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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
1
Department of Geography and Topographic Science, University of Glasgow,
Glasgow G12 8QQ, United Kingdom
2
School of Geography and Environmental Science, Monash University,
Melbourne VIC 3008, Australia
3
Department of Anthropology, 2424 Maile Way, Saunders 346, University of
Hawaii, Honolulu, Hawaii 96822
4Department 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 a
continuous record of sedimentation and paleoenvironments dating from about 2000 cal yr
B.C. Palynological data indicate that for much of the cal. 1st and 2nd millennia B.C. mangroves
dominated the regional vegetation, while extensively and regularly burnt grasslands dominated the local vegetation. Turbid, nutrient-rich standing water characterized the core locality,
perhaps suggesting a connection with rivers in the area. An abrupt change during the cal. 5th
to 6th centuries A.D. involved a dramatic reduction in grasslands and the expansion of secondary forest or re-growth taxa. These changes are synchronous with an abrupt decline in
the concentration of microscopic charcoal particles in the sediments, and the colonization of
the core locality by swamp forest plants. These changes are taken to indicate a shift in landuse strategies or, possibly, a period of land abandonment. The age for the construction of the
baray 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, so
its construction was fundamentally different from the methods used at the Angkorian capital
in northern Cambodia. 䉷 2003 Wiley Periodicals, Inc.
INTRODUCTION
The Mekong Delta of mainland Southeast Asia is famous as the heartland of one
of the earliest civilizations in the region. Called Funan by visiting Chinese dignitaries in the 3rd century A.D., it reputedly contained multiple urban centers between the 1st and 6th centuries A.D. (Pelliot, 1903; Coedès, 1968; Higham, 1989).
Documentary evidence provides one narrative of early state development in the
Mekong 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.
Geoarchaeology: An International Journal, Vol. 18, No. 3, 359– 393 (2003)
䉷 2003 Wiley Periodicals, Inc.
Published online in Wiley Interscience (www.interscience.wiley.com). DOI:10.1002/gea.10067
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BISHOP, PENNY, STARK, AND SCOTT
with Indian traders and Brahmins (Coedès, 1968; Jacques, 1979; Wheatley, 1983).
Some scholars have hypothesized that the region was important for its proximity
to 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 that
populations were attracted to the delta for its ideal combination of arable land,
reliable flooding regime, and potable water. Until recently, geopolitical factors have
limited archaeological and paleoenvironmental research to evaluate these interpretations.
Two possible early centers in the delta are now the subject of archaeological
investigation (Figure 1). One is the site of Oc Eo, in southern Vietnam, where
archaeologists from L’École Française d’Extrême Orient (EFEO) worked briefly in
the 1940s (Malleret, 1959, 1960, 1962). Vietnamese archaeologists resumed work in
the Vietnamese delta after 1975, and in the mid-1990s began collaborative research
with EFEO scholars (e.g., Manguin, 1998; Manguin and Vo Si Khai, 2000). The second site, in southern Cambodia, is Angkor Borei, a 300 ha walled and moated site
that has been the focus of research by the Lower Mekong Archaeological Project
since 1996 (e.g., Stark, 1998; Stark et al., 1999; Stark and Bong, 2001). Excavations
at several localities throughout the site have produced parallel dated sequences
and suggest that the site was first settled in the 4th century B.C. Settlement continued for at least a millennium, before the seat of power moved north in the 7th
century A.D. Archaeological research at Angkor Borei suggests that the region did
not experience subsequent abandonment but may have experienced pronounced
fluctuations in population and intensity of land use.
Systematic geoarchaeological and palynological research is now necessary to
complement this archaeological research by providing information on the geographical and environmental factors that facilitated and/or were associated with
early historic human settlement and land-use in the Mekong delta. The pace of
geoarchaeological and palynological research has increased in mainland Southeast
Asia (e.g., Maloney, 1992; Godley et al., 1993; Bishop and Godley, 1994; Kealhofer
and 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 Cambodia. This paper reports on the overall environmental setting of Angkor Borei
using sedimentological and palynological data from a sediment core extracted from
the site’s largest reservoir. A chronology for environmental and hydrological
change in the context of human-environment interactions is established, and the
age and mode of construction of the reservoir are discussed.
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GENERAL GEOMORPHOLOGICAL SETTING
The Angkor Borei area lies between 5 and 10 m above sea-level in the Mekong
Delta, 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 terraces, channels, and paleochannels with intervening backswamps and anthropo-
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Figure 1. Map of Cambodia showing the location of Angkor Borei in the Mekong Delta. Inset shows
regional setting. Map originally drafted by Jo Lynn Gunness
genic canals (Figures 2 and 3). The modern rivers are commonly leveed and separated by lower-lying backswamps, some of which are crossed by the faint traces
of the paleochannels, as well as by the dendritic drainage patterns of modern channels that drain the backswamps after the annual flood. The most clearly identifiable
paleochannels 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 stereoscopic
aerial photograph interpretation of Finnmap Oy’s 1:25,000 scale aerial photographs (flown in December
1992 and January 1993). The aerial photograph interpretation was transferred to the Takeo 1:50,000
topographic sheet, using a Bausch & Lomb Zoom Transfer Scope. Planimetric control in this transfer
was provided by superimposition of prominent landscape elements, including the Pol Pot era canals at
1-km spacings coincident with the national map grid. The canals mapped by Paris (1931, 1941) are
labeled according to his numbering. The supposed Funanese center of Oc Eo lies on a south-southeasterly continuation of the trace of canal 4, approximately 60 km from the southeast corner of this
map.
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Figure 3. Contour map of Angkor Borei city showing the locations of archaeological excavations (“Test
site”), 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 Drummond
from 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 the
horizontal was provided, therefore, by the intersections of the Pol Pot era canals at 1-km spacing on
the national map grid. Vertical control was provided by assuming that the surface of the monsoon flood
waters inundating the backswamps across the aerial photographs was at a uniform elevation, here
assumed 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; these
elevations will be corrected to absolute elevations when accurate survey control points become
available.
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The 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. The
western, northern, and eastern walls and moats are rectilinear, whereas the southern wall and moats are more sinuous, perhaps reflecting the exploitation of a palaeochanel remnant for the outer southern moat (Haruyama, 1998) (Figure 3). Other
prominent linear and sublinear traces in the Angkor Borei area and further south
have been mapped by Haruyama (1998) as paleochannels, but Paris (1931, 1941)
used aerial observations and photography to suggest that some of these are ancient
canals. He argued that the canals linked Angkor Borei and other major ancient
settlements to the south and southeast in the delta. Our more detailed mapping
using modern aerial photography and field reconnaissance confirms Paris’s mapping and reveals other linear traces that appear to be canals (Figure 2).
Other prominent forms of modification of local hydrology are the many reservoirs scattered throughout the city and adjacent areas (Figure 3). Ancient Khmers
used several different kinds of ponds and tanks to flank their houses and surround
their temples, including natural water features, artificial ponds, and large reservoirs
called baray. Baray are rectangular tanks constructed by mounding up earth to
form enclosing walls; these walls create the tank, and the floor of such reservoirs
is rarely very deep (Acker, 1998:9 – 10). Water management and the construction
of a variety of water control features are associated with the Khmer historical
sequence from the pre-Angkorian period (6th – 8th centuries A.D.), but rectangular
water control features called baray are most closely associated with the Angkorian
period that conventionally begins ca. A.D. 802.
Between the 9th and 14th centuries A.D., Angkorian kings ordered the construction 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 outside
the Angkorian core area north of the Tonle Sap Lake are smaller. The baray functions remain a matter of debate, and Higham (1989, 2001), Acker (1998), and Hayao
(1999) have provided recent commentary on this question. Whatever their function(s), baray are important in at least two contexts in the present study. First,
their construction must have required substantial collective effort, and therefore
yields insights into social organization at the time of construction. Second, baray
are bodies of standing water that normally have no inputs of water or sediment
other than from the atmosphere and the small amounts of runoff from the banks.
The baray therefore has considerable potential to retain an archive of environmental change since baray construction.
Two large reservoirs lie just outside the Angkor Borei city walls: a small one to
the 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 operations
in the baray, but connections between the two in the past are unknown. Work
described here focuses on a sediment core from this eastern baray.
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CORE 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 a
20 m x 20 m east – west/north – south grid; the shallow southern baray was surveyed
on foot. North – south and east – west cross-sections of the eastern baray, from
outside the embankments and through the vicinity of the coring localities, were
surveyed by theodolite (transit) and stadia tacheometry (Figure 4). A reconnaissance 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 m
length of 10 mm diameter steel rod with sharpened tip to detect the interface
between the baray sediments and the more resistant substrate. The shallow southern baray appears to have stored little sediment or its bed is so disturbed by human
activity and cattle trampling that probing does not detect an interface between the
baray infill sediments and the substrate. The eastern baray is underlain by up to
several 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 into
the bed, close to the center of the baray (Figures 3 and 4). A piston in the coring
pipe, tied off to a supporting tripod, helped to minimize sediment compaction,
which was approximately 11%. The recovered cores were cut into transportable
lengths, sealed and returned to the University of Glasgow.
Sedimentology Methods
Prior to opening the cores in Glasgow, they were X-rayed on an SMR Galaxy
15 kW machine at a range of settings to check for sedimentary structures. The
magnetic susceptibility of the cores was measured using a Bartington magnetic
susceptibility meter with an MS2C core logging sensor. The cores were then split,
photographed and described. The stratigraphy of core AB1 is identical to the upper
part of the adjacent core AB2. One split of each core was archived under refrigeration 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 principal
stratigraphic breaks identified during the description and analyses of the core. Samples for sedimentological analyses every 50 mm along the length of core AB2 were
analyzed at the University of Glasgow for the following properties: mean grain size,
sorting and percentage sand, silt and clay (by Beckman LS230 laser diffraction
particle size analyzer); pH (by electrochemical method); moisture content (by drying 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 linear
interpolation between the change-points, and a high-order polynomial model) were
considered 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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total 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 of
a random series of calendar ages at each dated depth. The change-points in the
change-point model were taken as the upper and lower depths of each of the dated
depth intervals. Model selection was carried out on the basis of the coefficients of
variation (% variation explained). For a given model, the 95% confidence interval(s)
for the slopes were calculated to allow testing of the hypothesis that the sedimentation rates were the same in different depth intervals.
The sedimentation rate is the inverse of the slope of the age-depth plot, and the
mineral sedimentation rate is a measure of the rate of accumulation of the sediment’s mineral component alone. The mineral sedimentation rate was calculated
for each dated depth interval in two ways: (1) by proportionately decreasing the
average sedimentation rate for that interval by the average organic matter content
for the interval; and (2) by calculating a mineral sediment accumulation rate for
the 50 mm depth interval represented by each individual sedimentology sample, by
adjusting the overall average sedimentation rate for the dated depth interval by the
organic content of each sample. All of these procedures, be they for average sedimentation rates for a dated depth interval or for the sedimentation rate indicated
by each individual sample interval, give only approximations of the respective true
sedimentation rates. This is because, first, the sedimentation rate for each dated
depth interval is assumed to be constant throughout that depth interval, and takes
no account of changes in the true sedimentation rate throughout the interval (which
would be revealed by more absolute age determinations within each dated depth
interval). Second, no measure is available of variable compaction throughout the
core. Such variable compaction can be syn-depositional and/or the result of the
coring process. Because of these compaction uncertainties, no correction was
made for the 11% compaction that occurred during coring.
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Microfossil Analysis
Samples for pollen and diatom analyses were taken at 50 – 150 mm intervals down
core AB2. Samples for pollen analysis were prepared following the methods in
Chivas et al. (2001: Appendix G), and the preparation of diatom samples followed
Battarbee (1986). The mounted pollen and diatom samples were analyzed using
conventional light microscopy at ⫻400 – 1000 magnifications. Pollen and spore taxonomy was based on published descriptions (Huang, 1972; Zhang et al., 1990; Tissot
et al., 1994; Wang et al., 1997) and, where possible, comparison with pollen reference samples. General pollen nomenclature follows Punt et al. (1994).
The diatom analysis was intended to provide a rapid assessment of changing
hydrological conditions at the baray locality over time, as indicated by changes in
the proportion of diatoms representative of particular habitat types (specifically,
planktonic versus periphytic taxa). Diatom taxa were therefore identified to genus
level only (Growns, 1999). Diatom taxonomy followed Krammer and Lange-Bertalot
(1986, 1988, 1991) with particular reference to the taxonomic revisions presented
by Round et al. (1990).
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Pollen 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 values
are expressed as a percentage of the entire diatom assemblage. Each microfossil
sequence was classified using the program CONISS (Grimm, 1987; square root
transformation, Euclidean distance, stratigraphically constrained, all unknown
types removed prior to analysis). Microscopic charcoal particle concentrations per
unit volume are based on the factor by which a known number of Lycopodium
marker-spores, introduced to the sample at the start of the procedure, are diluted
within the analyzed material.
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SEDIMENTOLOGY AND CHRONOLOGY OF CORE AB2: RESULTS
Radiocarbon Chronology and Stratigraphy
The radiocarbon chronology is, as expected, consistently older with depth (Table
I). The closeness of the radiocarbon ages bracketing the major stratigraphic breaks
means that the record probably does not contain major time breaks. The X-radiograph of core AB2 (Figure 5a) reveals bedding and other stratification, as well as
bioturbation structures, notably burrows and probably root casts. Faunal burrowing can result in disruption of bedding and significant amounts of vertical displacement of sediments, perhaps even to the extent of major mixing of the sediment
and confusion of the sedimentary record. However, the stratigraphic boundaries
identified both in the radiographs and visually (Figure 5a, b), in addition to the
clear visual evidence of bedding in the cores, and the coherence of the radiocarbon
chronology and the microfossil stratigraphy (see below), demonstrate that any bioturbation has not been sufficient to disrupt the stratigraphic integrity of the sediments in the core.
The record consists of four stratigraphic units (Figure 5). The basal unit, Unit
1, extending from the base of the core to 295 cm depth, consists of thinly laminated
muds 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 muddy
minerogenic 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 deposition
and a coarser-grained mineral fraction, a marked increase in organic content, most
notably 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 succeeded at about 18 cm depth (180 14C yr B.P.) by the muddier and more uniformly
grain size Unit 5.
Sedimentation Rates
The age-depth plot (Figure 6[a]) shows evidence of curvature (p ⬍ 0.01) and the
single straight line model of sedimentation is therefore discounted. Given the in-
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Sample
Code
(C14 Age)
Depth
(cm)
Lower
Upper
Prob.
Lower
Upper
Prob.
Lower
Upper
Prob.
Lower
Upper
Prob.
AA-36871
18
1653
1698
0.206
1723
1816
0.552
1836
1877
0.066
1916
1949
0.176
AA-39136
50
1223
1234
0.025
1235
1327
0.684
1345
1393
0.290
AA-36874
54
1040
1100
0.217
1112
1147
0.111
1151
1260
0.672
AA-47778
102
888
1039
0.991
1105
1107
0.001
1142
1150
0.009
AA-36872
138
438
456
0.038
461
517
0.105
520
645
0.857
AA-36873
140
415
577
0.981
580
591
0.019
AA-36875
299
⫺2192
⫺2173
0.026
⫺2145
⫺1916
0.974
All determinations were AMS determinations on bulk samples of sediment and organic matter. The 2␴ calibrated age ranges (calculated using Calib
v.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 calibration
curve and the probability (Prob.) that the calibrated calendar age lies within a particular interval.
b Calibrated B.C. ranges denoted as negative numbers.
a
RIGHT
AB2/018
(180 ⫾ 35)
AB2/050
(705 ⫾ 50)
AB2/054
(865 ⫾ 40)
AB2/102
(1040 ⫾ 45)
AB2/138
(1495 ⫾ 40)
AB2/140
(1570 ⫾ 40)
AB2/299
(3665 ⫾ 45)
2␴ Calibrated A.D./B.C.b Ranges
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Table I. Radiocarbon dates from core AB2, Angkor Borei.a
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Figure 5a– e. Core AB2, Angkor Borei: (a) line drawing of X-radiograph (B: burrows; V: void); (b) major
stratigraphic 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 measurements
correspond to high moisture contents); and (i) total sedimentation rates and minerogenic sedimentation
rates with 95% confidence intervals (see text for methods); 95% confidence intervals on minerogenic
sedimentation rates are very narrow below 18 cm depth (see Figures 6[b] and 6[c] for more detail on
sedimentation 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 changepoints (“dog-leg”); dotted line– high-order polynomial model. (b) Average sedimentation rates by dated
depth intervals for total sediment. (c) Average sedimentation rates by dated depth intervals for mineral
sediment only. For (b) and (c): Thick central line for each depth interval is the average rate and thin
lines define 95% confidence intervals on the average rate; 95% confidence limits below 140 cm depth are
narrower than the thickness of the line depicting the average rate. Dashed lines indicate the radiocarbondated boundaries between the stratigraphic units.
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Table II. Slopes, errors, and confidence intervals for a piece-wise linear fit for each of the dated depth
intervals (see text).
Depth (cm)
Slope
(cm/cal yr)
Estimated standard error
95% confidence
intervals
0– 18
⫺10.50
0.480
18– 50/54
⫺16.40
0.265
50/54– 102
⫺5.67
0.170
102– 138/140
⫺12.00
0.180
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138/140– 299
⫺15.7
0.050
⫺9.54– ⫺11.46 ⫺15.87– ⫺16.93 ⫺5.33– ⫺6.01 ⫺11.64– ⫺12.36 ⫺15.60– ⫺15.80
Total sedimentation rate
(mm a⫺1)
0.95
0.61
1.76
0.83
0.64
Average organic
matter content
0.137
0.305
0.588
0.502
0.046
Mineral sedimentation
rate (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 and
54 cm depths and at 138 and 140 cm depths, the most reasonable model of sedimentation is derived by combining the two dates in each pair and calculating a
piece-wise linear fit for each of the dated depth intervals. The slope of the agedepth plot for each depth interval is given in Table II with the estimated standard
errors and 95% confidence intervals on the slopes. These slopes are represented as
sedimentation rates in Figures 6(b) and 6(c) (see also Figure 5i). The mineral sedimentation 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 abundance was generally
satisfactory, from a minimum of 70 specimens (132 cm depth) to a maximum of
726 specimens (77 cm depth), with an average of 368 specimens per sample. Classification of the pollen and spore data (Figure 9) indicates eight sample groups or
zones 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 a
broadly similar pollen and spore assemblage (zones 1 – 3 and 6 – 7) are discussed
together. Charcoal particle concentrations, derived from the pollen samples, are
described here also. Note that upper sample in one zone and the lowermost sample
in the overlying zone (e.g., the shallowest and deepest samples, respectively, in
zone 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). Dashed
lines indicate the radiocarbon-dated boundaries between the stratigraphic units; for clarity, 95% confidence 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 microfossil 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 of
41.6% of the primary pollen sum. Rhizophora values decline gradually in the upper
part of zone 3, from 177 cm depth. Zone 1 is distinguished from the overlying zones
by its strong values of Aglaia-type pollen and lower taxon diversity (35 taxa in zone
1 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 representation at 207 cm depth to absence from the assemblage at 157 cm depth.
Similarly, the palm Areca-type, which is consistently represented below 188 cm
depth (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 throughout
zones 1 – 3; it is less common in zone 1 than in zones 2 or 3.
The nonarboreal pollen assemblage is entirely dominated by grasses (Poaceae
maintains 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. Chenopodiaceae 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 relatively
high values of Macaranga (33% of the primary pollen sum) and Uncaria (25% of
the primary pollen sum) pollen, and the decline in, or disappearance of, several
commonly recorded taxa (including Areca-type, Celtis, Elaeocarpus, and Eugenia).
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Combretaceae/Melastomataceae pollen is commonly recorded while Pinus, Rhizophora, and Urticaceae/Moraceae (triporate) are less common. Poaceae and Cyperaceae again dominate the non-arboreal pollen assemblage, and ferns are rare.
Charcoal concentrations fall to 2.8 ⫻ 105/cm3.
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Pollen Zone 5 (127 – 107 cm Depth)
Macaranga is the dominant arboreal pollen type in these samples. The previously
dominant 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/Melastomataceae pollen increases progressively up-core, and Eugenia and Uncaria
are both strongly represented. The abundance of Poaceae pollen declines substantially from the average of 71% in the stratigraphically lower zones, falling to reach
a minimum of 10% of the nonarboreal pollen sum at 107cm depth. Cyperaceae is
the most abundant herbaceous taxon, while the abundance of fern spores increases
markedly. Davalliaceae is the most commonly recorded fern, with the Psilamonolete group and Stenochlaena pallustris spores also very common. Charcoal concentrations are very low in these sediments, with an average value for the zone of
1.1 ⫻ 105/cm3.
Pollen Zones 6 – 7 (97 – 16 cm Depth)
Combretaceae/Melastomataceae pollen dominates these zones, becoming more
abundant 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, Diospyros, Glochidion, and Macaranga all commonly recorded.
Cyperaceae and Poaceae are codominant in zone 6, but Poaceae dominates zone
7. Psilamonolete spores are very common in these zones, increasing to reach a
maximum value of 56% at 27.5 cm depth. Both Stenochlaena pallustris and Lycopodium 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 representation
falls sharply to reach a minimum value of 3% at 58 cm depth. Charcoal particle
concentrations in zones 6 – 7 remain low relative to the early part of the record (an
average of 2.8 ⫻ 105/cm3), but show a slight increase through zones 6 – 7 to reach
a 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 primary pollen sum in these samples. Other common dry-land pollen types recorded
are Celtis, Dipterocarpus-type, Euphorbiaceae undifferentiated, Macaranga, and
Trema (not shown in Figure 8). The secondary pollen sum is dominated by Poaceae, with an average value of 65%, while Cyperaceae is also common at 10%. The
Psilamonolete group is the most common spore type in these sediments. Charcoal
particle concentrations remain low.
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Figure 10. Results of diatom analysis for core AB2. All genera expressed as a percentage of the total diatom assemblage.
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Diatoms
Diatom data are shown in Figure 10, and classification and zonation of these data
are given in Figure 11. Diatoms are not preserved in the samples at 307 and 297
cm 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 average
of 61% of the total diatom assemblage through the zone. Cyclotella is also common,
but declines as depth decreases. Pinnularia increases through the zone to reach
a maximum at 256 cm depth before declining slightly in the uppermost sample of
the zone. Other common genera are Achnanthes, Eunotia, Gomphonema, and
Navicula.
Diatom Zone 2 (237 – 147 cm Depth)
Aulacoseira remains the dominant genus, with an average value for the zone of
54%. The abundance of Cyclotella declines steadily as depth decreases, continuing
a trend observed in the previous zone. In contrast, the number of individuals within
Figure 11. Classification and zonation of diatom data from core AB2. Dotted lines represent zone
boundaries, dashed lines represent divisions within zones.
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the 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 demonstrate a discrete increase in abundance at 188 cm depth. Other common genera
recorded are Actinocyclus, Chaetoceros, Cymbella, Gomphonema, Navicula, and
Pinnularia.
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Diatom Zone 3 (132 – 16 cm Depth)
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, Diadesmis
is clearly the most abundant genus, reaching a maximum value of 55% at 37 cm
depth. In contrast, the previously dominant genus Aulacoseira is relatively poorly
represented, with an average of only 4% of the total assemblage. Eunotia increases
as depth decreases, reaching a maximum of 30% at 37 cm depth. Navicula increases
sharply 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 average
values of 44% and 23%, respectively. Navicula is subdominant, with Amphora, Aulacoseira, and Eunotia commonly recorded also.
DISCUSSION
The Stratigraphic Record
All of the sedimentological/stratigraphic boundaries coincide with zonal boundaries in the pollen and/or diatom records (Figure 12). The microfossil zonal boundary at 242 cm depth in both the pollen and diatom records is not recorded in the
sedimentological stratigraphic data, however, and the microfossil zonal boundaries
at 130 cm and 102 cm depths are found only in the pollen record. The microfossil
data, therefore, provide a more detailed stratigraphic subdivision of the core.
The diatom data indicate that the core locality has supported standing water for
the 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 natural water body that was subsequently exploited to create the baray.) The low
diatom levels at various intervals in the AB2 record are closely related to periods
of falling or low pH (Figure 5g) and are therefore probably related to chemical
dissolution of the silica frustules at these times rather than to mechanical degradation associated with shallow or dry periods. The indication of essentially continuous sedimentation throughout the full period of the core is consistent with the
generally short time periods encompassed by the paired radiocarbon determinations either side of the major stratigraphic boundaries (Table I). The only apparent
break in sedimentation, marked by roots and possible subaerial desiccation cracks
or pedal voids, is at 18 cm depth in the core and is suggestive of subaerial exposure
of 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 microfossil zone boundaries given here and in Figures 8– 11 are plotted midway between the sample depths
that define subjacent zones. The microfossil zone boundaries are therefore not precisely comparable to
the boundaries of the stratigraphic units which are defined on the basis of visual inspection and depth
measurements 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 clear
pattern of hydrological change over time. The absence of diatoms in the basal
samples at 307 and 297 cm depth is coincident with low pollen concentrations (an
average of 15,708 grains/cm3 against an average of 44,058 grains/cm3 for the total
sample population), the deposition of laminated sediments and relatively acid
chemistry (an average sediment pH of 4.7 between 290 and 300 cm depth). These
data suggest that the locality held shallow water on an episodic basis in the earliest
part of the AB2 record, possibly in association with seasonal floods as signaled by
the laminated sediments. The low sediment pH values may reflect water chemistry
changes during falling/evaporative phases in this seasonal hydrology. While diatoms are not preserved under these conditions of low pH and seasonal inundation,
pollens are preserved and are dominated by Aglaia-type, Rhizophora, and Poaceae.
This may reflect the presence of some form of humid forest in the area, with tidal
forest along streams and other watercourses, and grasses probably growing on and
around the locality.
The dominance of the tychoplanktonic genus Aulacoseira in diatom zones 1 and
2 signals a shift from periodic inundation to permanent standing water that was
highly turbid (Figures 10 and 13). This genus is common or dominant in the phytoplankton of large and nutrient rich rivers (Krammer and Lange-Bertalot, 1991;
Hötzel 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 of
holoplanktonic genera, particularly Cyclotella, corroborates the presence of a permanent water body. The early part of the record, then, intimates the occurrence of
a substantial hydrological change around 3500 14C yr B.P., from seasonal inundation
with laminated sedimentation and acid waters to a permanent full-lake phase dominated by mud sedimentation. The reasons for this change are unknown, but might
relate either to changes in the morphology of channels in the area, or to deliberate
damming of the locality by people. The steady decline in obligate planktonic genera
over time through diatom zones 1 and 2 is consistent with a gradual reduction in
water depth (that is, a restriction of planktonic habitat), presumably as a result of
the natural infilling of the basin.
In this early part of the record, the strong representation of Rhizophora pollen
and the presence of taxa such as Bruguiera, Nypa, Sonneratia, Xylocarpus, and
possibly Chenopodiaceae, are indicative of mangrove forests in the northern part
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of 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 summer
monsoon, but the abundance of Rhizophora pollen (between 8 – [38.9] – 55 % of the
primary pollen between 307 – 127 cm depth) argues strongly for the local occurrence of this community type. Crowley et al. (1994) found that Rhizophora values
comparable to those described from core AB2 (around 30%) indicate transitional
habitats on the landward margin of true tidal mangrove communities. Interestingly,
there is no clear evidence in the microfossil data to indicate saline surface water
around Angkor Borei. The relatively high values of Chenopodiaceae in pollen zones
2 and 3 may indicate higher regional salinity, but values are not sufficiently high to
indicate convincingly the occurrence of salt-marsh close to the city. The diatom
flora contemporary with occurrence of mangroves (diatom zones 1 – 2) indicate
fresh or possibly fresh-brackish, well-mixed, nutrient-rich waters. Indeed, there is
no compelling evidence that surface waters were highly saline at any time in the
AB2 record. It is probable, then, that mangrove communities were growing extensively along watercourses close to Angkor Borei during the Late Holocene. This
interpretation is consistent with a pollen record from the Tonle Sap Lake which
shows that, during the Holocene sea-level transgression, mangroves penetrated far
inland along the channels of the Mekong/Bassac Rivers and tributaries (Penny,
unpublished data). It is probable that the declining values of Rhizophora pollen in
zones 1 – 3 reflect the southward migration of mangrove forest in response to either
a regressive phase of sea-level change (Ta et al., 2001), possibly in combination
with flexural uplift of the northern Mekong Delta in a manner analogous to the
eastward tilting of the Ganges River delta (Blasco et al., 2001).
The most dramatic changes in the sedimentological and microfossil records occur at about 139 cm depth, at the abrupt transition between stratigraphic units 2
and 3, diatom zones 2 and 3, and pollen zones 3 and 4. This change is bracketed
by radiocarbon ages at 140 cm (AA-36 872) and 138 cm (AA-36873) depths (Table
I). At this transition, Aulacoseira is greatly reduced in abundance, and genera that
occur on various submerged (periphytic) or aerial (aerophilous) substrata become
dominant. Also at this time, Combretaceae/Melastomataceae pollen starts to increase, while secondary forest taxa, particularly Macaranga, become dominant.
An extremely rapid decline in the relative abundance of Poaceae is apparent, while
the representation of fern spores in the sediment increases markedly (Figure 14).
All of these changes are synchronous with a dramatic decline in the concentration
of microscopic charcoal particles being deposited in the sediment, intimating a
change in the local fire regime, probably to more infrequent burning. Moreover,
material being deposited in the basin at this time is predominantly organic (Figure
5f), presumably reflecting an increase in vegetation growth in and around the locality. This increase in organic sedimentation at the 140 – 138 cm transition is reflected in the increase in average total sedimentation rate this transition (Figure
6[b]) and a corresponding decrease in the average mineral sedimentation rate (Figure 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. Ferns
are epiphytic (closed squares), ground or twinning (closed triangles), and ungrouped ferns (open squares). The scale for charcoal contents
refers to the black plot; grey plot is a 5⫻ exaggeration of the measured values.
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detailed sedimentation rates), sedimentation rates by sample apparently become
much more variable in the organic unit (ca. 140 – 50 cm), fluctuating more widely
about 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 a
reduction in the frequency of burning at this time, or if the reduction of grasslands
is 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 the
fire-grass feedback loop (Vila et al., 2001), thereby permitting the expansion of
more fire-intolerant vegetation. The expansion of secondary forest taxa in pollen
zones 4 and 5 (particularly Macaranga and other Euphorbiaceae) seems to indicate
the colonization of areas that previously supported grasses under a regime of
regular burning. It is possible that these early successional changes may reflect
a 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 it
is 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 in
organic sedimentation from 139 cm depth, does not appear to have a substantial
herbaceous or aquatic component. There is only a very slight increase in Cyperaceae pollen at 132 and 127 cm depth, and none of the aquatic or littoral-swamp
plants demonstrates any increase in pollen values at this time. It is likely that
swamp forest trees, probably members of the Combretaceae/Melastomataceae
group, began to colonize the locality from this time. The dramatic increase in the
abundance of aerial diatoms at 132 cm depth is consistent with this development,
with genera such as Diadesmis and Luticola taking advantage of an effective expansion of habitat afforded them by the moist trunks of colonizing swamp forest
plants. Similarly, the marked increase in the representation of fern spores, particularly the epiphytic species Stenochlaena pallustris, is also consistent the development of moist, shaded conditions beneath the closed canopy of invading swamp
forest trees.
The dramatic shift in the diatom flora from the dominance of tycho- and holoplanktonic forms to periphyton dominance also indicates substantial hydrological
changes at this transition (Figure 13). Simplistically, the decline in abundance of
Aulacoseira is indicative of less turbid waters. The expansion of attached genera
might also indicate an increase in light availability to the bed associated with a fall
in turbidity. These changes may be the result of a decrease in water depth, or the
isolation of the locality from fluvial inputs, such as might be expected following
the abandonment or artificial closure of river channels.
Diatom preservation ceases from approximately 127 to 117 cm depth, most likely
as a result of post-mortem dissolution in response to increasing acidity in the highly
organic peat (Unit 3) (Ryves et al., 2001). Spores of the twinning fern Lygodium
microphyllum occur abundantly between 97 and 58 cm depth. This plant is known
to grow on exposed sites, dry slopes, and among exposed herbaceous swamp (Holttum, 1959; Tagawa and Iwatsuki, 1979). It is also an aggressive and invasive weed
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outside 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 forest
clearance in similar environments in Southeast Asia (Higham and Bannanurag,
1991:93 – 94). The increase in the representation of its spores, then, may reflect a
degree of “opening” of the canopy in the area around the locality, possibly in response to disturbance. This interpretation finds some support in the influx of sand
at about 100 cm depth and the higher rates of mineral sedimentation between 100
and 50 cm depth (the second highest average rates and the highest, and most
variable, individual rates in the whole core; Figures 6[c] and 7). The transition at
100 cm depth, between pollen zones 5 and 6, in fact marks the end of the transitional
period to full successional development of swamp forest, and coincides with a
further increase in organic production (peat) and faster decline in pH. The high
sedimentation rates, the suggestion of some opening of the canopy, and the influx
of sand at 100 cm depth might all be taken to indicate disturbance of the core
locality, but there is little evidence to support this contention in the full pollen
assemblage, save for the variable representation of Mallotus pollen in this part of
the sequence. Indeed, the strong evidence for swamp forest covering the locality
suggests the contrary.
Diatoms frustules begin to reappear in the record from 47 to 37 cm depth, probably reflecting the reduction in the acidity of lake deposits at these depths (Figure
5g). The character of the diatom assemblage appears to have changed very little
over the intervening ca. 800 14C years (i.e., between AA-36872 and AA-39136; Table
I), intimating a degree of stability over this time period.
Mineral sedimentation rates fall off very abruptly at the 50 cm boundary and fall
steadily to 18 cm depth (Figures 6[c] and 7), perhaps reflecting a combination of
well developed swamp forest at the locality and a cutting-off of the locality from
sediment influxes. Combretaceae/Melastomataceae pollen reaches peak abundance between 37 and 27.5 cm depths, suggesting that the parent species must have
been extremely common around the locality at this time, and throughout the local
area. The deposition of sands at 25 cm depth (Figure 5c) may indicate a degree of
disturbance close to the locality, such as the excavation of material for moat or
baray wall construction, or that high-energy floods maintained some connection
between the locality and rivers in the area.
The stratigraphy in the upper part of the core indicates the growth of herbaceous
plants directly on the core locality (evidenced by root penetration from a soil horizon at 18 cm depth), suggesting that the locality was either permanently shallow
or seasonally dry. The very poor representation of planktonic diatom genera in
these sediments supports this interpretation. The locality appears to have been
flooded thereafter, presumably through the deliberate redirection of surface waters
either from the east moat of the city or elsewhere. A clear decrease in the representation of Combretaceae/Melastomataceae pollen and increases in both grasses
and sedimentary charcoal particles above 7 cm depth are suggestive of clearing,
while the presence of the invasive exotic weed Mimosa pigra suggests regular
disturbance around the locality. This plant currently grows on the banks of the
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eastern 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 the
whole core, but also the most variable individual mineral sedimentation rates. The
sustained high values of individual sedimentation rates in the upper 18 cm of the
core are consistent with disturbance of the surrounding area and transport of sediment across the water body to the core locality, whereas the high but more variable
rates in the 100 – 150 cm depth interval are perhaps more consistent with pulses of
sediment deriving from natural flood events.
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Possible Evidence for Anthropogenic Activity
As described above, microscopic charcoal concentrations are substantially
greater in the lower half of the core, rising rapidly after the cessation of deposition
of the basal laminated muds at about 2000 cal. yr B.C. (299 cm depth). These higher
charcoal levels were sustained until the abrupt decline at the dramatic ecological/
stratigraphic change at 139 cm depth, which is bracketed by the radiocarbon determinations at 140 and 138 cm depths. The 95% calibrated range of these determinations is the 5th – 7th centuries A.D. (Table I), and the median ages of their
highest probability calibrated age ranges are A.D. 496 (140 cm) and A.D. 583 (138
cm). This late 5th to late 6th century A.D. change was associated with expansion
of swamp forest plants, peat formation, and, in the surrounding area, decreased
intensity of land-use (as signaled by the decreased charcoal concentrations, reduction in grasslands, and the expansion of regrowth taxa).
These data may be interpreted in several ways, in terms of changes both at the
core locality itself and more regionally. The development of swamp forest at the
core 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 of
swamp forest to establish. The more regional data, such as changes in the extent
of grasslands, inferred changes in the frequency of fires and the development of
secondary 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/6th
century A.D. After this time, the levels of land-use intensity never attained the pre5th/6th century A.D. levels except perhaps in the uppermost 18 cm of the AB2
record. Another interpretation, however, is that the changes in the regional pollen
and charcoal evidence signal a change away from land-use that employed extensive
burning. Such a change might have involved, for example, a move away from cultivation of rice in burned fields, with dry season burning, to flood recession cultivation 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 of
the Phnom Da art style from the mid-6th century A.D. (Phnom Da being a temple
about 1km south of Angkor Borei; Figures 1 and 2), as well as inscriptions from
the site from the early 7th century A.D., all suggest that Angkor Borei was still a
sufficiently important center to merit substantial economic and social investment
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until 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 than
in 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 sedimentation 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 continued
to flourish amid a landscape supporting a different (less intensive?) agriculture.
As already noted, charcoal levels after the 6th century A.D. never again attained
the pre-5th/6th century A.D. levels, but there is a steady, statistically significant
increase in charcoal concentrations up-core from the minimum concentration at
115 cm depth. A minor peak in charcoal concentration at 97 cm coincides with an
influx of sandy material to the core locality, perhaps signaling local anthropogenic
disturbance. As already noted, however, the highly variable rates of sedimentation
in the 100 – 50 cm interval might be taken to be more indicative of natural, flooddriven incursions of water and sediment to the coring locality (Figure 7).
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Baray Construction
The diatom data demonstrate that the core locality has been characterized by
standing water for the whole of the AB2 record between ca. 300 cm and 18 cm
depths. It seems, therefore, either that the locality has been an artificially constructed reservoir (baray) for essentially the entire period of the record, or that
baray construction at some point over the past 3000 – 4000 years simply represented the modification of an existing landform, in this case probably an abandoned
meander loop (oxbow lake). The paleochannel mapped by Haruyama (1998) as
intersecting the northeast corner of the baray is presumably the paleochannel that
was modified in baray construction. It is possible that this body of standing water
acted as a source of fresh water for the adjacent occupation site prior to baray
construction.
Exploitation of a pre-existing body of standing water in construction of the baray
is consistent with the baray floor being up to 4 m lower than the ground level
outside the reservoir (Figures 4b and 4c). The Angkor Borei baray is therefore
different 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 on
a gently sloping surface, would not have resulted in the relative elevations of baray
bed and baray exterior that are found at Angkor Borei. The exploitation of an
existing body of standing water to construct the baray presumably involved the
regularization of its outline and the construction of the baray walls. This approach
to baray construction may reflect a degree of pragmatism in the construction of
this reservoir enforced, perhaps, by limited labor and reflecting the nature of the
local environment. Moreover, water is more available in the delta (and for longer
during the year, reflecting the maintenance of flows to the delta by the draining of
Tonle Sap; MRCS/UNDP, 1998) than in other parts of Cambodia, and there may
have been less need at Angkor Borei to construct the major water retention features
found at Angkor.
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The 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 indicative of baray construction. It is reasonable to suggest that baray construction
would be signaled by open water and aquatic vegetation at the core locality, as
well as pioneer species and disturbance indicators in the local vegetation, indicative of the species growing on the newly constructed baray embankments. Increased sedimentation resulting from in-wash from the new embankments might
also be expected. As noted above, the major change signaled in the AB2 sediments
is in the late 5th to late 6th century A.D., corresponding to a de-intensification of
land-use and perhaps land abandonment, or possibly a change in land-use priorities
or techniques. Thereafter, until the stratigraphic break at 18 cm depth, the locality
was gradually colonized by closed, fern-rich swamp forest that was not cleared
until 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␴ calibrated
age 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 after
the break at 18 cm depth coincides with a decrease of Combretaceae/Melastomataceae and increases in the representation of dryland herbs, both of which changes
are suggestive of forest clearance. The strong presence of the invasive exotic weed
Mimosa pigra, currently growing on the banks of the eastern baray, is likewise
suggestive of disturbance and clearing. Disturbance and clearing are also consistent
with the onset and maintenance of sustained high mineral sedimentation rates at
the 18 cm stratigraphic boundary (Figures 6[c] and 7). The change across the 18
cm boundary in mineral sedimentation rates by individual sample is the second
largest individual increase recorded in the core, and coincides as we have seen
with the change to a core locality cleared of swamp forest and with a vegetation
indicative of disturbance. All of these data suggest that the baray walls date from
some time in the 17th to 19th century or, if the full 95% calibrated age range is
used, even later.
The only other time that the core locality could have been characterized by the
open water of a baray is prior to the late 5th to late 6th century A.D. change
recorded 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 late
6th century A.D., in which case it was abandoned in the late 5th to late 6th century
and was completely colonized by swamp forest. We cannot unequivocally exclude
this possibility at this stage, but there is no palaeobotanical evidence for disturbance around the core locality prior to the 5th/6th century changes that might
indicate the excavation and mounding-up of earthen walls (unlike the mid 17th to
early 19th centuries, when such evidence is unambiguously apparent). This, in combination 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 construction of the baray walls. The high rates of sedimentation in the 100 – 50 cm
depth interval might be suggestive of in-wash from baray walls, but the swamp
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vegetation at the core locality is completely inconsistent with the locality being an
artificial reservoir at this time.
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CONCLUSIONS
The Angkor Borei site is located in a fluvial-deltaic environment. Radiometric
dating of a sediment core from the eastern part of the city has revealed a long
depositional sequence dating from before the 2nd millennium B.C. Microfossil and
sedimentological analyses indicate that the core locality was originally a natural
depression, probably an abandoned meander loop. The east baray appears therefore to have been constructed by modifying a paleochannel remnant, and is thus
quite unlike the reservoirs of the Angkorian capitals to the northwest in both the
means of construction and, presumably, function. The date at which the walls of
the baray were constructed remains uncertain. However, we suggest that the most
convincing evidence points to a date sometime between the mid 17th to early 19th
century A.D., meaning that the east baray is essentially a modern feature and not
contemporary with the ancient Funan city of Angkor Borei. This date may reflect
the 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 sufficient
to feed large populations in the Mekong Delta, without recourse to large irrigation
networks (van Liere, 1980; Fox and Ledgerwood, 1999).
Microfossil data indicate that the site on which the city was founded in the 4th
century B.C. lay inland of southward-migrating mangrove forests that probably
occurred extensively in the area as a riparian forest. Substantial vegetational and
hydrological changes are apparent from the 5th to 6th century A.D., indicating a
period of changing land-use practices or priorities. The core locality was progressively invaded by swamp forest and was largely dry by A.D. 1735, after which time
it 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 cores
were imported into Great Britain under quarantine license #IMP/SOIL/19/1999. 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.
Thanks also to the following archaeologists for field assistance with the baray research during the 1999
and 2001 field seasons: Bong Sovath, Mitch Hendrickson, Sok Kimsan, Mâm Vannary, Chea Sopheary,
and Pich Thyda. We also thank Weipers School of Veterinary Science, University of Glasgow for the
core X-rays, and the British Geological Survey, Edinburgh, for use of the magnetometer. The following
Glasgow colleagues assisted us greatly, for which we are grateful: Peter Chung and Allen Jones (sedimentological analyses); Jane Drummond and Anne Dunlop (photogrammetry); and John Shearer, Mike
Shand, and Peter Chung (cartography and diagrams). The late Dr. Yasushi Kojo kindly translated Haruyama (1998) for us. We also thank Mitch Hendrickson for assistance with the hydrographic and crosssection surveys of the eastern baray, Dr. Pauline Reimer for advice on radiocarbon dating and manipulation of calibrated ages, and Dr. John Grindrod for discussion on mangrove ecology and palynology.
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Received April 2, 2002
Accepted for publication September 17, 2002
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