FROM SUSA TO ANURADHAPURA: RECONSTRUCTING ASPECTS OF TRADE AND EXCHANGE IN BITUMEN-COATED CERAMIC...

Archaeometry

49

, 3 (2007) 000–000 doi: 10.1111/j.1475-4754.2007.00347.x

*Received 13 June 2006; accepted 14 February 2007© University of Oxford, 2007

Blackwell Publishing LtdOxford, UKARCHArchaeometry0003-813X© University of Oxford, 2008XXX

ORIGINAL ARTICLES

Trade and exchange in bitumen-coated ceramic vesselsB. Stern

et al.

*Received 13 June 2006; accepted 14 February 2007

FROM SUSA TO ANURADHAPURA: RECONSTRUCTING ASPECTS OF TRADE AND EXCHANGE IN

BITUMEN-COATED CERAMIC VESSELS BETWEEN IRAN AND SRI LANKA FROM THE THIRD TO THE

NINTH CENTURIES AD

*

B. STERN,

1

J. CONNAN,

2

E. BLAKELOCK,

1

R. JACKMAN,

1

R. A. E. CONINGHAM

3

and C. HERON

1

1

Department of Archaeological Sciences, University of Bradford, Richmond Road, Bradford, West Yorkshire, BD7 1DP, UK

2

Laboratoire de Géochimie Bioorganique, Université Louis Pasteur, 25 rue Becquerel, 67200 Strasbourg, France

3

Department of Archaeology, Durham University, South Road, Durham, DH1 3LE, UK

In contrast with artefactual studies of long-distance trade and exchange in South Asia duringthe Prehistoric and Early Historic periods (Ardika

et al

. 1993; Gogte 1997; Krishnan andConingham 1997; Tomber 2000; Gupta

et al

. 2001; Ford

et al

. 2005), few scientificallyorientated analyses have focused on artefacts from the region’s Historic period. Duringexcavations at the ancient city of Anuradhapura, Sri Lanka, a number of buff ware ceramicswith a putative organic coating on the interior were recovered (Coningham 2006). Datedstylistically to between the third and ninth centuries

AD

, analysis of the coatings using gaschromatography–mass spectrometry (GC–MS) and stable isotope analysis (carbon anddeuterium) confirmed that the coatings are bitumen—an organic product associated withpetroleum deposits. There are no known bitumen sources in Sri Lanka, and biomarkerdistributions and isotopic signatures suggest that the majority of the samples appear to havecome from a single bitumen source near Susa in Iran. The relationship between the bitumencoatings and the vessels is discussed, and it is suggested that the coatings were used to sealpermeable ceramic containers to allow them to transport liquid commodities. This studyenhances our knowledge of networks of trade and exchange between Sri Lanka and westernAsia during Historic times.

KEYWORDS:

ANURADHAPURA, SRI LANKA, SUSA, IRAN, BITUMEN, RESIDUE ANALYSIS, GC–MS, TRADE, ARCHAEOLOGY, STABLE LIGHT ISOTOPES,

δ

13

C,

δ

D

© University of Oxford, 2008

INTRODUCTION

Between 1989 and 1994, a team of British and Sri Lankan archaeologists excavated trenchAnuradhapura Salgaha Watta 2 (ASW2) within the Citadel of Anuradhapura, the ancient capitalof Sri Lanka (Coningham 1999) (Fig. 1). The 9.5 m deep trench, measuring 10 m by 10 m,was designed to obtain a chronometrically dated sequence of structures and a periodizedcatalogue of artefacts, documenting the growth of a small Iron Age village into a medievalcapital (Coningham 1999, 81). Evidence of Historic trade and exchange with western Asiacomprised 33 sherds of Early Islamic Lustre ware (ninth to 10th centuries

ad

), one sherd of

2

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Figure 1 Sri Lanka’s topography (left) and a plan of the northwestern part of the city of Anuradhapura including the Citadel (right) (adapted from Coningham 1999, 13 and 29).

Trade and exchange in bitumen-coated ceramic vessels

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‘Imitation’ Lustre ware (ninth to 10th centuries

ad

), 78 sherds of White Tin Glaze ware, 11sherds of Lead Glazed ware (ninth to 13th centuries

ad

), 116 sherds of Sasanian-Islamic BlueGlazed ware and 59 sherds of a buff ware with an interior organic coating (Seely

et al

. 2006).The 59 buff ware sherds represent the heaviest category of imported ceramics from western

Asia (2232 g) and they are all unglazed, with a light tan to buff or red earthenware body withsand-tempered fabric. A large proportion of them have a black coating on the interior (veryoccasionally on the exterior as well), with the other side either plain or with a thin white slipcoating. Vessel types include jars with either of two bases, a flat circular one or a narrow flaringshaft, typical of an amphora. This category of ceramics has also been recovered elsewhere inthe Citadel of Anuradhapura (Ueyama and Nosaki 1993), as well as from the city of Sigiriyaand the ancient port site of Mantai, where Wijayapala and Prickett (1986, 17) identified it asSasanian or Early Islamic and attributed a date of between the fifth and ninth centuries

ad

. Asa whole, this category appears to belong to the well-known corpus of ~100 cm high thick-walledjars of ~35 cm width with a gently sloping shoulder, thickened rounded rim and pointed base,often referred to as ‘torpedo jars’ or ‘ring-necked handle-less storage jars’ (Kennet 2004, 84)(Figs 2 and 3). They are found throughout the Persian Gulf and Mesopotamia between theParthian and Abbasid periods (Kennet 2004, 84) and are frequently coated with an organicresidue, presumably for liquid storage (Tampoe 1989, 11). Priestman’s recent study of theOxford Williamson Collection has suggested that there are probably four main fabric groupsclearly identifiable by eye (Priestman pers. comm. 2006). In this study, we seek to identify thecoatings on the vessels and to provide further evidence as to the geographical origin of thevessels and their contents, and to extend our knowledge of trade and exchange between westernAsia and Sri Lanka in the Historic period.

ANURADHAPURA AND INDIAN OCEAN TRADE

The importance of Sri Lanka as a commercial trade centre was due to its geographical locationin the Indian Ocean, linking maritime trade routes between eastern and western Asia, combinedwith the presence of harbours and bays (Bopeachchi 1997, 379), including the lagoons aroundMantai, the primary port of the ancient kingdom (Prickett 1990b, 117). Indeed, Carswell andPrickett (1984) have suggested that Mantai played an important role between the western and

Figure 2 Special find 2627 buff ware ‘torpedo’ jar from Trench ASW2, Anuradhapura (line drawing by S. Cheshire).

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eastern sectors of the Indian Ocean, allowing goods to be transported on small vessels acrossthe shallow Palk Straits between India and Sri Lanka, and thus avoiding the long voyage roundthe island and through the dangerous Basse reefs to the south. Sri Lanka’s pivotal role wasalso enhanced by the presence of many products with a high export value, such as pearls, spices,ivory and semi-precious stone and objects. The evidence from ASW2 and Mantai suggests thatmaritime trade networks were first developed in the Iron Age with the presence of carnelianbeads from western India, and that these were augmented by lapis lazuli from Afghanistan andlivestock from India by the middle of the first millennium

bc

(Coningham 2002, 104). Thisbroad South Asian network was then expanded into western and eastern Asia at the beginningof the first millennium

ad,

with evidence of eastern Mediterranean glass vessels, Roman coinsand a number of other metal and ceramic objects with Graeco-Roman affinities (Coningham2002, 2006). As is clear above, this network continued to function into the Historic periodwith the presence of West and East Asian materials, the latter including Changsha paintedstoneware from southwestern China, Xing and Ding White wares from northern China, YueGreen ware from southeastern China and Coarse Grey Stoneware from China or northernVietnam (Seely

et al

. 2006). As will be evident from this list of materials, archaeologicalexcavations tend to only recover durable goods, and we have to anticipate the presence of spices,oils, wines, pastes, incense, woods and textiles. Indeed, many of these vessel categories wereprobably more important as receptacles or packaging for their valuable contents: in the wordsof Martha Prickett,

the physical remains alone provide an interesting but incomplete picture

(Prickett 1990a, 61). Finally, it should be noted that commerce was not the only mechanismfor the movement of materials: Anuradhapura was a centre for Buddhist pilgrims throughoutSouth and East Asia, as it contains seven of the island’s holiest places of pilgrimage, includingthe Bodhi tree, the Maricavatticetiya or Mirisavati stupa, the Mahathupa or Ruvanvalisayastupa, the Thuparama, the Abhayagiri vihara, the Jetavana vihara and the Selacetiya (Coningham1999).

Figure 3 An almost complete ‘torpedo’ jar from al-Qusur, Kuwait, from the eighth century AD (photograph courtesy of Dr Derek Kennet, Durham University).


5


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METHODOLOGY

Samples

Twenty-one sherds were selected for analysis (Table 1). The main criterion for sample selectionwas that they contained sufficient residue on the interior surface of the potsherds to permitGC–MS, stable light isotope and other geochemical analysis. However, due to the nature ofthe samples, not all analyses were possible (Table 2). Eight of the sherds were analysed at ElfAquitaine, Pau (France), and a further 13 sherds were analysed at the University of Bradford(UK) (Table 3).

Subsamples of the visible residues were taken using a solvent-cleaned spatula. Eachsubsample was dissolved in approximately 3 ml of dichloromethane:methanol (2:1, v/v), withultrasonication for 5 min to aid dissolution. Separation from the insoluble solid was aided bycentrifugation for 5 min (2000 rpm) and the solvent was decanted. This process was repeateda further two times combining the solvent soluble portion. The solvent was then removed bynitrogen blowdown to recover the total lipid extract.

Molecular characterization: GC–MS

The total lipid extracts were analysed by gas chromatography – mass spectrometry (GC–MS)in order to examine sterane (

m

/

z

217) and terpane (

m

/

z

191) patterns, which are generally usedto elaborate genetic parameters allowing the differentiation of various bitumen sources. For

Table 1 The find numbers, context, stratigraphic phase and a brief description of each sample

Find number Context Stratigraphic phase Description of sherd

8000 41 –8601 25 XCVII Body sherd, bitumen coating the interior surface8604 103 XCV Body sherd, bitumen coating the interior surface8605 25 XCVII Interior ribbed8606 134 XCV8612 15 CIII8615 74 XCIV Body sherd, bitumen coating the interior surface8617 124 XCV8618 14 CII8621 86 CX8628 4 CXIV Body sherd, very eroded, bitumen coating the interior surface8630 134 XCV8636 401 XCV8637 142 XCV Body sherd, bitumen coating the interior surface8639 256 XCV8641 316 XCV8642 368 XCV30101 4 CXIV Body sherd, bitumen coating the interior surface—

the exterior bears white slips with some areas of bitumen30102 4 XCV30105 4 CXIV Body sherd, bitumen coating the interior surface30112 4 CXIV Buff ware, base of amphora, bitumen coating the interior surface

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Table 2

Gross composition and biomarkers: these samples were analysed at Elf Aquitaine Pau (France)

Find number

Dichloromethaneextract

LC on 10 mg LC on 100 mg asp. on 100 mg Steranes Diasteranes

sat% aro% pol% sat% aro% res% asp% C

21

–C

22

C

27

–C

29

C

29

ααα

R

8601 7.46 4.6 7.1 88.3 3.4 5.4 19.2 72.0 Present Altered C

27

–C

28

Selectively altered Present8604 6.36 2.7 6.5 90.8 1.6 3.4 20.9 74.1 Traces Altered C

27

–C

28

Not altered Absent8615 0.85 9.5 14.2 76.3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.8628 0.63 11.5 9.2 79.3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.8637 3.29 10.8 9.5 79.7 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.30101 17.1 2.5 5.9 91.6 3.4 5.2 15.5 75.9 Traces Altered C

27

–C

28

Selectively altered Absent30105 32.9 1.4 5.0 93.6 0.9 2.6 17.7 78.8 Traces Altered C

27

–C

28

Selectively altered Absent30112 23.2 1.5 7.0 91.5 2.4 3.1 24.2 70.3 Traces Altered C

27

–C

28

Selectively altered Absent

Find number

Terpanes Degree of alteration

Biomarker ratios

αββ

steranes composition

Tricyclopol. Oleanane C

27

–C

35

hopanes OLN/C30

αβ

H Ts/Tm GCRN/C30

αβ

H C27Sdia/C29

ααα

R %C

27

%C

28

%C

29

8601 Present Absent Slightly altered 7 0.04 0.47 0.16 0.87 14.5 26.3 59.28604 Traces Absent Not altered 6b 0.02 0.11 0.27 0.02 11.8 23.2 65.08615 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.8628 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.8637 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.30101 Traces Absent Altered 8b 0.05 0.31 0.25 0.12 8.7 21.1 70.230105 Traces Absent Slightly altered 7 to 8b 0.03 0.2 0.33 0.07 6.3 19.6 74.130112 Traces Absent Altered 8b 0.04 0.37 0.39 0.05 4.0 25.4 70.6

n.d., No data obtained due to low sample size.


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those samples analysed in the UK, GC–MS was carried out using a Hewlett-Packard 5890series II GC connected to a 5972 series mass-selective detector. The splitless injector andinterface were maintained at 340

°

C. Helium was the carrier gas at constant inlet pressure. Thetemperature of the oven was programmed from 50

°

C (2 min) to 340

°

C (10 min) at 10

°

C min

−

1

.The GC was fitted with a 15 m

×

0.25 mm, 0.25

µ

m BP-1 phase fused silica column (SGE).The column was directly inserted into the ion source, where electron impact (EI) spectra wereobtained at 70 eV with full scan from

m

/

z

50 to 700. For samples analysed in France, GC–MSwas performed using a Hewlett-Packard 6890/5973 GC/MSD. The mass spectrometer wasoperated at 70 eV with a mass range of

m

/

z

50–600. The gas chromatograph was equippedwith a DB5 column (J&W, 60 m

×

0.25 mm, film thickness

=

0.1

µ

m) with a deactivatedpre-column (5 m

×

0.32 mm) between the injector and the chromatographic column toimprove the resolution. Helium was the carrier gas. The oven temperature was programmedfrom 40 to 300

°

C at 2

°

C min

−

1

and held at 300

°

C for 66 min.

Isotopic analysis:

δ

13

C and

δD

In order to gain information on the origin of the organic residue, the δ13C values (in ‰/PDB)must be acquired on a reliable organic fraction; for example, the asphaltene fraction, whichrepresents the bitumen. Values on bulk samples, as used by Schwartz et al. (1999), are unsuitableas they are extensively influenced by the carbonate content of the sample (Connan and Nishiaki2003, 283–306). δD values were also obtained despite the fact that this parameter, althoughpartly genetic, is mainly influenced by the degree of weathering of samples. To obtain the

Table 3 Stable isotopic data

Find number δ13Casp (‰) δDasp (‰)

8000* −27.7 −708601 −27.8 −828604 −28 −748605* −27.2 −728606* −27.8 −698612* −27.6 −698617* −27.0 −698618* −28.2 −668621* −27.7 −678628 −26.8† −81†8630* −26.4 −988636* −27.3 −698639* −26.8 −718641* −27.5 −698642* −27.9 −5930101 −27.7 −6030102* −27.8 −8230105 −27.8 −7030112 −27.7 −81

*Samples analysed at the University of Bradford, UK; the remainder of the

samples were analysed at Elf Aquitaine, Pau (France).

†δ13C and δD determinations of the bitumen fraction.

8 B. Stern et al.

© University of Oxford, 2007, Archaeometry 49, 3 (2007) 000–000

asphaltene fraction, 5 ml of n-hexane was added to a portion of the total lipid extract withultrasonication for 5 min. The asphaltene fraction precipitated during this process, withseparation assisted by centrifugation for 5 min (2000 rpm), and then the solvent was decanted.This process was repeated a further two times and the solvent removed by nitrogen blowdown.

For samples analysed in the UK, carbon isotope analysis required approximately 0.2 mgamounts of sample material weighed into tin capsules. Samples were flash combusted in acolumn containing (Cr2O3) and silvered cobalt (I) oxide held at a temperature of 1020°C andthe resultant gases reduced in a column of elemental copper at 680°C, and then passed througha water trap of magnesium perchlorate before being separated by the GC column prior tointroduction to the MS (Finnigan Delta Plus XL). The reference gas was standardized againstthe international standard PEF1. Methionine standards (δ13C – 26.6‰) were run as qualitycontrol check samples during batch analysis of the samples. For hydrogen isotope analysis,approximately 0.85 mg amounts of sample material were weighed into silver capsules. Thetechnique used for this analysis was elemental analyser – isotope ratio mass spectrometry(EA–IRMS). In this technique, the silver capsules are loaded into an automatic sampler. Theyare then dropped into a furnace at 1080°C and thermally decomposed to H2 and CO overglassy carbon. Any traces of water produced are removed by magnesium perchlorate, whileany traces of CO2 formed are removed via a Carbosorb trap. H2 is resolved by a packed columngas chromatograph held at 35°C. The resultant chromatographic peak enters the ion source ofthe IRMS, where it is ionized and accelerated. Gas species of different mass are separated ina magnetic field and then simultaneously measured on a Faraday cup universal collector array.For H2, masses 2 and 3 are monitored.

For samples analysed in France, the δ13C values on asphaltenes were measured by anon-line process that couples a Carlo-Erba 2500 elemental analyser with a Finnigan Delta Plusmass spectrometer via a Conflo II unit. The sample was transported by a helium flow (103.4kPa [15 psi], 86 ml min−1) and oxidized by pure oxygen (120.7 kPa [17.5 psi], 37 ml min−1)at a temperature of 1020°C in a furnace containing tungsten oxide on alumina. An on-linemagnesium perchlorate trap was used to remove the water produced by combustion. Thegenerated gases (CO2 and SO2) were separated at 90°C on a packed column and introduced intothe mass spectrometer. A pulse of reference gas (CO2) with known δ13C was injected before eachCO2 sample peak. Isotope values are provided with an uncertainty of ± 0.2‰/PDB for CO2.

δD values were determined by an off-line process. The sample was oxidized with copperoxide at 590°C for 15 h. The water produced by the oxidation was subsequently reduced withzinc at 470°C for 30 min and the resultant hydrogen introduced via a dual inlet into a FinniganDelta Plus mass spectrometer. A known standard is used to calculate the δD value of thesample. The NBS 22 standard was used to calibrate the instrument. Deuterium values are givenwith an uncertainty of ± 4‰/SMOW.

RESULTS AND DISCUSSION

GC–MS: The identification and origin of bitumen based on sterane–terpane biomarkers

All the samples examined here were identified as bitumen. The term bitumen is usually appliedto naturally occurring solid or liquid hydrocarbon deposits that are soluble in an organic solvent.It is a highly complex mixture of many classes of compounds and is comprised of three mainfractions; asphaltenes, resins (not to be confused with plant resins) and hydrocarbons. Thehydrocarbons contain certain molecules that have previously been used in the field of organic

Trade and exchange in bitumen-coated ceramic vessels 9


geochemistry as biomarkers (Killops and Killops 2005; Peters et al. 2005, 322–54). Mostoften, bitumen wells up from the ground in liquid or viscous form, although pure bitumen insolid form can be found in asphaltite veins (Connan and Deschesne 1996). Figure 4 illustratesthe locations of the archaeological samples and natural oil seeps discussed herein.

Bitumen, like other fossil fuels, contains a suite of characteristic molecules (biomarkers)that can be separated and identified by GC–MS. The distribution patterns of these biomarkerscan be used to help source the bitumen. Among the most commonly used biomarkers are thesteranes and terpanes. Steranes are a class of polycyclic alkanes derived from steroids bymoderate rearrangement. The second group of petroleum biomarkers are the terpanes. Themost important family within this group are the geohopanoids (Connan 1999, 39). In addition,there are other individual molecules that can help to identify the bitumen source. Gammaceraneis believed to be derived from a marine source and is a key biomarker seen in samplesfrom the Dead Sea and Jebel Bichri in Syria (Connan 1999, 46; Harrell and Lewan 2002, 287).Oleanane constitutes a unique genuine chronostratigraphic biomarker, as it provides keyinformation to trace an oil type back to a specific geological period. This molecule hasnever been observed in oils from Iraq and Syria, but is well known in many oils from Iran,in particular from the Khuzestan and Far provinces (Connan 1999, 40). The ratio between Tm(17α-22,29,30-trisnorhopane) and Ts (18α-22,29,30-trisnorneohopane) depends on the maturityof the bitumen, but it is also influenced by the lithology of the source.

As is usual in archaeological bitumen, biomarker fingerprints display various degrees ofdegradation, ranging from almost unaltered patterns (state 0) to highly altered patterns (state 8).At the highest level both steranes and terpanes are altered, but between states 0 and 7, terpanesare preserved and may tentatively be used to define genetic parameters reliably enough toidentify the origin of the bitumen. In our set of samples (Table 2), biomarkers were affectedand present degrees of alteration between 6 and 8 (‘b’ means that a selective degradation ofthe 29αααR sterane has taken place in the sterane group; such selectivity is a strong indicatorof biological degradation).

The GC–MS analysis confirms that most of the samples are very similar. Examples ofsterane and terpane distribution patterns are presented in Figure 5. Sample 8604 exhibits acharacteristic pattern with no obvious alteration, whereas sample 30112 illustrates a typicalpattern in which the hopane family has been severely altered. There was no trace of oleanane,which indicates that the Sri Lankan bitumen was unlikely to be from bitumen sources in the Farsor Khuzestan provinces of Iran (Fig. 4). The low abundance of gammacerane indicates that thesample is unlikely to have come from any sources of the Dead Sea or at Jebel Bichri in Syria.

The review of the regular sterane composition of the samples (Table 2) is represented ina ternary diagram (%C27, %C28 and %C29αββ steranes; Fig. 6). This confirms that the SriLankan archaeological samples (Fig. 6 (b)) are much more degraded than the Iranian oil seeps(Fig. 6 (a)). These weathering effects may have been produced during burial of the potsherds,after the coating of the bitumen layer. One should note in addition that the Sri Lanka datamatches those from cAkkaz, which also mostly originate from a coating of bitumen on theinterior face of potsherds and amphorae (Connan in press). However, from cAkkaz, bitumencontaining 18α-oleananes was identified and it has been proposed that bitumen fromMamatain, also in Luristan, was used in mixtures with others mainly from the area of Ain Gir– Dehluran. It is also noteworthy that these potsherds belong to the same date range; cAkkazis a Partho-Sasanian site in the harbour of present-day Kuwait. Some other archaeologicalsamples fall within the same zone: Susa, Tepe Ali Abad and Dehluran, all located in the Luristanprovince of Iran (Fig. 4).

10B

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rchaeometry 49, 3 (2007) 000–000

Figure 4 A map illustrating the locations of the archaeological samples (in italics) and natural oil seeps (normal font) discussed herein.



Fig

ure

5E

xam

ples

of

part

ial

mas

s ch

rom

atog

ram

s of

the

ter

pane

s (m

/z 1

91)

and

the

ster

anes

(m

/z 2

17)

for

sam

ple

find

num

bers

860

4 an

d 30

112:

(a)

m/z

191

te

rpan

es,

find

num

ber

8604

; (b

) m

/z 2

17 s

tera

nes,

find

num

ber

8604

; (c

) m

/z 1

91 t

erpa

nes,

find

num

ber

3011

2; (

d) m

/z 2

17 s

tera

nes,

find

num

ber

3011

2.

12 B. Stern et al.


Figure 7 shows the sterane and terpane ratios presented as the GCRN(gammacerane)/17α,21β-hopane versus Ts/Tm for these samples and the Iranian oil seeps. All the Sri Lankansamples fall within the area of Ain Gir, Dehluran, Siah-Kuh, Sultan oil seeps and Susa archaeo-logical samples. However, among the five Sri Lankan samples, find number 8604 (Fig. 7 (b),shown in bold) is the only one to have a reliable GCRN to 17α,21β-hopane (gammacerane/hopane) ratio. For the others, the obvious degradation of the hopane family has changed thegammacerane to hopane ratio, making it invalid as a true genetic parameter. Enrichment ofgammacerane generally occurs through biodegradation, as discussed in sample 30112 above(Fig. 5). Sample 8604 is therefore considered to have preserved bitumen, and falls close to theSultan oil seep as noted above.

Figure 6 Ternary diagrams of the percentage C27, C28 and C29αββ steranes for (a) the oil seeps of Iran, with some archaeological samples, and (b) the crude oils of Iran/Iraq, with Sri Lankan samples. Text in italics indicates archaeological samples, whilst normal font indicates natural oil seeps.



Figure 8 presents 18α-oleanane/17α,21β-hopane versus Ts/Tm. Samples from Sri Lankaexhibit 18α-oleanane/17α,21β-hopane values that are very low but not equal to zero. In fact,the degradation of hopanes revealed a peak with the retention time of oleanane, but this moleculehas not been unambiguously confirmed as such. Therefore, this ratio (Fig. 8 (b), y-axis) islikely to be equal to zero. Once again, the Sri Lankan samples fall in the area of Ain Gir, Dehluran,Siah-Kuh and the Sultan oil seeps, and close to some archaeological samples from cAkkaz.

Figure 9, which compares the Sri Lanka data (Fig. 9 (b)) to the Iranian oil seeps (Fig. 9 (a)),shows again that the archaeological samples match with the Ain Gir, Dehluran, Pol Dokhtarand Sultan oil seeps. One may notice that the most representative sample (8604) correspondsto the Pol Dokhtar and Sultan oil seeps. Sample 8601 is separate from the main group, withmore diasteranes and a higher Ts/Tm ratio.

Isotopic analysis: the source of bitumen based on δ13C and δD

Another way to source bitumen, which has been found to be complementary to the biomarkerapproach, is to use stable light isotope analysis to obtain values for δ13C and δD. In theirstudy of the bitumen in the Louvre Museum’s collection, Connan and Deschesne (1996, 115)showed that δ13C determinations of the asphaltene fraction, the least soluble fraction inpetroleum, provides a reliable genetic parameter as it is highly characteristic of the original oil.It is minimally altered for δ13C values (limited to 0.5‰/PDB) by the processes of biodegrada-tion, cleaning and oxidation, all of which can affect the molecular pattern—especially forarchaeological samples (Connan 1999, 119). However, the δD values are affected and can shifttowards heavier values.

Figure 7 Sterane and terpane ratios: GCRN/17α,21β-hopane versus Ts/Tm for (a) Iranian oil seeps, with some archaeological samples, and (b) Sri Lankan samples. Sample 8604, shown in bold, is the only one to have a reliable ratio. Text in italics indicates archaeological samples, whilst normal font indicates natural oil seeps.

14 B. Stern et al.


Figure 8 18α-oleanane/17α,21β-hopane versus Ts/Tm for (a) oil seeps with some archaeological samples and (b) Sri Lankan samples. Text in italics indicates archaeological samples, whilst normal font indicates natural oil seeps.

Figure 9 27Sdia/29ααR versus Ts/Tm for (a) oil seeps with some archaeological samples and (b) Sri Lankan samples. Text in italics indicates archaeological samples, whilst normal font indicates natural oil seeps.



For δD values, the Sri Lankan samples fall within the –98‰ to –59‰ range (Table 3),confirming that the bitumens are oxidized and biodegraded. Comparison of the Sri Lankandata with various bitumen sources available in antiquity from Iran (Fig. 10 (a)) shows that thesamples match the data found in the Ain Gir, Dehluran, Pol Dokthar and Sultan oil seeps. Thedata also match Susa bitumen, although one must only take into account that the data fromthe bitumen of Susa are of true bitumen mixed with organic matter and minerals, and are not dataon bitumen mastic, which corresponds to a natural rock extract. Bitumen mastic has sincebeen shown to be from rocks outcropping at 130 km from Susa (Connan and Deschesne 1998,2001) and not artificial bituminous mixtures as proposed initially (Connan and Deschesne1996).

In Connan and Deschesne’s (1996, 114–15) study of Susa artefacts, they identified threegroups of bitumens within the Susa area. The lack of oleanane and Ts to Tm ratios suggeststhat the Sri Lankan samples are from the type III family group. Therefore, the bitumen is mostlikely to have originated from the Luristan region.

DISCUSSION AND ARCHAEOLOGICAL IMPLICATIONS

Bitumen was a valuable commodity and has been used for a variety of purposes in westernAsia for over 40 000 years. Its first use may have been as an adhesive to fix handles to flinttools, but later it was used as mortar for constructing buildings, in jewellery, householdobjects, sculpture and mummification, and to waterproof baskets or pottery. Bitumen-linedceramics have been found at many archaeological sites; for example, at Ur in Iraq (Bilkadi1996, 107) and at Siraf and Susa in Iran (Tampoe 1989, 11; Connan and Deschesne 1996, 42–3).Natural bitumen deposits are widespread in western Asia, especially in the Zagros mountains

Figure 10 Isotopic data of the asphaltene fraction of bitumens from different deposits, plotted as δ13C (‰/PDB) versus δD (‰/SMOW), for (a) oil seeps and some archaeological samples from Iran, and (b) Sri Lankan samples and crude oils from Iran and Iraq.

16 B. Stern et al.


of Iran. Ancient peoples from northern Iraq, south-west Iran and the Dead Sea area used thisnatural resource extensively. Sri Lanka, on the other hand, has no known source of bitumen, sothe presence of bitumen-coated vessels must indicate trade, possibly with western Asia. Theevaluation of the bitumen source can provide valuable information on trade and culturalcontact. A study by Jacques Connan (1999, 39–41), using a combination of both GC–MS andstable light isotopes on archaeological bitumen from various phases of occupation at Tellel’Oueili, southern Mesopotamia, revealed that the source of bitumen varied through time. Thesource of the bitumen shifted from the Luristan and Khuzestan sources in 5800–4550 bc tonorthern Iraq between 4550 and 3700 bc. Later, in 3500–3200 bc, the bitumen was beingimported from the famous deposits at Hit-Abu Jir (Connan 1999, 39–41). Archaeologicalbitumens from Egypt and Canaan, dating to 3900–2200 bc, were analysed and linked to naturalasphalts from the Dead Sea area, increasing our knowledge of ancient trade routes (Connanet al. 1992). Analysis of the black residue on Egyptian mummies dating between 1000 bc andad 400 revealed that the bitumen found in some samples was geochemically similar to theDead Sea bitumen, and very different to the bitumen found in Iraq and Iran (Connan 1999).Bitumen associates with a mummy from the third intermediate period (c. 900 bc), which mostlikely came from the Gebel Zeit seep, near the Red Sea, on the Gulf of Suez (Harrell andLewan 2002). Another study showed that the bitumen used to paint ceramics from the LateNeolithic site of Tell Sabi Abyad in Syria came from at least two different sources in Iraq. Thisindicates that trade was taking place in either the bitumen used to paint the pots or the paintedpottery itself (Connan et al. 2004).

Both historical sources and archaeological evidence have shown that Sri Lanka played animportant role as a trade centre between the fourth and 10th centuries ad (Whitehouse andWilliamson 1973; De Silva 1981, 53–4), suggesting sea links between West, South, South-Eastand East Asia (Bopeachchi 1996). As noted above, Anuradhapura’s East Asian glazed ceramicssuggest links with southwestern, southeastern and northern China, perhaps northern Vietnamand its Early Islamic Lustre ware and White Tin Glaze ware links with Abbasid Iraq and Iran,and ‘Imitiation’ Lustre ware links with Khurasan in eastern Iran (Coningham 2006). Since SriLanka has no natural source of bitumen, this evidence is now augmented with the suggestionthat the buff ware ‘torpedo jars’ were transported into Sri Lanka with an internal depositof bitumen from western Asia. The molecular and isotopic data presented here extend theevidence to suggest a link between Sri Lanka and specific sources of bitumen. The majority ofthe samples of the bitumen most probably came from a source near Susa, possibly in theLuristan region of Iran. One explanation for the presence of bitumen deposits on the interiorsurfaces of the pottery is that bitumen was being traded between western Asia and Sri Lanka.Samerian and Assyrian documents refer to trade of bitumen in both solid form, traded byweight, and liquid form, sold by volume (Connan and Deschesne 1996, 30–1). However, it isunlikely that the buff ware vessels were used to trade bitumen, as in almost every case theinterior coating is a thin and homogeneous layer, as if deliberately applied. If bitumen wascontained in the vessel in quantity for decanting, then a more irregular and discontinuouspattern would have been expected. Finally, there is no evidence of bitumen in Sri Lanka beingused in other functions for which it is well known in West Asia—as a sealant or adhesive, forexample. Thus, we suggest that the interior surfaces of the vessels were coated with bitumenin order to waterproof and seal them. In addition, the bitumen used to line the interior ofthe buff ware vessels was most likely applied in its liquid form, as there are splashes on theexterior surface. This suggests that the pottery was coated using bitumen from a liquid source.The absence of polyaromatic hydrocarbons suggests that these samples have not been subjected



to thermal alteration. However, gentle heating to melt solid bitumen and make it flow may havebeen used, as under these mild thermal conditions polyaromatic compounds will not beformed in abundance. In addition, most of the low molecular weight aromatics may have beenlost due to weathering processes. A bitumen lining would allow the vessels to be used for thestorage or transport of liquid commodities. The lining of transport vessels is well known fromexamples of Roman amphorae from the eastern and western Mediterranean that were usedfor carrying wine, fish sauce and other commodities (Heron and Pollard 1988). There is alsoevidence for the lining of transport amphorae from both pre-Roman (fourth century bc; Beckand Borromeo 1990) and post-Roman (seventh century ad; Bass and van Doorninck 1982)contexts. Where molecular investigations have been undertaken from samples across thischronological range, the linings are almost exclusively derived from the Pinaceae family, eitherbled resins or heated wood tar and pitch derivatives (the most likely candidate is pine wood).

The production locations of the buff ware ‘torpedo jars’ are not known but could be closeto the natural bitumen sources, since transporting the vessels over great distances when emptyor transporting liquid bitumen to a pottery workshop is unlikely. However, it is also possiblethat the bitumen could have been imported to a ceramic production site near to the sea andlined there prior to transport to Sri Lanka. It is also plausible that the contents of the vesselswere procured from areas local to the bitumen sources or export sites, but only additional analysisof the sherds and other ‘torpedo jars’ will resolve this question of origin. The bitumen liningwould have acted as a good waterproofing layer, allowing liquids to be stored or transported.There are various reasons for the presence of buff ware ceramics at Anuradhapura. The first isthat the vessels themselves were being transported empty so that they could be used in thestorage or transportation of milk or other liquids within Sri Lanka (Weerakkody 1997, 86).The second, more likely, explanation is that the vessels were used to transport oils or winesfrom western Asia to South and South-East Asia, and that a number were received at Mantaiand then transported inland to the capital. It is equally possible that once they had beenemptied of their contents, the receptacles were then reused and became valued containers inthemselves.

CONCLUSIONS

Despite the fact that the bitumen from the buff ware ‘torpedo jars’ from Sri Lanka analysedherein are in general much more degraded samples than their oil seep counterparts, biomarkerratios as well as isotopic data indicate that the majority of the samples originate from a sourcenear Susa, in the Luristan region of Iran. Indeed, some of these samples are very much akin toarchaeological samples found in the slightly earlier Partho-Sasanian site of cAkkaz in Kuwait.This scientific evidence, together with archaeological evidence and documentary sources,confirms that Sri Lanka had extensive trade links with western Asia in the second half of thefirst millennium ad. The bitumen-lined vessels may have been used to store or transport avariety of perishable liquid products, such as oils, perfumed oils or wine (Brun 2000): how-ever, given the waterproofing layer, the chemical identification of these products is not easy todetermine. The nature of these contacts is still to be clarified, be they formal trade networks orinformal souvenir-collecting travellers, gift-bearing pilgrims (Prickett 1990c, 169) or even thepresence of communities of foreign traders at Anuradhapura, perhaps suggested by the discoveryof a Nestorian cross in the upper levels of the city (Hocart 1924, 52). The presence of ceramicsand even buff ware ceramics without bitumen linings may even suggest some informal modeof exchange, possibly in the form of offerings to the Buddhist temples at Anuradhapura.

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Certainly, whilst these vessels have been found at key urban sites such as Anuradhapura,Mantai and Sigiriya, two seasons of intensive survey and sample excavations within thehinterland of Anuradhapura have failed to find any examples amongst the rural sites ofAnuradhapura’s hinterland (Coningham et al. 2006). Finally, it is very interesting to note thatthe presence of long-distance traded goods from West and East Asia comes at a time whenstructures within the Citadel of Anuradhapura appear to be mainly of reused materials, whatParanavitana (1936, 3) termed vestiges of ephemeral mud structures, or squatter occupation.Indeed, most scholars would identify this period as one of steady decline, culminating with theend of the Anuradhapura kingdom, the abandonment of Anuradhapura as capital and thecapture of its last ruler, Mahindu V, by the Cholas armies of southern India in ad 1017 (De Silva1981, 26). However, it appears that the island’s international networks of trade and exchangewere powerful enough to continue functioning until the very end of the city itself, at which timeits merchants and traders presumably shifted their allegiances to other, more successful, emporia.

ACKNOWLEDGEMENTS

Thanks are due to Elf Aquitaine Production for their support in the analyses of samples intheir Scientific and Technical Center at Pau (France) and especially to Daniel Dessort, ThérèseTexier, André and Marie-Hélène Doumergue, Jean-Bernard Berrut, Nadine Loubère andYannick Poirier, who carried out the analyses of the samples. Derek Kennet and Seth Priestmanof the Department of Archaeology, Durham University, kindly provided comments on‘torpedo jars’ and Steve Cheshire illustrated the vessel from Anuradhapura. The excavationsat Anuradhapura were supported by the British Academy, the Sri Lankan Department ofArchaeology, the McDonald Institute for Archaeological Research, the Ancient India and IranTrust and the Society for South Asian Studies (the British Academy). The samples were fromsherds on temporary loan, courtesy of the Sri Lakan Department of Archaeology.

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