Luminescence chronology of the medieval citadel of Termez, Uzbekistan: TL dating of bricks masonries

Journal of Archaeological Science 34 (2007) 1402e1416 http://www.elsevier.com/locate/jas

Luminescence chronology of the medieval citadel of Termez, Uzbekistan: TL dating of bricks masonries Emmanuelle Vieillevigne*, Pierre Guibert, Franc¸oise Bechtel Institut de Recherche sur les Arche´oMATe´riaux-CRP2A, UMR 5060, CNRS/Universite´ Bordeaux 3, Maison de l’Arche´ologie, 33607 Pessac Cedex, France Received 18 July 2006; received in revised form 25 October 2006; accepted 31 October 2006

Abstract Among the dating methods in archaeology, thermoluminescence (TL) is widely developed. The present research concerns the construction chronology of the citadel of Termez (Uzbekistan) in the medieval period. On the methodological side along the TL study, we had to take into account two decisive factors that are expected to affect the accuracy: control of anomalous fading on the polymineral fraction (quartz and feldspars) and the effect of thermal treatments on the luminescence properties of the dated material. With regard to the determination of the annual dose, difficulties, associated with the evolution of the radiochemical composition of the dated samples and their environment over time, were overcome. The TL results, obtained on 24 bricks taken from 10 masonries essentially situated at the east of the citadel, are spread between the 11th and the 14th centuries. Averaged dates restrict the chronological interval to the 12th and 13th centuries for all structures. TL dating also allowed us to distinguish the various phases of construction of the citadel during the centuries, notably those of the fluvial wall. Moreover, our results appear to confirm, in an unquestionable way, that the citadel was not deserted after its sacking by the Mongols (in the 13th century). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Thermoluminescence dating; Bricks; Medieval architecture; Citadel of Termez; Uzbekistan; Bactriane; Islamic period

This paper deals with an application of luminescence dating to recent materials: medieval architecture. Few laboratories of luminescence dating are interested in TL or OSL applied to this field (Gallo et al., 1999; Bailiff and Holland, 2000; Greilich et al., 2002; Goedicke, 2003). This thermoluminescence study develops the study of medieval architecture and is applied to the medieval citadel of Termez (Uzbekistan). 1. The site and the project: a general presentation 1.1. Termez and the chronological issue The citadel of Termez is located along the Amu Darya river, in southern Uzbekistan. This building covers an area * Corresponding author. Tel.: þ33 557121085; fax: þ33 557124550. E-mail addresses: [email protected] (E. Vieillevigne), guibert@ u-bordeaux3.fr (P. Guibert), [email protected] (F. Bechtel). 0305-4403/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2006.10.030

of approximately 10 hectares. For a long time, Termez has been known as a major reference site for antic research in southern sovietic Central Asia. Systematic excavations carried out there by M. Masson between 1936 and 1938 enabled to establish the map of the antic and medieval city intra muros and to locate the built areas. The oldest level would belong to the Greco-Bactrian period. A lot of vestiges were attributed to the kuchan and medieval periods (Pougatchenkova, 2001b). In the 1970s, the Institute of Archaeology of Samarkand started research on the foundation of this city. After several excavation campaigns, evidences of the initial foundation of Termez by the Greco-Bactrians were found out at the southeastern part of the citadel, near the Amu Darya river. In 1994, an Uzbek-French archeological research team was created, under the direction of P. Leriche and C. Pidaev (Leriche, 2001; Leriche et al., 2001). Although the main aims of the new research group, which focused on a precise topographic map and on the archaeological understanding of

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the building construction phases, were altogether similar to those of the original research, specific attention has been devoted to the reassessment and further refinement of the chronological medieval sequence of the east corner of the citadel. Indeed, the attributed chronology of the different masonries’ phasing is based upon ceramic typology (including the typology based on brick size), architectural relations and few contemporary texts. It therefore seemed necessary to perform a series of dating upon samples that would be directly associated with well-defined archaeological contexts. From this perspective, the Termez’s thermoluminescence (TL) dating project has been developed. TL dates were obtained from fired bricks, extracted from 10 masonries, in various locations. Thus they are directly associated with the archaeological building of the citadel. 1.2. Fired bricks’ masonries from east corner of Termez: the structures and their context The majority of the masonries to be dated are mainly located in the excavations sector E. Three are located in the western part of the citadel: fluvial wall, the core and the belt of ‘‘west bastion’’ (Fig. 1). In the excavations of sector E, five architectural elements were sampled in 1999 and other two in 2002 (bastion 24 and curtain 23), the two latter masonries being archaeologically investigated in 2000. All structures belong to levels that are assigned to the medieval period, between the 9th and the 14th centuries. However, ceramic typology and an architectural relation study could not give a chronology for all masonries and so a luminescence dating programme was implemented in order to refine this chronological framework. The first TL sampling and on-site radioactivity measurements took place at Termez in September 1999 (by Franc¸oise Bechtel and Pierre Guibert); a second TL sampling occurred in September 2002 (by Emmanuelle Vieillevigne). Samples included bricks, as well as mortars and other bricks from their close surroundings (Table 1). 2. Thermoluminescence dating: summary of basic principles and techniques used The principles and general experimental aspects of TL dating are described in detail by Aitken (1985). To determine the TL age, two physical parameters must be evaluated, the equivalent dose and the annual dose, which are related as follows: TL age ¼ equivalent dose=annual dose In this work the equivalent dose has been evaluated using the fine-grain technique. The experimental protocol used was based on an additive dose procedure (first reading) carried out on naturally irradiated aliquots that were given additional laboratory doses, followed by regeneration experiments on annealed aliquots (regenerated TL) (details in Guibert et al., 1996). The equivalent dose was measured by integrating the TL curves (Fig. 2) between 300 and 450  C for example, this temperature interval corresponding to the plateau region

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(Fig. 3). The equivalent dose was calculated by polynomial approximation (Fig. 4): the data obtained with the regeneration procedure were used to define the growth function. The latter was fitted to the additive dose data points according to a slide method, taking into account the sensitivity change and/or the differences in mass between first series and second series aliquots. The annual dose was determined using different processes. On-site direct measurements of the environmental dose-rate were performed in 1999 using gamma spectroscopy (Canberra Inspector NaI) with an NaI:Tl scintillator. Spectral data (2k channel) are recorded in the 0e11 MeV energy range; it enables to separate the g region (0e3 MeV) from the pure cosmic one (above 3 MeV). The g dose-rate is determined according to two ways of exploitation: (i) as an energy integrator, considering the total g energy (0e3 MeV) deposited in NaI per time unit, (ii) as a spectrometer, measuring the areas of 40K (1461 keV), 214 Bi (1765 keV) and 208Tl (2615 keV) peaks for the respective radioactive families K, U and Th. Laboratory measurements of the radionuclide content of the TL samples and other materials taken from their surrounding medium were measured by low-background high-resolution gamma spectroscopy (Guibert and Schvoerer, 1991) to evaluate the a and b components of the annual dose, or the g components when necessary, using the conversion factors by Adamiec and Aitken (1998). Moreover, the cosmic dose-rate was estimated according to Prescott and Hutton’s data (1994), or directly measured with NaI spectrometer when it was significant compared to the archaeological situation of the brick sample being dated. Significant disequilibria of U-series in brick samples and in mortars were found and the relevant corrective method employed for annual dose determination (detailed in Guibert et al., 1997) is recalled below. The conditions of the second sampling campaign in 2002 did not enable to perform in situ gamma spectrometry measurements; consequently for the masonries concerned, the g dose rates were evaluated by a reconstruction technique according to the principles given in Guibert et al. (1998). 3. Thermoluminescence measurements 3.1. Characterization of materials The mineral composition of the TL bricks and the mortars was determined by optical petrography, SEMeEDS analysis and X-ray diffractometry. Quartz, calcite, hematite, potassium feldspar, plagioclase, pyroxene and mica were detected in bricks as crystalline phases. Mortars contain the same mineral assemblage, with charcoal, ceramic fragments and clay. The proportion of certain components differed according to the nature of the material: obviously, lime mortars are much richer in calcite than bricks: 45 wt% versus 10 wt% according to the data obtained by thermogravimetric analysis (Vieillevigne PhD, 2005).

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Fig. 1. Topographical map of the citadel of Termez showing the excavation areas: localisation of the tower ruins and of the west bastion (drawing by A. Colin and F. Ory, in Leriche et al., 1997). Location of the masonries in sector E, excavated by the Uzbek-French team, directed by C. Pidaev and P. Leriche (map represented in the excavation report of MAFOuz, 2000).

3.2. TL study: instruments and techniques After crushing and sieving samples of bricks, grains of diameter less than 40 mm were chemically and mechanically treated. Carbonate phases were removed by HCl (1 M) and organic

matter by washing with hydrogen peroxide. An additional treatment by immersion in diluted HF (0.5 M) in combination with HCl (1 M) was also performed to dissolve remaining clay materials that coated quartz or other TL crystals. This treatment enhanced the brightness of the luminescence and eliminated

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Table 1 Fragments of bricks for TL-dating and surrounding elements for gamma doserate evaluation Location of samples

Bricks dated

Tower ruins

BDX 6367, 6369, 6372, 6375, 6391, 6392 BDX 6405, 6406

Surrounding elements

Underlying alluvion: BDX 6376 3 lime mortars: BDX 6370, 6374, 6394 2 clayey mortars: BDX 6371, 6373 Wall 20 Sediment from a gamma-ray borehole: BDX 6407 Curtain 5a BDX 6411, 6412 Sediment from a gamma-ray borehole: BDX 6410 Curtain 13 BDX 6379, 1 lime mortar from a gamma-ray 6380, 6383 borehole: BDX 6384 Bastion 10 BDX 6385, 6387 1 lime mortar: BDX 6386 Curtain 23 BDX 8209, 8212 3 clayey mortars: BDX 8210, 8211, 8214 1 brick: BDX 8213 Bastion 24 BDX 8220 3 lime mortars: BDX 8216, 8219, 8221 1 clayey sediment: BDX 8235 2 bricks: BDX 8215, BDX 8218 Fluvial wall BDX 6388 1 lime mortar: BDX 6390 1 brick: BDX 6389 ‘West bastion’ 2 of the belt: BDX 1 lime mortar of the belt: BDX 6396 6395, 6397 3 of the core: BDX 1 lime mortar of the core: BDX 6402 6399, 6400, 6401

some clayous materials that can produce spurious luminescence signals while heating. The grains, ranging from 3 to 12 mm in diameter, were selected by successive sedimentations in acetone. Aliquots of the 3e12 mm polycrystalline powder were deposited (z1 mg each) on to brass discs. The TL curves were recorded from room temperature to 500  C in a ‘‘wet’’ nitrogen atmosphere (z96% N2, 4% H2O vapor), at a heating rate of 4  C s1, using an automatic TL reader that has been built in our laboratory. The addition of water in the nitrogen atmosphere prevents spurious signals generated by desorption of water or deshydroxylation from the silicate grain surface while TL heating. A preheat at 180  C during 2 min was applied to reduce

Fig. 2. Sets of TL curves obtained by an additive beta dose technique using fine grains (BDX 6388), after blackbody substraction. All aliquots were preheated at 180  C for 2 min. Groups of curves correspond to the following irradiations: (1) Natural, (2) N þ 3.2 Gy, (3) N þ 6.5 Gy, (4) N þ 9.7 Gy.

Fig. 3. Dose-plateau test (BDX 6388). A plateau domain is observed above 300  C. For further calculations, TL intensities have been integrated within the 300e445  C interval.

possible unstable components of the TL signal that could overlap the interesting signals. The TL emissions were detected by an EMI 9813/QKA phototube through a set of optical filters composed by 2 Schott BG12 and an anti-IR MTO Ta2. The overall detection spectral window extended from 350 to 450 nm. Irradiations were carried out using an 90Sre90Y b source (delivering a dose-rate of 0.09 Gy s1 in SiO2, or an equivalent material, the first January 2001) and an 241Am a source (delivering an alpha equivalent dose-rate of 0.55 Gy s1 in a 6 mm thick deposit of SiO2 or equivalent material). 3.3. Determination of the annealing temperature The determination of the regenerated TL growth curve requires the natural TL signal to be reset. Preliminary studies showed that it is necessary to find out annealing conditions

Fig. 4. Equivalent dose determination (BDX 6388). The growth curve parameters are deduced from regeneration experiments. A slide method taking into account the sensitivity changes (Guibert et al., 1996) is then applied to the ‘‘first reading’’ (Nat þ dose) experimental points. The equivalent dose is given by the intercept of the growth function with the dose axis.

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that best preserve the original luminescence properties of the archaeological material, which are related to its thermal history (Roque et al., 2004; Vieillevigne et al., 2004). It appears that different tested annealing temperatures can lead to significant modifications to the TL curves’ shapes. The growth of the regenerated signals with dose is then affected and so is the equivalent dose. In practice, the search for optimal annealing parameters is based upon a combined X-ray diffractione cathodoluminescenceethermoluminescence study of aliquots submitted to various annealing conditions that are compared to the original (not thermally treated in laboratory) archaeological material. Based on this TL study, the annealing conditions that produce the best scaling between the curves of first reading and those of regenerated TL are finally used. It must be noticed that a suitable thermal treatment for emptying TL traps has to be found for each sample being dated (Table 2). To determine the equivalent dose, two separate sets of aliquots were prepared from the same sample, the first set was used for the additive dose procedure on non-annealed material, and the second set for regenerating the TL signals, the grains having previously been annealed in air. 3.4. Fading studies Since feldspar is a component of the mineral assemblage at the fine-grain scale, a study of the stability of TL signals was necessary (Sanderson, 1988; Tyler and McKeever, 1988; Visocekas et al., 1994; Zink PhD, 1996). Two different hypotheses can occur: after a few days a stable level or an anomalous fading with no stable component can be observed. Concerning the Termez bricks, it was found that after laboratory irradiation and preheating at 180  C during 2 min, the high-temperature signals tended to initially slightly decrease, then to apparently become stable beyond 2 days (Fig. 5). Only one sample presented a high-temperature signal which tended to initially slightly increase, and then to apparently become stable beyond 2 days too (Vieillevigne et al., 2004). Another sample, BDX 6389, which presents a high content in K (Table 3) and K feldspars, did not present any stable high-temperature signals after a 28-day storage. Consequently, this brick has not been dated. Our experimental results showed that the existence of tunneling afterglow (and no stable TL) (Visocekas, 1985) was possible as well as a short term fading with stable level; the results are displayed in Fig. 5. Since the material in our samples contains a majority of quartz grains, known as to exhibit a stable TL, we chose this last model. Thus, all aliquots (natural plus dosed and regenerated) have been then stored for 2 days at room temperature after laboratory irradiation and before TL measurements in order to reduce the possible effects of the high-temperature TL’s fading on the age determination. However, if the model of anomalous fading is relevant, it gives an upper limit of the underestimate of the equivalent dose. In the case of brick BDX 6392, considered to be representative of the Termez bricks, this underestimate should be lower than 6% when applying a two-day delay between irradiation and TL recording.

Table 2 Medieval bricks at Termez Annealing temperature ( C)

b ED (Gy)

k-Value

Tower ruins BDX 6367 BDX 6369 BDX 6372 BDX 6375 BDX 6391 BDX 6392

600 950 900 850 850 950

2.83  0.17 3.60  0.28 4.16  0.19 3.36  0.25 3.54  0.36 3.04  0.31

0.077  0.006 0.084  0.009 0.071  0.004 0.098  0.009 0.102  0.012 0.085  0.010

Wall 20 BDX 6405 BDX 6406

700 900

3.80  0.21 4.04  0.20

0.059  0.002 0.074  0.001

Curtain 5a BDX 6411 BDX 6412

600 600

4.11  0.22 3.71  0.30

0.061  0.002 0.055  0.004

Curtain 13 BDX 6379 BDX 6380 BDX 6383

650 700 600

4.06  0.24 4.02  0.18 3.64  0.13

0.077  0.006 0.068  0.006 0.074  0.004

Bastion 10 BDX 6385 BDX 6387

700 950

3.76  0.26 4.36  0.31

0.096  0.009 0.109  0.012

Curtain 23 BDX 8209 BDX 8212

900 800

4.00  0.24 3.90  0.20

0.081  0.002 0.080  0.002

Bastion 24 BDX 8220

600

3.02  0.17

0.069  0.003

Fluvial wall BDX 6388

600

3.95  0.28

0.085  0.007

‘West bastion’ Belt BDX 6395 BDX 6397

950 900

3.31  0.13 3.23  0.31

0.087  0.006 0.085  0.009

Core BDX 6399 BDX 6400 BDX 6401

950 900 950

4.21  0.23 4.12  0.23 4.08  0.25

0.089  0.006 0.086  0.005 0.079  0.007

Sample

Equivalent dose values and statistical standard deviations determined from TL experiments. The systematic error relevant to the source calibration is 2% for b De. Annealing temperatures are indicated for every dated brick.

3.5. Equivalent dose measurement The Termez bricks exhibited a supralinear growth with b radiation in the low-dose region (below z3 Gy). Moreover, no change in the shape of the growth curve was observed between the two series of experiments, thus verifying the consistency of the applied annealing process. The results of equivalent dose measurements, k-value determination and associated standard deviation are reported in Table 2. Statistical uncertainty is generally below 6%, as a consequence of a low scatter of TL intensities.

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content. As a general trend, the richer in carbonated material the sample is, the lower the radioactivity (Fig. 7). More precisely, the Th/CaCO3 correlation appears to be different in bricks and in mortars. In Fig. 7, the extrapolation of the CaCO3/Th correlation line for bricks intercepts the CaCO3 axis at ca. 40% instead of 100%. The consumption of CaCO3 into high-temperature minerals while firing is a likely explanation for this. 4.3. Moisture content

Fig. 5. A check for the stability of TL (sample BDX 6392). Artificial beta doses have been added to natural irradiation. The remaining artificial TL is plotted as a function of the time of storage at room temperature. Intensities are normalized by the artificial TL signal recorded immediately after the end of laboratory irradiation (4-min delay). A tendency to a slight decrease is observed during the first days, after which the TL signal seems to be stabilized. For illustration, a fading curve fits the experimental data according to the tunnel recombination model (Visocekas et al., 1994) and is extrapolated up to the expected age of sample; the 2 limits at plus or minus one standard deviation have also been drawn.

4. Radiochemical characterization and annual dose 4.1. Radionuclide content The K, U and Th contents of TL samples and surrounding media, which were previously dried at room temperature, are given in Tables 3 and 4. The uranium content, denoted U(238U), is measured from 234Th and 235U g emissions, these nuclides being, respectively, in equilibrium and in constant ratio with the parent nuclide 238U. U(226Ra) is the equivalent uranium concentration deduced from 226Ra activity, considering the equilibrium between 238U and 226Ra. 226Ra activity was measured from 214Pb and 214Bi emissions. The experimental laboratory conditions used ensure that the equilibrium between radon and radium was obtained (Guibert and Schvoerer, 1991). Because of the weakness of 234Th and 235U emissions in comparison to those of 214Pb and 214Bi, the uncertainty that affects U(238U) is higher than that for U(226Ra). 4.2. The distribution of radioactivity Some relevant information can be drawn from the examination of the K, U and Th contents of bricks and mortars. A correlation between U(226Ra) and Th is observed for all samples (Fig. 6), as well as between U(238U) and Th. Those relationships indicate that U and Th series could be supported by a uniform mineral composition within the different kinds of samples (bricks, clayey mortars, sediments and alluvium) more or less diluted by inactive minerals (CaCO3, quartz,.). Lime mortars differ from other samples by having a lower radioactivity. According to the DTAeTG analysis (differential thermal analysisethermogravimetry) performed on TL samples and their surrounding materials, these differences in radiochemical composition are linked to the calcium carbonate

An examination of the as-sampled water content revealed large differences between the various sampled areas in Termez (Tables 3 and 4). In all sectors, except from the tower ruins, the structures had been totally exposed to a long drought period during months just before our fieldwork. Samples collected from these locations were thus entirely dry. In contrast, the material relevant to the tower ruins, embedded in swampy sediment and close to the Amu Darya river, exhibited an important natural moisture content. Saturation water content was measured in all samples. An average moisture content since the medieval structures were built was estimated using both data of the saturation values, and accounting for the (internal/external) position of the brick in the masonry and its relation with the Amu Darya river. The estimated water content of Termez samples, used in the age calculation, ranged between 40% and 75% of the saturation level. For the bricks sampled at the tower ruins, this water content was the natural moisture content, near the saturation level. 4.4. A study of uranium disequilibrium As shown in Tables 3 and 4, significant discrepancies between U(238U) and U(226Ra) were observed. They have been interpreted as the result of disequilibrium in the uranium series, since U(238U) was in most cases different from U(226Ra). Generally, in sediments or in porous media, disequilibria are a consequence of changes in the concentration of long-life nuclides, due to the solubility and thus the mobility of certain species such as uranium (234U, 235U, 238U in the redox state þVI) and/or radium (226Ra) (Gascoyne, 1982). In contrast, thorium is considered to be geochemically stable because of its very low solubility. In the present case at Termez, the disequilibrium could be interpreted as a consequence of water circulation or seepage in the masonries under oxidizing condition leading to a U ingress (U enrichment) or an Ra partial elimination. We have to take into account the proximity between the Amu Darya river and the various masonries to be dated. In addition, the sandy grounds around the citadel would have provided favourable conditions for water infiltrations. As important dosimetric differences can result from the two possibilities of disequilibrium (from U or from Ra) (Guibert et al., 1997), dating requires the predominant mechanism to be identified. For that purpose, 230 Th and 234U activities should have provided relevant information. However, g spectrometry is not routinely able to properly detect the corresponding low-energy and very weak g

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Table 3 Radiochemical composition and natural (as sampled) moisture content of fired bricks Sample dated

K (%)

U(238U) (ppm)

U(226Ra) (ppm)

Th (ppm)

Estimated ‘natural’ H2O (%)

Saturation level

Tower ruins BDX 6367 BDX 6369 BDX 6372 BDX 6375 BDX 6391 BDX 6392

2.598  0.032 2.214  0.027 2.372  0.029 2.616  0.034 2.617  0.034 1.979  0.025

3.86  0.20 3.53  0.19 4.61  0.21 2.98  0.20 2.98  0.20 2.42  0.14

3.380  0.036 3.874  0.038 4.573  0.043 3.146  0.038 3.147  0.038 3.214  0.035

10.41  0.11 11.19  0.11 13.22  0.13 10.77  0.12 10.76  0.12 10.04  0.11

26  2 27  3 25  3 19  3 22  2 13  2

26.1 30.5 27.8 24.2 22.5 15.2

Wall 20 BDX 6405 BDX 6406

2.313  0.029 2.230  0.033

3.20  0.14 4.03  0.19

2.925  0.028 3.195  0.041

9.42  0.09 10.11  0.13

10  3 12  3

19.4 23.1

Curtain 5a BDX 6411 BDX 6412

2.190  0.028 2.145  0.027

3.48  0.13 3.13  0.12

2.870  0.028 2.760  0.027

10.38  0.10 10.18  0.10

94 73

21.9 18.2

Curtain 13 BDX 6379 BDX 6380 BDX 6383

2.246  0.029 2.309  0.028 2.502  0.033

3.26  0.15 2.85  0.13 2.74  0.15

3.031  0.034 2.870  0.030 2.388  0.033

10.34  0.11 9.53  0.10 8.89  0.11

10  3 82 13  3

21.4 15.7 18.7

Bastion 10 BDX 6385 BDX 6387

2.327  0.030 1.985  0.026

2.86  0.15 3.29  0.15

2.806  0.034 3.624  0.038

9.91  0.11 11.33  0.12

13  3 12  5

18.1 24.3

Curtain 23 BDX 8209 BDX 8212 BDX 8213

2.121  0.028 2.169  0.032 2.286  0.032

3.53  0.15 3.14  0.18 3.70  0.16

3.377  0.034 3.036  0.039 3.924  0.040

11.01  0.12 10.46  0.13 13.01  0.14

94 73 13  2

21.7 18.2 32.7

Bastion 24 BDX 8215 BDX 8218 BDX 8220

1.841  0.026 1.942  0.023 1.859  0.029

3.38  0.15 3.55  0.10 3.57  0.17

2.935  0.033 3.511  0.025 3.070  0.039

9.81  0.11 11.09  0.09 9.61  0.12

73 12  3 10  4

17.4 28.8 26.1

Fluvial wall BDX 6388 BDX 6389

2.862  0.036 3.947  0.048

3.32  0.16 2.91  0.14

3.284  0.038 2.618  0.032

10.68  0.12 9.31  0.11

18  3 11  2

24.8 14.1

‘West bastion’ Belt BDX 6395 BDX 6397

2.155  0.026 2.235  0.032

2.97  0.15 3.76  0.20

3.599  0.034 3.168  0.037

9.44  0.10 10.15  0.12

16  3 19  3

21.3 25.9

Core BDX 6399 BDX 6400 BDX 6401

2.448  0.036 2.683  0.037 2.286  0.038

4.73  0.23 3.72  0.20 3.44  0.25

3.203  0.041 3.035  0.037 2.468  0.043

10.98  0.14 10.10  0.12 8.52  0.14

13  3 16  3 17  3

17.2 21.3 22.7

The K, U and Th contents (dry material) have been determined by low-background gamma spectrometry. Uncertainties are one counting standard deviation ; for overall error estimation, the following systematic uncertainties, expressed as estimated relative standard deviations, 0.5% for K, 2% for U(226Ra) and Th, 4% for U(238U), have to be quadratically summed (Bechtel et al., 1997). Note the significant discrepancies between U(238U) and U(226Ra), meaning the existence of a disequilibrium in the U-series.

emission. Furthermore, more appropriate techniques such as alpha spectrometry or thermo-ionization mass spectrometry (TIMS) were not available. Nevertheless, it is possible to determine which element generates disequilibrium thanks to a statistical analysis of the U, Ra and Th distribution amongst the analyzed samples. The basic principle of our analysis is that the disequilibrium

originates in a geochemical alteration. For an archaeological material assumed to be originally in equilibrium, successive leachings will cause variability in contents of the element(s) affected by alteration, according to variable burial and hydrological conditions. The most altered element is then expected to have the largest variability in content and this feature leads to the identification of that element.

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Table 4 Radiochemical composition of environmental elements associated to the material to be dated: lime mortars (LM), clayey mortars (CM), sediments (SD) and alluvial deposits of Amu Darya (AD) Environmental elements

K (%)

U(238U) (ppm)

U(226Ra) (ppm)

Th (ppm)

Tower ruins BDX 6370 BDX 6371 BDX 6373 BDX 6374 BDX 6376 BDX 6394

LM CM CM LM AD LM

1.535  0.023 1.649  0.012 1.552  0.010 1.493  0.022 1.827  0.017 1.277  0.020

4.63  0.23 1.72  0.17 1.78  0.15 2.62  0.14 2.33  0.17 3.15  0.14

1.655  0.032 1.873  0.017 1.876  0.016 1.730  0.031 2.933  0.029 1.778  0.030

4.92  0.10 6.63  0.05 6.53  0.05 5.62  0.09 10.70  0.09 4.96  0.09

Wall 20 BDX 6407

SD

1.943  0.029

2.97  0.16

2.502  0.035

8.59  0.12

Curtain 5a BDX 6410

SD

1.982  0.030

2.91  0.16

2.544  0.036

8.66  0.12

Curtain 13 BDX 6384

LM

1.393  0.020

2.19  0.18

3.304  0.039

4.36  0.09

Bastion 10 BDX 6386

LM

1.650  0.023

2.76  0.17

2.025  0.029

4.20  0.08

Curtain 23 BDX 8210 BDX 8211 BDX 8214

CM CM CM

1.874  0.028 1.905  0.027 1.895  0.027

3.37  0.16 2.71  0.14 2.90  0.14

2.544  0.035 2.473  0.032 2.309  0.032

8.67  0.12 9.29  0.11 8.42  0.11

Bastion 24 BDX 8216 BDX 8219 BDX 8221 BDX 8235

LM LM LM SD

1.774  0.026 1.709  0.026 1.664  0.028 1.923  0.029

5.83  0.16 6.11  0.16 5.35  0.22 3.76  0.20

1.844  0.030 1.817  0.030 1.657  0.033 2.533  0.035

5.86  0.10 5.86  0.10 5.64  0.11 8.41  0.12

Fluvial wall BDX 6390

LM

3.618  0.045

2.48  0.13

1.559  0.027

4.58  0.08

‘West bastion’ Belt BDX 6396

LM

0.853  0.023

3.86  0.21

1.703  0.044

3.27  0.12

Core BDX 6402

LM

1.563  0.026

6.42  0.18

1.858  0.033

4.38  0.10

The alluvial deposits presented contents close to those of the bricks. The high potassium content of the samples of fluvial wall must be noticed. The disequilibrium in U-series is visible too, especially noticeable for lime mortars.

Applying this analysis to the Termez bricks, the U(238U)/Th and U(226Ra)/Th ratios were examined. The dilution effect of calcium carbonate on absolute content is neutralized by calculating the U/Th ratio. The results in Table 5 show that, after correction for random analytical uncertainties, the U(238U)/ Th ratio is the most variable: a variance of 1.20  103 against 0.47  103 for U(226Ra)/Th. Thus, weak movements of uranium appear to be at the origin of the observed disequilibrium. According to the fact that U(238U) is higher than U(226Ra) in most samples, the general tendency is a U partial enrichment. Fig. 8 illustrates this conclusion: a cluster of points is stretched along the U(238U)/Th axis. This disequilibrium remains at a weak amplitude, at least regarding the studied bricks. For the Termez mortars, we have to distinguish both types of mortars: lime and clayey ones. Because of their fewer carbonate contents, the latter present a higher average radioactivity. Lime

mortars present a very high variance in U (8.6  102 against 0.6  102 for U(226Ra)/Th), and clayey mortars present the same characteristics but to a lesser extent (145  105 for U(238U)/Th against 7  105 for U(226Ra)/Th) (Table 5). Thus, lime mortars seem to be subjected to the disequilibrium already observed in bricks and one of the samples could have also undergone an Ra alteration giving the observed Ra measurement dispersion (Fig. 9). Clayey mortars, as for the bricks, seem to have been subjected to disequilibrium less than lime mortars (Fig. 9). Since the radionuclide concentration was gradually changing, the determination of the annual dose enables to estimate U(238U) and U(226Ra) average contents during time for each sample. The averaged values depend on the disequilibrium kinetics, and the cumulative absorbed dose will be obviously different if two extreme situations, early or recent disequilibrium,

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we will consider the following lists of nuclides according to the type of disequilibrium:  ‘‘head’’ of series e from 238U to 234U, and 235U;  ‘‘end’’ of series e from 230Th to 206Pb (including and from 231Th to 207Pb.

226

Ra),

For every sample, the average concentration through time in the U-head, denoted U*U, is given by UU ¼ 1=2½Uð238 UÞ þ Uð226 RaÞ since the content in U isotopes has been gradually varying from U(226Ra) to its actual value, U(238U). The average concentration in the U-end, denoted U*Ra , is given by URa ¼ Uð226 RaÞ Fig. 6. Correlation between U(226Ra) and 232Th. The link between the radioelement contents indicates that actinides could be supported by a uniform mineral assemblage, the lime mortars excepted. Variations of U(226Ra) and Th contents (and K) are a consequence of the CaCO3 concentration inverse variations mainly: A, bricks; -, clayey mortars and sediments; ,, lime mortars; >, alluvion of the Amu Darya river.

The lack of knowledge about the real kinetics leads to a systematic uncertainty that was taken into account in the error estimation. In terms of U content, its maximal extent is equal to

are taken into account. Limits in annual dose estimation can then be defined. At Termez, although there is no independent data to indicate early or late disequilibrium onset, there is relevant information. The climate has not dramatically changed since the medieval period and groundwater movements are expected to have been constant over time at a given location. As a consequence, we consider that a progressive uranium variation is a mid-term hypothesis and a likely behaviour (equivalent to a linear variation in U content). To evaluate dose rates from analytical data in the case of disequilibria requires a separation of U-series nuclides into two groups (Guibert et al., 1997). For example, at Termez,

However, if the model of progressive variation in uranium is inappropriate, the consequences on the TL dates are of less importance. Indeed, for brick BDX 6399 (Table 3), which presents the greatest disequilibrium, the choice of the model leads to a variation of around 20 years for the TL ages. If we applied the ‘‘Early Uptake model’’ to this brick, i.e. U*U ¼ U(238U) and U*Ra ¼ U(226Ra), its age is 807 years. Conversely, if we applied the ‘‘late Uptake model’’, i.e. U*U ¼ U(226Ra) and U*Ra ¼ U(226Ra), its age is 827 years. With the chosen model of progressive variation of uranium, the age of brick BDX 6399 is 819 years (Table 10).

D ¼ jUð238 UÞ  U U j

4.5. The environmental dose-rate Field measurements by gammametry and laboratory determinations of gamma dose-rate from analytical data obtained by low-background gamma spectrometry, have contributed to the evaluation of the environmental dose-rate. Since gammametry measurements were not applied to the curtain 23 and the bastion 24, the evaluation of the gamma dose-rate required reconstruction of the close environment of samples, details of which are presented in Vieillevigne (PhD, 2005). Calculation of g dose rate by reconstruction is primarily based on a geometrical simplification of the ‘‘universe’’ seen by the sample to be dated. We distinguish three types of radioactive materials: (i) the brick being dated, (ii) mortar that seals all bricks in the studied masonry, (iii) other bricks of the wall. These three elementary materials compose three distinct media: Fig. 7. Correlation between 232Th and CaCO3 contents. The differences in radiochemical composition are linked to the calcium carbonate content (symbols as in Fig. 6).

(i) the external medium made of surrounding bricks and mortars: the mass concentration of the elementary materials (mortar and ‘‘other bricks’’) is calculated from

E. Vieillevigne et al. / Journal of Archaeological Science 34 (2007) 1402e1416

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Table 5 Determination of the main cause of disequilibrium in U-series, using a variability study of the U(238U)/Th and U(226Ra)/Th ratios Bricks 238

U(

U)/Th

Lime mortars 226

U(

Ra)/Th

U(

238

U)/Th

Clayey mortars U(

226

Ra)/Th

U(238U)/Th

U(226Ra)/Th

Mean value Standard deviation Relative standard deviation (%)

0.329 0.039 11.9

0.307 0.023 7.4

0.887 0.312 35.1

0.369 0.079 21.5

0.320 0.047 14.5

0.284 0.011 3.7

Variance (V) Variance due to analytical random errors (Van) Net variance (V  Van)

148  105 28  105

50  105 3  105

8750  105 193  105

568  105 13  105

185  105 40  105

9  105 2  105

120  105

47  105

8557  105

555  105

145  105

7  105

238

226

The variance of U( U)/Th being much greater than that of U( Ra)/Th implies that U is the most altered element and is very likely to be at the disequilibrium origin. For lime mortars, we excluded one of them because of its great content of Ra, very different from those of the 48 other samples.

the average dimensions of the bricks and from the average spatial period of the ‘‘brick þ mortar’’ unit, using on-site measurements and photographs (the unit cell is composed by a brick covered by half thickness of mortar). (ii) the mortar layer that covers the dated brick on every side, whose thickness is taken equal to half the distance between two consecutive bricks in order to ensure continuity with the external medium, (iii) the dated brick.

these ones, the concentration of the three radioactive media is calculated (calculating the differences in mass of each medium). By weighting each spherical element (radius r) with a factor proportional to the dose transmission (through a sphere of radius r), the equivalent content of the dated brick, mortar and external medium is calculated. It defines the infinite equivalent medium, whose K, U, Th contents are then known.

A set of points are virtually defined in the portion of brick that has been sampled in laboratory to extract the TL grains. For each point, the current one becomes the center of concentric spheres (with radius varying from 1 to 50 cm: 1, 2, 3, 4, 5, 7, 9, 12, 15, 20, . 50 cm). In any spherical volume, the mass concentrations of the three media (external, mortar, brick being dated) are calculated. By using the difference between two consecutive spheres, spherical elements are defined. In

Table 6 reports the composition of the radioactive materials taken into account in g dose-rate calculation of the two samples dated at curtain 23 (BDX 8209 and 8212); cosmic and environmental dose rates are also given. The external medium is composed of 69.9% ‘‘brick material’’ (average content of samples BDX 8209, 8212 and 8213), and 30.1% ‘‘mortar material’’ (average content of samples BDX 8211, 8214). Table 7 regroups the same kind of data relative to the bastion 24. The external medium is quite different because of the

Fig. 8. Study of the disequilibrium in the U-series for the Termez bricks. The greater scatter of the U(238U)/Th values in comparison with those of U(226Ra)/ Th illustrates the fact that disequilibrium is due to mobility of uranium rather than radium. The latter is quite weak as the cluster is located around the equilibrium line.

4.6. The environmental dose-rate at curtain 23, bastion 24 and other masonries

Fig. 9. Study of the disequilibrium in U-series of mortars and sediments. Here appears a tendency to a slight disequilibrium for the clayey mortars (-), and to a higher disequilibrium for the lime mortars (,).

E. Vieillevigne et al. / Journal of Archaeological Science 34 (2007) 1402e1416

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Table 6 Environmental dose-rate determination at curtain 23 (in mGy a1) Components of the g reconstruction

Environment of brick BDX 8209

Environment of brick BDX 8212

Equivalent concentration (%)

Infinite matrix g dose-rate

Equivalent concentration (%)

Infinite matrix g dose-rate

Dated brick Mortars BDX 8211, 8214 External medium

41.4 8.3 50.3

1.308  0.042 1.065  0.036 1.271  0.048

35.0 7.9 57.2

1.275  0.042 1.065  0.036 1.271  0.048

1.269  0.042 0.150  0.015 1.419  0.045

Gamma dose-rate Cosmic dose-rate Environmental dose-rate

1.257  0.043 0.150  0.015 1.407  0.045

For these reconstruction using analytical results, the different elements contributing to the g radiation are sorted in three components: the contribution of the dated brick, an average content corresponding to the nearby mortars and another content of the average outside surroundings. Their respective weight in the total g doserate has been calculated according to Guibert et al.’s technique (1998).

larger thickness of mortar in this masonry. It is composed of 53.5% ‘‘brick material’’ (average content of samples BDX 8215 and 8220), and 46.5% ‘‘mortar material’’ (average content of samples BDX 8216, 8221). The gamma dose rates were measured by on-site gammametry for the other masonries (Table 8). 4.7. Annual dose assessment Table 9 includes dose-rate data relevant to the dated bricks. The total annual dose of brick fragments varies within a 4.1e 5.1 mGy a1 range. 5. TL-dating results and discussion The TL results are given in Table 10 and illustrated by Fig. 10. Two types of uncertainties are given. We accounted for random uncertainties such as those generated by the scatter of the TL curves and counting statistics in gamma spectroscopy. Systematic uncertainties regroup those generated by calibration uncertainties of a and b sources used for TL experiments, those related to K, U and Th determination by gamma spectroscopy (radionuclide half-lives, geometric factor, approximations in self-absorption corrections, in efficiency function determination), estimated errors on archaeological Table 7 Environmental dose-rate determination at bastion 24 (in mGy a1) Components of the g reconstruction

Environment of brick BDX 8220 Equivalent concentration (%)

Infinite matrix g dose-rate

Dated brick Mortars BDX 8211, 8214 External medium

33.1 10.8 56.0

1.138  0.037 0.817  0.029 0.973  0.032

Gamma dose-rate Cosmic dose-rate Environmental dose-rate

1.010  0.032 0.150  0.015 1.160  0.036

For these reconstruction using analytical results, the different elements contributing to the g radiation are sorted in the three same components as those used for the curtain 23: the contribution of the dated brick, an average content corresponding to the nearby mortars and another content of the average outside surroundings. Their respective weight in the total g dose-rate has been calculated according to Guibert et al.’s technique (1998).

moisture content, on environmental dose-rate determination by reconstruction and on effects of disequilibrium kinetics on annual dose. All uncertainties are combined in quadrature. General observations can be drawn from the data summarized in Table 10. Random uncertainties (standard deviations) on TL ages are rather low e less than 5% in most cases e as a consequence of a low dispersion of TL intensities and regular behaviour of a majority of samples. However, we notice that statistical uncertainties on De measurements still form, in most cases, the main contribution to overall uncertainty. Moreover, the estimated error on ‘‘archaeological’’ water content plays an important part in systematic uncertainties due to the high porosity of the bricks. For the tower ruins, archeologically considered to be the medieval port, the six TL ages are not consistent with the expected dispersion of results given by statistical uncertainties. The TL ages of bricks BDX 6369 and 6372 are younger than the four others, although no anomaly of the TL properties was observed during the experiments, i.e. fading was not detected and growth curves fitted correctly. We supposed that the two bricks BDX 6369 and 6372 may have been re-used but a c2 test confirmed that only one (BDX 6372) provided anomalous results with respect to the assumption of contemporaneity. Indeed, c2 value is 27.8 when we include BDX 6372

Table 8 Environmental dose-rate determinations at all the dated masonries, except those of the curtain 23 and the bastion 24 (in mGy a1) Masonry

Associated dated samples

Environmental dose-rate (NaI:Tl)

Tower ruins

Wall 20 Curtain 5a Curtain 13 Bastion 10 Fluvial wall

BDX BDX BDX BDX BDX BDX BDX BDX

6369 6375 6392 6406 6412 6380, 6383 6387

1.037  0.030 1.085  0.040 1.002  0.040 1.363  0.040 1.407  0.045 1.097  0.060 1.207  0.050 1.246  0.030

‘West bastion’ Belt Core

BDX 6395, 6397 BDX 6399, 6400, 6401

1.086  0.040 1.366  0.066

6367, 6372, 6391, 6405, 6411, 6379, 6385, 6388

The values are obtained by in situ measurements (4p geometry).

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Table 9 Dose-rate determinations (in mGy a1) of the studied Termez’s bricks Sample dated

Alpha dose-rate (eq. b)

Beta dose-rate

Internal dose-rate

External dose-rate

Total dose-rate

Tower ruins BDX 6367 BDX 6369 BDX 6372 BDX 6375 BDX 6391 BDX 6392

1.05  0.10 1.25  0.15 1.28  0.11 1.36  0.15 1.39  0.19 1.21  0.18

2.24  0.06 2.04  0.07 2.31  0.08 2.35  0.08 2.30  0.06 2.01  0.10

3.29  0.14 3.29  0.19 3.58  0.16 3.71  0.19 3.68  0.21 3.22  0.25

1.04  0.03 1.04  0.03 1.09  0.04 1.09  0.04 1.00  0.04 1.00  0.04

4.33  0.16 4.33  0.21 4.67  0.20 4.80  0.23 4.69  0.22 4.22  0.26

Wall 20 BDX 6405 BDX 6406

0.80  0.09 1.09  0.13

2.27  0.09 2.24  0.13

3.08  0.15 3.33  0.24

1.36  0.04 1.36  0.04

4.44  0.18 4.69  0.26

Curtain 5a BDX 6411 BDX 6412

0.88  0.11 0.78  0.13

2.23  0.14 2.21  0.11

3.12  0.23 2.99  0.19

1.41  0.05 1.41  0.05

4.52  0.26 4.40  0.22

Curtain 13 BDX 6379 BDX 6380 BDX 6383

1.13  0.12 0.94  0.10 0.87  0.08

2.26  0.08 2.30  0.06 2.26  0.09

3.39  0.17 3.24  0.12 3.13  0.15

1.10  0.06 1.10  0.06 1.10  0.06

4.49  0.21 4.34  0.15 4.23  0.18

Bastion 10 BDX 6385 BDX 6387

1.28  0.15 1.78  0.24

2.21  0.07 2.13  0.12

3.49  0.18 3.90  0.31

1.21  0.05 1.21  0.05

4.70  0.22 5.11  0.37

Curtain 23 BDX 8209 BDX 8212

1.30  0.15 1.20  0.12

2.25  0.10 2.27  0.08

3.56  0.21 3.47  0.17

1.42  0.05 1.41  0.05

4.97  0.26 4.88  0.20

Bastion 24 BDX 8220

0.98  0.12

1.98  0.11

2.96  0.21

1.16  0.04

4.12  0.24

Fluvial wall BDX 6388

1.22  0.13

2.55  0.08

3.77  0.18

1.25  0.03

5.01  0.21

‘West bastion’ Belt BDX 6395 BDX 6397

1.25  0.13 1.17  0.15

2.11  0.09 2.10  0.09

3.37  0.19 3.27  0.20

1.09  0.04 1.09  0.04

4.45  0.22 4.36  0.22

Core BDX 6399 BDX 6400 BDX 6401

1.36  0.20 1.19  0.15 1.18  0.17

2.41  0.20 2.45  0.11 2.33  0.16

3.77  0.38 3.64  0.22 3.50  0.30

1.37  0.07 1.37  0.07 1.37  0.07

5.14  0.40 5.00  0.25 4.87  0.32

Uncertainties are one standard deviation. They include statistical and systematic components.

(c2 range: 1.6e9.2 with 80% of probability) (CEA, 1978). The weighted average date for the five other samples is 1290  31 AD with a c2 equal to 3.4 (c2 range: 1.1e7.8). Ages have been averaged using a classical procedure: the weighting coefficient that affects every TL age is equal to the inverse of statistical variance. These vestiges are probably not the ruins of the medieval port, existing at the end of the 12th century (Gelin and Tonnel, 2001). Moreover, they have been built after the destruction by Genghis Khan in 1220 (Vieillevigne et al., 2004). Wall 20 has been dated by two bricks, exhibiting concordant ages. The averaged TL date is 1145  50 AD. Curtain 5a is running on from the curtain 13. These two masonries are strictly numerically contemporaneous. The first has an averaged TL date of 1119  60 AD, and the second of

1115  37 AD. Bastion 10, resting on curtain 5a, presents an averaged TL date of 1178  46 AD. It is highly likely then that bastion 10 was built later than both masonries 5a and 13. Curtain 23 is dated by two bricks and bastion 24 by one, sampled at its belt. A second brick that was sampled at the latter masonry showed an anomalous dose-plateau test and its result has not been taken into account. In the same way as the previous walls, the averaged TL date of curtain 23 (1202  44 AD) and the date of brick BDX 8220 sampled at bastion 24 (1270  57 AD) enable us to conclude that bastion 24 is posterior. Only one brick, BDX 6388, taken from fluvial wall, was dated because of the remarkable anomalous fading of the other (BDX 6389) leading us to give up its dating. So this wall has a TL date of 1215  51 AD.

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Table 10 TL-dating results of medieval bricks from the Termez citadel Masonry and laboratory reference

TL age  total uncertainty (a)

Statistical uncertainty (a)

Estimated systematic uncertainty (a)

TL date (AD)

Averaged TL date (AD)

Tower ruins BDX 6367 BDX 6369 BDX 6372 BDX 6375 BDX 6391 BDX 6392

656  37 833  57 893  44 703  46 759  61 723  64

31 49 32 39 57 53

20 28 30 24 22 36

1348  37 1171  57 1111  44 1301  46 1245  61 1281  64

1290  31 (BDX 6372 excluded)

Wall 20 BDX 6405 BDX 6406

855  57 862  63

48 44

31 44

1149  57 1142  63

1145  50

Curtain 5a BDX 6411 BDX 6412

908  69 843  79

51 70

47 36

1096  69 1161  79

1119  60

Curtain 13 BDX 6379 BDX 6380 BDX 6383

906  54 928  45 860  42

42 36 27

34 28 33

1098  54 1076  45 1144  42

1115  37

Bastion 10 BDX 6385 BDX 6387

802  51 854  63

43 46

27 42

1202  51 1150  63

1178  46

Curtain 23 BDX 8209 BDX 8212

805  60 800  51

50 42

33 28

1199  60 1204  51

1202  44

Bastion 24 BDX 8220

734  57

43

37

1270  57

Fluvial wall BDX 6388

789  51

44

25

1215  51

‘West bastion’ Belt BDX 6395 BDX 6397

744  40 742  60

25 53

31 29

1260  40 1261  60

1260  38

Core BDX 6399 BDX 6400 BDX 6401

819  69 823  52 838  65

34 39 41

60 34 51

1185  69 1181  52 1166  65

1178  54

TL ages are given in years before 2004. Uncertainties are one standard deviation. Final weighted average dates and relevant uncertainties are reported.

‘‘West bastion’’ is dated by two different masonries. The first, the belt of the bastion, has an averaged TL date of 1260  38 AD. For the second, the core of this structure, the averaged TL date is 1178  54 AD. We could conclude that the first bastion presented only a rectangular core, and that its belt was built in a second phase, probably after the Genghis Khan sacking. When we compare the dates of the belts of bastions 24 and ‘‘west’’, we could assume that these two masonries are strictly contemporaneous. The date of the wall 23 seems to be inserted between the dates of the two phases of the fluvial wall. Moreover, brick BDX 6388, sampled at fluvial wall, seems to correspond to a restoration. Its dating (1215  51 AD) is neither contemporaneous to the first phase of the fluvial wall, the rectangular bastions (1178  54 AD for the ‘‘west

bastion’’), nor to the second phase, the semicircular belts (1260  38 AD for the same bastion). 6. Conclusion Most TL ages obtained at Termez have a low statistical standard deviation, around or less than 5%, leading to averaged dates that exhibit a satisfactory precision for drawing out some general trends about the site’s chronological sequence. So, differences in age between the investigated structures appear significant. The tower ruins, the belts of bastions 24 and ‘‘west’’ are the oldest: their averaged TL dates place them in the second half of the 13th century. The fluvial wall, the curtain 23 and the core of ‘‘west bastion’’ and 10 were built previously,

E. Vieillevigne et al. / Journal of Archaeological Science 34 (2007) 1402e1416

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to Guy Salignie`re and Pierre Selva, technician and engineer at the IRAMAT-CRPAA, for their very significant contribution to that work. References

Fig. 10. Dating results for medieval bricks from the Termez citadel. The segments represent one standard deviation including statistical and estimated systematic uncertainties.

at some point between the second half of the 12th century and the first half of the 13th century. So we can deduce that the defensive wall around the citadel was built in two distinct phases: the first corresponds to a wall with rectangular bastions, in agreement with the tradition of the Hellenistic defensive architecture (Pougatchenkova, 2001a), the second phase consisted of building semicircular masonry bastions around the rectangular bastions, apparently after the destruction by Genghis Khan in 1220. The curtains 13, 5a and wall 20 are the most recent of all: their averaged TL dates attach these masonries to the first half of the 12th century. The TL campaign at Termez leads us to revisit the initial chronology suggested by the archaeologists (Vieillevigne PhD, 2005). Previously, the medieval citadel was expected to belong to the 9the14th century interval. TL dates, and particularly the averaged TL dates, offer a narrower range, between the beginning of the 12th century and the end of the 13th century. These medieval fortifications were therefore built quite quickly. All dynasties in power (Karakhanides, Ghazne´vides, Khorezmchahs, Mongols) seem to have been involved in their construction or restoration.

Acknowledgements This work was financially supported by the following institutions: Ministe`re franc¸ais des Affaires e´trange`res, Conseil re´gional d’Aquitaine, Universite´ de Bordeaux 3 and CNRS. The authors wish to thank Pierre Leriche (AURORHE, UMR 8546-9 CNRS/ Ecole Normale Supe´rieure, Paris) for the proposal of this TL study on the citadel of Termez. Special thanks

Adamiec, G., Aitken, M.J., 1998. Dose-rate conversion factors: update. Ancient-TL 16 (2), 37e49. Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Bailiff, I.K., Holland, N., 2000. Dating bricks of the last two millennia from Newcastle upon Tyne: a preliminary study. Radiation Measurements 32, 615e619. Bechtel, F., Guibert, P., Schvoerer, M., Vartanian, E., Fa€ın, J., Miallier, D., Montret, M., Pilleyre, Th., Sanzelle, S., Bahain, J.J., Falgue`res, C., Tripier, J., Poupeau, G., Mercier, N., Valladas, H., 1997. Evaluation de l’incertitude de mesure de la dose annuelle en datation par luminescence (TL, OSL) et par RPE: une expe´rience d’intercomparaison a` la Grotte XVI, Ce´nac et Saint-Julien, Dordogne. Revue d’Arche´ome´trie 21, 21e27. CEA, 1978. Statistique applique´e a` l’exploitation des mesures, t.1. Masson, Paris. Gallo, N., Fieni, L., Martini, M., Sibilia, E., 1999. Building archaeology, 14C and thermoluminescencea: two examples comparison. In: Actes du Colloque ‘‘C14 et Arche´ologie’’, III congre`s international, Lyon 6e10 avril 1998, Me´moire de la Socie´te´ Pre´historique Franc¸aise, tome XXVI et Supple´ment de la Revue d’Arche´ome´trie, pp. 425e431. Gascoyne, M., 1982. Geochemistry of the actinides and their daughters. In: Ivanovitch, M., Harmon, R.S. (Eds.), Uranium Series Disequilibrium: Applications to Environmental Problems. Clarendon Press, Oxford, pp. 33e55. Gelin, M., Tonnel, B., 2001. Les fortifications fluviales de la citadelle de Termez. Re´sultats des travaux de 1994 a` 1996, in: La Bactriane au carrefour des routes et des civilisations de l’Asie centrale: Termez et les villes de Bactriane-Tokharestan, Actes du colloque de Termez 1997, Collection La Bibliothe`que d’Asie centrale. Maisonneuve et Larose-IFEAC, Paris, pp. 101e121. Goedicke, C., 2003. Dating historical calcite mortar by blue OSL: results from known age samples. Radiation Measurements 37 (4e5), 409e416. Greilich, S., Glasmacherich, U.A., Wagner, G.A., 2002. Spatially resolved detection of luminescence: a unique tool for archaeochronometry. Naturwissenschaften 89, 371e375. Guibert, P., Schvoerer, M., 1991. TL-dating: low background gamma spectrometry as a tool for the determination of the annual dose. Nuclear Tracks Radiation Measurements 18 (1e2), 231e238. Guibert, P., Vartanian, E., Bechtel, F., Schvoerer, M., 1996. Non linear approach of TL response to dose: polynomial approximation. Ancient-TL 14 (2), 7e14. Guibert, P., Bechtel, F., Schvoerer, M., 1997. De´se´quilibre des se´ries de l’uranium, implications sur la dose annuelle en datation par thermoluminescence: une e´tude a` la Grotte XVI, Ce´nac et Julien, Dordogne (France). Quaternaire 8, 377e389. Guibert, P., Bechtel, F., Schvoerer, M., Mu¨ller, P., Balescu, S., 1998. A new method for gamma dose-rate estimation in heterogeneous media. Radiation Measurements 29 (5), 561e572. Leriche, P., Pidaev, C., Abdoullaev, K., Gentelle, P., 1997. Bilan de la campagne 1997 de la MAFOuz de Bactriane. Bulletin of Asia Institute 11, 17e52. Leriche, P., 2001. Termez antique et me´die´vale. La Bactriane au carrefour des routes et des civilisations de l’Asie centrale: Termez et les villes de Bactriane-Tokharestan, Actes du colloque de Termez 1997, Collection La Bibliothe`que d’Asie centrale. Maisonneuve et Larose-IFEAC, Paris, pp. 75e99. Leriche, P., Pidaev, C., Gelin, M., Abdoullaev, K., 2001. La Bactriane au carrefour des routes et des civilisations de l’Asie centrale: Termez et les villes de Bactriane-Tokharestan, Actes du colloque de Termez 1997, Collection La Bibliothe`que d’Asie centrale. Maisonneuve et Larose-IFEAC, Paris. Pougatchenkova, G., 2001a. Histoire des recherches arche´ologiques en Bactriane septentrionale. Re´gion du Sourkhan Darya, Ouzbe´kistan (jusqu’a` la cre´ation de la MAFOuz B). La Bactriane au carrefour des routes et des civilisations de l’Asie centrale: Termez et les villes de

1416

E. Vieillevigne et al. / Journal of Archaeological Science 34 (2007) 1402e1416

Bactriane-Tokharestan, Actes du colloque de Termez 1997, Collection La Bibliothe`que d’Asie centrale. Maisonneuve et Larose-IFEAC, Paris, pp. 23e34. Pougatchenkova, G., 2001b. C’e´tait hier, c’e´tait il y a longtemps (Souvenirs d’une participante a` l’expe´dition arche´ologique de Termez en 1938). La Bactriane au carrefour des routes et des civilisations de l’Asie centrale: Termez et les villes de Bactriane-Tokharestan, Actes du colloque de Termez 1997, Collection La Bibliothe`que d’Asie centrale. Maisonneuve et Larose-IFEAC, Paris, pp. 37e46. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term time variations. Radiation Measurements 23 (2e3), 497e500. Roque, C., Guibert, P., Vartanian, E., Vieillevigne, E., Bechtel, F., 2004. Changes in luminescence properties induced by thermal treatments;

a case study at Sipan and Trujillo Moche sites (Peru). Radiation Measurements 38 (1), 119e126. Sanderson, D.C.W., 1988. Fading of TL in feldspars: characteristics and corrections. Nuclear Tracks and Radiation Measurements 14 (1e2), 155e161. Tyler, S., McKeever, S.W.S., 1988. Anomalous fading of TL in oligoclase. Nuclear Tracks and Radiation Measurements 14 (1e2), 149e154. Vieillevigne, E., Guibert, P., Bechtel, F., Leriche, P., 2004. Thermoluminescence et chronologie de l’architecture me´die´vale: datation de briques de la citadelle de Termez en Ouzbe´kistan. Revue d’Arche´ome´trie 28, 43e55. Visocekas, R., 1985. Tunneling radiative recombination in labradorite: its association with anomalous fading. PACT 3, 258e265. Visocekas, R., Spooner, N.A., Zink, A., Blanc, P., 1994. Tunnel afterglow, fading and infrared emission in thermoluminescence of feldspars. Radiation Measurements 23, 377e385.