemporary desert surface deposit shows the potential of using the Ti–Li and the Ti–H centres in quartz for dating fossil sediments from similar environments. Depending on the fitting procedure and statistical calculations, the mean palaeodose based on both signals ranges between 11731 Gy and 076 Gy. For Middle and Early Pleistocene contexts, where the potential of single grain ESR dating is highest, a comparable absolute offset from the real age and an error of this magnitude would be neglegible. It was found that the systematic offset from zero age is caused by anomalously behaving dose response curves, the cause of which remains undetermined so far. At this moment it is not clear whether it is feasible to use the sum of both signal intensities for dating fossil deposits. If not, this would mean for instance that either of both defects is not suitable for dating, or that there is a mutual influence. Not in the least, the different bleaching behaviour might pose problems related to resetting in some sedimentary environments. Further investigations on bleaching behaviour, response to gamma-irradiation and thermal stability of both signals are needed to support this approach.

Acknowledgements We would like to express our gratitude to P.M. Vermeersch and P. Van Peer from the Belgian Middle Egypt Prehistoric Project (KULeuven) for having

ARTICLE IN PRESS K. Beerten, A. Stesmans / Quaternary Science Reviews 24 (2005) 223–231

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