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Luminescence dating of canal sediments from Angkor Borei, Mekong Delta, Southern Cambodia
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Quaternary Geochronology 2 (2007) 322–329 www.elsevier.com/locate/quageo
Research paper
Luminescence dating of canal sediments from Angkor Borei, Mekong Delta, Southern Cambodia David C.W. Sandersona,, Paul Bishopb, Miriam Starkc, Sally Alexandera, Dan Pennyd a
Scottish Universities Environmental Research Centre, East Kilbride G75 OQF, UK b Department of Geographical and Earth Sciences, University of Glasgow, UK c Department of Anthropology, University of Hawaii, HI, USA d School of Geosciences, University of Sydney, Australia Received 12 October 2005; accepted 10 May 2006 Available online 14 August 2006
Abstract Work to apply luminescence dating to archaeological sites in the Lower Mekong Delta has continued with a programme aimed at dating ancient canal sediments and brick monuments in the vicinity of ancient city of Angkor Borei. Following the successful application of OSL dating to the Paris 2 canal near Angkor Borei further fieldwork and analysis has been undertaken. The infill and substrate of the larger Paris 4 canal connecting Angkor Borei to Oc Eo, some 80 km to the south in Vietnam has been sampled and subjected to luminescence analysis. Field spectroscopy and underwater bleaching experiments were also conducted in the Baray and Angkor Borei in 2004. The results show that both illumination intensities and spectral distributions are severely altered by as little as 1.5 m of turbid water, and that OSL bleaching rates for both quartz and feldspars are reduced. Since quartz resetting is heavily dependant on the UV components in daylight, which have preferentially attenuated the effects of turbid water on OSL zeroing rates are especially marked. The new data from the Paris 4 canal, which has been dated by OSL to be between the first millenium BC and the late first millenium AD are significant to understanding the archaeological development of the Fu Nan state in the Lower Mekong Delta, and the sequence of development of the canal network linking inland agrarian sites and coastal trading centres. r 2006 Elsevier Ltd. All rights reserved. Keywords: Optical stimulated luminescence dating; Canal sediments; Archaeology; Early state formation in SE Asia; Underwater bleaching of luminescence; Quartz; Feldspars
1. Introduction
2. Background
Starting in 2001 a programme applying luminescence dating to sediments and ceramics from archaeological sites in the Lower Mekong Delta has been underway at the Scottish Universities Environmental Research Centre. Following the work reported at Reno (Sanderson et al., 2003; Bishop et al., 2004) which provided the first luminescence dates for an ancient canal in SE Asia, here we present further results from canal sediments including a study of underwater bleaching in turbid tropical water. Work which has been undertaken to date architectural bricks will be reported in more detail elsewhere.
The work has been centred around the ancient settlement of Angkor Borei (Fig. 1) in Southern Cambodia and forms a contribution to the Lower Mekong Archaeological Project (LoMAP), which was initiated in the mid-1990s to conduct archaeological field survey and excavation in the area (Stark, 2001, 2003, 2004; Stark et al., 1999; Stark and Sovath, 2001). The landscape setting comprises low lying land in the delta, used extensively for rice cultivation, and subject to monsoonal flooding, interspersed with higher terrace remnants and local bedrock outcrops which have been, and remain, the focus for settlement. Many of these ancient settlements are linked by traces of ancient canals (Paris, 1929, 1931). The area has been associated with the so-called Fu Nan state inferred historically from
Corresponding author. Tel.: +44 1355 270110; fax: +44 1355 229898.
E-mail address: [email protected] (D.C.W. Sanderson). 1871-1014/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2006.05.032
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Fig. 1. Location map showing of the area around Angkor Borei in the Mekong delta. Archaeological sites are indicated by dots, with linear canal traces numbered.
Chinese diplomatic texts, and considered archaeologically to combine both agrarian and trading elements, based on rice cultivation and strategic links with coastal trade between India and China (Pelliot, 1903; Coedes, 1968; Higham, 2002). The ancient city of Angkor Borei occupies an elevated location, and has evidently been an important regional centre for more than 2000 years. An area of more than 300 ha has been enclosed by an elaborate series of banks and moats, including a brick-capped monumental wall. The enclosed area is of outstanding archaeological wealth. It also supports a thriving modern population leading to conservation pressure and opportunities for investigating sites exposed by building and quarrying operations. A similar situation is encountered in the surrounding area, where a series of sites, including monuments showing exposed brick-work survive. A priority for the LoMAP project is to record the locations and state of preservation of monuments in the area, in combination with archae-
ologists from the Royal University of Fine Arts in Phnom Penh, and the Cambodian Ministry of Culture. Both the canal network and the brick architecture are of interest to luminescence dating. In the vicinity of Angkor Borei canals 1, 2 and 3 seem to form a local network linking settlements on higher ground to the regional centre at Angkor Borei (Fig. 1). Canal 4 extends some 80 km south to Oc Eo, in Vietnam, which was an important trading port during the Fu Nan period. It is of interest to establish the chronologies of the construction, utilisation and abandonment of the canals as well as palaeo-environmental records of the post-abandonment infill sediments. Was the local network in the agrarian interior established at an earlier period than the external trading links, as potentially implied by the extensive later first millenium AD material recovered at Oc Eo by Malleret (1959–63), and the discovery of first millenium BC material during excavations in Angkor Borei (Stark et al., 1999; see also Bishop et al., 2004)? Or does the more sinuous trace of the
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long-distance (and larger) Paris 4 canal reflect a greater age for the Paris 4 canal? The answers to these questions are of interest to archaeology in providing insight into the mechanisms for early state formation in the region. Luminescence analysis of 30 small profiling samples from thre Paris 2 canal infill was used successfully to identify the canal base, and to characterise the sedimentary sequence (Sanderson et al., 2003; Bishop et al., 2004). The quartz SAR method on nine tube samples of the infill provided the basis for dating the more slowly accumulating layers of the upper canal sediments to the later first millenium AD. These OSL dates have been corroborated by 14-C dating (Bishop et al., 2004). It has also been noted that the pollen evidence from canal 2 correlates with the pollen sequence determined by Bishop et al. (2003) from the east baray at Angkor Borei; a palaeo-channel remnant, reconfigured as an artificial reservoir. In particular the disappearance of mangrove in the mid-first millennium AD, reflecting a combination of coastal retreat associated with late-Holocene sea level regression and possible anthropogenic activity, is mirrored in canal 2 at depths and OSL ages that match the baray chronology. Deeper infill samples, close to the natural substrate, were more problematic however, showing evidence of mixed ages, with highly variable quartz SAR results from both normal and small aliquots (Spencer et al., 2003). The basal age (i.e. the age of canal digging) has been estimated as belonging to the late first millenium BC, based on F ratios of the lowest age components of the small aliquot results. However the results from such disturbed layers cannot be regarded as providing the basis for a definitive canal constructional date. Based on a small number of observations from polymineral and feldspar fractions it was noted that the apparent ages from feldspars in lower layers of canal 2 seemed to be less affected by substrate mixing (Sanderson et al., 2003), raising the question as to whether the feldspars may be more readily reset under the illumination conditions of turbid tropical water. In the 2004 field season a section was excavated across the southern reaches of the Paris 4 canal, some 40 km south of Angkor Borei. Two sets of profiling samples and a further set of tube samples for OSL dating were recovered from this trench. A series of experiments was also conducted in the East Baray at Angkor Borei to investigate underwater illumination spectra and bleaching rates for quartz and feldspars. The results of these investigations are reported in this paper. Brick monuments have been sampled in all three field seasons, with nine samples taken from the eastern section of the monumental wall at Angkor Borei in 2001; a further six smaller field monuments sampled in 2004, and additional samples from sites in Angkor Borei collected in 2005. Laboratory reports on the luminescence dates obtained from these samples, and from fabric analyses (Paterson, 2005) have been prepared, and the results will be reported in detail elsewhere.
3. Underwater spectrometry and bleaching experiments in the East Baray, Angkor Borei The aim of these experiments was to evaluate the hypothesis that the poorly bleached and mixed response from basal quartz samples in the Paris 2 canal was in part due to the lack of UV/blue daylight components in illumination under water with a high suspended sediment load. The experiment had two parts: optical spectroscopy (or spectral radiometry) to evaluate spectral distributions of daylight under water, and bleaching experiments using both quartz and feldspars to assess rates of bleaching under real field conditions. A body of relatively still water without excessive sedimentary disturbance during the experiment was selected. The East Baray at Angkor Borei (Bishop et al., 2003) was chosen for this work as it is used for fish cultivation and does not have the motorised water traffic of the canal or river paths in the area. Optical spectra were recorded with an Ocean Optics USB2000 spectrometer with a 2 m fibre optic probe, waterproofed, by inserting the probe tip in a glass tube, surrounding the fibre sheath with plastic l ducting, and sealing the spectrometer unit in a large zip-seal bag. An experiment was conducted to determine the spectralattenuation of the glass tube by recording interleaved spectra with and without the tube over the probe tip. It can be seen from Table S1 that attenuation in the near UV ranged from 3% to 5%, from 2% to 3% in the visible region, and from 1% to 2% in the near IR. At the baray the spectrometer probe was attached to a bamboo pole marked with 25 cm intervals and positioned successively in three orientations (pointing upwards, sideways and downwards). Spectra were recorded while both lowering and raising the spectrometer in 25 cm intervals between the surface and a water depth of 150 cm. The dark signal was recorded also. Surface temperatures exceeded 35 1C, and it is possible that the dark signals increased over the course of the experiment. However the data (Fig. 2), reveal a 2, 3 order of magnitude attenuation of the daylight spectrum in moving from 0 to 150 cm depth. At peak daylight wavelengths intensities are reduced to approximately 5% of surface levels through 150 cm of this moderately turbid water. By contrast the UV wavelengths, and some IR components, are attenuated more severely leaving less than 1% of surface intensities over the same depth range. These spectral distribution changes with depth may well influence the relative bleaching rates of different luminescence minerals, as a result of differences between OSL excitation or bleaching spectra. The bleaching experiments utilised pairs of irradiated quartz and feldspar samples deposited on stainless steel discs held within waterproof Petri-dishes and exposed to light in the baray for 1 day and 10 days. The quartz used was from pure separates from the Paris 2 canal, irradiated at SUERC to an equivalent dose of approximately 40 Gy. The feldspars were dispensed from a bulk supply of IAEA F1, which had been gamma irradiated to a 100 Gy dose
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recorded from both minerals at all depths for at least one of the exposure periods. Luminescence analysis with a Riso DA 15 reader at SUERC was used to evaluate equivalent doses (Fig. 3). As expected the bleaching rates and proportions of stored dose lost are considerably lower at depth for both minerals. It is notable that the quartz samples bleach more rapidly than feldspars for the surface exposures, where illumination fields contain higher UV intensities. No attempt has been made at this stage to separate different decay components
beforehand. All samples were transported to and from Cambodia in dark covers, and accompanied by transit dosimeters. For field exposure the samples were mounted under dark plastic bags onto a weighted wooden frame constructed to expose samples at depths of 0–5, 50, 100 and 150 cm, and attached to lengths of stout bamboo cane driven into the sedimentary bed of the baray. The paireddisc experiments were initiated on 26 February 2004. Two of the sample sets were inadvertently dislodged and lost from the experiment during deployment, but data were
Fig. 2. Underwater spectral radiometry measurements at Angkor Borei Baray.
10 Day Initial value
Initial value
Feldspar IR
Initial Feldspar Post IR Blue value
0
0
20
20
20
40 60 80 100 120 140 160 0.01
Depth below surface/cm
0
Depth below surface/cm
Depth below surface/cm
Quartz
1 Day
40 60 80 100 120 140
0.1
1
10
Equivalent Dose/ Gy
100
160 0.01
40 60 80 100 120 140
0.1
1
10
Equivalent Dose / Gy
100
160 0.01
0.1
1
10
100
Equivalent Dose /Gy
Fig. 3. Underwater bleaching experiments at Angkor Borei Baray in 2004. Irradiated quartz (40 Gy) and feldspar (100 Gy) samples were exposed to natural light for 1 and 10 days, respectively, at depths of 0, 50, 100 and 150 cm from the water surface.
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in the OSL data, and therefore part of this initial advantage to the quartz system arises as a result of the faster and simpler structure to CW OSL decay curves from quartz. At depth however the situation may be different. The quartz samples show roughly two orders of magnitude greater residual OSL signals at 150 cm depth compared with surface samples, whereas the feldspar system appears to be responding to the lower fractional change of the main parts of the visible illumination spectrum. It thus appears that the feldspar system may be more rapidly bleached than quartz under conditions of severe daylight attenuation and the shift to longer wavelengths of the moderately turbid underwater environment. Thus a case can be made for continuing to investigate feldspar properties under such conditions, particularly for layers exhibiting mixed-age quartz results. 4. Luminescence dating of the Paris 4 canal section Trenching and augering of the Paris 4 canal trace some 40 km south of Angkor Borei point to a canal more than 70 m wide at this location. The excavated portion, approximately 30 m long, representing slightly less than half the canal width on the canal’s central western side. The stratigraphy was initially classified into six units from the soil surface, some of which may be combined for final analysis. The luminescence data reported here are referred to this initial field stratigraphy. The canal trench was notable for the recovery of a remarkable assemblage of more than 100 wood fragment, both with and without cut marks and mainly from an organic-rich layer at approximately 1–1.3 m depth. The fill also contained pottery sherds, mainly concentrated in the lower layers, but also represented in the organic-rich layer. A metal artefact was also recovered as were two brick fragments, one of which has been dated by OSL to 180 AD. A sand-rich basal layer, particularly represented in the eastern half of the trench, overlies the layer identified as the lowest layer from which ceramic artefacts were recovered. Two profiles of 15 small samples were taken at distances of 17.88 m (profile 1) and 0.5 m (profile 2) from the eastern end of the trench to study the luminescence stratigraphy and to identify the canal base. Bulk tube samples for OSL dating runs were also recovered from five depths at each of the two profiles. All samples were collected in plastic tubes, protected from light exposure, sealed to retain moisture, and shipped to the UK accompanied by transit dosimeters. The transit doses determined arising from air transport including X-ray inspection ranged from 20 to 40 mGy. Sample preparation followed the procedures outlined by Sanderson et al. (2003). Profiling samples were sieved and small numbers of discs of coarse polymineral and etched quartz were recovered and used for simple equivalent dose measurements in the Riso DA 15 system. The results of ED determination are presented in Figs. 4 and 5 together with the stratigraphic units assigned in the field. Taken
together the profiling results show very clear identification of the canal base as corresponding to the interface between field units V and VI. This is entirely consistent with the artefact distributions observed, and provides a further confirmation of the value of luminescence profiling in identifying stratigraphic discontinuities. The upper layers show somewhat simpler depth dependence than observed from the Paris 2 canal in earlier work. Nonetheless there are indications that the transition between basal layers and the lower sedimentary fills may be less clear in profile 1 than in profile 2, and that there appears to be material in the upper part of layer V and lower part of layer IV with a higher equivalent dose than that underlying it; implying a possible inclusion of older material (e.g. from the canal banks). A similar feature was noted in the Paris 2 section and tentatively attributed to anthropogenic influence. Further up the section of profile 1, in the layer II/III interface zone there is some evidence, particularly marked in the TL profile but also present on both quartz and polymineral signals of higher ED’s again possibly implying mixed age material. Between these layers however the profiles do imply favourable conditions for luminescence dating. The bulk samples were processed in a more conventional manner for luminescence dating, with water content determination, high-resolution gamma spectrometry and thick source beta counting (Sanderson, 1988), and preparation of both quartz and feldspar extracts for dose determination. The quartz samples were used for SAR analysis, which forms the basis of the dates estimated so far. Dose rates were evaluated using dose rate conversion factors (Aitken, 1983), absorbed dose fractions (Mejdahl, 1979) and cosmic-ray estimates (Prescott and Hutton, 1994). The procedure includes a nuclide specific analysis of the gamma-ray emitters from K, U, and Th, a reconciliation of field and laboratory dose rates, and of measured and calculated beta dose rate, where available, and an analysis of the uncertainties of the reference materials, the dose rate conversion constants, and the assumed water contents. Table S2 shows the radionuclide concentrations determined from the 10 tube samples, arranged in vertical stratigraphic order from the top (SUTL1712–1716 are in the position of profile 1; SUTL1717–1721a in the position of profile 2). It is notable that the majority of samples have very typical U, Th and K contents for terrestrial sediments. Concentrations of K, U and Th decline with increasing depth, particularly in profile 2, for reasons that are not immediately apparent. Since the relatively insoluble Th content is also affected, there seems no reason to interpret this in terms of in situ leaching from the older layers. Rather it seems to be related to the relative concentrations of sand and clay in the layers themselves. The quartz extracts were subjected to SAR analysis in a similar manner to that used for the Paris 2 canal. The sequence constructed five-point dose–response curves for each sample, with recycling ratio checks, IR response checks, zero cycle checks, and four groups of discs
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Paris 4 Canal 2004 Trench - Profile 1 (17.88 m) Quartz
Field Stratigraphy
Polymineral 0
0 Quartz
TL
post IR-OSL
IRSL
I/II
100
II/III III/IV IV/V
Depth /cm
Depth /cm
Depth /cm
Depth /cm
50
150
V/VI 200 0
10 20 30 40 50
0
50
100 150 200
0
Equivalent Dose /Gy
Equivalent Dose /Gy
50
100
150
0
50 100 150 200 250 300 Equivalent Dose /Gy
Equivalent Dose /Gy
Fig. 4. Luminescence profile 1 (17.88 m west of centre) from the 2004 Paris 4 canal trench.
Paris 4 Canal 2004 Trench - Profile 2 (0.8 m) Polymineral
Quartz
Field Stratigraphy
0 Quartz
IRSL
post IR OSL
TL
I/II
Depth /cm
50
II/III 100 III/IV IV/V 150
V/VI 200 0
10
20
30
Equivalent Dose/Gy
40
0 200 400 600 8001000 Equivalent Dose /Gy
0
50
100
150
200
Equivalent Dose /Gy
0
50
100
150
Equivalent Dose/Gy
Fig. 5. Luminescence profile 2 (0.88 m west of centre) from the 2004 Paris 4 canal trench.
subjected to 220, 240, 260 and 280 1C preheating. Based on profiling data the samples were measured to 5, 10 and 20 Gy doses; two runs were also extended to 30 Gy retrospectively. The quartz from canal 4, in keeping with prior experience of quartz from the area shows a high sensitivity to radiation (typically 104, 105 photon counts/ Gy) with very good SAR properties. As indicated in Table 1 the recycling ratios, IR response and zero cycle behaviour were extremely good. Sensitivity changes were small per cycle, but in any case should be taken into
account by the test-dose normalisation within SAR analysis. The reproducibility of regenerated data within individual samples was excellent, with standard errors on 16 disc runs of better than 1% in most cases. The scatter in natural OSL ratios from disc to disc is typically an order of magnitude greater, albeit confined to 3–5% uncertainties for the majority of samples, rising to 8% (TL1721a) and 12.5% (TL1715) for those samples in close proximity to the stratigraphic boundaries of the substrate. It is thought likely that the 3–5% uncertainty level reflects partly the
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328 Table 1 SAR performance indicators Sample
Standard error on OSL ratios (%)
Sensitivity change (%) per cycle
IR response (%) of OSL signal
Zero level as normalised OSL
Natural
Regen.
Recycling ratio
SUTL1712 SUTL1713 SUTL1714 SUTL1715 SUTL1716
3.05 4.6 4.63 12.5 2.32
0.6 0.5 0.7 0.8 0.5
0.99970.007 1.00070.004 1.00270.005 0.99970.009 0.98870.005
1.76 2.01 1.55 1.65 2.55
0.14 0.05 0.09 0.08 0.21
0.00670.001 0.00570.001 0.00670.002 0.00670.001 0.00670.001
SUTL1717 SUTL1718 SUTL1719 SUTL1720 SUTL1721a
4.2 6.8 4.9 5.4 8.8
0.6 1.6 0.8 1.2 1.2
0.97470.011 1.00670.009 1.00370.006 1.00470.009 1.00970.006
0.18 2.42 1.20 1.39 1.23
0.19 0.30 0.20 0.35 0.33
0.01770.007 0.00870.004 0.00870.003 0.01370.003 0.01570.003
differences between natural and laboratory irradiation, and partly the possible variations in underlying dose response from different aliquots. With this in mind it was decided to use a composite dose response curve derived from all samples to estimate equivalent doses (Fig. S1). The dose distributions were examined, and a small number of outlying observations excluded from further analysis. Table S3 summarises the equivalent doses estimated from both unweighted and weighted analyses, together with measured water contents, effective dose rates (assuming a 10% uncertainty in water content), and estimated ages based on the weighted mean equivalent dose data. Table 2 summarises the luminescence dates achieved in this manner together with the positions and stratigraphic locations of each sample. The results imply a regional substrate laid down approximately 9000 years ago (TL1716) and intermixed into basal canal infill samples TL1715 and 1721a, both of which show higher variations in single aliquot data. Both sets of samples show OSL dates in the early first millennium BC (TL1714 top of unit V; TL1718 lower III; TL1719 unit IV) which seem to be stratigraphically coherent. The upper layers imply first millennium AD infilling up to the later parts of the first millennium associated with the Angkorian period. Returning to the question of the quartz–feldspar relationship in turbid waters a small series of density separates was prepared from two samples in each position representing upper and lower parts of the canal. Table S4 shows a comparison of equivalent dose estimates, based on post-IR blue OSL measurements from these separates, with the corresponding quartz SAR doses. With the exception of TL1715 where the low-density fractions do seem to give a lower dose estimate than the quartz, and the sample originated from a mixed-age layer in close contact to the substrate, the results do not seem to differ very much between fractions. One interpretation of this is that the both quartz and feldspar fractions are for the most part well bleached in this suite of samples, although further examination of the density separates from these and other samples may merit attention.
Table 2 Quartz SAR OSL dates for the 2004 Paris 4 canal trench Sample
Position (m) W. of centre
Depth (cm)
Layer
Date
SUTL1712 SUTL1713 SUTL1714 SUTL1715 SUTL1716
17.88 17.88 17.88 17.88 17.88
53 110 127 178 190
Top of II Base of II Top of V Base of V VI
810765 AD 607125 BC 8107160 BC 54707440 BC 70307520 BC
SUTL1717 SUTL1718 SUTL1719 SUTL1720 SUTL1721a
0.8 0.8 0.8 0.8 0.8
53 95 138 158 178
II III Lower III IV/V V/VI
100790 AD 840755 AD 7907140 BC 6607160 BC 29007270 BC
5. Conclusions To summarise the work reported here, the application of luminescence dating to archaeological ceramics and sediments from the Lower Mekong delta has continued following the successful luminescence dating of the Paris 2 canal section reported previously. Additional fieldwork has been undertaken both in connection with dating bricks, which will be reported in detail elsewhere, and in extending the dating of canals. Field bleaching and spectrometry experiments conducted under in situ conditions in SE Asia have confirmed the hypotheses advanced in our earlier paper. There is compelling evidence of both attenuation and preferential loss of the UV components of daylight under water with a moderate suspended sediment load leading to much reduced OSL bleaching rates for both quartz and feldspar systems. The data also imply that the assumption that quartz will normally be more completely reset may not be satisfied under conditions of turbid water. New data from the larger long distance canal 4 that links Angkor Borei—arguably the agrarian capital of the Fu Nan ‘state’, with the coastal settlement of Oc Eo in Vietnam and hence the longer range trade network, have been produced.
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Luminescence profiling has again clearly characterised the stratigraphic sequence, identifying the contact between the canal infill and substrate, and pointing to layers in the infill that may include older material, such as re-deposited bank material. The quartz SAR results provide the basis for a chronology of the canal infill. Luminescence properties of the material appear to be extremely favourable, and although there is still some evidence of mixed-age material the data can be used to produce an infill chronology with origins in the first millenium BC leading through to the late first millennium AD. The possibility that the earlier of these dates may be influenced by incorporation of some redeposited sediments should be borne in mind, in which case further examination of small aliquots, single grains and/or feldspars may be worthwhile. Wood and ceramics from within the canal section should provide independent checks on the sedimentary chronology. Nonetheless the internal consistency of the new OSL data sets is high, and they do provide an initial chronology for the Paris 4 canal and hence the longer range network. It remains difficult to judge the relationship between the regional and local canal networks, mainly as a result of the more complex basal stratigraphy encountered in the Paris 2 canal section. However in the 2005 field season an additional series of trenches across the local network was excavated. When these samples have been analysed it may be possible to assess the developmental sequence of this important canal network in the Lower Mekong Delta. Acknowledgements The research reported here was funded by National Geographic Research Grant #6087–97. Our special thanks go to Cambodian colleagues, including Minister of Culture Princess Norodom Bopha Devi for permission to undertake research, and to Under Secretary of State Chuch Phoeurn for collaboration in research. The assistance of colleagues in Glasgow and East Kilbride is also gratefully acknowledged; in particular Peter Chung (sedimentological analyses), Mike Shand (cartography and diagrams) and Lorna Carmichael (sample preparation). Editorial handling by: R. Roberts Appendix A. Supplementary materials Supplementary data associated with this article can be found in the online version at: doi:10.1016/j.quageo.2006.05.032.
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Bishop, P., Penny, D., Stark, M., Scott, E.M., 2003. A 3.5 ka record of palaeoenvironments and human occupation at Angkor Borei, Mekong Delta, Southern Cambodia. Geoarchaeology 18 (3), 359–393. Bishop, P., Sanderson, D.C.W., Stark, M.T., 2004. OSL and radiocarbon dating of a pre-Angkorian canal in the Mekong delta, Southern Cambodia. Journal of Archaeological Science 31, 319–336. Coedes, G., 1968. The Indianised States of Southest Asia. East-West Centre Press, Honolulu. Higham, C., 2002. Early Cultures of Mailand South East Asia. River Books, Bangkok. Malleret, L., 1959–63. Archaeologie du Delta du Mekong, vol. 4, L’Ecole Francaise d’Extreme Orient, Paris (4 vols.). Mejdahl, V., 1979. Thermoluminescence dating: beta dose attenuation in quartz grains. Archaeometry 21, 61–72. Paris, P., 1929. Anciens canaux reconnus sur photographies aeriennes dans les provinces de Ta-Keo, Chau-Doc, Long Xuyen et Rach-Gia. Bulletin de l’Ecole Francais d’Extreme Orient 29, 221–224. Paris, P., 1931. Anciens canaux reconnus sur photographies aeriennes dans les provinces de Ta-Kev et de Chau-Doc. Bulletin de l’Ecole Francais d’Extreme Orient 31, 221–224. Paterson, M.C., 2005. Brick Technology and use at Ancient Angkor Borei, Cambodia, MA Honours Dissertation, Department of Archaeology, University of Glasgow, 76pp. Pelliot, P., 1903. Le Fou Nan. Bulletin de l’Ecole Francais d’Extreme Orient 2, 248–333. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term time variations. Radiation Measurements 23, 497–500. Sanderson, D.C.W., 1988. Thick Source Beta Counting: a rapid method for measuring beta dose rates. Nuclear Tracks 14, 203–207. Sanderson, D.C.W., Bishop, P., Stark, M.T., Spencer, J.Q., 2003. Luminescence dating of anthropogenically reset canal sediments from Angkor Borei, Mekong delta, Cambodia. Quaternary Science Reviews 22, 1111–1121. Spencer, J.Q., Sanderson, D.C.W., Deckers, K., Sommerville, A.A., 2003. Assessing mixed dose distributions in young sediments using small aliquots and a simple two-step SAR procedure: the F statistic as a diagnostic tool. Radiation Measurements 37, 425–431. Stark, M.T., 2001. Some preliminary results of the 1999–2000 archaeological field investigations at Angkor Borei, Takeo Province. Journal of Khmer Studies 2, 19–35. Stark, M.T., 2003. Angkor Borei and the Archaeology of the Mekong Delta. In: Khoo, J.C.M. (Ed.), Art & Archaeology of Fu Nan: Pre-khmer Kingdom of the Lower Mekong Valley. The South East Asian Ceramic Society, Orchid Press, Bangkok, pp. 87–106 (Chapter III). Stark, M.T., 2004. Pre-Angkorian and Angkorian Cambodia. In: Glover, I., Bellwood, P. (Eds.), Southeast Asia: From Prehistory to History. Routledge, Curzon, pp. 89–119. Stark, M.T., Sovath, B., 2001. Recent research on emergent complexity in Cambodia’s Mekong Delta. In Indo-Pacific Prehistory Association Bulletin 21, 85–98. Stark, M.T., et al., 1999. Results of the 1995–96 archaeological field investigations at Angkor Borei, Cambodia. Asian Perspectives 39 (1), 7–36.
Further reading References Aitken, M.J., 1983. Dose rate data in S.I. units. PACT 9, 69–76.
OECD, 1994. JEF-PC—a personal computer programme for displaying nuclear data from the Joint Evaluated File Library. OECD, Nuclear Energy Agency, Paris.
Quaternary Geochronology 2 (2007) 322–329 www.elsevier.com/locate/quageo
Research paper
Luminescence dating of canal sediments from Angkor Borei, Mekong Delta, Southern Cambodia David C.W. Sandersona,, Paul Bishopb, Miriam Starkc, Sally Alexandera, Dan Pennyd a
Scottish Universities Environmental Research Centre, East Kilbride G75 OQF, UK b Department of Geographical and Earth Sciences, University of Glasgow, UK c Department of Anthropology, University of Hawaii, HI, USA d School of Geosciences, University of Sydney, Australia Received 12 October 2005; accepted 10 May 2006 Available online 14 August 2006
Abstract Work to apply luminescence dating to archaeological sites in the Lower Mekong Delta has continued with a programme aimed at dating ancient canal sediments and brick monuments in the vicinity of ancient city of Angkor Borei. Following the successful application of OSL dating to the Paris 2 canal near Angkor Borei further fieldwork and analysis has been undertaken. The infill and substrate of the larger Paris 4 canal connecting Angkor Borei to Oc Eo, some 80 km to the south in Vietnam has been sampled and subjected to luminescence analysis. Field spectroscopy and underwater bleaching experiments were also conducted in the Baray and Angkor Borei in 2004. The results show that both illumination intensities and spectral distributions are severely altered by as little as 1.5 m of turbid water, and that OSL bleaching rates for both quartz and feldspars are reduced. Since quartz resetting is heavily dependant on the UV components in daylight, which have preferentially attenuated the effects of turbid water on OSL zeroing rates are especially marked. The new data from the Paris 4 canal, which has been dated by OSL to be between the first millenium BC and the late first millenium AD are significant to understanding the archaeological development of the Fu Nan state in the Lower Mekong Delta, and the sequence of development of the canal network linking inland agrarian sites and coastal trading centres. r 2006 Elsevier Ltd. All rights reserved. Keywords: Optical stimulated luminescence dating; Canal sediments; Archaeology; Early state formation in SE Asia; Underwater bleaching of luminescence; Quartz; Feldspars
1. Introduction
2. Background
Starting in 2001 a programme applying luminescence dating to sediments and ceramics from archaeological sites in the Lower Mekong Delta has been underway at the Scottish Universities Environmental Research Centre. Following the work reported at Reno (Sanderson et al., 2003; Bishop et al., 2004) which provided the first luminescence dates for an ancient canal in SE Asia, here we present further results from canal sediments including a study of underwater bleaching in turbid tropical water. Work which has been undertaken to date architectural bricks will be reported in more detail elsewhere.
The work has been centred around the ancient settlement of Angkor Borei (Fig. 1) in Southern Cambodia and forms a contribution to the Lower Mekong Archaeological Project (LoMAP), which was initiated in the mid-1990s to conduct archaeological field survey and excavation in the area (Stark, 2001, 2003, 2004; Stark et al., 1999; Stark and Sovath, 2001). The landscape setting comprises low lying land in the delta, used extensively for rice cultivation, and subject to monsoonal flooding, interspersed with higher terrace remnants and local bedrock outcrops which have been, and remain, the focus for settlement. Many of these ancient settlements are linked by traces of ancient canals (Paris, 1929, 1931). The area has been associated with the so-called Fu Nan state inferred historically from
Corresponding author. Tel.: +44 1355 270110; fax: +44 1355 229898.
E-mail address: [email protected] (D.C.W. Sanderson). 1871-1014/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2006.05.032
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Fig. 1. Location map showing of the area around Angkor Borei in the Mekong delta. Archaeological sites are indicated by dots, with linear canal traces numbered.
Chinese diplomatic texts, and considered archaeologically to combine both agrarian and trading elements, based on rice cultivation and strategic links with coastal trade between India and China (Pelliot, 1903; Coedes, 1968; Higham, 2002). The ancient city of Angkor Borei occupies an elevated location, and has evidently been an important regional centre for more than 2000 years. An area of more than 300 ha has been enclosed by an elaborate series of banks and moats, including a brick-capped monumental wall. The enclosed area is of outstanding archaeological wealth. It also supports a thriving modern population leading to conservation pressure and opportunities for investigating sites exposed by building and quarrying operations. A similar situation is encountered in the surrounding area, where a series of sites, including monuments showing exposed brick-work survive. A priority for the LoMAP project is to record the locations and state of preservation of monuments in the area, in combination with archae-
ologists from the Royal University of Fine Arts in Phnom Penh, and the Cambodian Ministry of Culture. Both the canal network and the brick architecture are of interest to luminescence dating. In the vicinity of Angkor Borei canals 1, 2 and 3 seem to form a local network linking settlements on higher ground to the regional centre at Angkor Borei (Fig. 1). Canal 4 extends some 80 km south to Oc Eo, in Vietnam, which was an important trading port during the Fu Nan period. It is of interest to establish the chronologies of the construction, utilisation and abandonment of the canals as well as palaeo-environmental records of the post-abandonment infill sediments. Was the local network in the agrarian interior established at an earlier period than the external trading links, as potentially implied by the extensive later first millenium AD material recovered at Oc Eo by Malleret (1959–63), and the discovery of first millenium BC material during excavations in Angkor Borei (Stark et al., 1999; see also Bishop et al., 2004)? Or does the more sinuous trace of the
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long-distance (and larger) Paris 4 canal reflect a greater age for the Paris 4 canal? The answers to these questions are of interest to archaeology in providing insight into the mechanisms for early state formation in the region. Luminescence analysis of 30 small profiling samples from thre Paris 2 canal infill was used successfully to identify the canal base, and to characterise the sedimentary sequence (Sanderson et al., 2003; Bishop et al., 2004). The quartz SAR method on nine tube samples of the infill provided the basis for dating the more slowly accumulating layers of the upper canal sediments to the later first millenium AD. These OSL dates have been corroborated by 14-C dating (Bishop et al., 2004). It has also been noted that the pollen evidence from canal 2 correlates with the pollen sequence determined by Bishop et al. (2003) from the east baray at Angkor Borei; a palaeo-channel remnant, reconfigured as an artificial reservoir. In particular the disappearance of mangrove in the mid-first millennium AD, reflecting a combination of coastal retreat associated with late-Holocene sea level regression and possible anthropogenic activity, is mirrored in canal 2 at depths and OSL ages that match the baray chronology. Deeper infill samples, close to the natural substrate, were more problematic however, showing evidence of mixed ages, with highly variable quartz SAR results from both normal and small aliquots (Spencer et al., 2003). The basal age (i.e. the age of canal digging) has been estimated as belonging to the late first millenium BC, based on F ratios of the lowest age components of the small aliquot results. However the results from such disturbed layers cannot be regarded as providing the basis for a definitive canal constructional date. Based on a small number of observations from polymineral and feldspar fractions it was noted that the apparent ages from feldspars in lower layers of canal 2 seemed to be less affected by substrate mixing (Sanderson et al., 2003), raising the question as to whether the feldspars may be more readily reset under the illumination conditions of turbid tropical water. In the 2004 field season a section was excavated across the southern reaches of the Paris 4 canal, some 40 km south of Angkor Borei. Two sets of profiling samples and a further set of tube samples for OSL dating were recovered from this trench. A series of experiments was also conducted in the East Baray at Angkor Borei to investigate underwater illumination spectra and bleaching rates for quartz and feldspars. The results of these investigations are reported in this paper. Brick monuments have been sampled in all three field seasons, with nine samples taken from the eastern section of the monumental wall at Angkor Borei in 2001; a further six smaller field monuments sampled in 2004, and additional samples from sites in Angkor Borei collected in 2005. Laboratory reports on the luminescence dates obtained from these samples, and from fabric analyses (Paterson, 2005) have been prepared, and the results will be reported in detail elsewhere.
3. Underwater spectrometry and bleaching experiments in the East Baray, Angkor Borei The aim of these experiments was to evaluate the hypothesis that the poorly bleached and mixed response from basal quartz samples in the Paris 2 canal was in part due to the lack of UV/blue daylight components in illumination under water with a high suspended sediment load. The experiment had two parts: optical spectroscopy (or spectral radiometry) to evaluate spectral distributions of daylight under water, and bleaching experiments using both quartz and feldspars to assess rates of bleaching under real field conditions. A body of relatively still water without excessive sedimentary disturbance during the experiment was selected. The East Baray at Angkor Borei (Bishop et al., 2003) was chosen for this work as it is used for fish cultivation and does not have the motorised water traffic of the canal or river paths in the area. Optical spectra were recorded with an Ocean Optics USB2000 spectrometer with a 2 m fibre optic probe, waterproofed, by inserting the probe tip in a glass tube, surrounding the fibre sheath with plastic l ducting, and sealing the spectrometer unit in a large zip-seal bag. An experiment was conducted to determine the spectralattenuation of the glass tube by recording interleaved spectra with and without the tube over the probe tip. It can be seen from Table S1 that attenuation in the near UV ranged from 3% to 5%, from 2% to 3% in the visible region, and from 1% to 2% in the near IR. At the baray the spectrometer probe was attached to a bamboo pole marked with 25 cm intervals and positioned successively in three orientations (pointing upwards, sideways and downwards). Spectra were recorded while both lowering and raising the spectrometer in 25 cm intervals between the surface and a water depth of 150 cm. The dark signal was recorded also. Surface temperatures exceeded 35 1C, and it is possible that the dark signals increased over the course of the experiment. However the data (Fig. 2), reveal a 2, 3 order of magnitude attenuation of the daylight spectrum in moving from 0 to 150 cm depth. At peak daylight wavelengths intensities are reduced to approximately 5% of surface levels through 150 cm of this moderately turbid water. By contrast the UV wavelengths, and some IR components, are attenuated more severely leaving less than 1% of surface intensities over the same depth range. These spectral distribution changes with depth may well influence the relative bleaching rates of different luminescence minerals, as a result of differences between OSL excitation or bleaching spectra. The bleaching experiments utilised pairs of irradiated quartz and feldspar samples deposited on stainless steel discs held within waterproof Petri-dishes and exposed to light in the baray for 1 day and 10 days. The quartz used was from pure separates from the Paris 2 canal, irradiated at SUERC to an equivalent dose of approximately 40 Gy. The feldspars were dispensed from a bulk supply of IAEA F1, which had been gamma irradiated to a 100 Gy dose
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recorded from both minerals at all depths for at least one of the exposure periods. Luminescence analysis with a Riso DA 15 reader at SUERC was used to evaluate equivalent doses (Fig. 3). As expected the bleaching rates and proportions of stored dose lost are considerably lower at depth for both minerals. It is notable that the quartz samples bleach more rapidly than feldspars for the surface exposures, where illumination fields contain higher UV intensities. No attempt has been made at this stage to separate different decay components
beforehand. All samples were transported to and from Cambodia in dark covers, and accompanied by transit dosimeters. For field exposure the samples were mounted under dark plastic bags onto a weighted wooden frame constructed to expose samples at depths of 0–5, 50, 100 and 150 cm, and attached to lengths of stout bamboo cane driven into the sedimentary bed of the baray. The paireddisc experiments were initiated on 26 February 2004. Two of the sample sets were inadvertently dislodged and lost from the experiment during deployment, but data were
Fig. 2. Underwater spectral radiometry measurements at Angkor Borei Baray.
10 Day Initial value
Initial value
Feldspar IR
Initial Feldspar Post IR Blue value
0
0
20
20
20
40 60 80 100 120 140 160 0.01
Depth below surface/cm
0
Depth below surface/cm
Depth below surface/cm
Quartz
1 Day
40 60 80 100 120 140
0.1
1
10
Equivalent Dose/ Gy
100
160 0.01
40 60 80 100 120 140
0.1
1
10
Equivalent Dose / Gy
100
160 0.01
0.1
1
10
100
Equivalent Dose /Gy
Fig. 3. Underwater bleaching experiments at Angkor Borei Baray in 2004. Irradiated quartz (40 Gy) and feldspar (100 Gy) samples were exposed to natural light for 1 and 10 days, respectively, at depths of 0, 50, 100 and 150 cm from the water surface.
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in the OSL data, and therefore part of this initial advantage to the quartz system arises as a result of the faster and simpler structure to CW OSL decay curves from quartz. At depth however the situation may be different. The quartz samples show roughly two orders of magnitude greater residual OSL signals at 150 cm depth compared with surface samples, whereas the feldspar system appears to be responding to the lower fractional change of the main parts of the visible illumination spectrum. It thus appears that the feldspar system may be more rapidly bleached than quartz under conditions of severe daylight attenuation and the shift to longer wavelengths of the moderately turbid underwater environment. Thus a case can be made for continuing to investigate feldspar properties under such conditions, particularly for layers exhibiting mixed-age quartz results. 4. Luminescence dating of the Paris 4 canal section Trenching and augering of the Paris 4 canal trace some 40 km south of Angkor Borei point to a canal more than 70 m wide at this location. The excavated portion, approximately 30 m long, representing slightly less than half the canal width on the canal’s central western side. The stratigraphy was initially classified into six units from the soil surface, some of which may be combined for final analysis. The luminescence data reported here are referred to this initial field stratigraphy. The canal trench was notable for the recovery of a remarkable assemblage of more than 100 wood fragment, both with and without cut marks and mainly from an organic-rich layer at approximately 1–1.3 m depth. The fill also contained pottery sherds, mainly concentrated in the lower layers, but also represented in the organic-rich layer. A metal artefact was also recovered as were two brick fragments, one of which has been dated by OSL to 180 AD. A sand-rich basal layer, particularly represented in the eastern half of the trench, overlies the layer identified as the lowest layer from which ceramic artefacts were recovered. Two profiles of 15 small samples were taken at distances of 17.88 m (profile 1) and 0.5 m (profile 2) from the eastern end of the trench to study the luminescence stratigraphy and to identify the canal base. Bulk tube samples for OSL dating runs were also recovered from five depths at each of the two profiles. All samples were collected in plastic tubes, protected from light exposure, sealed to retain moisture, and shipped to the UK accompanied by transit dosimeters. The transit doses determined arising from air transport including X-ray inspection ranged from 20 to 40 mGy. Sample preparation followed the procedures outlined by Sanderson et al. (2003). Profiling samples were sieved and small numbers of discs of coarse polymineral and etched quartz were recovered and used for simple equivalent dose measurements in the Riso DA 15 system. The results of ED determination are presented in Figs. 4 and 5 together with the stratigraphic units assigned in the field. Taken
together the profiling results show very clear identification of the canal base as corresponding to the interface between field units V and VI. This is entirely consistent with the artefact distributions observed, and provides a further confirmation of the value of luminescence profiling in identifying stratigraphic discontinuities. The upper layers show somewhat simpler depth dependence than observed from the Paris 2 canal in earlier work. Nonetheless there are indications that the transition between basal layers and the lower sedimentary fills may be less clear in profile 1 than in profile 2, and that there appears to be material in the upper part of layer V and lower part of layer IV with a higher equivalent dose than that underlying it; implying a possible inclusion of older material (e.g. from the canal banks). A similar feature was noted in the Paris 2 section and tentatively attributed to anthropogenic influence. Further up the section of profile 1, in the layer II/III interface zone there is some evidence, particularly marked in the TL profile but also present on both quartz and polymineral signals of higher ED’s again possibly implying mixed age material. Between these layers however the profiles do imply favourable conditions for luminescence dating. The bulk samples were processed in a more conventional manner for luminescence dating, with water content determination, high-resolution gamma spectrometry and thick source beta counting (Sanderson, 1988), and preparation of both quartz and feldspar extracts for dose determination. The quartz samples were used for SAR analysis, which forms the basis of the dates estimated so far. Dose rates were evaluated using dose rate conversion factors (Aitken, 1983), absorbed dose fractions (Mejdahl, 1979) and cosmic-ray estimates (Prescott and Hutton, 1994). The procedure includes a nuclide specific analysis of the gamma-ray emitters from K, U, and Th, a reconciliation of field and laboratory dose rates, and of measured and calculated beta dose rate, where available, and an analysis of the uncertainties of the reference materials, the dose rate conversion constants, and the assumed water contents. Table S2 shows the radionuclide concentrations determined from the 10 tube samples, arranged in vertical stratigraphic order from the top (SUTL1712–1716 are in the position of profile 1; SUTL1717–1721a in the position of profile 2). It is notable that the majority of samples have very typical U, Th and K contents for terrestrial sediments. Concentrations of K, U and Th decline with increasing depth, particularly in profile 2, for reasons that are not immediately apparent. Since the relatively insoluble Th content is also affected, there seems no reason to interpret this in terms of in situ leaching from the older layers. Rather it seems to be related to the relative concentrations of sand and clay in the layers themselves. The quartz extracts were subjected to SAR analysis in a similar manner to that used for the Paris 2 canal. The sequence constructed five-point dose–response curves for each sample, with recycling ratio checks, IR response checks, zero cycle checks, and four groups of discs
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Paris 4 Canal 2004 Trench - Profile 1 (17.88 m) Quartz
Field Stratigraphy
Polymineral 0
0 Quartz
TL
post IR-OSL
IRSL
I/II
100
II/III III/IV IV/V
Depth /cm
Depth /cm
Depth /cm
Depth /cm
50
150
V/VI 200 0
10 20 30 40 50
0
50
100 150 200
0
Equivalent Dose /Gy
Equivalent Dose /Gy
50
100
150
0
50 100 150 200 250 300 Equivalent Dose /Gy
Equivalent Dose /Gy
Fig. 4. Luminescence profile 1 (17.88 m west of centre) from the 2004 Paris 4 canal trench.
Paris 4 Canal 2004 Trench - Profile 2 (0.8 m) Polymineral
Quartz
Field Stratigraphy
0 Quartz
IRSL
post IR OSL
TL
I/II
Depth /cm
50
II/III 100 III/IV IV/V 150
V/VI 200 0
10
20
30
Equivalent Dose/Gy
40
0 200 400 600 8001000 Equivalent Dose /Gy
0
50
100
150
200
Equivalent Dose /Gy
0
50
100
150
Equivalent Dose/Gy
Fig. 5. Luminescence profile 2 (0.88 m west of centre) from the 2004 Paris 4 canal trench.
subjected to 220, 240, 260 and 280 1C preheating. Based on profiling data the samples were measured to 5, 10 and 20 Gy doses; two runs were also extended to 30 Gy retrospectively. The quartz from canal 4, in keeping with prior experience of quartz from the area shows a high sensitivity to radiation (typically 104, 105 photon counts/ Gy) with very good SAR properties. As indicated in Table 1 the recycling ratios, IR response and zero cycle behaviour were extremely good. Sensitivity changes were small per cycle, but in any case should be taken into
account by the test-dose normalisation within SAR analysis. The reproducibility of regenerated data within individual samples was excellent, with standard errors on 16 disc runs of better than 1% in most cases. The scatter in natural OSL ratios from disc to disc is typically an order of magnitude greater, albeit confined to 3–5% uncertainties for the majority of samples, rising to 8% (TL1721a) and 12.5% (TL1715) for those samples in close proximity to the stratigraphic boundaries of the substrate. It is thought likely that the 3–5% uncertainty level reflects partly the
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328 Table 1 SAR performance indicators Sample
Standard error on OSL ratios (%)
Sensitivity change (%) per cycle
IR response (%) of OSL signal
Zero level as normalised OSL
Natural
Regen.
Recycling ratio
SUTL1712 SUTL1713 SUTL1714 SUTL1715 SUTL1716
3.05 4.6 4.63 12.5 2.32
0.6 0.5 0.7 0.8 0.5
0.99970.007 1.00070.004 1.00270.005 0.99970.009 0.98870.005
1.76 2.01 1.55 1.65 2.55
0.14 0.05 0.09 0.08 0.21
0.00670.001 0.00570.001 0.00670.002 0.00670.001 0.00670.001
SUTL1717 SUTL1718 SUTL1719 SUTL1720 SUTL1721a
4.2 6.8 4.9 5.4 8.8
0.6 1.6 0.8 1.2 1.2
0.97470.011 1.00670.009 1.00370.006 1.00470.009 1.00970.006
0.18 2.42 1.20 1.39 1.23
0.19 0.30 0.20 0.35 0.33
0.01770.007 0.00870.004 0.00870.003 0.01370.003 0.01570.003
differences between natural and laboratory irradiation, and partly the possible variations in underlying dose response from different aliquots. With this in mind it was decided to use a composite dose response curve derived from all samples to estimate equivalent doses (Fig. S1). The dose distributions were examined, and a small number of outlying observations excluded from further analysis. Table S3 summarises the equivalent doses estimated from both unweighted and weighted analyses, together with measured water contents, effective dose rates (assuming a 10% uncertainty in water content), and estimated ages based on the weighted mean equivalent dose data. Table 2 summarises the luminescence dates achieved in this manner together with the positions and stratigraphic locations of each sample. The results imply a regional substrate laid down approximately 9000 years ago (TL1716) and intermixed into basal canal infill samples TL1715 and 1721a, both of which show higher variations in single aliquot data. Both sets of samples show OSL dates in the early first millennium BC (TL1714 top of unit V; TL1718 lower III; TL1719 unit IV) which seem to be stratigraphically coherent. The upper layers imply first millennium AD infilling up to the later parts of the first millennium associated with the Angkorian period. Returning to the question of the quartz–feldspar relationship in turbid waters a small series of density separates was prepared from two samples in each position representing upper and lower parts of the canal. Table S4 shows a comparison of equivalent dose estimates, based on post-IR blue OSL measurements from these separates, with the corresponding quartz SAR doses. With the exception of TL1715 where the low-density fractions do seem to give a lower dose estimate than the quartz, and the sample originated from a mixed-age layer in close contact to the substrate, the results do not seem to differ very much between fractions. One interpretation of this is that the both quartz and feldspar fractions are for the most part well bleached in this suite of samples, although further examination of the density separates from these and other samples may merit attention.
Table 2 Quartz SAR OSL dates for the 2004 Paris 4 canal trench Sample
Position (m) W. of centre
Depth (cm)
Layer
Date
SUTL1712 SUTL1713 SUTL1714 SUTL1715 SUTL1716
17.88 17.88 17.88 17.88 17.88
53 110 127 178 190
Top of II Base of II Top of V Base of V VI
810765 AD 607125 BC 8107160 BC 54707440 BC 70307520 BC
SUTL1717 SUTL1718 SUTL1719 SUTL1720 SUTL1721a
0.8 0.8 0.8 0.8 0.8
53 95 138 158 178
II III Lower III IV/V V/VI
100790 AD 840755 AD 7907140 BC 6607160 BC 29007270 BC
5. Conclusions To summarise the work reported here, the application of luminescence dating to archaeological ceramics and sediments from the Lower Mekong delta has continued following the successful luminescence dating of the Paris 2 canal section reported previously. Additional fieldwork has been undertaken both in connection with dating bricks, which will be reported in detail elsewhere, and in extending the dating of canals. Field bleaching and spectrometry experiments conducted under in situ conditions in SE Asia have confirmed the hypotheses advanced in our earlier paper. There is compelling evidence of both attenuation and preferential loss of the UV components of daylight under water with a moderate suspended sediment load leading to much reduced OSL bleaching rates for both quartz and feldspar systems. The data also imply that the assumption that quartz will normally be more completely reset may not be satisfied under conditions of turbid water. New data from the larger long distance canal 4 that links Angkor Borei—arguably the agrarian capital of the Fu Nan ‘state’, with the coastal settlement of Oc Eo in Vietnam and hence the longer range trade network, have been produced.
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Luminescence profiling has again clearly characterised the stratigraphic sequence, identifying the contact between the canal infill and substrate, and pointing to layers in the infill that may include older material, such as re-deposited bank material. The quartz SAR results provide the basis for a chronology of the canal infill. Luminescence properties of the material appear to be extremely favourable, and although there is still some evidence of mixed-age material the data can be used to produce an infill chronology with origins in the first millenium BC leading through to the late first millennium AD. The possibility that the earlier of these dates may be influenced by incorporation of some redeposited sediments should be borne in mind, in which case further examination of small aliquots, single grains and/or feldspars may be worthwhile. Wood and ceramics from within the canal section should provide independent checks on the sedimentary chronology. Nonetheless the internal consistency of the new OSL data sets is high, and they do provide an initial chronology for the Paris 4 canal and hence the longer range network. It remains difficult to judge the relationship between the regional and local canal networks, mainly as a result of the more complex basal stratigraphy encountered in the Paris 2 canal section. However in the 2005 field season an additional series of trenches across the local network was excavated. When these samples have been analysed it may be possible to assess the developmental sequence of this important canal network in the Lower Mekong Delta. Acknowledgements The research reported here was funded by National Geographic Research Grant #6087–97. Our special thanks go to Cambodian colleagues, including Minister of Culture Princess Norodom Bopha Devi for permission to undertake research, and to Under Secretary of State Chuch Phoeurn for collaboration in research. The assistance of colleagues in Glasgow and East Kilbride is also gratefully acknowledged; in particular Peter Chung (sedimentological analyses), Mike Shand (cartography and diagrams) and Lorna Carmichael (sample preparation). Editorial handling by: R. Roberts Appendix A. Supplementary materials Supplementary data associated with this article can be found in the online version at: doi:10.1016/j.quageo.2006.05.032.
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Further reading References Aitken, M.J., 1983. Dose rate data in S.I. units. PACT 9, 69–76.
OECD, 1994. JEF-PC—a personal computer programme for displaying nuclear data from the Joint Evaluated File Library. OECD, Nuclear Energy Agency, Paris.