Optically stimulated luminescence (OSL) dating investigations of rock and underlying soil from three case studies

Journal of Archaeological Science 34 (2007) 1659e1669 http://www.elsevier.com/locate/jas

Optically stimulated luminescence (OSL) dating investigations of rock and underlying soil from three case studies A. Vafiadou a,b, A.S. Murray a, I. Liritzis b,* a

Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University, Risø National Laboratory, DK-4000 Roskilde, Denmark b University of the Aegean, Department of Mediterranean Studies, Laboratory of Archaeometry, 1 Democratias Avenue, Rhodes 85100, Greece Received 31 July 2006; received in revised form 29 November 2006; accepted 5 December 2006

Abstract Three rock samples and associated underlying surface (floor) soils of geoarchaeological significance from Greece, Sweden and a modern surface stone-sample from a Danish site were investigated using OSL dating. Thin slice, sub-samples, from drilled core surfaces were prepared. A single-aliquot regenerative-dose (SAR) protocol was used on whole rock slices to estimate the laboratory equivalent dose. Laboratory tests showed that the SAR protocol successfully corrected for sensitivity changes and that a known laboratory dose could be measured accurately. The luminescence signals from quartz and feldspar stimulated by blue light and from feldspar under IR stimulation were employed in equivalent dose calculations. Only IR signals showed measurable fading on a laboratory timescale. Laboratory tests showed that daylight bleaching of the rock surfaces is rapid, and that the light-exposed region extended into the solid rock. The geoarchaeological ages obtained for the rocks and soils were in satisfactory agreement with independent age estimates. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: OSL; Luminescence; Dating; Rock; Granite; Soil; Bleaching; Neolithic

1. Introduction Geoarchaeological materials often encountered in archaeological contexts can be regarded as potential chronometers. Often archaeological materials, such as tools, artefacts and other remains, are either too sparse or too valuable to be destructively sampled for dating or characterization. However, geological materials found in archaeological contexts, such as soil floors, pebbles, tephra deposits, burnt clay, and raw obsidian, may provide extremely useful information on the dating of cultural phases, characterization, trade exchange, and the dating of geological events related to the archaeology of the settlement. In this paper we examine the potential of luminescence dating applied to soil floors and overlying pebbles. Optically stimulated luminescence (OSL) can be used to determine the time elapsed since certain minerals, such as

* Corresponding author. E-mail address: [email protected] (I. Liritzis). 0305-4403/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jas.2006.12.004

quartz and feldspar were last exposed to daylight. It is now widely used in the dating of geological sediments such as aeolian, marine and fluvial sand and muds, loess, and colluvial materials over the last w200 ky (recently reviewed by Murray and Olley, 2002; see also Liritzis, 2000; Liritzis et al., 2002). It also has many applications in archaeology and anthropology (Roberts, 1997; Liritzis and Galloway, 1999; Liritzis et al., 1996; Liritzis, 1994, 2002; Theocaris et al., 1997). As well as dating the geological sediments providing the burial context in archaeological deposits, it can be used directly to date artefacts such as ceramics, pot boilers, fireplaces, and even rock art (Roberts et al., 1996). In OSL dating, the amount of trapped charge within the crystal structure is used as a measure of time; this charge accumulates as a result of the exposure to the natural radiation flux. The trapped charge can be emptied by exposure to heat or light; exposure to daylight empties previously trapped charge, and the longer this exposure the more complete the zeroing or bleaching process. In the laboratory the same process is used to measure the trapped charge population e when the crystal is

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exposed to visible or infrared (IR) light under controlled conditions, the release of charge is accompanied by release of ultraviolet (UV) and visible light, as the charge recombines and gives up energy. Because this light (luminescence) is stimulated by exposure to light it is called optically stimulated luminescence. The trapped charge population is measured in terms of the dose absorbed by the crystal lattice, and methods for measuring this dose have undergone rapid development in the last 10 years. Many measurements are now based on so-called single-aliquot methods (Duller, 1991, 1995; Liritzis, 1995; Liritzis et al., 1994, 1997a, 2001; Murray et al., 1997a; Murray and Wintle, 2000), where all the measurements needed to determine a dose are made on one sub-sample (aliquot) of the material to be dated. Determining the age of stone structures and artefacts (tools, monoliths, buildings, cairns, field walls, etc.) using physical methods is notoriously difficult. Age estimates almost always use material associated with the construction period, rather than material directly from the fabric of the construction, in particular by association with 14C datable material; in many cases appropriate organic debris is either not available, or the association is insecure. We have investigated an alternative strategy, which uses OSL to date the last time a stone surface was exposed to daylight. If construction subsequently buried this surface, then this approach should provide a direct method for dating the time of construction. Rocks contain the same minerals that are used for dating geological and archaeological sediments, and many rock surfaces are exposed to daylight for very long periods of time before being exploited, often for much longer than the time for which sediments were exposed during transport. Thus it is likely that in many cases, all the mineral grains in at least some of the surfaces of stones used by ancient peoples will have been exposed to daylight sufficiently to have been completely zeroed, and thus provide an accurate chronometer. This project is a first attempt to date rock surfaces, and their underlying soil surfaces, by using quartz and feldspar (Habermann et al, 2000; Greilich et al., 2002). It builds on earlier experience of the luminescence signals from limestone rocks (Liritzis et al., 1996, 1997b; Liritzis and Galloway, 1999). We describe the IR and blue light stimulated luminescence characteristics of three rocks (two from archaeological sites, one from a modern cobbled farmyard). The time since last daylight exposure is calculated for the two older samples, and compared with independent age control based on 14C dating, and on OSL dating of the soil surfaces immediately underlying the rocks. 2. Samples, sample preparation and equipment Three rock samples were used in this study. Two were from archaeological sites: sample MYK-R (lab. code 003017a) comes from a Neolithic settlement at Ftelia, Mykonos, Greece, and sample SWE-R (lab. code 003015) comes from a burial mound in Rojningsrose, Ska˚ne, Sweden. The third sample DEN-R (lab. code 003020) was a cobble from a farmyard in eastern Zealand, Denmark. The first two samples had

associated soil samples (MYK-S, lab. code 003017, and SWE-S, lab. code 003016, respectively), collected between 0 and 3 cm below the bottom surface of the rocks. A further soil sample SWE-SR (lab. code 003015a), was scraped (in the laboratory) directly from a 0.5 cm thick soil layer adhering to the bottom surface of the rock SWE-R. Soil samples SWE-S and SWE-RS are the same. Sample SWE-RS is the soil beneath the rock that was stuck on the rock. All samples for dating were collected at night, except for DEN-R, which had been exposed to daylight for an extended period before sampling. From a visual inspection of fresh sections, sample MYK-R is granite with potassium- and calcium rich feldspar and quartz; sample SWE-R is an ultramafic rock with veins of quartz and occasional heavy minerals, and sample DEN-R is a quartz metamorphic rock with minor feldspar (Jain, private communication). Sub-samples for luminescence measurements were taken by drilling cores from the rock surface using a diamond tipped coring drill, 6 mm internal diameter. One millimeter thick slices were then cut from the cores using a low speed diamond tipped saw. By examining the luminescence sensitivity and signal, it was found necessary to wash the surface slices in 10% HCl for 10 min and etch in 45% HF for 10 min, to remove weathering products. From the surface slices would be used to obtain the burial age, and so it is important that the OSL signal is unaffected by the effects of burial (e.g. weathering products, diffusion of impurities, etc.). No acid treatment was necessary on inner slices. Soil samples MYK-S, SWE-S and SWE-SR were sieved to recover the 90e250 mm, 180e250 mm and 90e250 mm grain sizes, respectively. These fractions were then washed in 10% HCl and 10% H2O2 to remove carbonates and organics, etched in concentrated HF for 40 min to remove any feldspar and finally washed again in 10% HCl. Measurements of the whole rock slices were made in an automated Risø TL/OSL reader equipped with blue LED (w50 mW/cm2 at 470  30 nm) and IR laser (w500 mW/ cm2 at 830 nm) stimulation sources (Bøtter-Jensen et al., 2000). Luminescence was detected through 9 mm of U-340 filter, and the reader is fitted with an internal 90Sr/90Y beta source delivering 0.024 Gy/s. Individual grains of quartz from the soil samples were measured using the single-grain attachment to the Risø TL/OSL reader (Duller et al., 2000). This unit uses a stimulation source consisting of a 532 nm laser delivering about 40 W/cm2 to 180e212 mm grains mounted in one hundred holes, 300 mm diameter by 300 mm deep, drilled in the surface of a 10 mm diameter aluminium sample disc. Detection optics is the same as in the standard reader, and the beta source in this unit delivers 0.10 Gy/s. A single-aliquot regenerative-dose (SAR) protocol was used for the measurement of the equivalent dose De for all samples: for the separated quartz grains, in the form described by Murray and Wintle (2000); for the whole rock slices, in the form described by Wallinga et al. (2000). These measurement sequences are summarised in Table 1.

A. Vafiadou et al. / Journal of Archaeological Science 34 (2007) 1659e1669

0.85  0.05 for the rock slices (Mejdahl, 1979), because of the larger mean grain size in the whole rock slices (as observed under a microscope). In every case the alpha dose rate was ignored since the samples were etched in HF. The differences in radioactivity between rock and soil under extreme assumptions of the relative contributions of rock and soil to the total dose rate give a 7% uncertainty in the total dose rate; this is the uncertainty used in the beta dosimetry in further calculations. Note that the dosimetry given in Table 2 is calculated for quartz grains, i.e. no allowance is made for possible localised internal radioactivity such as can exist in potassium-rich feldspar grains (Bøtter-Jensen et al., 1996). The validity of this assumption is discussed in Section 9.

Table 1 Outline of SAR protocols Step

Soil (i.e. quartz)

Whole rock slices (i.e. mixed quartz and feldspar)

1.

Give dose Di (¼0 Gy if i ¼ 0, i.e. natural measurement)

2.

Preheat

3.

Measure OSL

4.

Give test dose

5.

Heat

6.

Measure OSL

7.

Repeat steps 1e6 for a range of regeneration doses including a zero dose and a repeated regeneration dose

200  C for 10 s 50 s blue light at 125  C (¼Lbi)

100 s IR at 125  C (¼Lri) 50 s blue light at 125  C (¼Lbi) Dt 160  C

50 s blue light at 125  C (¼Tbi)

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100 s IR at 125  C (¼Tri) 50 s blue light at 125  C (¼Tbi)

Calculate Li/Ti for natural and regenerated points and plot as a function of laboratory dose, Di. For the sample DEN-R the preheat temperature in step 2 is 240  C for 10 s and the heat temperature in step 5 is 210  C.

4. OSL results for soil samples The two archaeological samples (MYK-S and SWE-S) both have associated radiocarbon ages. These are summarised in Table 3. To provide further age control on the two archaeological rock samples, the associated soil samples were analysed using the single-grain attachment to the Risø reader. In this approach, the soil samples were sieved again to retrieve grains 180e 250 mm in diameter and the SAR protocol used to generate OSL signals from these quartz grains; these signals were then analysed in the usual manner (Murray and Roberts, 1997). Fig. 1 presents the radial distributions (Galbraith et al., 1999) of these grains for the three soil samples: 21 grains (Fig. 1c) MYK-S associated with rock sample MYK-R (43 grains were examined), 46 grains (Fig. 1b) SWE-S associated with rock sample SWE-R (92 grains were examined), and 51 grains (Fig. 1a) SWE-SR which was scraped from the surface of rock sample SWE-R (103 grains were examined). In all three cases there may be some evidence for some contamination by recent exposure to daylight (grains with doses w0 Gy), especially in the case of sample SWE-S (Fig. 1b). The equivalent doses (De) from these ‘exposed’ grains have been ignored in calculating the average values of De shown in Fig. 1, and estimates of De with errors greater than 20% were also rejected. Using the dosimetry derived from Table 2 and the assumptions in Section 3, the apparent time since last exposure to daylight of these buried soil surfaces is given in Table 3.

3. Dosimetry High resolution gamma-spectrometry was used to provide the information on the dosimetry of the samples which is summarised in Table 2. Rock sample SWE-R was assumed to be sufficiently large (w30 cm diameter) that 50% of both the beta and gamma dose rates to the surface slice were generated internally, and the other 50% from the surrounding soil. The range of beta particles in soil is only about 1 mm and so we assume that in all the soil samples the entire beta dose is derived from the soil. This may not be valid for sample SWE-SR, because the soil was actually scraped from the rock surface. The gamma dose rate in both SWE-S and SWE-SR is assumed to be 50% from the rock and 50% from the soil, because of the size of the rock sample. The Greek rock sample MYK-R was about 10 cm in diameter. Fifty percent of the beta dose rate was assumed to come from the rock and the other 50% from the soil (the two samples are in direct contact and the size of the rock sample justifies this hypothesis). The gamma dose rate to the rock MYK-R, and the soil MYK-S were assumed to come entirely from the soil, since the rock is small comparing to the range of the gamma rays (Photo 1). The beta dose attenuation correction for the grain size used in soil analyses is 0.93  0.05 and Table 2 Summary of dosimetry for all five rocks and soil samples Sample SWE-R SWE-SR SWE-S MYK-R MYK-S

U, Bq kg1

238

Ra, Bq kg1

226

Th, Bq kg1

232

K, Bq kg1

40

Water content, %

Dry beta dose rate, Gy ky1

Dry gamma dose rate, Gy ky1

94

9.1  0.3

9.0  0.3

69  4

0

0.31  0.02

0.23  0.01

43  8

34.7  0.7

27.5  0.6

833  18

25

2.57  0.07

1.25  0.03

56  7 50  7

53.5  0.7 58.8  0.8

63.9  0.8 104.0  1.1

1392  24 1108  20

0 20

4.33  0.16 3.91  0.14

2.27  0.05 2.58  0.05

The gamma-spectrometry calibration, etc. is described in Murray et al. (1987). Conversion factors from activity concentrations to dose rate are taken from Olley et al. (1997) and from Liritzis and Kokkoris (1992, see also Liritzis et al., 2001). 1 Bq 238U/kg ¼ 0.081 ppm U, 1 Bq 232Th/kg ¼ 0.246 ppm Th, 1 Bq 40K/kg ¼ 32.3 ppm K. The water content is expressed as dry weight of sediment.

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Photo 1. Rock (MYK-R) and underlying Neolithic soil floor (MYK-S) from Ftelia, Mykonos settlement.

The apparent soil ages for MYK-S and SWE-S are consistent with the expected age based on the radiocarbon ages, but the age obtained for sample SWE-SR is significantly younger than the expected 7 ky from 14C. It is possible that this reflects a dosimetry problem; the dose rates in the soil SWE-SR and rock SWE-R are very different (see Table 2). Only soil sample SWE-SR was taken in direct contact with the corresponding rock sample. If 50% of both the beta and gamma dose rates are derived from the overlying rock, the age of SWS-SR increases to about 6.5 ky, consistent with the age for SWE-S and the age expected from 14C. Because of this uncertainty, and in view of the agreement between the remaining samples and the expected ages, we accept the known age of emplacement of w7 ky for both rock samples MYK-R and SWE-R. 5. OSL decay curves and dose response of rock samples The OSL decay curves of inner (non-surface) slices (w25 mm from the surface), using IR and blue light stimulation were first measured. The signals from IR stimulation can be confidently associated with feldspar; the signals from blue stimulation result from the stimulation of both feldspar and quartz (Spooner, 1994). The OSL decay curves are presented

Table 3 Quartz OSL and radiocarbon ages for Swedish and Greek soil samples Sample

SWE-SR

SWE-S

MYK-S

De (Gy) 9.5  0.9 (n ¼ 51) 19  3 (n ¼ 46) 41  3 (n ¼ 21) Dose rate (Gy/ky) 2.40  0.13 2.6  0.2 5.5  0.2 OSL age (ky) 3.95  0.43 7.31  1.28 7.45  0.61 Radiocarbon age (ky) 7.10  0.16a 6.6  7.0b n, Denotes the number of grains used in the calculation of De. (Calculated cosmic ray contributions are included in the dose rate data.) The cosmic dose rate for the case of samples SWE-SR and SWE-S is 0.10  0.05 Gy/ky and for sample MYK-S 0.13  0.03 Gy/ky. a C-14 date is calibrated age span using Stuiver and Kra (1986; see Lindeman, 2001). Hardwood sample came from near the bottom of the cairn (ID no 1079, RAA 158). Soil sample comes within about 0.5 m of the C-14 sample. b C-14 dates were made at NCPR Democritos, Athens (see, Sampson, 2002).

Fig. 1. Equivalent dose measurements from (a) sample SWE-RS, (b) sample SWE-S, and (c) sample MYK-S plotted as radial plots. Open diamonds e all data. Open triangles e accepted data.

on semi-log plots in Fig. 2. Slices from sample SWE-R (ultramafic, Fig. 2a) usually gave a significant blue signal, with a weak IR response. In contrast, the IR response from sample MYK-R (granite, Fig. 2b) was typically about 10 times stronger than the blue, although both IR and blue light stimulation

A. Vafiadou et al. / Journal of Archaeological Science 34 (2007) 1659e1669 2

Normalised OSL

SWE-R

Blue 100

1

0 0

20

40

60

80

100

Stimulation time (s) IR

(a)

OSL Intensity (a.u.)

100x103

Normalised OSL

10

MYK-R

IR

2

1

0 0

10x103

20

40

60

80

100

Stimulation time (s)

Blue

(b) 1x103

Normalised OSL

DEN-R 1000 IR

1,5 1,0 0,5 0,0 0

20

40

60

80

100

Stimulation time (s) Blue

100

(c) 0

10

20

30

40

50

Stimulation time (s) Fig. 2. Typical natural OSL signals from inner slices from rock samples: (a) SWE-R, ultramafic, (b) MYK-R, granite, and (c) DEN-R, quartz metamorphic. Insets show blue light stimulated linear modulation curves from slices, which have been bleached, irradiated (100 Gy), preheated and IR stimulated (see text). The dashed lines are data from inset (c) normalised to the later parts of the curve.

provided strong signals. Sample DEN-R (quartz metamorphic, Fig. 2c) gave a strong IR signal and no significant rapidly decaying blue response, despite the fact that the rock was, from visual inspection, dominated by quartz (Jain, private communication). Slices from sample SWE-R exhibited large variations in OSL sensitivity from slice to slice; slices from the other two rocks were considerably more reproducible.

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In an attempt to distinguish whether the blue stimulated signals were derived mainly from quartz, or whether there was also a detectable feldspar contribution, we used the technique of linear modulation (LM; Bulur et al., 2000) of the blue light source, after a standard preheat and IR stimulation. In this stimulation method the blue light power is increased from zero, linearly with time, to full power. Then the most optically sensitive OSL signals appear as peaks in the LM curve at low power (short times), and OSL signals that are insensitive to light appear later. For sample DEN-R the preheat temperature was 240  C and the reading temperature 125  C, and for samples MYK-R and SWE-R 200  C and the reading temperature 125  C. The observed stimulation curves are shown in the insets to Fig. 2 as solid lines. The LM curves for both samples SWE-R and MYK-R (Figs. 2a and b) show a significant optically sensitive signal near the beginning of the stimulation period which can be confidently attributed to quartz, followed by a strong slowly-varying component, which could derive from either quartz or feldspar. Sample DEN-R only shows a slowly-varying component, which is probably dominated by feldspar (Fig. 2c). By normalising the least light sensitive region of the blue response feldspar-dominated curve of Fig. 2c to the corresponding regions of the mixed (i.e. quartz and feldspar) curves of Figs. 2a and b (see dashed lines in Fig. 2a and b insets), we can derive a minimum quartz contribution to the blue stimulated OSL, after IR stimulation, of about 70%. (This is a lower limit, because the quartz signal must also contribute a significant but unknown amount to the slowly-varying component.) As a further test of the origin of the OSL in our polymineral rock samples, pulse anneal curves were measured on a slice of each rock. The three slices were first heated to 400  C for 10 s to empty any stored charge, and given a dose of 100 Gy. An IR pulse anneal curve was then constructed using 0.1 s stimulation pulses (0.5 s for SWE-R, because of the weaker signal) following each 25  C increase in preheat temperature to a maximum temperature of 450  C, starting at 125  C. The sample was held at each temperature for 10 s before cooling, to help ensure temperature equilibrium. (The IR stimulated OSL was measured at 125  C, and the stimulation laser power was reduced by 50% to minimise the reduction of OSL by the stimulation itself.) The data for each sample slice are shown in Fig. 3a. The blue light stimulated pulse anneal curves were measured in a similar manner using the blue light source, except that all slices were exposed to the full power IR laser for 100 s at 125  C before beginning the measurement of the pulse anneal curve. (Stimulation was at 125  C, and the blue light source was reduced to about 3% of full power.) These data are shown in Fig. 3b. None of the curves (Figs. 3a and b) are typical of published data for quartz or feldspar, in that there is considerable signal remaining after heating to 320  C for 10 s. We attribute this observation to the 1 mm thickness of our samples, compared to the 100e200 mm grains used in most published measurements (Bøtter-Jensen et al., 1993; Li and Chen, 2001; Wintle and Murray, 1998; Duller, 1994c; Tso et al., 1996). Nevertheless, a thermally unstable component below 200  C is clearly visible in the IR pulse anneal curve for SWE-R. This appears to be

A. Vafiadou et al. / Journal of Archaeological Science 34 (2007) 1659e1669

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with dose or repeated measurement cycles. One important test of the success of this approach is the ability to re-measure a low dose point in the growth curve after the measurement of a high dose point. The ratios between the first and the last dose points derived from this ‘‘recycling test’’ are summarised in Table 4; these values are considered to be acceptable, close to unity, (with the exception of the IR response from SWE-R; the large uncertainty on this result reflects the weak signals from IR from this sample).

1

Normalised OSL

(a)

0 100

30 200

300

400

(a) SWE-R

500

Temperature (°C) 1 20

Normalised OSL

(b)

10

0 0

200

300

400

500

Temperature (°C) Fig. 3. Pulse anneal curve; stimulation using (a) IR, and (b) blue light sources (see text for experimental details). Solid line MYK-R, dotted line SWE-R, long dash line DEN-R.

similar to that described by Clarke and Rendell (1997a,b), and it contributes almost all of the IR stimulated luminescence in this sample. In contrast, the IR OSL in the other two samples appears to be uniformly distributed over the temperature region 150  C to about 400  C. All of these observations are consistent with feldspar being the source of the IR OSL. The blue stimulated signal from SWE-R is also affected by low temperature heating, consistent with the observation above that feldspar contributes to the blue stimulated OSL signal. The other two samples are more similar, although the data from MYK-R are systematically higher than those from DEN-R around 300  C, consistent with the presence of a quartz contribution (it should be noted that these blue OSL measurements reflect the behaviour of both the fast and the slow components from quartz and feldspar, because no background is subtracted from these pulse annealed OSL signals). 6. Dose response curves

1000

500

1000

1500

(b) MYK-R

40

Corrected OSL (Li/Ti)

0 100

500

30

20

10

0 0

1500

150

(c) DEN-R

100

50

0 0

200

400

600

Dose (Gy)

Single-aliquot regenerative-dose (SAR) protocols (Murray and Wintle, 2000; Wallinga et al., 2000, see Table 1) were used to measure the dose response curves of slices from each rock sample (Fig. 4). The SAR procedure uses the response to a fixed test dose to correct for changes in sensitivity

Fig. 4. SAR growth curves for single slices of rock samples (a) SWE-R, ultramafic, (b) MYK-R, granite, (c) DEN-R, quartz metamorphic. The filled circles represent the sensitivity corrected blue response and the open circles the IR response. The growth curves have been fitted with an expression of the form y ¼ a(1  ebx). Li/Ti is defined in Table 1.

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Table 4 Recycling ratio and recuperation values for three rock samples Sample

SWE-R (n ¼ 13)

MYK-R (n ¼ 13)

DEN-R (n ¼ 1)

Stimulation wavelength

BLUE

IR

BLUE

IR

IR

Recycling ratio Recuperation (Gy)

1.12  0.05 1.3  0.7

1.26  0.11 0.3  0.6

1.07  0.02 0.9  0.4

0.99  0.01 0.2  0.4

0.99  0.03 3.76  0.04

n, Denotes the number of aliquots used. These aliquots are those used for the De measurements. The uncertainties are standard errors on the mean.

30

(a)

SWE-R initial dose

20

10

0

-10 100

10

1 25

MYK-R

(b) initial dose

20

Apparent dose (Gy)

A second test of the successful application of the protocol is the response to a zero regeneration dose. This response should ideally be zero; in practice, thermal transfer during preheating usually results in a finite sensitivity corrected OSL signal. This can be expressed as a dose, by interpolating onto the fitted growth curve. These doses, termed ‘‘recuperation values’’ are also summarised in Table 4. All values are small compared to likely doses received over a few thousand years, especially those obtained using blue stimulation. The growth curve of the IR stimulated signals from sample SWE-R (ultramafic) (Fig. 4a) show significant increase beyond 1300 Gy, as would be expected from a feldspar-dominated sample. The blue response of this sample saturates at much lower doses (about 250 Gy) and is much more typical of quartz. The blue and IR responses from sample MYK-R (granite) (Fig. 4b) are very similar in shape, and both saturate at about 600 Gy. Since the IR signals are very unlikely to be derived from quartz, it may be that the blue response from this sample also has a significant feldspar contribution. The IR growth curve from sample DEN-R is typical of feldspar (Fig. 4c). At this stage we conclude that feldspar signals are present in all three samples, although the IR OSL signal is weaker than the blue stimulated signal in the case of sample SWER. The blue response may be dominated by quartz in sample SWE-R, but probably not in sample MYK-R.

15 10 5 0

7. Bleaching

-5 1

It is important to know how quickly and completely the OSL signals are bleached when the rock is exposed to daylight. We first investigated the rate of bleaching using up to 13 individual slices of each rock sample. These were all bleached by placing under a sunlight simulator lamp (Ho¨nle Sol 2) for 1 h, and then given a dose of 50 Gy (DEN-R) and 20 Gy (SWE-R and MYK-R). These irradiated slices were exposed to winter daylight (55 N) for different periods of time (between 1 and 8000 s), before measuring the apparent residual dose using the SAR protocol. In all cases, the apparent doses decreased rapidly (Fig. 5), reaching a stable value between 100 and 1000 s of exposure. The average values are listed in Table 5 (final bleached dose, average from the last four slices); there is no significant difference between the final blue light and IR stimulation values for each sample. These residual doses, especially those for samples SWE-R and DEN-R, may be significant compared with doses likely to be received during burial, since the final bleached doses for these samples were expected to be close to zero (Table 5). Thus, a further test was undertaken to determine whether these

10

50

100

1000

DEN-R

initial dose

(c) 40

30

20

10

0 1

10

100

1000

10000

daylight exposure (s) Fig. 5. The bleaching rate curves: (a) sample SWE-R, (b) sample MYK-R, and (c) sample DEN-R. The error bars in two of the graphs are hidden by symbols. The filled circles are for the blue stimulated OSL and the open circles for the IR stimulated OSL. The fit functions are chosen just to present the modulation of the apparent doses.

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Table 5 Residual doses after daylight bleaching of the rock slices for each of the three rock samples Sample

Final bleached dose (n ¼ 4)a Inner slices (n ¼ 3) Surface slice (n ¼ 1)

MYK-R

DEN-R

BLUE

IR

BLUE

IR

IR

Dose (Gy)

Dose (Gy)

Dose (Gy)

Dose (Gy)

Dose (Gy)

3.5  1.0 48  19 2.30  0.17

52 3.3  1.1 0.38  0.03

0.54  0.16 33  4 0.67  0.004

0.2  0.5 18.0  0.8 2.2  0.01

1.81  0.02 170  40 1.42  0.05

Refers to the last four data points in each of the graphs in Fig. 5.

data were affected by thermal transfer, despite the low preheat temperatures (160  C for blue signal and 210  C for IR). Three slices of each sample were bleached using blue light in the OSL reader, given a dose of 100 Gy, and then exposed to 1000 s of daylight. The IR and blue light stimulated OSL from all samples were then measured at 125  C without preheating. The slices were then preheated and measured using the SAR protocol in the usual manner. The results did not show any significant thermal transfer. The other important aspect of bleaching in nature is the degree to which daylight penetrates the rock surface (Liritzis et al., 1997b; Liritzis and Galloway, 1999). A section of whole rock was broken off the main sample in the darkroom and three slices taken from the previously unexposed inner face (>3 cm from the nearest old surface; the average measured dose for each sample is given in Table 5 e inner slices). The inner (previously unexposed to light) surface was then exposed to daylight for 14 days (9 days for DEN-R) and a 20 mm long core extracted from the fresh rock surface. This core was then cut into 1 mm slices. Fig. 6 presents the apparent dose in each slice as a function of depth into the light-exposed rock surface. The scatter in both the blue and IR stimulated data from the ultramafic sample SWE-R is considerable, probably because of the unhomogeneity concerning the minerals between the slices, and the IR data seem unrealistically small, with a dose in the middle of the rock of only 3.3  1.1 Gy. There are individual layers with surprisingly small doses, even in the blue stimulated data (e.g. at 12 mm, Fig. 6a). Apparent doses in the granite sample MYK-R increase more smoothly with depth. The similarity in values of De with depth suggests that the effect of light penetration is similar for both the IR and blue light stimulated signals. However, for the blue stimulated data, the mean dose of 18 Gy at about 15 mm into the rock is still only about 50% of the dose measured before light exposure (33 Gy). In the case of the IR data, the mean dose at 15 mm is consistent with the previously measured dose of 18 Gy (Fig. 6b). The apparent light penetration in sample DEN-R is different, with the IR derived data remaining low (at about 10 Gy) for more than 5 mm into the surface, before increasing again. The previously measured dose in the inner part of this rock was 170  40 Gy, and the rate of increase at 15 mm may be consistent with this. All values for the surface slices in this experiment are consistent with those found in the daylight bleaching experiment

160

(a) SWE-R 120

80 centre Blue dose=48±19 Gy 40 centre IR dose=3.3±1.1 Gy 0 0

5

10

15

40

(b) MYK-R

35

Apparent dose (Gy)

a

SWE-R

centre Blue dose =33±4 Gy

30 25 20 centre IR dose =18.0±0.8 Gy

15 10 5 0 0

5

10

15

200

(c) DEN-R

centre dose =170±40 Gy

150

100

50

0 0

5

10

15

Depth (mm) Fig. 6. Daylight penetration in rock: (a) sample SWE-R, (b) sample MYK-R, and (c) sample DEN-R. The filled circles show the blue response and the open circles the IR response. The centre dose was measured on slices cut from recently broken inner portion of the rock (around 5.0 cm from the surface).

A. Vafiadou et al. / Journal of Archaeological Science 34 (2007) 1659e1669

using individual slices (Fig. 5, and ‘surface slice’ and ‘final bleached dose’ in Table 5).

Table 7 Outline for the protocol used for the fading test Step

8. Dose recovery and fading experiments A minimum requirement in the measurement of dose using SAR is that a known dose given to the sample can be successfully recovered. It is important that the treatment given prior to administering the known dose must be kept to a minimum; in particular, preheating must be avoided (Wallinga et al., 2002). Five previously unused slices of each sample were bleached under the daylight simulator lamp for 1 h, and each was then given a known dose (40.5 Gy for sample SWE-R, 27.2 Gy for sample MYK-R, 10.2 Gy for sample DEN-R) and then measured using the SAR procedures out line earlier. Table 6 compares these known doses with those subsequently measured using SAR. All measured doses (except that obtained using IR stimulation for sample SWE-R) are within experimental uncertainty (given as the standard error) of the known dose administered in the laboratory. As a further test of our ability to accurately measure a known dose, we conducted fading tests on previously unused slices from all our samples. We first zeroed the luminescence signal of several slices by heating them to 350  C, administered a dose of 20 Gy and then stored them for 3 months at room temperature. The sequence followed for this measurement is presented in Table 7. The final fading value is expressed as a ratio of the measured to the given dose, and the results are given in Table 8. The blue OSL stimulated signal does not fade (all the fading values are close to or indistinguishable from unity), but that from IR fades significantly (50% for SWE-R and DEN-R). The wide range in fading values in the case of sample SWE-R and IR stimulation (0.90, 0.01, 0.65) is remarkable. The standard error (se) on the fading measurement for sample DEN-R is large because the blue signal was very weak. 9. Equivalent doses and derived ages The 13 individual blue stimulated estimates of De for sample MYK-R (granite, see Table 9) gave a relative standard deviation of about 35%, but there was no evidence for a non-unimodal distribution. In contrast, the relative standard deviation of the estimates from sample SWE-R (ultramafic) was 70% and there was distinct evidence for a bi-modal distribution. (No estimates of De are available for sample DEN-R, because it did not give a significant blue light stimulated fast component in the OSL signal.)

1667

SWE-R

MYK-R

DEN-R



240  C for 10 s

1.

Preheat

200 C for 10 s

2.

Measure OSL

3.

Give test dose, Dt

20 Gy

4.

Measure TL

to 160  C (heating rate 5  C)

5.

Measure OSL

6.

Give dose

7.

Repeat steps 1e6 for three times e cycles

100 s IR at 125  C (¼Lri) 50 s blue light at 125  C (¼Lbi) 5 Gy

2 Gy to 210  C (heating rate 5  C)

100 s IR at 125  C (¼Lri) 50 s blue light at 125  C (¼Lbi) 20 Gy

The OSL ages are also given in Table 9, calculated using the dosimetry data of Table 2. The De estimates in Table 9 are calculated using the blue light stimulated signals. These calculations are based on the assumption that the blue light stimulated signals (from which the De estimates of Table 9 are calculated) are derived primarily from quartz. It is assumed that quartz dominates these blue signals, in which case the internal dose rate is negligible compared to the external dose rate, and so the average dose rate derived from bulk rock measurements is appropriate. Had feldspar contributed significantly to the blue stimulated OSL signal, some contribution from the internal potassium concentration in those feldspar grains that contain potassium would need to be taken into account. For a typical potassium-rich grain, this would increase the dose rate by up to about 30%, but (a) there is definitely a significant quartz contribution to the blue signal (from Fig. 2, probably >70%), and (b) only some of the feldspar grains contributing to the blue signal would contain elevated concentrations of potassium. At this stage in our investigations, this assumption is not considered critical. There may be a small overestimate of the OSL age of sample MYK-R (granite; 7.8  0.7 ky), and the MYK-S age, (7.45  0.61 ky), when compared with the independent 14C age of 6.6e7.1 ky (Table 3), but the uncertainties on the rock and soil ages make it difficult to be sure. Such an overestimate for the rock sample might result from the assumption that the signal is derived almost entirely from quartz, or from incomplete bleaching. The result from sample SWE-R (7.6  1.5 ky; Table 9) is indistinguishable from the radiocarbon result (7.10  0.16 ky). The 20% error suggests that the rock was not completely Table 8 Fading result (measured/given dose) for the three rock samples

Table 6 Dose recovery experiment

Sample

Sample

SWE-R

MYK-R

DEN-R

Stimulation wavelength

BLUE

IR

BLUE

IR

IR

Known dose (Gy) Mean dose recovered (Gy)

40.5 39  2

40.5 29  2

27.2 26.7  0.8

27.2 27.3  0.4

10.2 10.9  1.0

SWE-R

MYK-R

DEN-R

Stimulation BLUE wavelength

IR

BLUE

IR

IR

Aliquot 1 Aliquot 2 Aliquot 3 Mean value

0.90 0.01 0.65 0.52  0.27

1.03 0.95 1.01 1.00  0.02

0.87 0.87 0.91 0.88  0.01

0.60 0.55 0.52 0.56  0.02

1.04 1.17 1.01 1.07  0.05

A. Vafiadou et al. / Journal of Archaeological Science 34 (2007) 1659e1669

1668

Table 9 Estimated De and ages for the rock samples

Acknowledgments

Sample

SWE-R

MYK-R

De (Gy) Dose rate (Gy/ky) Date (ky)

12.9  2.5 (n ¼ 13) 1.7  0.04 7.6  1.5

45  4 (n ¼ 13) 5.72  0.19 7.8  0.7

n, Denotes the number of individual surface slices used to derive the De. (Calculated cosmic ray contributions are included in the dose rate data.)

bleached, a conclusion that is supported by the bi-modal distribution of doses. There will never be any guarantee that stones used to make burial mounds were adequately exposed to light before final burial (Liritzis et al., 1997b). 10. Conclusion This work has shown that two geoarchaeological materials frequently encountered in archaeological excavation sites, i.e. soil floors and pebbles, can be effectively dated by OSL, thus providing ages related to the last occupation of prehistoric settlements. Our detailed investigation have shown that: (a) Solar penetration in granitic rocks, for the test exposure of 14 days, has resulted in significant bleaching down to 12e 15 mm. All samples were bleached at 5 mm depth. This has important implications for the dating of carved granitic or quartz/feldspar-based rock types, often used for ancient constructions, and supports earlier tests on limestone rocks (e.g. marble) (Liritzis et al., 2002; Liritzis and Galloway, 1999). (b) Dose recovery and fading tests both suggested strongly that the IR stimulated signals are not of value in dating these rock surfaces, because these tests indicate that the dose absorbed during burial cannot be accurately measured. However, these tests also indicated that the blue signal, thought to come largely, but not exclusively, from quartz, is likely to give a more accurate measurement of dose. (c) For the samples from Mykonos, both the rock and sediment blue OSL ages (MYK-R and MYK-S) of 7.8  0.7 ky and 7.45  0.61 ky (Tables 3 and 9) were expected to give the same age, as was observed. Radiocarbon dates from this layer and from a higher layer give an age span between 6.6 and 7.0 ky (Table 3). Both OSL ages are indistinguishable from this 14C age range. (d) At the Swedish site e samples SWE-R, SWE-SR and SWE-S e the rock age is again consistent with the 14C age, but one of the soil samples markedly underestimates its known age. We attribute this underestimate to uncertainties in the beta dosimetry of sample SWE-SR. (e) It is likely that dose distribution techniques used to examine incompletely bleached sediments (see Fig. 1, and e.g. Murray and Olley, 1999) will be of value in some samples. By taking many slices from the entire surface of a rock sample, and examining the distribution of apparent doses in these slices, that portion of the rock surface which was best exposed last time to daylight (i.e. gives the lowest estimates of De) could be identified.

AV thanks the Eleni Nakou Foundation for a scholarship to visit Denmark, the State Scholarship Foundation of Greece (IKY). We thank Prof. A. Sampson for providing the Mykonos samples, and Dr. P. Skoglund of the Department of Archaeology, Lund University, Sweden for the SWE-R and SWE-S samples. The work was part of a large project funded by Inrerreg II (2000e2001). Appendix The settlement in Ftelia bay is of the most significant Late Neolithic excavation sites in the Cyclades. It consist of several trenches yielding abundance of pottery finds, obsidian blades and flakes, animal bones, copper (bronze) artifacts (tools and jewellery). It has been certified the occurrence of thick deposits and four distinctive architectural phases. A large part of the settlement is under sea to a depth of around 4 m, and the estimated area is about 6500 m2. Pottery varies from open to closed vases, strainers-perforated sherds, four legged vessels, asymmetrical vases, cheese-pot types, spouts (feeders), vases of ‘rechaud type’, impressed wares, incised, pierced handles, with impressions on rim, white on dark painted and polychrome ware, clay figurines, miscellaneous stone artifacts (smoothed stones, axes, pumice stones, pendants), small clay finds and clay objects, clay anchor-shaped objects, incised symbols on sherds, bone finds (see, Sampson, 2002). The finds and their analyses point to an agricultural economy based on land cultivation (e.g. barley and legumes) and animal breeding (ship and goat). The pottery types ascribed the settlement to the Late Neolithic I period, supported also by typological comparison with Saliagos (near Paros) and Zas (at Naxos) at Cyclades, TsangriArapi phase at Thessaly. Ten calibrated radiocarbon dates of charcoal ranged between 4500 and 5100 BC (see Facorellis and Maniatis, in Sampson, 2002, pp. 309e315). Mykonos centered in the Aegean and in the eastern edge of the chain formed by the northern Cycladic islands favours journeys towards the islands of the eastern Aegean and the coast of Asia Minor. Although with some similarities with other Aegean islands as well as mainland Greece pottery here have independent characteristics. Settlements during this period in the Aegean seem to situate on the edge of promontories orientated to the north. References Bulur, E., Bøtter-Jensen, L., Murray, A.S., 2000. Optically stimulated luminescence from quartz measured using the linear modulation technique. Radiation Measurements 32, 407e411. Bøtter-Jensen, L., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiation Measurements 32, 523e528. Bøtter-Jensen, L., Jungner, H., Mejdahl, V., 1993. Recent developments of OSL techniques for dating quartz and feldspars. Radiation Protection Dosimetry 47, 643e648.

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