- Email: [email protected]
The use of AMS 14C dating to explore issues of occupation and demise at the medieval city of Angkor, Cambodia
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 259 (2007) 388–394 www.elsevier.com/locate/nimb
The use of AMS 14C dating to explore issues of occupation and demise at the medieval city of Angkor, Cambodia Dan Penny a, Quan Hua b
b,*
, Christophe Pottier c, Roland Fletcher d, Mike Barbetti
d,e
a School of Geosciences, University of Sydney, NSW 2006, Australia Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia c Ecole Franc¸aise d’Extreˆme Orient, P.O. Box 93300, Siem Reap, Cambodia d Department of Archaeology, University of Sydney, NSW 2006, Australia e Radiogenic Isotope Laboratory, University of Queensland, Brisbane, QLD 4072, Australia
Available online 2 March 2007
Abstract Angkorian temples are characterised by one or more encircling moats that are excavated into the alluvial substrate. As a key part of the overall design of the temple, the moats are important symbolically and are presumed to be contemporaneous with the associated temple. They also represent important depositional basins for sediment and other materials and can therefore yield vertical profiles of sediment that has accumulated since the moat was originally excavated. Unconformities in these moat profiles can be dated absolutely using smallsample, high-precision AMS radiocarbon techniques. These unconformities are likely to represent periodic re-excavation or maintenance of the moat and therefore indicate the presence of large, presumably centrally organised workforces. In some instances, presumed anthropogenic unconformities occur centuries after Angkor was supposedly abandoned. In this way, radiocarbon dates themselves are being used as a proxy indicator of cultural activity and are being used to challenge the historiography of Angkor’s famous demise. Ó 2007 Elsevier B.V. All rights reserved. PACS: 01.75.+m; 07.75.+h Keywords: AMS; Radiocarbon dating; Archaeology; Angkor; Cambodia
1. Introduction Angkor was the capital of a vast medieval empire that incorporated most of mainland Southeast Asia at its zenith in the 12/13th centuries AD (Fig. 1). Angkor itself – a succession of temples established between the 9th and 15th centuries AD surrounded by a vast penumbra of low-density residential settlement – is now known to be the largest low-density pre-industrial city on Earth [1]. Despite its size, power and ostentatious wealth [2], 19th century European visitors found most of the Angkor region utterly abandoned. Unfortunately, the historiography of Angkor’s final centuries is extremely speculative due to the lack of contemporary inscriptions and monumental construction. *
Corresponding author. Tel.: +61 2 9717 3671; fax: +61 2 9717 9265. E-mail address: [email protected] (Q. Hua).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.02.095
Consequently, the reasons for Angkor’s demise and the timing and processes of Angkor’s eventual abandonment, remain unknown. AMS 14C dating has been used to date key periods in the history of the urban complex at Angkor [3] and establish chronologies for palynological records used to study land-use change at Angkor [4]. This paper focuses on the use of AMS 14C dating of sediment sequences from temple moats to identify unconformities in the moat stratigraphy that can be related to initial excavation or occasional re-excavation of the moats. This is important in terms of the demise of Angkor because clear unconformities may indicate when (presumably) large and centrally organised work forces were present at Angkor, but also when such activities ceased, potentially revealing patterns of continuity or abandonment that are invisible in the archaeological and historic records. In this way, small-sample radiocarbon
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Fig. 1. Map of central and southern Angkor, showing the location of sites mentioned in the text and the core locations. Mapping and ground verification of southern Angkor by Pottier [18]. Digitisation by Evans [19]. Spot heights and contours from the Japan International Co-operation Agency.
dating becomes a valuable proxy indicator of cultural activity. 2. Angkorian temples and temple moats The design of Angkorian temples was intended to represent the Hindu universe in microcosm. Thus, the moats and walled enclosures that surround the temple represent the encircling oceans and mountains around the continent of Jambudvipa (the raised central platform), upon which the sacred Mt Meru (the temple proper) and the house of gods sits. The moats are therefore central to the conceptual design and superstructure of the temples and their excavation is presumed to be coeval with the construction of the temple or shrine which they encircle. Evidence from archaeological excavations conducted by the Mission Arche´ologique Franco-Khme`re sur l’Ame´nagement du Territoire Angkorien (MAFKATA) between 2000 and 2005 demonstrate unequivocally that the initial excavation of temple moats is coeval with the construction of the associated temple and that the material excavated
from moats was used to elevate the central mound or platform upon which the temple was built. At the site of Prei Khmeng, for example, excavation revealed the stratigraphy of the mound on which the 6–7th century temple was built [5]. The raised platform or mound upon which the temple was constructed sits atop an Iron Age occupation site characterised by a continuous succession of habitat and burial layers from the 1st to the 6th century AD. The large artificial mound incorporates obviously re-worked materials including human bones and Iron Age artefacts, indicating the soils used to build the mound were directly excavated from around the site by creating the encircling moat. Excavation of the mound revealed a remarkable inverted cultural stratigraphy overlying the Iron Age layers. Additionally, the absence of such inverted stratigraphy in the occupation mounds located outside of the moat suggests that all material excavated from the moat was used in the construction of the temple platform. The excavations also showed that temple moats were occasionally cleaned out or re-excavated, in some instances linked with larger, centrally organised works on the temple
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and its precincts. At Prasat Trape´ang Phong, for instance, evidence for the re-excavation of the moat has been found [5]. Architectural remains cleared in 1936 and artefacts discovered during recent excavations reveal that the foundation of the site may date as early as the ‘Pre-Angkorian’ period, sometime around the 7–8th century AD and that the site remained occupied until the later Angkorian period (around the 13th century AD). In 2004, a 2 m by 2 m test pit was excavated in the southwest corner of the temple’s moat in order to collect potential organic materials to date the foundation of the temple. However, the bottom layer of the depositional sequence incorporated various potsherds, including some dark brown glazed stoneware characteristic of the later Angkorian period (post 11th century AD) and showing obviously that the moat has been cleaned out, if not partially re-excavated, during the Angkorian period. In that particular case, the work undertaken in the temple’s moat may have been associated with a large works program which fundamentally modified the original site configuration, in particular with the excavation of two large trape´ang (reservoirs or water tanks) and the widening of the temple’s north moat.
NaOCl. The pre-treated pollen samples were washed with deionised water and dried in an oven at 60 °C for 2 days. They were then combusted using the sealed-tube technique in the presence of pre-cleaned CuO and Ag wire and converted to graphite using the H2/Fe method [12]. AMS 14C measurements were carried out using the ANTARES facility at ANSTO [13]. The 14C/13C ratio of each sample was measured relative to the NIST standard of oxalic acid I (HOxI). The 14C age of each sample was calculated after correcting for backgrounds (accelerator and chemistry) and isotopic fractionation using measured d13C. Calibration to convert radiocarbon ages to calender ages was carried out using IntCal04 data set [14] and CALIB program version 5.0.2. [15]. 4. Results and discussion Radiocarbon results from several Angkorian moats are presented in Table 1, together with their calibrated age ranges at 2r. Stratigraphic information is provided in Fig. 3. 4.1. Dating temple construction
3. Materials and methods Sediment cores were taken from temple moats using a Kullenburg-type percussion corer [6] operated from a floating platform. In most instances multiple cores were taken and correlated using the volume magnetic susceptibility [7] profile of each core in order to demonstrate that a representative sample of the stratigraphy had been obtained. Sampling sites for this study are the moats of Angkor Wat, Angkor Thom, Bakong, Preah Khan and Chau Srei Vibol and the reservoir of Srah Srang (Fig. 1). Cores were split longitudinally in the laboratory and the sediments were described and photographed. In all cases it is possible to distinguish the transition from substrate to moat or reservoir infill, either visually (a change in the colour and texture of the sediment), an increase in the loss of weight following ignition at 550 oC [8], or a change in the response of the sediment to a weak magnetic field, reflecting an increase in the concentration of magnetic minerals [7,9] (Fig. 2). All of these indicators reflect a change in the primary source of materials, from a regional alluvial source to a localised supply of biomass from aquatic flora and fauna that become established within the moats and reservoirs following their excavation. The ability to identify unequivocally the transition from substrate to infill means that the timing of the excavation – or re-excavation – can be determined by radiocarbon. Samples for dating were removed from the split cores and sporpolleniferous microfossils were extracted for dating following the method of Brown et al. [10]. This is a modification of standard chemical pre-treatment used in Quaternary palynology [11] where the digestion of plant cellulose – normally achieved by acetolysis – is achieved by repeated washes in 2% w:w sodium hypochlorite
The ability to date the contact between organic moat infill and the alluvial substrate, which should represent the initial excavation of the moat, provides the ability to date absolutely the construction of the associated temple. Additionally, most of the major temples have stylistic or epigraphic ages – often very specific – that enable comparison with the absolute ages for the excavation of the moat. In some cases the historic and absolute radiocarbon ages are in agreement. For example, core AT/01/04/B, from the southern part of the moat surrounding the late 12th century AD walled city of Angkor Thom (Fig. 1), has two dates (OZH173 and OZI294; Fig. 3) that bracket the boundary between moat infill and natural substrate (ca. 44 cm depth), indicating the moat was excavated between the late 11th and early 13th centuries AD. By contrast, however, two dates (OZG481 and OZH177) immediately above the original excavation surface in the moat of the Bakong temple (which occurs at 210 cm depth in core BK/07/02/A; Fig. 1), return ages of 1270 ± 40 and 1250 ± 50 14C yrs BP, or ca. 660–890 cal yrs AD [4]. The epigraphic age for the completion of the temple is 881 AD, at the extreme end of the 2r probability distribution for the absolute ages. It is probable that the moat was excavated and began to accumulate autochthonous organic sediment several decades if not a century before the temple was finally dedicated, implying that an earlier structure may have apparent on that site during the 7th century [16]. 4.2. Evidence of re-excavation In many instances radiocarbon dating of the contact between moat infill and natural substrate indicates that
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Fig. 2. An example of the ways in which various stratigraphic and sedimentological data can be used to identify excavation surfaces. In this core from the reservoir of Srah Srang (SS/07/02/A; Fig. 1), initial excavation is indicated by a change in the colour and texture of the sediment at ca. 22 cm depth, subtle changes in the volume magnetic susceptibility values and the preservation of plant microfossils. A second anthropogenic unconformity occurs at 12 cm depth, indicated by a fining of mineral sediments and increase in the amount of organic carbon in the sediments and an increase in the abundance of pollen from aquatic plants. Lithological Unit I (0–12 cm depth) is a brown organic sandy clay deposit. Unit II (12–22 cm depth) is pale brown clay sand deposit. Unit III (22–32 cm depth) is a pale yellow, firm, oxidised, sandy clay deposit.
the radiocarbon age is significantly younger than the historic age for the temple. For example, a core (AW/01/03/ B, 23 cm total length) extracted from the moat of the famous temple of Angkor Wat (Fig. 1), which has an historic age in the early 12th century AD, revealed an acute contact between the organic infill and the mineral substrate at 17.5 cm depth. A sample (OZH181) taken for dating at 16–17 cm depth, immediately above this contact surface, returned an age of 245 ± 40 14C yrs BP, or 1517–1806 AD at 2r, post-dating the construction of the temple by more than 400 years. Such a major discrepancy between the historic age for the temple complex and the date by which sediment began to accumulate in its moat is problematic if one accepts that the moat is central to the design of the temple and must be contemporary with its construction. If one rejects the notion that the moat was a latter addition, then the most plausible explanation is that there exists an anthropogenic unconformity caused by the excavation of all or part of the moat some time between the
16th and 18th centuries. Critically, this occurs after Angkor was reputedly sacked by invading Thai forces (1430–1431 AD) and subsequently abandoned. In the case of Angkor Wat, ‘post-Angkorian’ occupation is known archaeologically and historically as it was the location of renewed decoration work in the later 16th century when Angkor was again the royal seat of power [17]. This new evidence indicates that the moat of Angkor Wat was probably re-excavated at that time, presumably to increase the depth or capacity of the moat, or to remove encroaching aquatic vegetation. Similar evidence of periodic re-excavation is apparent at the late 12th century AD (1191) temple of Preah Khan. A 37 cm core of sediment was extracted from the southern part of the western arm of the moat (Fig. 1). The contact between infill and substrate occurred at 30 cm depth and a sample (OZG487), taken at 27–28 cm depth, returned an age of 515 ± 35 14C yrs BP, or 1320–1445 cal yrs AD at 2r. This implies that the moat was re-excavated some
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Table 1 Results of AMS radiocarbon dating Lab. code
Core code (see Figs. 1 and 3)
Depth in core (cm)
Radiocarbon age (years BP ± 1r)
OZH179
CSV/01/04/A
25–26
355 ± 30
OZG477 OZG478
SS/07/02/A SS/07/02/A
12.5–13.5 10.5–11.5
335 ± 35 60 ± 35
OZG479
SS/07/02/A
20.5–21.5
540 ± 35
OZG481
BK/07/02/A
207–208
1270 ± 40
OZG487
PK/07/02/B
27–28
515 ± 35
OZH170
AT/01/03/B
26–27
520 ± 40
OZH173
AT/01/04/B
42–43
845 ± 35
OZH177 OZH181
BK/07/02/A AW/01/03/B
208–210 16–17
OZI294
AT/01/04/B
45–46
1250 ± 50 245 ± 40
955 ± 50
Calibrated age range AD (2r probability range) 1454–1529 [0.467] 1540–1634 [0.533] 1469–1642 [1.0] 1691–1729 [0.244] 1810–1923 [0.727] 1312–1358 [0.350] 1387–1439 [0.650] 662–828 [0.947] 838–866 [0.053] 1320–1350 [0.163] 1391–1445 [0.837] 1316–1355 [0.22] 1388–1447 [0.78] 1051–1082 [0.067] 1152–1265 [0.918] 668–884 [1.0] 1517–1595 [0.199] 1618–1684 [0.438] 1734–1806 [0.291] 993–1187 [0.991] 1199–1206 [0.009]
Figures in square brackets indicate the proportion of the probability range represented by each intersection of the probability curve.
Fig. 3. Diagram showing stratigraphic boundaries and known unconformities in cores shown in Fig. 1. *A section of core BK/07/02/A is omitted here in order to show the relationship between the excavation surface and the radiocarbon samples.
200 years after the temple was complete. A 46 cm length core (CSV/01/04/A) taken from the moat of the 11th
century temple complex of Chau Srei Vibol, approximately 17 km to the east of central Angkor, revealed that the infill/
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substrate interface (26 cm depth) dates to 355 ± 30 14C yrs BP (OZH179), or 1454–1634 cal yrs AD at 2r. This postdates the stylistic age of the temple by approximately four to five hundred years and, like Angkor Wat, suggests some form of re-excavation in the post-Angkorian period when the city was supposedly abandoned. In some instances, multiple unconformities are apparent. At the 10th century reservoir of Srah Srang, a sample (OZG479) taken immediately above the excavation surface (22 cm depth) returned an age of 540 ± 35 14C yrs BP, or 1312–1439 cal yrs AD at 2r, around 400 years after the historical age for the creation of the reservoir and more than one century after the site was remodelled by king Jayavarman VII. In the same core, two dates (OZG478 and OZG477) bracket a boundary at 12 cm depth between pale yellow-grey sandy clay (19.5–12 cm depth) and overlying pale brown organic mud (12–0 cm depth). There is a marked fining of average mineral particle size across this boundary, an increase in the proportion of organic carbon and an increase in the abundance of pollen from aquatic plants (particularly Nymphoides), all of which are indicative of an increase in, or stabilisation of, water levels within the reservoir (Fig. 2). OZG478, which dates the initiation of this ‘full reservoir’ phase, returned an age of 60 ± 35 14C yrs BP, or 1691–1923 cal yrs AD at 2r (73% of the 2r probability distribution lies with the range 1810–1923 AD), indicating that this boundary represents a modern unconformity, possibly related to the repopulation of the landscape by Khmer farmers or clearance of the site by French archaeologists in the early 20th century. Sample OZG477 (12.5–13.5 cm depth), which dates the upper boundary of the Angkorian-aged sediments, returned an age of 335 ± 35 14C yrs BP, or 1469–1642 cal yrs AD at 2r, indicating that the most recent re-excavation of the reservoir removed the most of the ‘post-Angkorian’ aged sediment at that location. While multiple cores are correlated in the field according to their magnetic stratigraphy, thus ensuring a representative stratigraphy has been recovered, the spatial extent of episodic re-excavation can not be determined unequivocally in the absence of more extensive stratigraphic survey. For example, it is not possible to claim that the moat of Preah Khan temple was excavated in its entirety in the late 14th century AD, but that some parts of the moat certainly were. The spatial heterogeneity of sediments within the moats is demonstrated at Angkor Thom. Core AT/01/04/ B, discussed above, was taken approximately 300 m west of the central causeway on the southern side of the moat of Angkor Thom. The excavation of the moat at this location is in good agreement with the epigraphic age for the construction of the city and it has a continuous moat-infill sequence to the present day. A 30 cm core taken approximately 200 m to the east of the causeway (AT/01/03/B), however, revealed an acute, horizontal contact between organic moat sediment and mineral substrate at 28.3 cm depth. The mineral material is a typical graded bed, with a thin (<4 mm) lens of grey clay overlying pale-yellow sand. A sample (OZH170) taken for dating at 26–27 cm
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depth, immediately above this graded mineral deposit, returned an age of 520 ± 40 14C yrs BP, or 1316–1447 AD at 2r, around two centuries after the moat was originally excavated, according to the dates from core AT/01/ 04/B, some 500 m to the west. Be that as it may, periodic re-excavation of the temple moats and reservoirs, even if spatially heterogeneous, imply the active maintenance and modification of cultural features in Angkor that, in many instances, clearly post-date the supposed demise of the city. 5. Conclusion AMS 14C dating of sediment cores extracted from several temple moats and reservoirs at Angkor indicates that, in some cases, sedimentation was initiated some time after the historic (epigraphic) and/or stylistic age for the associated monument – as much as half a millennium in some cases. We argue that, because the moats are unequivocally part of the cosmological design of the monuments and can be presumed to coeval with the construction of the associated monument, this dissonance between the historic and absolute ages indicates the episodic excavation of temple moats and reservoirs. If this is indeed the case, then large, perhaps even centrally organised work forces may have been present in Angkor as much as two centuries after the city was supposedly abandoned. Acknowledgements This research is part of the Greater Angkor Project, a collaborative research project by the University of Sydney, Ecole Franc¸aise d’Extreˆme-Orient and APSARA, the Cambodian government body responsible for the management of the Angkor World Heritage site. Funding is provided by the Australian Research Council (Discovery Projects Grant # 0558130) and the Australian Institute of Nuclear Science and Engineering (AINSE Grant # 03/ 091P and 05/134). References [1] R. Fletcher, D. Evans, M. Barbetti, D. Penny, T. Heng, S. Im, C. Khieu, S. Tous, C. Pottier, Udaya 4 (2003) 107. [2] T.-K. Chou, The Customs of Cambodia, The Siam Society, Bangkok, 2001. [3] U. Zoppi, M. Barbetti, R. Fletcher, Q. Hua, R.K. Chhem, C. Pottier, M. Watanasak, Nucl. Instr. and Meth. B 223–224 (2004) 681. [4] D. Penny, C. Pottier, R.J. Fletcher, M. Barbetti, D. Fink, Q. Hua, Antiquity 80 (2006) 599. [5] C. Pottier, Mission Arche´ologique Franco-Khme`re sur l’Ame´nagement du Territoire Angkorien (MAFKATA) Rapport pre´liminaire sur la campagne de fouilles 2004, APSARA-MAE-EFEO, Siem Reap, Cambodia, 2004. [6] J.R. Glew, J.P. Smol, W.M. Last, Sediment core collection and extrusion, in: W.M. Last, J.P. Smol (Eds.), Tracking Environment Change Using Lake Sediments, Basin Analysis, Coring and Chronological Techniques, Vol. 1, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2001, p. 73.
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[7] J.A. Dearing, Environmental Magnetic Susceptibility: Using the Bartington MS2 System, Chi Publishing, Kenilworth, UK, 1999. [8] W.E. Dean, J. Sed. Petrol. 44 (1974) 242. [9] D.C.W. Sanderson, P. Bishop, M.T. Stark, J. Spencer, Quat. Sci. Rev. 22 (2003) 1111. [10] T.A. Brown, G.W. Farwell, P.M. Grootes, F.H. Schmidt, Radiocarbon 34 (1992) 550. [11] P.D. Moore, J.A. Webb, M.E. Collinson, Pollen Analysis, second ed., Blackwell Science, Oxford, 1991. [12] Q. Hua, G.E. Jacobsen, U. Zoppi, E.M. Lawson, A.A. Williams, A.M. Smith, M.J. McGann, Radiocarbon 43 (2001) 275. [13] D. Fink, M. Hotchkis, Q. Hua, G. Jacobsen, A.M. Smith, U. Zoppi, D. Child, C. Mifsud, H. van der Gaast, A. Williams, M. Williams, Nucl. Instr. and Meth. B 223–224 (2004) 109. [14] P.J. Reimer, M.G.L. Baillie, E. Bard, A. Bayliss, J.W. Beck, C.J.H. Bertrand, P.G. Blackwell, C.E. Buck, G.S. Burr, K.B. Cutler, P.E.
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Damon, R.L. Edwards, F.G. Fairbanks, M. Friedrich, T.P. Guilderson, A. Hogg, K.A. Hughen, B. Kromer, G. McCormac, S. Manning, C. Bronk Ramsey, R.W. Reimer, S. Remmele, J.R. Southon, M. Stuiver, S. Talamo, F.W. Taylor, J. van der Plicht, C.E. Weyhenmeyer, Radiocarbon 46 (2004) 1029. M. Stuiver, P.J. Reimer, Radiocarbon 35 (1993) 215. C. Pottier, BEFEO 83 (1996) 318. B.P. Groslier, Angkor et le Cambodge au XVIe`me sie`cle d’apre`s les sources portugaises et espagnole, Annales du Muse´e Guimet, Bibliothe`que d’e´tude 63, PUF, Paris, 1958. C. Pottier, Carte archeologique de la region d’Angkor Zone Sud, Histoire et archaeologie du Monde Indien, Paris, Universite Paris III – Sorbonne Nouvelle, 1999. D. Evans, Pixels, Ponds and People: Urban Form at Angkor from Radar Imaging, Unpublished BA (Hons.) Thesis, Department of Archaeology, University of Sydney, 2002.
Nuclear Instruments and Methods in Physics Research B 259 (2007) 388–394 www.elsevier.com/locate/nimb
The use of AMS 14C dating to explore issues of occupation and demise at the medieval city of Angkor, Cambodia Dan Penny a, Quan Hua b
b,*
, Christophe Pottier c, Roland Fletcher d, Mike Barbetti
d,e
a School of Geosciences, University of Sydney, NSW 2006, Australia Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia c Ecole Franc¸aise d’Extreˆme Orient, P.O. Box 93300, Siem Reap, Cambodia d Department of Archaeology, University of Sydney, NSW 2006, Australia e Radiogenic Isotope Laboratory, University of Queensland, Brisbane, QLD 4072, Australia
Available online 2 March 2007
Abstract Angkorian temples are characterised by one or more encircling moats that are excavated into the alluvial substrate. As a key part of the overall design of the temple, the moats are important symbolically and are presumed to be contemporaneous with the associated temple. They also represent important depositional basins for sediment and other materials and can therefore yield vertical profiles of sediment that has accumulated since the moat was originally excavated. Unconformities in these moat profiles can be dated absolutely using smallsample, high-precision AMS radiocarbon techniques. These unconformities are likely to represent periodic re-excavation or maintenance of the moat and therefore indicate the presence of large, presumably centrally organised workforces. In some instances, presumed anthropogenic unconformities occur centuries after Angkor was supposedly abandoned. In this way, radiocarbon dates themselves are being used as a proxy indicator of cultural activity and are being used to challenge the historiography of Angkor’s famous demise. Ó 2007 Elsevier B.V. All rights reserved. PACS: 01.75.+m; 07.75.+h Keywords: AMS; Radiocarbon dating; Archaeology; Angkor; Cambodia
1. Introduction Angkor was the capital of a vast medieval empire that incorporated most of mainland Southeast Asia at its zenith in the 12/13th centuries AD (Fig. 1). Angkor itself – a succession of temples established between the 9th and 15th centuries AD surrounded by a vast penumbra of low-density residential settlement – is now known to be the largest low-density pre-industrial city on Earth [1]. Despite its size, power and ostentatious wealth [2], 19th century European visitors found most of the Angkor region utterly abandoned. Unfortunately, the historiography of Angkor’s final centuries is extremely speculative due to the lack of contemporary inscriptions and monumental construction. *
Corresponding author. Tel.: +61 2 9717 3671; fax: +61 2 9717 9265. E-mail address: [email protected] (Q. Hua).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.02.095
Consequently, the reasons for Angkor’s demise and the timing and processes of Angkor’s eventual abandonment, remain unknown. AMS 14C dating has been used to date key periods in the history of the urban complex at Angkor [3] and establish chronologies for palynological records used to study land-use change at Angkor [4]. This paper focuses on the use of AMS 14C dating of sediment sequences from temple moats to identify unconformities in the moat stratigraphy that can be related to initial excavation or occasional re-excavation of the moats. This is important in terms of the demise of Angkor because clear unconformities may indicate when (presumably) large and centrally organised work forces were present at Angkor, but also when such activities ceased, potentially revealing patterns of continuity or abandonment that are invisible in the archaeological and historic records. In this way, small-sample radiocarbon
D. Penny et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 388–394
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Fig. 1. Map of central and southern Angkor, showing the location of sites mentioned in the text and the core locations. Mapping and ground verification of southern Angkor by Pottier [18]. Digitisation by Evans [19]. Spot heights and contours from the Japan International Co-operation Agency.
dating becomes a valuable proxy indicator of cultural activity. 2. Angkorian temples and temple moats The design of Angkorian temples was intended to represent the Hindu universe in microcosm. Thus, the moats and walled enclosures that surround the temple represent the encircling oceans and mountains around the continent of Jambudvipa (the raised central platform), upon which the sacred Mt Meru (the temple proper) and the house of gods sits. The moats are therefore central to the conceptual design and superstructure of the temples and their excavation is presumed to be coeval with the construction of the temple or shrine which they encircle. Evidence from archaeological excavations conducted by the Mission Arche´ologique Franco-Khme`re sur l’Ame´nagement du Territoire Angkorien (MAFKATA) between 2000 and 2005 demonstrate unequivocally that the initial excavation of temple moats is coeval with the construction of the associated temple and that the material excavated
from moats was used to elevate the central mound or platform upon which the temple was built. At the site of Prei Khmeng, for example, excavation revealed the stratigraphy of the mound on which the 6–7th century temple was built [5]. The raised platform or mound upon which the temple was constructed sits atop an Iron Age occupation site characterised by a continuous succession of habitat and burial layers from the 1st to the 6th century AD. The large artificial mound incorporates obviously re-worked materials including human bones and Iron Age artefacts, indicating the soils used to build the mound were directly excavated from around the site by creating the encircling moat. Excavation of the mound revealed a remarkable inverted cultural stratigraphy overlying the Iron Age layers. Additionally, the absence of such inverted stratigraphy in the occupation mounds located outside of the moat suggests that all material excavated from the moat was used in the construction of the temple platform. The excavations also showed that temple moats were occasionally cleaned out or re-excavated, in some instances linked with larger, centrally organised works on the temple
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and its precincts. At Prasat Trape´ang Phong, for instance, evidence for the re-excavation of the moat has been found [5]. Architectural remains cleared in 1936 and artefacts discovered during recent excavations reveal that the foundation of the site may date as early as the ‘Pre-Angkorian’ period, sometime around the 7–8th century AD and that the site remained occupied until the later Angkorian period (around the 13th century AD). In 2004, a 2 m by 2 m test pit was excavated in the southwest corner of the temple’s moat in order to collect potential organic materials to date the foundation of the temple. However, the bottom layer of the depositional sequence incorporated various potsherds, including some dark brown glazed stoneware characteristic of the later Angkorian period (post 11th century AD) and showing obviously that the moat has been cleaned out, if not partially re-excavated, during the Angkorian period. In that particular case, the work undertaken in the temple’s moat may have been associated with a large works program which fundamentally modified the original site configuration, in particular with the excavation of two large trape´ang (reservoirs or water tanks) and the widening of the temple’s north moat.
NaOCl. The pre-treated pollen samples were washed with deionised water and dried in an oven at 60 °C for 2 days. They were then combusted using the sealed-tube technique in the presence of pre-cleaned CuO and Ag wire and converted to graphite using the H2/Fe method [12]. AMS 14C measurements were carried out using the ANTARES facility at ANSTO [13]. The 14C/13C ratio of each sample was measured relative to the NIST standard of oxalic acid I (HOxI). The 14C age of each sample was calculated after correcting for backgrounds (accelerator and chemistry) and isotopic fractionation using measured d13C. Calibration to convert radiocarbon ages to calender ages was carried out using IntCal04 data set [14] and CALIB program version 5.0.2. [15]. 4. Results and discussion Radiocarbon results from several Angkorian moats are presented in Table 1, together with their calibrated age ranges at 2r. Stratigraphic information is provided in Fig. 3. 4.1. Dating temple construction
3. Materials and methods Sediment cores were taken from temple moats using a Kullenburg-type percussion corer [6] operated from a floating platform. In most instances multiple cores were taken and correlated using the volume magnetic susceptibility [7] profile of each core in order to demonstrate that a representative sample of the stratigraphy had been obtained. Sampling sites for this study are the moats of Angkor Wat, Angkor Thom, Bakong, Preah Khan and Chau Srei Vibol and the reservoir of Srah Srang (Fig. 1). Cores were split longitudinally in the laboratory and the sediments were described and photographed. In all cases it is possible to distinguish the transition from substrate to moat or reservoir infill, either visually (a change in the colour and texture of the sediment), an increase in the loss of weight following ignition at 550 oC [8], or a change in the response of the sediment to a weak magnetic field, reflecting an increase in the concentration of magnetic minerals [7,9] (Fig. 2). All of these indicators reflect a change in the primary source of materials, from a regional alluvial source to a localised supply of biomass from aquatic flora and fauna that become established within the moats and reservoirs following their excavation. The ability to identify unequivocally the transition from substrate to infill means that the timing of the excavation – or re-excavation – can be determined by radiocarbon. Samples for dating were removed from the split cores and sporpolleniferous microfossils were extracted for dating following the method of Brown et al. [10]. This is a modification of standard chemical pre-treatment used in Quaternary palynology [11] where the digestion of plant cellulose – normally achieved by acetolysis – is achieved by repeated washes in 2% w:w sodium hypochlorite
The ability to date the contact between organic moat infill and the alluvial substrate, which should represent the initial excavation of the moat, provides the ability to date absolutely the construction of the associated temple. Additionally, most of the major temples have stylistic or epigraphic ages – often very specific – that enable comparison with the absolute ages for the excavation of the moat. In some cases the historic and absolute radiocarbon ages are in agreement. For example, core AT/01/04/B, from the southern part of the moat surrounding the late 12th century AD walled city of Angkor Thom (Fig. 1), has two dates (OZH173 and OZI294; Fig. 3) that bracket the boundary between moat infill and natural substrate (ca. 44 cm depth), indicating the moat was excavated between the late 11th and early 13th centuries AD. By contrast, however, two dates (OZG481 and OZH177) immediately above the original excavation surface in the moat of the Bakong temple (which occurs at 210 cm depth in core BK/07/02/A; Fig. 1), return ages of 1270 ± 40 and 1250 ± 50 14C yrs BP, or ca. 660–890 cal yrs AD [4]. The epigraphic age for the completion of the temple is 881 AD, at the extreme end of the 2r probability distribution for the absolute ages. It is probable that the moat was excavated and began to accumulate autochthonous organic sediment several decades if not a century before the temple was finally dedicated, implying that an earlier structure may have apparent on that site during the 7th century [16]. 4.2. Evidence of re-excavation In many instances radiocarbon dating of the contact between moat infill and natural substrate indicates that
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Fig. 2. An example of the ways in which various stratigraphic and sedimentological data can be used to identify excavation surfaces. In this core from the reservoir of Srah Srang (SS/07/02/A; Fig. 1), initial excavation is indicated by a change in the colour and texture of the sediment at ca. 22 cm depth, subtle changes in the volume magnetic susceptibility values and the preservation of plant microfossils. A second anthropogenic unconformity occurs at 12 cm depth, indicated by a fining of mineral sediments and increase in the amount of organic carbon in the sediments and an increase in the abundance of pollen from aquatic plants. Lithological Unit I (0–12 cm depth) is a brown organic sandy clay deposit. Unit II (12–22 cm depth) is pale brown clay sand deposit. Unit III (22–32 cm depth) is a pale yellow, firm, oxidised, sandy clay deposit.
the radiocarbon age is significantly younger than the historic age for the temple. For example, a core (AW/01/03/ B, 23 cm total length) extracted from the moat of the famous temple of Angkor Wat (Fig. 1), which has an historic age in the early 12th century AD, revealed an acute contact between the organic infill and the mineral substrate at 17.5 cm depth. A sample (OZH181) taken for dating at 16–17 cm depth, immediately above this contact surface, returned an age of 245 ± 40 14C yrs BP, or 1517–1806 AD at 2r, post-dating the construction of the temple by more than 400 years. Such a major discrepancy between the historic age for the temple complex and the date by which sediment began to accumulate in its moat is problematic if one accepts that the moat is central to the design of the temple and must be contemporary with its construction. If one rejects the notion that the moat was a latter addition, then the most plausible explanation is that there exists an anthropogenic unconformity caused by the excavation of all or part of the moat some time between the
16th and 18th centuries. Critically, this occurs after Angkor was reputedly sacked by invading Thai forces (1430–1431 AD) and subsequently abandoned. In the case of Angkor Wat, ‘post-Angkorian’ occupation is known archaeologically and historically as it was the location of renewed decoration work in the later 16th century when Angkor was again the royal seat of power [17]. This new evidence indicates that the moat of Angkor Wat was probably re-excavated at that time, presumably to increase the depth or capacity of the moat, or to remove encroaching aquatic vegetation. Similar evidence of periodic re-excavation is apparent at the late 12th century AD (1191) temple of Preah Khan. A 37 cm core of sediment was extracted from the southern part of the western arm of the moat (Fig. 1). The contact between infill and substrate occurred at 30 cm depth and a sample (OZG487), taken at 27–28 cm depth, returned an age of 515 ± 35 14C yrs BP, or 1320–1445 cal yrs AD at 2r. This implies that the moat was re-excavated some
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Table 1 Results of AMS radiocarbon dating Lab. code
Core code (see Figs. 1 and 3)
Depth in core (cm)
Radiocarbon age (years BP ± 1r)
OZH179
CSV/01/04/A
25–26
355 ± 30
OZG477 OZG478
SS/07/02/A SS/07/02/A
12.5–13.5 10.5–11.5
335 ± 35 60 ± 35
OZG479
SS/07/02/A
20.5–21.5
540 ± 35
OZG481
BK/07/02/A
207–208
1270 ± 40
OZG487
PK/07/02/B
27–28
515 ± 35
OZH170
AT/01/03/B
26–27
520 ± 40
OZH173
AT/01/04/B
42–43
845 ± 35
OZH177 OZH181
BK/07/02/A AW/01/03/B
208–210 16–17
OZI294
AT/01/04/B
45–46
1250 ± 50 245 ± 40
955 ± 50
Calibrated age range AD (2r probability range) 1454–1529 [0.467] 1540–1634 [0.533] 1469–1642 [1.0] 1691–1729 [0.244] 1810–1923 [0.727] 1312–1358 [0.350] 1387–1439 [0.650] 662–828 [0.947] 838–866 [0.053] 1320–1350 [0.163] 1391–1445 [0.837] 1316–1355 [0.22] 1388–1447 [0.78] 1051–1082 [0.067] 1152–1265 [0.918] 668–884 [1.0] 1517–1595 [0.199] 1618–1684 [0.438] 1734–1806 [0.291] 993–1187 [0.991] 1199–1206 [0.009]
Figures in square brackets indicate the proportion of the probability range represented by each intersection of the probability curve.
Fig. 3. Diagram showing stratigraphic boundaries and known unconformities in cores shown in Fig. 1. *A section of core BK/07/02/A is omitted here in order to show the relationship between the excavation surface and the radiocarbon samples.
200 years after the temple was complete. A 46 cm length core (CSV/01/04/A) taken from the moat of the 11th
century temple complex of Chau Srei Vibol, approximately 17 km to the east of central Angkor, revealed that the infill/
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substrate interface (26 cm depth) dates to 355 ± 30 14C yrs BP (OZH179), or 1454–1634 cal yrs AD at 2r. This postdates the stylistic age of the temple by approximately four to five hundred years and, like Angkor Wat, suggests some form of re-excavation in the post-Angkorian period when the city was supposedly abandoned. In some instances, multiple unconformities are apparent. At the 10th century reservoir of Srah Srang, a sample (OZG479) taken immediately above the excavation surface (22 cm depth) returned an age of 540 ± 35 14C yrs BP, or 1312–1439 cal yrs AD at 2r, around 400 years after the historical age for the creation of the reservoir and more than one century after the site was remodelled by king Jayavarman VII. In the same core, two dates (OZG478 and OZG477) bracket a boundary at 12 cm depth between pale yellow-grey sandy clay (19.5–12 cm depth) and overlying pale brown organic mud (12–0 cm depth). There is a marked fining of average mineral particle size across this boundary, an increase in the proportion of organic carbon and an increase in the abundance of pollen from aquatic plants (particularly Nymphoides), all of which are indicative of an increase in, or stabilisation of, water levels within the reservoir (Fig. 2). OZG478, which dates the initiation of this ‘full reservoir’ phase, returned an age of 60 ± 35 14C yrs BP, or 1691–1923 cal yrs AD at 2r (73% of the 2r probability distribution lies with the range 1810–1923 AD), indicating that this boundary represents a modern unconformity, possibly related to the repopulation of the landscape by Khmer farmers or clearance of the site by French archaeologists in the early 20th century. Sample OZG477 (12.5–13.5 cm depth), which dates the upper boundary of the Angkorian-aged sediments, returned an age of 335 ± 35 14C yrs BP, or 1469–1642 cal yrs AD at 2r, indicating that the most recent re-excavation of the reservoir removed the most of the ‘post-Angkorian’ aged sediment at that location. While multiple cores are correlated in the field according to their magnetic stratigraphy, thus ensuring a representative stratigraphy has been recovered, the spatial extent of episodic re-excavation can not be determined unequivocally in the absence of more extensive stratigraphic survey. For example, it is not possible to claim that the moat of Preah Khan temple was excavated in its entirety in the late 14th century AD, but that some parts of the moat certainly were. The spatial heterogeneity of sediments within the moats is demonstrated at Angkor Thom. Core AT/01/04/ B, discussed above, was taken approximately 300 m west of the central causeway on the southern side of the moat of Angkor Thom. The excavation of the moat at this location is in good agreement with the epigraphic age for the construction of the city and it has a continuous moat-infill sequence to the present day. A 30 cm core taken approximately 200 m to the east of the causeway (AT/01/03/B), however, revealed an acute, horizontal contact between organic moat sediment and mineral substrate at 28.3 cm depth. The mineral material is a typical graded bed, with a thin (<4 mm) lens of grey clay overlying pale-yellow sand. A sample (OZH170) taken for dating at 26–27 cm
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depth, immediately above this graded mineral deposit, returned an age of 520 ± 40 14C yrs BP, or 1316–1447 AD at 2r, around two centuries after the moat was originally excavated, according to the dates from core AT/01/ 04/B, some 500 m to the west. Be that as it may, periodic re-excavation of the temple moats and reservoirs, even if spatially heterogeneous, imply the active maintenance and modification of cultural features in Angkor that, in many instances, clearly post-date the supposed demise of the city. 5. Conclusion AMS 14C dating of sediment cores extracted from several temple moats and reservoirs at Angkor indicates that, in some cases, sedimentation was initiated some time after the historic (epigraphic) and/or stylistic age for the associated monument – as much as half a millennium in some cases. We argue that, because the moats are unequivocally part of the cosmological design of the monuments and can be presumed to coeval with the construction of the associated monument, this dissonance between the historic and absolute ages indicates the episodic excavation of temple moats and reservoirs. If this is indeed the case, then large, perhaps even centrally organised work forces may have been present in Angkor as much as two centuries after the city was supposedly abandoned. Acknowledgements This research is part of the Greater Angkor Project, a collaborative research project by the University of Sydney, Ecole Franc¸aise d’Extreˆme-Orient and APSARA, the Cambodian government body responsible for the management of the Angkor World Heritage site. Funding is provided by the Australian Research Council (Discovery Projects Grant # 0558130) and the Australian Institute of Nuclear Science and Engineering (AINSE Grant # 03/ 091P and 05/134). References [1] R. Fletcher, D. Evans, M. Barbetti, D. Penny, T. Heng, S. Im, C. Khieu, S. Tous, C. Pottier, Udaya 4 (2003) 107. [2] T.-K. Chou, The Customs of Cambodia, The Siam Society, Bangkok, 2001. [3] U. Zoppi, M. Barbetti, R. Fletcher, Q. Hua, R.K. Chhem, C. Pottier, M. Watanasak, Nucl. Instr. and Meth. B 223–224 (2004) 681. [4] D. Penny, C. Pottier, R.J. Fletcher, M. Barbetti, D. Fink, Q. Hua, Antiquity 80 (2006) 599. [5] C. Pottier, Mission Arche´ologique Franco-Khme`re sur l’Ame´nagement du Territoire Angkorien (MAFKATA) Rapport pre´liminaire sur la campagne de fouilles 2004, APSARA-MAE-EFEO, Siem Reap, Cambodia, 2004. [6] J.R. Glew, J.P. Smol, W.M. Last, Sediment core collection and extrusion, in: W.M. Last, J.P. Smol (Eds.), Tracking Environment Change Using Lake Sediments, Basin Analysis, Coring and Chronological Techniques, Vol. 1, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2001, p. 73.
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