a3.5karecordofpaleoenvironments ... · surveyed by theodolite (transit)and stadia tacheometry...

Geoarchaeology: An International Journal, Vol. 18, No. 3, 359–393 (2003)� 2003 Wiley Periodicals, Inc.Published online in Wiley Interscience (www.interscience.wiley.com). DOI:10.1002/gea.10067

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A 3.5 ka Record of Paleoenvironments

and Human Occupation at Angkor Borei,

Mekong Delta, Southern Cambodia

Paul Bishop,1 Dan Penny,2, Miriam Stark,3 and Marian Scott4

1Department of Geography and Topographic Science, University of Glasgow,

Glasgow G12 8QQ, United Kingdom2School of Geography and Environmental Science, Monash University,

Melbourne VIC 3008, Australia3Department of Anthropology, 2424 Maile Way, Saunders 346, University of

Hawaii, Honolulu, Hawaii 968224Department of Statistics, University of Glasgow, Glasgow G12 8QW, United

Kingdom

Microfossil and sedimentological data from a 3.1 m core extracted from a reservoir (baray)at the ancient Cambodian settlement of Angkor Borei in the Mekong Delta have provided acontinuous record of sedimentation and paleoenvironments dating from about 2000 cal yrB.C. Palynological data indicate that for much of the cal. 1st and 2ndmillennia B.C.mangrovesdominated the regional vegetation, while extensively and regularly burnt grasslands domi-nated the local vegetation. Turbid, nutrient-rich standingwater characterized the core locality,perhaps suggesting a connection with rivers in the area. An abrupt change during the cal. 5thto 6th centuries A.D. involved a dramatic reduction in grasslands and the expansion of sec-ondary forest or re-growth taxa. These changes are synchronous with an abrupt decline inthe concentration of microscopic charcoal particles in the sediments, and the colonization ofthe core locality by swamp forest plants. These changes are taken to indicate a shift in land-use strategies or, possibly, a period of land abandonment. The age for the construction of thebaray is interpreted to be in the 17th–19th centuries, but this dating remains speculative.Construction of the Angkor Borei baray exploited a preexisting body of standing water, soits construction was fundamentally different from the methods used at the Angkorian capitalin northern Cambodia. � 2003 Wiley Periodicals, Inc.

INTRODUCTION

The Mekong Delta of mainland Southeast Asia is famous as the heartland of oneof the earliest civilizations in the region. Called Funan by visiting Chinese digni-taries in the 3rd century A.D., it reputedly contained multiple urban centers be-tween the 1st and 6th centuries A.D. (Pelliot, 1903; Coedes, 1968; Higham, 1989).Documentary evidence provides one narrative of early state development in theMekong Delta that focuses on kings, missions to and from China, and of contact

D. Penny is now with the School of Geosciences, University of Sydney, Sydney NSW 2006, Australia.

BISHOP, PENNY, STARK, AND SCOTT

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top of textbase of textwith Indian traders and Brahmins (Coedes, 1968; Jacques, 1979; Wheatley, 1983).

Some scholars have hypothesized that the region was important for its proximityto the South China Sea and its growing international maritime trade network (e.g.,Hall, 1982, 1985). Others, such as Ng (1979: 267) and van Liere (1980), believe thatpopulations were attracted to the delta for its ideal combination of arable land,reliable flooding regime, and potable water. Until recently, geopolitical factors havelimited archaeological and paleoenvironmental research to evaluate these inter-pretations.

Two possible early centers in the delta are now the subject of archaeologicalinvestigation (Figure 1). One is the site of Oc Eo, in southern Vietnam, wherearchaeologists from L’Ecole Francaise d’Extreme Orient (EFEO) worked briefly inthe 1940s (Malleret, 1959, 1960, 1962). Vietnamese archaeologists resumed work inthe Vietnamese delta after 1975, and in the mid-1990s began collaborative researchwith EFEO scholars (e.g., Manguin, 1998; Manguin and Vo Si Khai, 2000). The sec-ond site, in southern Cambodia, is Angkor Borei, a 300 ha walled and moated sitethat has been the focus of research by the Lower Mekong Archaeological Projectsince 1996 (e.g., Stark, 1998; Stark et al., 1999; Stark and Bong, 2001). Excavationsat several localities throughout the site have produced parallel dated sequencesand suggest that the site was first settled in the 4th century B.C. Settlement con-tinued for at least a millennium, before the seat of power moved north in the 7thcentury A.D. Archaeological research at Angkor Borei suggests that the region didnot experience subsequent abandonment but may have experienced pronouncedfluctuations in population and intensity of land use.

Systematic geoarchaeological and palynological research is now necessary tocomplement this archaeological research by providing information on the geo-graphical and environmental factors that facilitated and/or were associated withearly historic human settlement and land-use in the Mekong delta. The pace ofgeoarchaeological and palynological research has increased in mainland SoutheastAsia (e.g., Maloney, 1992; Godley et al., 1993; Bishop and Godley, 1994; Kealhoferand Piperno, 1994; Bishop et al., 1996; Kealhofer, 1996; Penny et al., 1996; Maxwell,2001; Penny, 2001), but no work has been undertaken previously in southern Cam-bodia. This paper reports on the overall environmental setting of Angkor Boreiusing sedimentological and palynological data from a sediment core extracted fromthe site’s largest reservoir. A chronology for environmental and hydrologicalchange in the context of human-environment interactions is established, and theage and mode of construction of the reservoir are discussed.

GENERAL GEOMORPHOLOGICAL SETTING

The Angkor Borei area lies between 5 and 10 m above sea-level in the MekongDelta, with the town itself located on a terrace that has been mapped as marine(Haruyama, 1998) (Figures 1 and 2). The area is characterized by a series of ter-races, channels, and paleochannels with intervening backswamps and anthropo-

PALEOENVIRONMENTS AT ANGKOR BOREI, SOUTHERN CAMBODIA

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL 361

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Figure 1. Map of Cambodia showing the location of Angkor Borei in the Mekong Delta. Inset showsregional setting. Map originally drafted by Jo Lynn Gunness

genic canals (Figures 2 and 3). The modern rivers are commonly leveed and sep-arated by lower-lying backswamps, some of which are crossed by the faint tracesof the paleochannels, as well as by the dendritic drainage patterns of modern chan-nels that drain the backswamps after the annual flood. The most clearly identifiablepaleochannels are located mainly in the vicinity of modern rivers (Figure 2).

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Figure 2. Map of the Angkor Borei area showing canals and paleochannels, based on stereoscopicaerial photograph interpretation of Finnmap Oy’s 1:25,000 scale aerial photographs (flown in December1992 and January 1993). The aerial photograph interpretation was transferred to the Takeo 1:50,000topographic sheet, using a Bausch & Lomb Zoom Transfer Scope. Planimetric control in this transferwas provided by superimposition of prominent landscape elements, including the Pol Pot era canals at1-km spacings coincident with the national map grid. The canals mapped by Paris (1931, 1941) arelabeled according to his numbering. The supposed Funanese center of Oc Eo lies on a south-south-easterly continuation of the trace of canal 4, approximately 60 km from the southeast corner of thismap.



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Figure 3. Contour map of Angkor Borei city showing the locations of archaeological excavations (“Testsite”), and the eastern baray coring locality. The contour map was constructed by Mr. John Shearer,using 7127 elevation points derived photogrammetrically by Mrs. Anne Dunlop and Dr. Jane Drummondfrom Finnmap Oy aerial photographs (1:25,000; December 1992, Roll 29, Strip 100, photos 7067, 7068,and 7069). Reliable survey control points are not available for the Angkor Borei area (Lieven Geerinck,Mekong River Commission, personal communication, March 1999). Photogrammetric control in thehorizontal was provided, therefore, by the intersections of the Pol Pot era canals at 1-km spacing onthe national map grid. Vertical control was provided by assuming that the surface of the monsoon floodwaters inundating the backswamps across the aerial photographs was at a uniform elevation, hereassumed to be 2 m above sea level. All elevations on the contour map are therefore internally consistent(within the errors of the photogrammetry) but must not be taken to be absolute elevations; theseelevations will be corrected to absolute elevations when accurate survey control points becomeavailable.


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top of RHbase of RHtop of textbase of textThe modern town of Angkor Borei is surrounded by the remains of a brick wall

on a linear mound and the remains of moats inside and outside the wall. Thewestern, northern, and eastern walls and moats are rectilinear, whereas the south-ern wall and moats are more sinuous, perhaps reflecting the exploitation of a pa-laeochanel remnant for the outer southern moat (Haruyama, 1998) (Figure 3). Otherprominent linear and sublinear traces in the Angkor Borei area and further southhave been mapped by Haruyama (1998) as paleochannels, but Paris (1931, 1941)used aerial observations and photography to suggest that some of these are ancientcanals. He argued that the canals linked Angkor Borei and other major ancientsettlements to the south and southeast in the delta. Our more detailed mappingusing modern aerial photography and field reconnaissance confirms Paris’s map-ping and reveals other linear traces that appear to be canals (Figure 2).

Other prominent forms of modification of local hydrology are the many reser-voirs scattered throughout the city and adjacent areas (Figure 3). Ancient Khmersused several different kinds of ponds and tanks to flank their houses and surroundtheir temples, including natural water features, artificial ponds, and large reservoirscalled baray. Baray are rectangular tanks constructed by mounding up earth toform enclosing walls; these walls create the tank, and the floor of such reservoirsis rarely very deep (Acker, 1998:9–10). Water management and the constructionof a variety of water control features are associated with the Khmer historicalsequence from the pre-Angkorian period (6th–8th centuries A.D.), but rectangularwater control features called baray are most closely associated with the Angkorianperiod that conventionally begins ca. A.D. 802.

Between the 9th and 14th centuries A.D., Angkorian kings ordered the construc-tion of these baray as monuments that accompanied temples (Groslier, 1979;Higham, 1989:325–329; Acker, 1998; Freeman et al., 1998). Baray vary in size—the largest example being 8 km by 2 km (Acker, 1998)—but baray found outsidethe Angkorian core area north of the Tonle Sap Lake are smaller. The baray func-tions remain a matter of debate, and Higham (1989, 2001), Acker (1998), and Hayao(1999) have provided recent commentary on this question. Whatever their func-tion(s), baray are important in at least two contexts in the present study. First,their construction must have required substantial collective effort, and thereforeyields insights into social organization at the time of construction. Second, barayare bodies of standing water that normally have no inputs of water or sedimentother than from the atmosphere and the small amounts of runoff from the banks.The baray therefore has considerable potential to retain an archive of environ-mental change since baray construction.

Two large reservoirs lie just outside the Angkor Borei city walls: a small one tothe south and a larger one adjacent to the settlement’s east wall moat (Figure 3).The east moat and baray are currently connected as part of fish farming operationsin the baray, but connections between the two in the past are unknown. Workdescribed here focuses on a sediment core from this eastern baray.



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top of textbase of textCORE AB2 SEDIMENTOLOGY, MICROFOSSIL ANALYSES, AND

CHRONOLOGY: METHODS

Survey and Core Collection

Water depth in the eastern baray was surveyed by sounding from a canoe on a20 m x 20 m east–west/north–south grid; the shallow southern baraywas surveyedon foot. North–south and east–west cross-sections of the eastern baray, fromoutside the embankments and through the vicinity of the coring localities, weresurveyed by theodolite (transit) and stadia tacheometry (Figure 4). A reconnais-sance survey of sediment thickness was undertaken in both the southern baray(on foot) and eastern baray (from a canoe) by probing the bed of each with a 5 mlength of 10 mm diameter steel rod with sharpened tip to detect the interfacebetween the baray sediments and the more resistant substrate. The shallow south-ern baray appears to have stored little sediment or its bed is so disturbed by humanactivity and cattle trampling that probing does not detect an interface between thebaray infill sediments and the substrate. The eastern baray is underlain by up toseveral meters of sediment. Two cores, AB1 (1.34 m long) and AB2 (3.11 m long),were recovered from a pontoon, by driving 50 mm diameter PVC water pipe intothe bed, close to the center of the baray (Figures 3 and 4). A piston in the coringpipe, tied off to a supporting tripod, helped to minimize sediment compaction,which was approximately 11%. The recovered cores were cut into transportablelengths, sealed and returned to the University of Glasgow.

Sedimentology Methods

Prior to opening the cores in Glasgow, they were X-rayed on an SMR Galaxy15 kW machine at a range of settings to check for sedimentary structures. Themagnetic susceptibility of the cores was measured using a Bartington magneticsusceptibility meter with an MS2C core logging sensor. The cores were then split,photographed and described. The stratigraphy of core AB1 is identical to the upperpart of the adjacent core AB2. One split of each core was archived under refrig-eration at � 5�C, and the second splits were sampled for a range of analyses.

The seven radiocarbon samples from AB2 were aimed at dating the principalstratigraphic breaks identified during the description and analyses of the core. Sam-ples for sedimentological analyses every 50 mm along the length of core AB2 wereanalyzed at the University of Glasgow for the following properties: mean grain size,sorting and percentage sand, silt and clay (by Beckman LS230 laser diffractionparticle size analyzer); pH (by electrochemical method); moisture content (by dry-ing at 65�C for 24 h); and organic content (by loss on ignition at 375�C for 12 h).

Sedimentation Rates

Three age-depth models (a single linear model, a change-point model with linearinterpolation between the change-points, and a high-order polynomial model) wereconsidered for both total sedimentation and mineral sedimentation only (giving a


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Figure 4. (a) The eastern baray showing the locations of cores AB1 and AB2. (b) East–west and north–south cross-sections of the eastern baray on the section lines shown in a. (c) Same as b but with 5�vertical exaggeration.



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top of textbase of texttotal of six age-depth models). In all cases, the calibrated age distributions for each

radiocarbon determination were used as the basis of a Monte Carlo simulation ofa random series of calendar ages at each dated depth. The change-points in thechange-point model were taken as the upper and lower depths of each of the dateddepth intervals. Model selection was carried out on the basis of the coefficients ofvariation (% variation explained). For a given model, the 95% confidence interval(s)for the slopes were calculated to allow testing of the hypothesis that the sedimen-tation rates were the same in different depth intervals.

The sedimentation rate is the inverse of the slope of the age-depth plot, and themineral sedimentation rate is a measure of the rate of accumulation of the sedi-ment’s mineral component alone. The mineral sedimentation rate was calculatedfor each dated depth interval in two ways: (1) by proportionately decreasing theaverage sedimentation rate for that interval by the average organic matter contentfor the interval; and (2) by calculating a mineral sediment accumulation rate forthe 50 mm depth interval represented by each individual sedimentology sample, byadjusting the overall average sedimentation rate for the dated depth interval by theorganic content of each sample. All of these procedures, be they for average sed-imentation rates for a dated depth interval or for the sedimentation rate indicatedby each individual sample interval, give only approximations of the respective truesedimentation rates. This is because, first, the sedimentation rate for each dateddepth interval is assumed to be constant throughout that depth interval, and takesno account of changes in the true sedimentation rate throughout the interval (whichwould be revealed by more absolute age determinations within each dated depthinterval). Second, no measure is available of variable compaction throughout thecore. Such variable compaction can be syn-depositional and/or the result of thecoring process. Because of these compaction uncertainties, no correction wasmade for the 11% compaction that occurred during coring.

Microfossil Analysis

Samples for pollen and diatom analyses were taken at 50–150 mm intervals downcore AB2. Samples for pollen analysis were prepared following the methods inChivas et al. (2001: Appendix G), and the preparation of diatom samples followedBattarbee (1986). The mounted pollen and diatom samples were analyzed usingconventional light microscopy at �400–1000 magnifications. Pollen and spore tax-onomy was based on published descriptions (Huang, 1972; Zhang et al., 1990; Tissotet al., 1994; Wang et al., 1997) and, where possible, comparison with pollen refer-ence samples. General pollen nomenclature follows Punt et al. (1994).

The diatom analysis was intended to provide a rapid assessment of changinghydrological conditions at the baray locality over time, as indicated by changes inthe proportion of diatoms representative of particular habitat types (specifically,planktonic versus periphytic taxa). Diatom taxa were therefore identified to genuslevel only (Growns, 1999). Diatom taxonomy followed Krammer and Lange-Bertalot(1986, 1988, 1991) with particular reference to the taxonomic revisions presentedby Round et al. (1990).


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top of RHbase of RHtop of textbase of textPollen and spore taxon values are here expressed as a percentage of either a

primary sum (all trees, shrubs, and other woody plants) or a secondary sum (herbs,aquatic plants, and ferns likely to be of local origin). Individual diatom taxon valuesare expressed as a percentage of the entire diatom assemblage. Each microfossilsequence was classified using the program CONISS (Grimm, 1987; square roottransformation, Euclidean distance, stratigraphically constrained, all unknowntypes removed prior to analysis). Microscopic charcoal particle concentrations perunit volume are based on the factor by which a known number of Lycopodiummarker-spores, introduced to the sample at the start of the procedure, are dilutedwithin the analyzed material.

SEDIMENTOLOGY AND CHRONOLOGY OF CORE AB2: RESULTS

Radiocarbon Chronology and Stratigraphy

The radiocarbon chronology is, as expected, consistently older with depth (TableI). The closeness of the radiocarbon ages bracketing the major stratigraphic breaksmeans that the record probably does not contain major time breaks. The X-radio-graph of core AB2 (Figure 5a) reveals bedding and other stratification, as well asbioturbation structures, notably burrows and probably root casts. Faunal burrow-ing can result in disruption of bedding and significant amounts of vertical displace-ment of sediments, perhaps even to the extent of major mixing of the sedimentand confusion of the sedimentary record. However, the stratigraphic boundariesidentified both in the radiographs and visually (Figure 5a, b), in addition to theclear visual evidence of bedding in the cores, and the coherence of the radiocarbonchronology and the microfossil stratigraphy (see below), demonstrate that any bio-turbation has not been sufficient to disrupt the stratigraphic integrity of the sedi-ments in the core.

The record consists of four stratigraphic units (Figure 5). The basal unit, Unit1, extending from the base of the core to 295 cm depth, consists of thinly laminatedmuds with low pH, which pass abruptly upwards into the overlying unit, Unit 2,about 3500 14C yr B.P. This Unit 2, extending from 295 to 139 cm depth, is a muddyminerogenic unit of uniform grain size and generally neutral to slightly acid pH,succeeded abruptly at 139 cm (ca. 1500 14C yr B.P.) by an organic-rich peaty unit.This Unit 3 is characterized by increasingly acid conditions during its depositionand a coarser-grained mineral fraction, a marked increase in organic content, mostnotably at 100 cm depth (ca. 1000 14C yr B.P.), and two pulses of sandy material.This organic depositional phase ended at about 50 cm depth, ca. 800 14C yr B.P.,with a return to a sandy mud of varying grain size (Unit 4), which itself was suc-ceeded at about 18 cm depth (180 14C yr B.P.) by the muddier and more uniformlygrain size Unit 5.

Sedimentation Rates

The age-depth plot (Figure 6[a]) shows evidence of curvature (p � 0.01) and thesingle straight line model of sedimentation is therefore discounted. Given the in-

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Table I. Radiocarbon dates from core AB2, Angkor Borei.a

SampleCode

(C14 Age)LabCode

Depth(cm)

2� Calibrated A.D./B.C.b Ranges

Lower Upper Prob. Lower Upper Prob. Lower Upper Prob. Lower Upper Prob.

AB2/018

(180 � 35)AA-36871 18 1653 1698 0.206 1723 1816 0.552 1836 1877 0.066 1916 1949 0.176

AB2/050

(705 � 50)AA-39136 50 1223 1234 0.025 1235 1327 0.684 1345 1393 0.290

AB2/054

(865 � 40)AA-36874 54 1040 1100 0.217 1112 1147 0.111 1151 1260 0.672

AB2/102

(1040 � 45)AA-47778 102 888 1039 0.991 1105 1107 0.001 1142 1150 0.009

AB2/138

(1495 � 40)AA-36872 138 438 456 0.038 461 517 0.105 520 645 0.857

AB2/140

(1570 � 40)AA-36873 140 415 577 0.981 580 591 0.019

AB2/299

(3665 � 45)AA-36875 299 �2192 �2173 0.026 �2145 �1916 0.974

a All determinations were AMS determinations on bulk samples of sediment and organic matter. The 2� calibrated age ranges (calculated using Calibv.4.2) are given in terms of the Lower and Upper limits of the calendar year age intervals for which the 2� C14 age interval intersects the calibrationcurve and the probability (Prob.) that the calibrated calendar age lies within a particular interval.b Calibrated B.C. ranges denoted as negative numbers.


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Figure 5a–e. Core AB2, Angkor Borei: (a) line drawing of X-radiograph (B: burrows; V: void); (b)majorstratigraphic boundaries identified by visual inspection (V: void); (c–e) sedimentological data.



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Figure 5f–i. Core AB2, Angkor Borei: (f–g) sedimentological data (cont’d); (h) magnetic susceptibility(volume susceptibility; given in dimensionless SI units; negative magnetic susceptibility measurementscorrespond to high moisture contents); and (i) total sedimentation rates and minerogenic sedimentationrates with 95% confidence intervals (see text for methods); 95% confidence intervals on minerogenicsedimentation rates are very narrow below 18 cm depth (see Figures 6[b] and 6[c] for more detail onsedimentation rates).

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Figure 6. Core AB2: (a) Age–depth plots for different sedimentation models (see text): dashed line—single linear model; unbroken line—change-point model with linear interpolation between the change-points (“dog-leg”); dotted line–high-order polynomial model. (b) Average sedimentation rates by dateddepth intervals for total sediment. (c) Average sedimentation rates by dated depth intervals for mineralsediment only. For (b) and (c): Thick central line for each depth interval is the average rate and thinlines define 95% confidence intervals on the average rate; 95% confidence limits below 140 cm depth arenarrower than the thickness of the line depicting the average rate. Dashed lines indicate the radiocarbon-dated boundaries between the stratigraphic units.



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top of textbase of textTable II. Slopes, errors, and confidence intervals for a piece-wise linear fit for each of the dated depth

intervals (see text).

Depth (cm) 0–18 18–50/54 50/54–102 102–138/140 138/140–299

Slope(cm/cal yr)

�10.50 �16.40 �5.67 �12.00 �15.7

Estimated stan-dard error

0.480 0.265 0.170 0.180 0.050

95% confidenceintervals

�9.54–�11.46 �15.87–�16.93 �5.33–�6.01 �11.64–�12.36 �15.60–�15.80

Total sedimen-tation rate(mm a�1)

0.95 0.61 1.76 0.83 0.64

Average organicmatter con-tent

0.137 0.305 0.588 0.502 0.046

Mineral sedi-mentationrate (mm a�1)

0.82 0.42 0.73 0.41 0.61

significant difference between the two ages in the paired determinations at 50 and54 cm depths and at 138 and 140 cm depths, the most reasonable model of sedi-mentation is derived by combining the two dates in each pair and calculating apiece-wise linear fit for each of the dated depth intervals. The slope of the age-depth plot for each depth interval is given in Table II with the estimated standarderrors and 95% confidence intervals on the slopes. These slopes are represented assedimentation rates in Figures 6(b) and 6(c) (see also Figure 5i). The mineral sed-imentation rate indicated by each 50 mm sample is given in Figure 7.

Pollen and Charcoal

Ninety-eight pollen and spore taxa were identified, representing 60 families.These data are plotted stratigraphically in Figure 8. Pollen abundancewas generallysatisfactory, from a minimum of 70 specimens (132 cm depth) to a maximum of726 specimens (77 cm depth), with an average of 368 specimens per sample. Clas-sification of the pollen and spore data (Figure 9) indicates eight sample groups orzones that are used here as a framework for description. For ease of description,this framework is not rigidly adhered to, and zones that are considered to have abroadly similar pollen and spore assemblage (zones 1–3 and 6–7) are discussedtogether. Charcoal particle concentrations, derived from the pollen samples, aredescribed here also. Note that upper sample in one zone and the lowermost samplein the overlying zone (e.g., the shallowest and deepest samples, respectively, inzone 3 and the overlying zone 4) may be up to 150 mm apart. This does not rep-


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Figure 7. Mineral sedimentation rates for each 50 mm sample interval (see text for methods). Dashedlines indicate the radiocarbon-dated boundaries between the stratigraphic units; for clarity, 95% confi-dence intervals not included but are the same width as in Figure 6(c).

resent a break in the record but simply reflects the sampling interval for the mi-crofossil analyses (50–150 mm).

Pollen Zones 1–3 (307–147 cm Depth)

Rhizophora pollen is the dominant type in these zones, with an average value of41.6% of the primary pollen sum. Rhizophora values decline gradually in the upperpart of zone 3, from 177 cm depth. Zone 1 is distinguished from the overlying zonesby its strong values of Aglaia-type pollen and lower taxon diversity (35 taxa in zone1 compared with 80 in zones 2 and 3).Elaeocarpus and Glochidion pollen are more strongly represented in zone 3

(above 217 cm depth), while Sonneratia caseolaris declines from a maximum rep-resentation at 207 cm depth to absence from the assemblage at 157 cm depth.Similarly, the palm Areca-type, which is consistently represented below 188 cmdepth (an average value of 7.3% of the primary pollen sum), is absent in the upper

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Figure 8. Results of pollen and charcoal analysis for core AB2. All taxa expressed as a percentage of either the primary or secondary sums(see text for details). The scale for charcoal contents refers to the black plot; grey plot is a 5� exaggeration of the measured values.


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Figure 9. Classification and zonation of pollen data from core AB2.

part of zone 3. Macaranga is consistently represented at low values throughoutzones 1–3; it is less common in zone 1 than in zones 2 or 3.

The nonarboreal pollen assemblage is entirely dominated by grasses (Poaceaemaintains an average value of 71.4% of the secondary pollen sum for zones 1–3)with sedges (Cyperaceae) consistently represented as a sub-dominant family. Chen-opodiaceae pollen is also consistently represented, though less common in zone 1.Of the ferns, Stenochlaena pallustris is the most abundant, particularly in zone 1.

Charcoal values are relatively high and highly variable throughout zones 1–3,increasing from the base of the record to peaks at 267 cm depth, 225 cm depth,and 167 cm depth. Average particle concentration for zones 1–3 is 1.8 � 106/cm3.

Pollen Zone 4 (132 cm Depth)

This single sample is identified as a discrete zone on the basis of the relativelyhigh values of Macaranga (33% of the primary pollen sum) and Uncaria (25% ofthe primary pollen sum) pollen, and the decline in, or disappearance of, severalcommonly recorded taxa (includingAreca-type,Celtis, Elaeocarpus, andEugenia).



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top of textbase of textCombretaceae/Melastomataceae pollen is commonly recorded while Pinus, Rhi-

zophora, and Urticaceae/Moraceae (triporate) are less common. Poaceae and Cy-peraceae again dominate the non-arboreal pollen assemblage, and ferns are rare.Charcoal concentrations fall to 2.8 � 105/cm3.

Pollen Zone 5 (127–107 cm Depth)

Macaranga is the dominant arboreal pollen type in these samples. The previouslydominant Rhizophora declines sharply and is absent at 117 cm depth and only 3%of the pollen sum at 107 cm depth. The relative abundance of Combretaceae/Me-lastomataceae pollen increases progressively up-core, and Eugenia and Uncariaare both strongly represented. The abundance of Poaceae pollen declines substan-tially from the average of 71% in the stratigraphically lower zones, falling to reacha minimum of 10% of the nonarboreal pollen sum at 107cm depth. Cyperaceae isthe most abundant herbaceous taxon, while the abundance of fern spores increasesmarkedly. Davalliaceae is the most commonly recorded fern, with the Psilamono-lete group and Stenochlaena pallustris spores also very common. Charcoal con-centrations are very low in these sediments, with an average value for the zone of1.1 � 105/cm3.

Pollen Zones 6–7 (97–16 cm Depth)

Combretaceae/Melastomataceae pollen dominates these zones, becoming moreabundant as depth decreases to reach a maximum value of 64% at 17 cm depth.Celtis is codominant, most markedly in zone 6, with Areca-type, Calamus, Dios-pyros, Glochidion, and Macaranga all commonly recorded.

Cyperaceae and Poaceae are codominant in zone 6, but Poaceae dominates zone7. Psilamonolete spores are very common in these zones, increasing to reach amaximum value of 56% at 27.5 cm depth. Both Stenochlaena pallustris and Lyco-podium microphyllum spores are common in zone 6 but fall sharply in zone 7.Similarly, Davalliaceae is common in the early part of zone 6, but its representationfalls sharply to reach a minimum value of 3% at 58 cm depth. Charcoal particleconcentrations in zones 6–7 remain low relative to the early part of the record (anaverage of 2.8 � 105/cm3), but show a slight increase through zones 6–7 to reacha maximum value of 5.2 � 105/cm3 at 27.5 cm depth.

Pollen Zone 8 (7.5–2 cm Depth)

Combretaceae/Melastomataceae, Pinus and Mimosa pigra dominate the pri-mary pollen sum in these samples. Other common dry-land pollen types recordedare Celtis, Dipterocarpus-type, Euphorbiaceae undifferentiated, Macaranga, andTrema (not shown in Figure 8). The secondary pollen sum is dominated by Po-aceae, with an average value of 65%, while Cyperaceae is also common at 10%. ThePsilamonolete group is the most common spore type in these sediments. Charcoalparticle concentrations remain low.


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Figure 11. Classification and zonation of diatom data from core AB2. Dotted lines represent zoneboundaries, dashed lines represent divisions within zones.

Diatoms

Diatom data are shown in Figure 10, and classification and zonation of these dataare given in Figure 11. Diatoms are not preserved in the samples at 307 and 297cm depths, and so the description of diatom assemblages begins at 287 cm depth.

Diatom Zone 1 (287–247 cm Depth)

Aulacoseira dominates the diatom flora in these samples, maintaining an averageof 61% of the total diatom assemblage through the zone. Cyclotella is also common,but declines as depth decreases. Pinnularia increases through the zone to reacha maximum at 256 cm depth before declining slightly in the uppermost sample ofthe zone. Other common genera are Achnanthes, Eunotia, Gomphonema, andNavicula.


Aulacoseira remains the dominant genus, with an average value for the zone of54%. The abundance of Cyclotella declines steadily as depth decreases, continuinga trend observed in the previous zone. In contrast, the number of individuals within


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top of RHbase of RHtop of textbase of textthe genus Eunotia increases to a maximum value of 20% at 167 cm depth, before

decreasing sharply to 2% at 147 cm depth. Both Luticola and Diadesmis demon-strate a discrete increase in abundance at 188 cm depth. Other common generarecorded are Actinocyclus, Chaetoceros, Cymbella, Gomphonema, Navicula, andPinnularia.


Many of the samples in this zone (117–47 cm depth) are barren of diatoms.However, in those samples that do contain preserved diatom frustules, Diadesmisis clearly the most abundant genus, reaching a maximum value of 55% at 37 cmdepth. In contrast, the previously dominant genus Aulacoseira is relatively poorlyrepresented, with an average of only 4% of the total assemblage. Eunotia increasesas depth decreases, reaching a maximum of 30% at 37 cm depth.Navicula increasessharply in abundance at 16/17 cm depth.

Diatom Zone 4 (7.5–2 cm Depth)

Pinnularia and Gyrosigma are the dominant genera in this zone, with averagevalues of 44% and 23%, respectively. Navicula is subdominant, with Amphora, Au-lacoseira, and Eunotia commonly recorded also.

DISCUSSION

The Stratigraphic Record

All of the sedimentological/stratigraphic boundaries coincide with zonal bound-aries in the pollen and/or diatom records (Figure 12). The microfossil zonal bound-ary at 242 cm depth in both the pollen and diatom records is not recorded in thesedimentological stratigraphic data, however, and the microfossil zonal boundariesat 130 cm and 102 cm depths are found only in the pollen record. The microfossildata, therefore, provide a more detailed stratigraphic subdivision of the core.

The diatom data indicate that the core locality has supported standing water forthe whole of the AB2 record between ca. 300 cm and 18 cm depths. (For clarity,we foreshadow here our later conclusion that the core locality was initially a nat-ural water body that was subsequently exploited to create the baray.) The lowdiatom levels at various intervals in the AB2 record are closely related to periodsof falling or low pH (Figure 5g) and are therefore probably related to chemicaldissolution of the silica frustules at these times rather than to mechanical degra-dation associated with shallow or dry periods. The indication of essentially contin-uous sedimentation throughout the full period of the core is consistent with thegenerally short time periods encompassed by the paired radiocarbon determina-tions either side of the major stratigraphic boundaries (Table I). The only apparentbreak in sedimentation, marked by roots and possible subaerial desiccation cracksor pedal voids, is at 18 cm depth in the core and is suggestive of subaerial exposureof the sediment surface at this time.



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Figure 12. Locations of boundaries of stratigraphic, pollen and diatom zones in core AB2. The micro-fossil zone boundaries given here and in Figures 8–11 are plotted midway between the sample depthsthat define subjacent zones. The microfossil zone boundaries are therefore not precisely comparable tothe boundaries of the stratigraphic units which are defined on the basis of visual inspection and depthmeasurements of the core, and the more closely spaced sampling for the sedimentological data.

The diatom data, when grouped into habitat types (Figure 13), reveal a clearpattern of hydrological change over time. The absence of diatoms in the basalsamples at 307 and 297 cm depth is coincident with low pollen concentrations (anaverage of 15,708 grains/cm3 against an average of 44,058 grains/cm3 for the totalsample population), the deposition of laminated sediments and relatively acidchemistry (an average sediment pH of 4.7 between 290 and 300 cm depth). Thesedata suggest that the locality held shallow water on an episodic basis in the earliestpart of the AB2 record, possibly in association with seasonal floods as signaled bythe laminated sediments. The low sediment pH values may reflect water chemistrychanges during falling/evaporative phases in this seasonal hydrology. While dia-toms are not preserved under these conditions of low pH and seasonal inundation,pollens are preserved and are dominated byAglaia-type,Rhizophora, and Poaceae.This may reflect the presence of some form of humid forest in the area, with tidalforest along streams and other watercourses, and grasses probably growing on andaround the locality.

The dominance of the tychoplanktonic genus Aulacoseira in diatom zones 1 and2 signals a shift from periodic inundation to permanent standing water that washighly turbid (Figures 10 and 13). This genus is common or dominant in the phy-toplankton of large and nutrient rich rivers (Krammer and Lange-Bertalot, 1991;Hotzel and Croome, 1996), and its presence here may indicate more permanent


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Figure 13. Diatom genera (%) from core AB2 grouped into habitat types.

hydrological connections with rivers in the area. The relatively strong presence ofholoplanktonic genera, particularly Cyclotella, corroborates the presence of a per-manent water body. The early part of the record, then, intimates the occurrence ofa substantial hydrological change around 3500 14C yr B.P., from seasonal inundationwith laminated sedimentation and acid waters to a permanent full-lake phase dom-inated by mud sedimentation. The reasons for this change are unknown, but mightrelate either to changes in the morphology of channels in the area, or to deliberatedamming of the locality by people. The steady decline in obligate planktonic generaover time through diatom zones 1 and 2 is consistent with a gradual reduction inwater depth (that is, a restriction of planktonic habitat), presumably as a result ofthe natural infilling of the basin.

In this early part of the record, the strong representation of Rhizophora pollenand the presence of taxa such as Bruguiera, Nypa, Sonneratia, Xylocarpus, andpossibly Chenopodiaceae, are indicative of mangrove forests in the northern part



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top of textbase of textof the Mekong Delta. It may be that a proportion of these pollen types were blown

north from coastal mangrove communities in the Mekong Delta during the summermonsoon, but the abundance of Rhizophora pollen (between 8–[38.9]–55 % of theprimary pollen between 307–127 cm depth) argues strongly for the local occur-rence of this community type. Crowley et al. (1994) found that Rhizophora valuescomparable to those described from core AB2 (around 30%) indicate transitionalhabitats on the landward margin of true tidal mangrove communities. Interestingly,there is no clear evidence in the microfossil data to indicate saline surface wateraround Angkor Borei. The relatively high values of Chenopodiaceae in pollen zones2 and 3 may indicate higher regional salinity, but values are not sufficiently high toindicate convincingly the occurrence of salt-marsh close to the city. The diatomflora contemporary with occurrence of mangroves (diatom zones 1–2) indicatefresh or possibly fresh-brackish, well-mixed, nutrient-rich waters. Indeed, there isno compelling evidence that surface waters were highly saline at any time in theAB2 record. It is probable, then, that mangrove communities were growing exten-sively along watercourses close to Angkor Borei during the Late Holocene. Thisinterpretation is consistent with a pollen record from the Tonle Sap Lake whichshows that, during the Holocene sea-level transgression, mangroves penetrated farinland along the channels of the Mekong/Bassac Rivers and tributaries (Penny,unpublished data). It is probable that the declining values of Rhizophora pollen inzones 1–3 reflect the southward migration of mangrove forest in response to eithera regressive phase of sea-level change (Ta et al., 2001), possibly in combinationwith flexural uplift of the northern Mekong Delta in a manner analogous to theeastward tilting of the Ganges River delta (Blasco et al., 2001).

The most dramatic changes in the sedimentological and microfossil records oc-cur at about 139 cm depth, at the abrupt transition between stratigraphic units 2and 3, diatom zones 2 and 3, and pollen zones 3 and 4. This change is bracketedby radiocarbon ages at 140 cm (AA-36 872) and 138 cm (AA-36873) depths (TableI). At this transition, Aulacoseira is greatly reduced in abundance, and genera thatoccur on various submerged (periphytic) or aerial (aerophilous) substrata becomedominant. Also at this time, Combretaceae/Melastomataceae pollen starts to in-crease, while secondary forest taxa, particularly Macaranga, become dominant.An extremely rapid decline in the relative abundance of Poaceae is apparent, whilethe representation of fern spores in the sediment increases markedly (Figure 14).All of these changes are synchronous with a dramatic decline in the concentrationof microscopic charcoal particles being deposited in the sediment, intimating achange in the local fire regime, probably to more infrequent burning. Moreover,material being deposited in the basin at this time is predominantly organic (Figure5f), presumably reflecting an increase in vegetation growth in and around the lo-cality. This increase in organic sedimentation at the 140–138 cm transition is re-flected in the increase in average total sedimentation rate this transition (Figure6[b]) and a corresponding decrease in the average mineral sedimentation rate (Fig-ure 6[c]). In detail (and remembering the cautionary notes above concerning the

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Figure 14. Pollen taxa grouped into community or habitat type, with each taxon expressed as a percentage of the relevant pollen sum. Fernsare epiphytic (closed squares), ground or twinning (closed triangles), and ungrouped ferns (open squares). The scale for charcoal contentsrefers to the black plot; grey plot is a 5� exaggeration of the measured values.



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top of textbase of textdetailed sedimentation rates), sedimentation rates by sample apparently become

much more variable in the organic unit (ca. 140–50 cm), fluctuating more widelyabout the average value than in the preceding unit.

It is unclear if the decline in grasses at the ca. 139 cm transition is related to areduction in the frequency of burning at this time, or if the reduction of grasslandsis a response to some other mechanism that reduced fuel loads around the site.The latter would lead to a reduction in the frequency of fires by disrupting thefire-grass feedback loop (Vila et al., 2001), thereby permitting the expansion ofmore fire-intolerant vegetation. The expansion of secondary forest taxa in pollenzones 4 and 5 (particularlyMacaranga and other Euphorbiaceae) seems to indicatethe colonization of areas that previously supported grasses under a regime ofregular burning. It is possible that these early successional changes may reflecta period of land abandonment, or perhaps a deintensification of existing land-use.If this is the case, the absence of Lagerstroemia pollen is curious, given that itis the key taxon in successional forest for this region (Rollet, 1972; Blanc et al.,2000).

The vegetation that developed at the core locality, indicated by the increase inorganic sedimentation from 139 cm depth, does not appear to have a substantialherbaceous or aquatic component. There is only a very slight increase in Cypera-ceae pollen at 132 and 127 cm depth, and none of the aquatic or littoral-swampplants demonstrates any increase in pollen values at this time. It is likely thatswamp forest trees, probably members of the Combretaceae/Melastomataceaegroup, began to colonize the locality from this time. The dramatic increase in theabundance of aerial diatoms at 132 cm depth is consistent with this development,with genera such as Diadesmis and Luticola taking advantage of an effective ex-pansion of habitat afforded them by the moist trunks of colonizing swamp forestplants. Similarly, the marked increase in the representation of fern spores, partic-ularly the epiphytic species Stenochlaena pallustris, is also consistent the devel-opment of moist, shaded conditions beneath the closed canopy of invading swampforest trees.

The dramatic shift in the diatom flora from the dominance of tycho- and holo-planktonic forms to periphyton dominance also indicates substantial hydrologicalchanges at this transition (Figure 13). Simplistically, the decline in abundance ofAulacoseira is indicative of less turbid waters. The expansion of attached generamight also indicate an increase in light availability to the bed associated with a fallin turbidity. These changes may be the result of a decrease in water depth, or theisolation of the locality from fluvial inputs, such as might be expected followingthe abandonment or artificial closure of river channels.

Diatom preservation ceases from approximately 127 to 117 cm depth, most likelyas a result of post-mortem dissolution in response to increasing acidity in the highlyorganic peat (Unit 3) (Ryves et al., 2001). Spores of the twinning fern Lygodiummicrophyllum occur abundantly between 97 and 58 cm depth. This plant is knownto grow on exposed sites, dry slopes, and among exposed herbaceous swamp (Holt-tum, 1959; Tagawa and Iwatsuki, 1979). It is also an aggressive and invasive weed


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top of RHbase of RHtop of textbase of textoutside its natural range (Mirsky, 1999), is known to be a weed of fallow rice fields

in the region (Roder et al., 1995), and has been used as an indicator of forestclearance in similar environments in Southeast Asia (Higham and Bannanurag,1991:93–94). The increase in the representation of its spores, then, may reflect adegree of “opening” of the canopy in the area around the locality, possibly in re-sponse to disturbance. This interpretation finds some support in the influx of sandat about 100 cm depth and the higher rates of mineral sedimentation between 100and 50 cm depth (the second highest average rates and the highest, and mostvariable, individual rates in the whole core; Figures 6[c] and 7). The transition at100 cm depth, between pollen zones 5 and 6, in fact marks the end of the transitionalperiod to full successional development of swamp forest, and coincides with afurther increase in organic production (peat) and faster decline in pH. The highsedimentation rates, the suggestion of some opening of the canopy, and the influxof sand at 100 cm depth might all be taken to indicate disturbance of the corelocality, but there is little evidence to support this contention in the full pollenassemblage, save for the variable representation of Mallotus pollen in this part ofthe sequence. Indeed, the strong evidence for swamp forest covering the localitysuggests the contrary.

Diatoms frustules begin to reappear in the record from 47 to 37 cm depth, prob-ably reflecting the reduction in the acidity of lake deposits at these depths (Figure5g). The character of the diatom assemblage appears to have changed very littleover the intervening ca. 800 14C years (i.e., between AA-36872 and AA-39136; TableI), intimating a degree of stability over this time period.

Mineral sedimentation rates fall off very abruptly at the 50 cm boundary and fallsteadily to 18 cm depth (Figures 6[c] and 7), perhaps reflecting a combination ofwell developed swamp forest at the locality and a cutting-off of the locality fromsediment influxes. Combretaceae/Melastomataceae pollen reaches peak abun-dance between 37 and 27.5 cm depths, suggesting that the parent species must havebeen extremely common around the locality at this time, and throughout the localarea. The deposition of sands at 25 cm depth (Figure 5c) may indicate a degree ofdisturbance close to the locality, such as the excavation of material for moat orbaray wall construction, or that high-energy floods maintained some connectionbetween the locality and rivers in the area.

The stratigraphy in the upper part of the core indicates the growth of herbaceousplants directly on the core locality (evidenced by root penetration from a soil ho-rizon at 18 cm depth), suggesting that the locality was either permanently shallowor seasonally dry. The very poor representation of planktonic diatom genera inthese sediments supports this interpretation. The locality appears to have beenflooded thereafter, presumably through the deliberate redirection of surface waterseither from the east moat of the city or elsewhere. A clear decrease in the repre-sentation of Combretaceae/Melastomataceae pollen and increases in both grassesand sedimentary charcoal particles above 7 cm depth are suggestive of clearing,while the presence of the invasive exotic weed Mimosa pigra suggests regulardisturbance around the locality. This plant currently grows on the banks of the



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top of textbase of texteastern baray, suggesting continuity with the modern flora. Mineral sedimentation

rates are consistently high in this upper unit (Figures 6[c] and 7), unlike in the 100–50 cm interval, which has the highest individual mineral sedimentation rates of thewhole core, but also the most variable individual mineral sedimentation rates. Thesustained high values of individual sedimentation rates in the upper 18 cm of thecore are consistent with disturbance of the surrounding area and transport of sed-iment across the water body to the core locality, whereas the high butmore variablerates in the 100–150 cm depth interval are perhaps more consistent with pulses ofsediment deriving from natural flood events.

Possible Evidence for Anthropogenic Activity

As described above, microscopic charcoal concentrations are substantiallygreater in the lower half of the core, rising rapidly after the cessation of depositionof the basal laminated muds at about 2000 cal. yr B.C. (299 cm depth). These highercharcoal levels were sustained until the abrupt decline at the dramatic ecological/stratigraphic change at 139 cm depth, which is bracketed by the radiocarbon de-terminations at 140 and 138 cm depths. The 95% calibrated range of these deter-minations is the 5th–7th centuries A.D. (Table I), and the median ages of theirhighest probability calibrated age ranges are A.D. 496 (140 cm) and A.D. 583 (138cm). This late 5th to late 6th century A.D. change was associated with expansionof swamp forest plants, peat formation, and, in the surrounding area, decreasedintensity of land-use (as signaled by the decreased charcoal concentrations, reduc-tion in grasslands, and the expansion of regrowth taxa).

These data may be interpreted in several ways, in terms of changes both at thecore locality itself and more regionally. The development of swamp forest at thecore locality indicates either a natural successional change in the local vegetation,or a change in land-use or management of the water body and the allowing ofswamp forest to establish. The more regional data, such as changes in the extentof grasslands, inferred changes in the frequency of fires and the development ofsecondary forest, may be taken to indicate that the peak levels of land use intensity,and presumably also occupation of Angkor Borei, occurred prior to the 5th/6thcentury A.D. After this time, the levels of land-use intensity never attained the pre-5th/6th century A.D. levels except perhaps in the uppermost 18 cm of the AB2record. Another interpretation, however, is that the changes in the regional pollenand charcoal evidence signal a change away from land-use that employed extensiveburning. Such a change might have involved, for example, a move away from cul-tivation of rice in burned fields, with dry season burning, to flood recession culti-vation of rice. The recovery of several 5th or 6th century sculptures from the site(see Dalsheimer and Manguin [1998] for discussion of dating), the emergence ofthe Phnom Da art style from the mid-6th century A.D. (Phnom Da being a templeabout 1km south of Angkor Borei; Figures 1 and 2), as well as inscriptions fromthe site from the early 7th century A.D., all suggest that Angkor Borei was still asufficiently important center to merit substantial economic and social investment


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top of RHbase of RHtop of textbase of textuntil the mid-7th century A.D. Therefore, we would expect to observe a major

decline one to two centuries later, in the mid to late 7th century A.D., rather thanin the 5th/6th century A.D., as suggested by the data presented here. It is possible,or even likely, therefore, that the patterns evident in the palaeobotanical and sed-imentation records reflect the early abandonment of the area east of the city walls,perhaps as a result of changing land-use priorities, while the city itself continuedto flourish amid a landscape supporting a different (less intensive?) agriculture.

As already noted, charcoal levels after the 6th century A.D. never again attainedthe pre-5th/6th century A.D. levels, but there is a steady, statistically significantincrease in charcoal concentrations up-core from the minimum concentration at115 cm depth. A minor peak in charcoal concentration at 97 cm coincides with aninflux of sandy material to the core locality, perhaps signaling local anthropogenicdisturbance. As already noted, however, the highly variable rates of sedimentationin the 100–50 cm interval might be taken to be more indicative of natural, flood-driven incursions of water and sediment to the coring locality (Figure 7).

Baray Construction

The diatom data demonstrate that the core locality has been characterized bystanding water for the whole of the AB2 record between ca. 300 cm and 18 cmdepths. It seems, therefore, either that the locality has been an artificially con-structed reservoir (baray) for essentially the entire period of the record, or thatbaray construction at some point over the past 3000–4000 years simply repre-sented the modification of an existing landform, in this case probably an abandonedmeander loop (oxbow lake). The paleochannel mapped by Haruyama (1998) asintersecting the northeast corner of the baray is presumably the paleochannel thatwas modified in baray construction. It is possible that this body of standing wateracted as a source of fresh water for the adjacent occupation site prior to barayconstruction.

Exploitation of a pre-existing body of standing water in construction of the barayis consistent with the baray floor being up to 4 m lower than the ground leveloutside the reservoir (Figures 4b and 4c). The Angkor Borei baray is thereforedifferent from traditional Khmer baray, such as the East and West baray at Angkor,in that the construction technique for the latter, the mounding-up of the walls ona gently sloping surface, would not have resulted in the relative elevations of baraybed and baray exterior that are found at Angkor Borei. The exploitation of anexisting body of standing water to construct the baray presumably involved theregularization of its outline and the construction of the baray walls. This approachto baray construction may reflect a degree of pragmatism in the construction ofthis reservoir enforced, perhaps, by limited labor and reflecting the nature of thelocal environment. Moreover, water is more available in the delta (and for longerduring the year, reflecting the maintenance of flows to the delta by the draining ofTonle Sap; MRCS/UNDP, 1998) than in other parts of Cambodia, and there mayhave been less need at Angkor Borei to construct the major water retention featuresfound at Angkor.



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top of textbase of textThe core record should signal the construction of a baray wall, which should

therefore be datable. This approach relies on identifying changes in the water,vegetation, and/or sediments at the core locality that can be interpreted as indic-ative of baray construction. It is reasonable to suggest that baray constructionwould be signaled by open water and aquatic vegetation at the core locality, aswell as pioneer species and disturbance indicators in the local vegetation, indica-tive of the species growing on the newly constructed baray embankments. In-creased sedimentation resulting from in-wash from the new embankments mightalso be expected. As noted above, the major change signaled in the AB2 sedimentsis in the late 5th to late 6th century A.D., corresponding to a de-intensification ofland-use and perhaps land abandonment, or possibly a change in land-use prioritiesor techniques. Thereafter, until the stratigraphic break at 18 cm depth, the localitywas gradually colonized by closed, fern-rich swamp forest that was not cleareduntil 18 cm depth. The radiocarbon determination from 18 cm depth has a 2� (95%)calibrated age of A.D. 1653–1949. There is a 76% probability that this 2� calibratedage lies in the range A.D. 1653–1816 (mid 17th to early 19th century; median age� A.D. 1735; Table I). The reestablishment of standing water at the locality afterthe break at 18 cm depth coincides with a decrease of Combretaceae/Melastoma-taceae and increases in the representation of dryland herbs, both of which changesare suggestive of forest clearance. The strong presence of the invasive exotic weedMimosa pigra, currently growing on the banks of the eastern baray, is likewisesuggestive of disturbance and clearing. Disturbance and clearing are also consistentwith the onset and maintenance of sustained high mineral sedimentation rates atthe 18 cm stratigraphic boundary (Figures 6[c] and 7). The change across the 18cm boundary in mineral sedimentation rates by individual sample is the secondlargest individual increase recorded in the core, and coincides as we have seenwith the change to a core locality cleared of swamp forest and with a vegetationindicative of disturbance. All of these data suggest that the baray walls date fromsome time in the 17th to 19th century or, if the full 95% calibrated age range isused, even later.

The only other time that the core locality could have been characterized by theopen water of a baray is prior to the late 5th to late 6th century A.D. changerecorded at 139 cm depth, when highly turbid standing water occupied the locality.It is, of course, possible that the baray was constructed prior to the late 5th to late6th century A.D., in which case it was abandoned in the late 5th to late 6th centuryand was completely colonized by swamp forest. We cannot unequivocally excludethis possibility at this stage, but there is no palaeobotanical evidence for distur-bance around the core locality prior to the 5th/6th century changes that mightindicate the excavation and mounding-up of earthen walls (unlike the mid 17th toearly 19th centuries, when such evidence is unambiguously apparent). This, in com-bination with the relative elevations of the lake bed and the surrounding floodplain(see above), does not support an early (i.e., pre 5th/6th century) date for the con-struction of the baray walls. The high rates of sedimentation in the 100–50 cmdepth interval might be suggestive of in-wash from baray walls, but the swamp


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artificial reservoir at this time.

CONCLUSIONS

The Angkor Borei site is located in a fluvial-deltaic environment. Radiometricdating of a sediment core from the eastern part of the city has revealed a longdepositional sequence dating from before the 2nd millennium B.C. Microfossil andsedimentological analyses indicate that the core locality was originally a naturaldepression, probably an abandoned meander loop. The east baray appears there-fore to have been constructed by modifying a paleochannel remnant, and is thusquite unlike the reservoirs of the Angkorian capitals to the northwest in both themeans of construction and, presumably, function. The date at which the walls ofthe baray were constructed remains uncertain. However, we suggest that the mostconvincing evidence points to a date sometime between the mid 17th to early 19thcentury A.D., meaning that the east baray is essentially a modern feature and notcontemporary with the ancient Funan city of Angkor Borei. This date may reflectthe fact that large water storage features were not required in this part of Cambodia,as well as supporting the contention that recession-rice agriculture was sufficientto feed large populations in the Mekong Delta, without recourse to large irrigationnetworks (van Liere, 1980; Fox and Ledgerwood, 1999).

Microfossil data indicate that the site on which the city was founded in the 4thcentury B.C. lay inland of southward-migrating mangrove forests that probablyoccurred extensively in the area as a riparian forest. Substantial vegetational andhydrological changes are apparent from the 5th to 6th century A.D., indicating aperiod of changing land-use practices or priorities. The core locality was progres-sively invaded by swamp forest and was largely dry by A.D. 1735, after which timeit was artificially flooded and, in all probability, the baray walls were constructed.

The research reported here was funded by National Geographic Research Grant #6087-97; the coreswere imported into Great Britain under quarantine license #IMP/SOIL/19/1999. Our special thanks go toCambodian colleagues, including Minister of Culture Princess Norodom Bopha Devi for permission toundertake research, and to Under Secretary of State Chuch Phoeurn for collaboration in research.Thanks also to the following archaeologists for field assistance with the baray research during the 1999and 2001 field seasons: Bong Sovath, Mitch Hendrickson, Sok Kimsan, Mam Vannary, Chea Sopheary,and Pich Thyda. We also thank Weipers School of Veterinary Science, University of Glasgow for thecore X-rays, and the British Geological Survey, Edinburgh, for use of the magnetometer. The followingGlasgow colleagues assisted us greatly, for which we are grateful: Peter Chung and Allen Jones (sedi-mentological analyses); Jane Drummond and Anne Dunlop (photogrammetry); and John Shearer, MikeShand, and Peter Chung (cartography and diagrams). The late Dr. Yasushi Kojo kindly translated Har-uyama (1998) for us. We also thank Mitch Hendrickson for assistance with the hydrographic and cross-section surveys of the eastern baray, Dr. Pauline Reimer for advice on radiocarbon dating and manip-ulation of calibrated ages, and Dr. John Grindrod for discussion on mangrove ecology and palynology.

REFERENCES

Acker, R. (1998). New geographical test of the hydraulic thesis at Angkor. South East Asia Research, 6,5–47.



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and palaeohydrology (pp. 527–570). Chichester: Wiley.Bishop, P., & Godley, D. (1994). Holocene palaeochannels, north central Thailand: Ages, significance

and palaeoenvironmental indications. The Holocene, 4, 32–41.Bishop, P., Hein, D., & Godley, D. (1996). Was medieval Sisatchanalai like modern Bangkok: Flooded

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Received April 2, 2002

Accepted for publication September 17, 2002

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