TL dating of Sri Lankan archaeological sites

QuaternaO" Geochronology (QuaternaO" Science Reviews), Vol. 13, pp. 585-588, 1994. Copyright © 1993, Elsevier Science Ltd. Printed in Great Britain, All rights reserved. 0277-3791/94 $26.00

Pergamon

0277-3791(94)E0062-F

TL DATING OF SRI LANKAN ARCHAEOLOGICAL

SITES

Mohan Abeyratne*

Quaternary Dating Research Centre, RSPAS, Australian National University, Canberra, ACT 0200, Australia TL dating in archaeology was initiated in Sri Lanka in 1985 at the research laboratories of the UNESCO Sri Lanka Cultural Triangle Project. Initially three test programs were carried out to see how well this dating method works in Sri Lanka: (1) Dating of bricks and pottery of historically well established structures and contexts. (2) Dating a series of samples: (a) bricks from different phases of Mirisewetiya stupa, (b) pottery from different contexts of citadel, Anuradhapura. (3) Heated sediments from the prehistoric human occupation site of Beli-Cave, Kitulgala (Abeyratne, 1994). All TL age estimates agree well with 14C chronologies or with the historical dates given in the ancient chronicle, Mahavamsa (the great chronicle of Ceylon) (Geiger and Bode, 1960). INTRODUCTION The first TL date for Sri Lanka was obtained in England, by the dating of a fired rock crystal of Bellan Baddipalassa (Wintle and Oakley, 1972), and a second set of TL dates was obtained in France by Francois (1980), who dated the so-called King Dutugemunu ashes. In 1985 TL dating in archaeology was established in Sri Lanka at the Research Laboratory of the UNESCO - Sri Lanka Cultural Triangle Project. Initially three test programs were carried out (Fig. 1 shows the location of the sites):

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(I) Dating of historically well-established layers or contexts including: (a) Northern ayake (altar) of the first brick line of the Jetavanaramaya stupa Anuradhapura (Ratnayake, 1984) (stupa or the dagaba, as it is called in Sinhala, is an integral feature in a Buddhist Monastery and is supposed to contain a small particle of the corporeal remains of Buddha or a saint). (b) Bricks from Dutugemunu maluwa (terrace of the stupa) of the Mirisewetiya stupa, Anuradhapura. (c) First brick line 30 cm below the soil, at the centre of the western side of the Nipena Viharaya (Buddhist Monastery), Polonnaruwa. (d) Potsherd from the test pit %w~J8/7/7 of Sigiriya (Bandaranayake, 1984). (2) Dating a series of samples from: (a) Different phases of Mirisewetiya stupa, Anuradhapura: vahalkada (the gate way) south; inner dome of Dutugemunu; inner pesawa (basal ring of the stupa) Dutugemunu south; external pesawa of Gajaba east and south.

*Permanent address: Central Cultural Fund, 212/1, B a u d d h a l o k a Mawatha, C o l o m b o 7. Sri Lanka.

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FIG. 1. Location of sites. (1) A n u r a d h a p u r a ; (2) Polonnaruwa; (3) Sigiriya: (4) Bell-Cave, Kitulgala.

(b) Pottery from various contexts of AMP (Anuradhapura Mahapali site), citadel, Anuradhapura (Deraniyagala, 1992). (3) Dating heated sediments of Beli-Cave at Kitulgala where all layers contained microliths, a geometric form that did not come into prominence in Europe until 12 ka BP (see Abeyratne, submitted).

586

M. Abeyratne

TL DATING TL dating has the potential to provide reliable dates for many archaeological materials; burnt flints (Valladas et al., 1987a, b, 1988), burnt bricks (Goedicke et al., 1983) and sediments (Singhvi and Mejdahl, 1985; Berger, 1988; Wintle, 1993). For further details on TL dating methods see Aitken (1985). In TL dating an age estimate is derived from the relationship: Age(years) = Dose, DE(Gy)/Dose R a t e , / ) (Gy/year) The dose, D E, a sample has received since it was heated is generated by the decay of radioactive elements (mainly uranium, thorium plus their decay products, and potassium) in the sample and its surroundings. The dose rate (/)) is derived from the chemical analysis of the radioactive elements in the sample and the surrounding material. The bricks and pottery samples for the first and second test programs were collected from 30 cm or more below the ground surface. In the case of heated sediments a special sampling procedure was adapted (Abeyratne, submitted).

Experimental 100--160 ~m diameter quartz grains were extracted by dry sieving, etching with conc. HCI acid, then washed with water, alcohol and acetone. The grains were then etched with conc. HF acid for 30 min. Finally, the grains were floated in bromoform to extract pure quartz grains and magnetically separated if necessary (after microscopic examination). The TL measurements were performed on a Valladas Brou TL apparatus, heating rate of 10°C/sec. The TL signals were recorded with a RTXP 2232B photomultiplier through a H 326 c (M.T.O.) blue-pass filter and an M.T.O. heat rejecting filter. Laboratory irradiations were conducted with a Sr-90/Y-90 beta source delivering 2.71 Gy/min. The samples were checked for (i) non-linear dose response; (ii) inhomogeneity; (iii) insufficient heat treatment; and (iv) anomalous fading, and the samples that passed the above criteria were used for the dose estimations (Abeyratne, submitted). Initially the dose was determined by the additive dose method (Aitken, 1985). As most archaeological heated material shows supralinearity, a second glow response was measured in order to estimate the magnitude of this influence. For those samples which exhibited supralinearity the necessary corrections were made. The corrected dose value derived from the above procedure was checked by the normalisation technique suggested by Valladas and Gillot (1978) for the determination of the final dose (see Abeyratne,

submitted). In the case of dating bricks it was assumed that the sample and the environment have the same concentration of radioactive elements (as there are more than five layers of bricks surrounding the samples)

and therefore the beta and gamma doses were evaluated from the brick sample, whilst in sediments, the dose rate was measured using the bulk sediment (Abeyratne, submitted). In pottery, the beta dose rate was evaluated from the pot sherd and the gamma dose rate was measured using the surrounding soil. In bricks and pottery uranium and thorium were analysed using thick source alpha counting (the samples were rejected if there was a difference of 3% or more between the sealed and unsealed samples), and potassium was analysed using flame photometry. The average water content for all samples were assumed to be in the range of 10 _+ 5% and the corrections for the dose rate were made according to the equations given by Bowman (1976). The cosmic dose rate in the case of sediments was presumed to be negligible because of more than 20 m of bedrock above the excavation site. Similarly, in all brick samples the cosmic ray dose rate was also presumed to be negligible because of more than 25-200 m of bricks overlying the samples. The cosmic ray contribution in the case of pottery dating was taken as 170 ~Gy/year corresponding to a burial depth of 1-2 m (Prescott and Hutton, 1988). The beta attenuation factors were taken from Mejdahl (1979) and secular equilibrium has been assumed. The error of the dose rate and age estimates were calculated according to Aitken and Alldred (1972). In the case of sediment dating, errors were calculated with the Age program (Grfin, unpublished).

RESULTS The results of the dating test program (1) are shown in Table 1 illustrating both historical and TL dates. Table 2 shows the TL and historical dates for TABLE 1. TL and historical dates for the samples of the test program (1) (historical dates were taken from Geiger and Bode, 1960) Site name

Dose (Gy)

Dose rate (mGy/year)

Jetavanaramaya5.07-+0.21 Sigiriya 4.21+0.17 Mirisewctiya 6.90-+0.47 Polonnaruwa 2.65-+0.11

TL date

Historic date

2.89-+0.17 243-+ 125 A.D. 290-+ 14A.D. 2.81-+0.16 493___105 A.D. 487-+8A.D. 3.18-+0.22 177_+-210B.C. 149-+12B.C. 3.11+0.20 l143-+65A.D. 1201-+50A.D.

TABLE 2, TL and historic dates for the Mirisewetiya stupa (historical dates were taken from Geiger and Bode, 1960) Phase of the stupa VS IDDE IPDS DM EPGE EPGS

Dose (Gy) 5.26+0.20 6.41+0.28 7.42_+0.32 6.90_+0.47 7.03+0.22 8.11+0.32

Dose rate (mGy/year)

TL date

2.51_+0.10 101_+ll5B.C. 3.04+0.12 115-+124 B.C. 3.44+0.15 163-+132 B.C. 3.18-+0.22 175-+210 B.C. 3.75_+0.13 123-+87 A.D. 4.35-+0.13 130-+92 A.D.

Historic date 149+12 B.C. 149-+12 B.C. 149-+12 B.C. 149+12 B.C. 123-+!1 A.D. 123-+11 A.D.

VS=vahalkada south, 1DDE = inner dome of Dutugemunu, IPDS = inner pesawa Dutugemunu south, DM = Dutugemunu maluwa, EPGE = External pesawa Gaiaba east, EPGS = external pesawa Gajaba south.

TL Dating of Sri Lankan Archaeological Sites

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FIG. 2. Plot of ~4C (charcoal) and TL age estimates vs. context numbers for Beli-lena Cave(from Abeyratne, submitted).

TABLE 3. TL age estimates (pottery) and the calibrated 14C age estimates (charcoal) for the different layers for the citadel, Anuradhapura

Layer no

Dose (Gy)

Dose rate (mGy/year)

TL age 14C age estimates estimates (years) (BP)

72

--

--

--

2413_+22

72

8.60_+0.34

3.75_+0.2

2290_+150

--

75

--

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2469_+24

75

--

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2596_+ 106

75

9.21_+0.27

3.80_+0.21

2410+150

--

75

9.56-+0.38

3.96-+0.23

2410_+ 170

--

75

8.90+0.37

3.59-+0.18

2480-+ 160

--

the bricks belonging to the different phases of the Mirisewetiya stupa. Table 3 indicates the ~4C and T L age estimates (pottery) for the different layers of citadel, Anuradhapura. The results of the plot of ~4C (charcoal) and T L age estimates (heated sediments) in the different contexts for the Beli Cave are shown in Fig. 2.

except one 14C result in layer 75 (Table 3). The T L age estimates of heated sediments agree very well with the conventional t4C (see Deraniyagala, 1992) dates (see Fig. 2).

CONCLUSIONS The three T L test programs in Sri Lanka show that it is possible to obtain reliable age estimates on archaeologically heated samples, provided that rigorous quality control is applied. Excellent agreement has been obtained between either historical ages or with ~4C chronology with the T L dates or age estimates for the fired pottery and bricks. ~4C ages on charcoal also correspond closely with the T L age estimates on heated sediments from Beli-Cave. The T L dates at Beli-Cave further demonstrate that in Sri Lanka prehistoric man produced microlithic-shaped artefacts 15,000 years earlier than in Europe.

ACKNOWLEDGEMENTS DISCUSSION The results of the test program (1) shown in Table 1, using pottery and brick samples from four different sites, indicate that all T L age estimates agree reasonably well with the historical dates. In the case of the different phases of the Mirisewetiya stupa (Table 2), the T L results coincide with the ages anticipated from the historical chronologies. For dating pottery of citadel excavations, the T L age estimates correspond closely to the calibrated ~4C ages (see Deraniyagala, 1992),

I wish to thank R. Griin, QDRC, Canberra, for comments. I am grateful to R. Silva, S. Bandaranayake and H.A. Ratnayake, Central Cultural Fund, Colombo, S.U. Deraniyagala and W. Wijepala, Dept. of Archaeology, Colombo, for their encouragement and help with the sample collection. R. Roberts, Dept. of Prehistory, Canberra, and S. Robertson, QDRC, Canberra, made many helpful comments. The radiocarbon age estimates were provided by the Physical Research Laboratory (Ahmedabad, India), British Museum (London), Goethe University (Frankfurt) and Beta Analytical Inc. (U.S.A.). Neutron activation analyses were carried out at ANSTO, Lucas Heights.

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REFERENCES Abeyratne, M. (1994). Thermoluminescence dating of archaeologically heated sediments at Beli-Cave, a prehistoric site in Sri Lanka. Quaternary Geochronology

(Quaternary Science Reviews) (this issue). Aitken, M.J. (1985). Thermoluminescence Dating. Academic Press, London, New York, 359 pp. Aitken, M.J. and Alldred, J.C. (1972). The assessment of error limits in thermoluminescence dating. Archaeometry, 14, 257-267. Bandaranayake, S. (1984). Sigiriya Project. First Archaeological Excavation Report (January-September 1982). Colombo, Central Cultural Fund, Ministry of Cultural Affairs, 188 pp. Berger, G.W. (1988). Dating Quaternary events by luminescence. In: Easterbrook, D.J. (ed.), Dating Quaternary Sediments. Geological Society of America Special Paper, 227, 13-50. Bowman, S.G.E. (1976). Thermoluminescence dating: the evaluation of radiation dosage. Unpublished Ph.D. Thesis, University of Oxford, Oxford. Deraniyagala, S.U. (1992). The Pre-history of Sri Lanka, An Ecological Perspective. Memoir volume 8, Department of Archaeological Survey, Government of Sri Lanka, 813pp. Francois, H. (1980). Dating of King Dutugemunu ashes (pers. commun. ). Geiger, W. and Bode, M.H. (1960). The Mahawamsa, or The Great Chronicle of Ceylon. Ceylon Government Information Department, Colombo, 323pp. Goedicke, C., Slusallek, K. and Kubelik, M. (1983). TL analysis of Palladio, s Villa Rotonda: An interim report. PACT, 9, 245. Mejdahl, V. (1979). Thermoluminescence dating: Beta dose attenuation in quartz grains. Archaeometry, 21, 61-72.

Prescott, J.R. and Hutton, J.T. (1988). Cosmic ray and gamma ray dosimetry for TL and ESR dating. Nuclear Tracks, 14, 223-227. Ratnayake, H.A. (1984). Jetavanaramaya Research Report,

First Archaeological Excavation and Research Report (January-June 1982). Central Cultural Fund, Ministry of Cultural Affairs, Colombo, 92 pp. Singhvi, A.K. and Mejdahl, V. (1985). Thermoluminescence dating of sediments. Nuclear Tracks and Radiation Measurements, 10, 137-161. Valladas, G. and Gillot, P.Y. (1978). Dating of the Olby lava flow using heated quartz pebbles: some problems. PACT, 2, 141-149. Valladas, H., Chadelle, J.P., Geneste, J.M., Joron, J.L., Meignen, L. and Texier, P.J. (1987a). Datations par la thermoluminescence de gisements Mousteriens du sud de la France. L'Anthropologie, 91,211-226. Valladas, H., Joron, J.L., Valladas, G., Arensburg, B., BarYosef, O., Belfer-Cohen, A., Goldberg, P., Laville, H., Meignen, L., Rak, Y., Tchernov, E., Tillier, A.M. and Vandermeersch, B. (1987b). Thermoluminescence dates for the Neanderthal burial site at Kebara in Israel. Nature, 330, 159-160. Valladas, H., Reys, J.L., Joron, J.L., Valladas, G., BarYosef, O. and Vandermeersch, B. (1988). Thermoluminescence dating of Mousterian 'Proto-Cro-Magnon' remains from Israel and the origin of modern man. Nature, 331, 614-616. Wintle, A.G. (1993). Luminescence dating of aeolian sands: An overview. In: Pye, K. (ed.), The Dynamics and Environmental Context of Aeolian Sedimentary Systems. Geological Society Special Publication No. 72, pp. 49--58. Wintle, A.G. and Oakley, K.P. (1972). Thermoluminescence dating of fired rock crystal from Bellan Bandi-palassa, Ceylon. Archaeometry, 14, 277-279.