DSpace at VNU: Recent sedimentation and sediment accumulation rates of the Ba Lat prodelta (Red River, Vietnam)

Journal of Asian Earth Sciences 29 (2007) 545–557
www.elsevier.com/locate/jaes

Recent sedimentation and sediment accumulation rates
of the Ba Lat prodelta (Red River, Vietnam)
G.D. van den Bergh a,¤, W. Boer a, M.A.S. Schaapveld b, D.M. Duc c, Tj.C.E. van Weering a
a

Royal Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands
b
Shell Todd Oil Services Ltd., Private Bag 2035, 4620 New Plymouth, New Zealand
c
Ha Noi University of Science, 334 NguyenTrai, Thanh Xuan, Ha Noi, Viet Nam
Received 8 September 2003; received in revised form 21 February 2005; accepted 2 March 2006

Abstract
The Ba Lat River is the major distributary of the Red River system in North Vietnam. To assess the recent to subrecent depositional
processes in the Ba Lat prodelta, a detailed sediment analysis was conducted. Bottom samples were collected during two Weld surveys, one
in the dry season (winter) and one in the wet season (summer). A steep frontal prodelta slope is characterized by very rapid sedimentation
(tens of cm per year) of muddy sediments under inXuence of the turbid river plume. Beyond direct inXuence of the river plume the bottom
slope decreases and bottom transport by the prevailing southward directed currents becomes important. Coarse-grained tempestites alternate with the dominating muddy sediments. Downcore changes in the 234Th activities indicate that the subaqueous delta progrades to the
southwest, with erosion and reworking of older sediments occurring north of the present outlet. The southwestward progradation is also
encountered in the trend of 210Pb activities indicating that this process has continued for at least 100 years. Avulsion of the Ba Lat outlet
in 1973 has led to a decrease in sedimentation rates north of the Ba Lat outlet.
© 2006 Elsevier Ltd. All rights reserved.
Keywords: Recent sedimentation; Red River Delta; Vietnam; 210Pb dating; 234Th analysis; Prodelta; Tempestites

1. Introduction
The Ba Lat is the main distributary of the Red River system in North Vietnam (Fig. 1). The coastal plain near the
Ba Lat outlet has accreted over a distance of 23 km during
the last 500 years, and is build up by an alternation of fossil

beach-spit systems with back barrier swamp deposits in
between (Thanh et al., 1997). The river discharge follows a
clear seasonal pattern reXecting the variation in rainfall
under the constraint of a monsoonal climate. Besides seasonal diVerences, the inter-annual variation in suspended
sediment transport varies between 30 and 120 million tons
per year passing Son Tay. Environmental changes, both
anthropogenic as well as natural induced changes, have a

*

Corresponding author. Tel.: +31 222 369 394; fax: +31 222 319 674.
E-mail address: (G.D. van den Bergh).

1367-9120/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2006.03.006

high potential to aVect the coastal zone of the Ba Lat Delta.
The main Ba Lat channel debauched at its present position
before 1938 and after 1973. From 1938 until 1971 it entered
the sea 10 km more to the north, when during a severe Xood
in August 1971 its location shifted to a position south of the
present outlet. During the typhoon Kate in 1973 the frontal
sand barrier broke through and the main outlet started to
occupy its present position (Thanh et al., 1997).
There are indications that the frequency of typhoons
aVecting the coast of Vietnam has increased during the second half of the 20th century (Thanh et al., 1997). Furthermore, the construction between 1979 and 1994 of the Hoa
Binh Dam in one of the three major tributaries of the Red
River (Fig. 1A), has nearly halved the average annual suspended sediment concentration at Son Tay gauging station
(van Maren, 2004). These are some of the factors that are
expected to inXuence the densely populated coastal zone of

the Red River in the coming decades, and the development


546

G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

Fig. 1. (A) Map of Vietnam with location of the study area and (B) map of the study area showing a sub-division of the Ba Lat prodelta (various shaded
zones), based on the acoustic study of van den Bergh et al. (2006). Also shown are the positions of bottom sampling stations. Hatched area represents the
zone of maximum accumulation, where the most recent acoustic unit has a thickness in excess of 2 m.

of sustainable coastal zone management becomes increasingly necessary. Integrated coastal zone studies provide useful information not only in documenting modern terrestrial
and marine environments, but also in understanding erosion patterns within geological and historical contexts (Milliman et al., 1987).
Our main objective within the framework of the Red
River Delta Research Program was to gain an understanding of the sedimentological processes that govern the development of the present Ba Lat prodelta. The study is based
on the analysis of bottom samples recovered from the Ba
Lat prodelta. The sedimentological and geochemical
imprints in the sedimentary record were analyzed in order
to reconstruct the processes that resulted in the present
conWguration of the prodelta on a 100-years time scale.
Analysis of the downcore 210Pb activity has been performed on a number of gravity cores, to assess spatial variability in recent accumulation rates on a »100 years time
scale. Analysis of excess 234Th has been applied on box core
samples, in order to determine seasonal diVerences in deposition, re-suspension, and mixing-processes of the surface
sediments. Other parameters that were analyzed are the
composition, grain-size distributions and organic carbon
and nitrogen contents of the sediments. A study on acoustic
facies analysis and prodelta geometry is presented elsewhere in this volume (van den Bergh et al., 2006). For the
hydrodynamical and climatic conditions governing the
delta development the reader is referred to van Maren and
Hoekstra (2004, 2005) and van Maren et al. (2004).

2. Delta setting
Based on the acoustic study carried out by van den Bergh
et al. (2006), a morphogenetic subdivision of the study area
was made (Fig. 1B). This subdivision consists of: (1) delta
front, (2) prodelta, (3) Gulf of Tonkin Shelf, and (4) erosional
shoreface zones. The delta front fringes the delta plain and is
marked by a slope break between 5 and 7 m water depth at
the transition with the prodelta. The delta front has not been
sampled in the course of this study. The prodelta forms a relatively steep muddy slope that merges in to the shelf of the
Gulf of Tonkin around the 30 m isobath. The recent prodelta
deposits are recognizable on the acoustic proWles as a dark
band of high reXectivity with multiple sub-bottom reXectors,
which become weaker and gradually converge with the bottom reXector in oVshore direction. Based on bottom gradients, bottom proWle, and the relative position with respect to
the Ba Lat River mouth, the prodelta can be subdivided into:
(A) the Northern Prodelta, (B) the Frontal Prodelta, and (C)
the Southern Prodelta (Fig. 1). The Frontal Prodelta, located
adjacent to the Ba Lat mouth, is only 6 km wide and has the
steepest bottom gradient of »6.5 m/km between 12 and 23 m
water depth. Towards the South the prodelta rapidly widens
to more than 20 km across and bottom gradients decrease to
»1.5 m/km. The Southern Prodelta is detached from the
coast. Towards the North the prodelta widens as well, but
near coastal bottom gradients remain relatively steep
(»4.5 m/km). The Northern Prodelta is also detached from
the coast by a 2–3 km wide erosional shoreface.


G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

3. Methods
In 2000 Weld campaigns where conducted in February–
March and July–August, during the dry and wet monsoon,
respectively. Bottom sampling stations were selected based
on a preliminary study of the shallow penetrating acoustic
proWles (van den Bergh et al., 2006). A gravity corer with a
length of 2 m and a diameter of 9 cm was used. Gravity coring stations are shown in Fig. 1, and a list of all cores with
their coordinates is presented in Table 1. At the Weld station
cores were split, photographed and described macroscopically. Magnetic susceptibility was measured at 1 cm intervals with a handheld Bartington MS2E1 surface sensor.
Sub-samples with known volume were taken at 5-cm intervals for standard Dry Bulk Density (DBD) measurements.
The top part of each core was more intensely sampled for
210
Pb analysis. Then the cores were covered with plastic foil
and sealed in plastic and stored horizontally at a temperature of 5–7 °C. The intact core halves were shipped to the
NIOZ in the Netherlands for X-ray photograpy and further
analysis (granulometry, XRF, Corg and C/N ratios).
The XRF Cortex-corescanner, is a non-destructive,
semi-quantitative logging instrument for major element
determination (Jansen et al., 1998). The elements Fe and Ca
were analyzed at 1-cm intervals. Stereo X-ray photographs
were made of selected intervals. Based on a study of the
photographs, additional sample levels for grain-size and,
for cores 2 and 15, Corg and C/N ratio analyses, were
selected besides standard intervals of 5 cm.

547

Grain-size analyses were performed on 15 cores using
a Coulter LS 230 analyser. Approximately 0.1 g of sediment was weighted in glass beakers and mixed with 15 ml
of tap water. The samples were put in an ultrasonic bath

for 5 minutes. Then the sample was passed through a
2 mm sieve and measured under continued ultrasonic
treatment.
Organic carbon and nitrogen contents were determined
on samples from cores 2 and 15, using a Carlo Erba NA1500 series 2 Nitrogen Carbon Sulphur Analyser. Sample
treatment and analysis was according to the method of Verardo et al. (1990). The data are presented as weight percentages of organic carbon and nitrogen versus depth and C/N
versus depth.
Measurements for 210Pb analysis were made following
the methods outlined in Boer et al. (2006). Coarse-grained
intervals, recognized by the dry bulk densities or macroscopic descriptions, were omitted for 210Pb analysis. The
best model Wts through the data points were calculated
using the Constant Flux and Constant Sedimentation
(CF–CS) model (Appleby and OldWeld, 1992; Boer et al.,
2006). For gravity cores 6, 10 and 15 the supported 210Pb
activity, as deWned by 226Ra, was determined by analyzing
the 226Ra activity using gamma spectrometry, according
to the method outlined in van den Bergh et al., 2003). In
several cores with very high accumulation rates, supported 210Pb activities were not reached at the base of the
core. If no 226Ra-based supported activities were available, model Wts were obtained with pre-deWned supported

Table 1
List of Red River Delta gravity cores sampled in 2000
Core

Sampling date

(UTM zone 48Q)
Easting

Northing

1
2
3
6
7
8
8b
9
10
11
12
13
15
16
18
21
22
23
25
26
27
29
30
31
32
33

March 12
March 12

March 12
March 13
March 13
March 13
March 13
March 13
March 18
March 18
July 22
August 6
August 6
August 6
July 22
July 22
July 22
August 6
August 6
August 6
August 6
August 7
August 7
August 7
August 27
August 27

672225
671893
670983
655824
652584

648589
647683
645558
674960
676499
675985
666934
658934
654725
673363
675408
687998
656885
651707
662166
665910
681869
683876
678874
678113
676988

2232724
2233551
2234186
2216655
2219909
2223832
2225116
2227512

2249079
2239790
2233428
2223748
2222861
2226651
2228744
2254562
2251230
2224829
2212073
2214096
2220296
2244712
2244781
2251021
2225223
2227083

a

Core not located along acoustic transect: water depth is estimated.

Water depth (m)

Core length (cm)

26
25
21,3

24,6
21.3
16
13
10,7
20
27.5
29.5
25
22
17
29.5
17.5
28
20
30a
28.5
27.5
29.5
30
22
32
31.5

69
162
155
161
115
Sandy bottom

Sandy bottom
§15 stiV clay
74
86
105
29.5
141
111
147.5
121.5
Sandy bottom
139
131.5
135
56
141.5
141.5
167.5
34
25


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G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

activities of 36.4 and 41.3 Bq kg¡1, based on the minimum
and maximum 226Ra-based supported activities measured
in other cores. The development of a surface mixed layer
(SML) was not always evident in core proWles. In case of

doubt solution Wts were forced both with and without
SML, and the calculated variables resulting from the various solutions are presented as a range in tables and
Wgures.
In four cores (2, 10, 12 and 18) vertical grain-size Xuctuations appeared to result in highly irregular 210Pb activity
plots, due to rather variable speciWc surface areas in these
samples. In these cores 210Pb activities were measured on
the <25 m fractions.
Boxcores for 234Th analysis were retrieved (Fig. 1B)
using a cylindrical boxcorer with an inner diameter of 12
and 38 cm length. Some stations were sampled during
both seasons in order to assess seasonal variability. Only
cores were used that showed no signs of disturbance
upon retrieval. On ship deck an 11 cm-diameter plastic
pipe was pushed into the sediment and sealed at both
ends after the water on the surface of the sediment had
been removed. Boxcores were transported vertically to
the Weld station, where they were sliced. The freeze–dried
samples were analysed for 234Th using -spectrometry
following the procedure as mentioned in Schmidt et al.
(2002).
4. Results
The most recent depositional unit that is recognizable on
the acoustics has been mapped separately as Unit Z1 (van
den Bergh et al., 2006: Fig. 6). The area of major recent
accumulation, where Unit Z1 has its thickest development
of >2 m, is indicated hatched in Fig. 1B. This area of major
accumulation covers the steep frontal delta slope and a
large proximal area of the Southern Prodelta. Gravity cores
3, 15, 16 and 23 are located in this area of major accumulation. Stations 1, 2, and 12 cover the more distal areas of the
Frontal Prodelta. Stations 6–8, 13–14, and 24–27 are

located in the distal parts of the Southern Prodelta. At stations 8 and 8B stiV, Wne-grained sandy sediment was
retrieved in the core catcher.
Cores 10–11, 21 and 29–31 penetrate the Northern
Prodelta. Several short cores (stations 22, 18, 32 and 33)
sampled the surface sediments of the Gulf of Tonkin
Shelf. At station 22 coring failed but some coarse sandy
material was recovered in the core catcher. Station 18 is
located in an oVshore depression, which is bounded to the
west by a SSW-NNE trending fault with surface expression at the bottom and to the east by a convex ridge
(Fig. 1B).
The erosional coastal zones were not sampled, but are
clearly revealed on the acoustic proWles (van den Bergh
et al., 2006: Fig. 3A). At station 9, located between the Hai
Hau coast and the Southern Prodelta, multiple coring
attempts resulted in the retrieval of only 15 cm of stiV, wellconsolidated mud. The acoustic proWle that runs along this

station shows inclined sub-bottom reXectors that are truncated by the bottom reXector, indicating erosion of older
prodelta deposits.
At 12 stations boxcores were retrieved, either during the
dry season, the wet season, or during both seasons
(Fig. 1B). The boxcore retrieved at station 3 during the dry
season at 15.5 m water depth showed fresh elongated parallel scours of several mm deep at the surface, indicating erosion by strong bottom currents.
4.1. Sediment characterization
Data compilations of lithology and various analyzed
parameters are shown in Figs. 2A–C and 7. Muddy sediments from the Ba Lat prodelta can be easily distinguished
from the contrasting sandy deposits covering the Gulf of
Tonkin Shelf. The modern prodelta deposits have a reddish
brown color, whereas the sandy shelf de posits have a
greenish gray color, are coarser-grained, and contain abundant shell debris.
The prodelta sediments contain quartz, feldspar and

mica as major grain types in the coarse silt and Wne sandy
fractions. Silt-sized detrital grains are mostly covered
with a reddish brown coating of Fe-oxides. XRD analysis
on bulk samples indicated the presence of 14 Å clay
minerals, mostly chlorite. Kaolonite is probably also
present in the clay fraction, besides hematite. Calcite
shows very weak intensities on the XRD diagrams.
The contrasting color diVerence between the prodelta
muds and the sandy shelf deposits is caused by relatively
low amounts of Fe-oxides in the latter. The XRF measurements demonstrate the relatively low amount of Fe minerals in these sandy shelf deposits (e.g. Fig. 2B: base of core
18), and the low Fe contents also correlate well with very
low magnetic susceptibility values. No hematite could be
demonstrated to be present in the sandy shelf samples.
Instead, pyrite is more prominently present in these sediments. At stations along the distal margin of the prodelta,
where cores penetrated the muddy prodelta deposits into
the underlying shelf sands, the transition between both sediment types was always found gradual and marked by a 10–
20 cm thick interval of heavily bioturbated and mottled
sediments.
The prodelta deposits consist predominantly of muddy
sediments with a few thin (<5 cm) coarser-grained intercalations. Median grain-sizes are mostly between 5 and 10 m,
whereas the thin-bedded coarser-grained intercalations
have 90% of their volume in the range smaller than 100 m
(very Wne sandy silt). Notable are the upwards-coarsening
trends at the top of cores from the Northern Prodelta
(cores 10, 21, 31, 11, 21 and 12). Cores located on the Frontal prodelta (core 2) and in shallower water (cores 10 and
21) show the most pronounced jumps in grain-size. Cores
located more distally on the Southern Prodelta (cores 6, 7
and 25) have the most homogeneous grain-size distributions of very Wne silt. Carbonate building organisms were
overall quite rare.

G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

The thin silt and sandy silt layers frequently exhibit parallel lamination. Occasionally cross-lamination (current
ripples), erosional bases, and Wning-upwards trends are
developed. In the more oVshore cores coarser layers usually
constitute less than 4% of the total core length. The coarsergrained layers tend to become more frequent in cores
located in shallower water, varying between 5 and 21% of
the total core length. The sandy layers may consist of relatively well-sorted sand or silt, and occasionally contains
abundant plant remains (e.g. core 2, 143–145 cm depth).

549

4.2. 210Pb analysis
210

Pb dating is an important tool widely used to assess
spatial variability in recent accumulation rates on a »100
years time scale (Krishnawami et al., 1980; Benninger et al.,
1997; Fuller et al., 1999; Chague-GoV et al., 2000; van den
Bergh et al., 2003; Boer et al., 2006). 210Pb (with a half-life
of 22.3 years) is produced by the decay of atmospheric
222
Rn and is removed from the atmosphere as fallout. In the
marine environment it rapidly adheres to the surface of

A

B

Fig. 2. Data compilations of gravity cores from the Ba Lat prodelta, showing lithology, downcore values of DBD, magnetic susceptibility, grainsize, and
for some cores Ca and Fe contents and Corg and C/N ratios. (A) Cores from the Northern Prodelta. (B) Cores from the Frontal Prodelta. (C) Cores from
the Southern Prodelta (next page). For the grain size columns dots represent median values and squares represent the grain size of the 90% percentile.
Gray bands in the right columns of core 15 correspond with tempestites.


550

G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

C

Core 15
Lithology

Core 6
DBD & magsus
CGS units

0

100 1

50

Grainsize

XRF

(µm)

10 100 1000 0

20

40

C/N ratio

%C org

Ca% (of
total counts - Fe)

0

1

2

3

4 0

0

20

0

20

20

40

40

Depth (cm)

0

60

Depth (cm)

10

80

120

120

140

140

0.5

1

1.5

80
85
90
0.0
0.1
Fe% of total counts
%N org

0.2

1

(µm)
10 100 1000

80

100

DBD (g.cm-3)

CGS units
50
100

60

100

0

Grain size

DBD & magsus
Lithology

160

0.0 0.5 1.0 1.5
-3

DBD (g.cm )

Fig. 2 (continued )

sediment particles, to become incorporated into accumulating sediments.
Grain-size Xuctuations have an eVect on the 210Pb activities. Smaller grains result in higher speciWc surface area and
thus in a higher potential to capture 210Pb (Eisma et al.,
1989). Variable grain-sizes aVected the outcome of the 210Pb
activity proWles of cores 2, 10, 12 and 18 negatively. Examination of grain-size distributions showed that coarser-grained
samples usually showed major peaks in the coarse silt or Wne
sand fractions (Fig. 3A). Using the <25 m fraction for measuring 210Pb activities, negative eVects from grain-size Xuctuations were largely reduced (Fig. 3B). Even when only the
<25 m fractions was used, some marked shifts in activity
remain (e.g. cores 2, 10, Fig. 4). These Xuctuations presumably reXect highly variable activities of the accumulating sediment, in particular for cores located relatively close to the
river mouth. Erosional events may also account for some of
the discontinuities in the proWles. This is probably the case in
core 2, where a sharp decrease in activity occurs between 15

and 20 cm core depth at the top of a silty interval.
The proWles of total downcore 210Pb activities for the 12
analyzed cores are shown in Fig. 4. The model results of
supported activities (Csupp), inventories, extrapolated activities at the surface (C0), accumulation rates ( ), mixing rates
(Db) and depths of the Surface Mixed Layer (Zmix) are summarized in Table 2. The accumulation rates are presented in
cm yr¡1.
Surface Mixed Layers (SML) only occur at stations
located along the outer margin of the prodelta (stations 12,
25 and 26), reaching a thickness of up to 30 cm. These two
stations have the lowest recorded sedimentation rates of
<1 cm yr¡1. Station 25 is the furthest away from the Ba Lat
River mouth. An accumulation rate of between 0.7 and
1.6 cm yr¡1 was obtained, depending on an interpretation
with or without SML.

Highest accumulation rates of more than 3 cm yr¡1 were
found at the Frontal Prodelta (stations 1–3) and the proximal part of the Southern Prodelta (station 15). Station 3,
located on the steep slope of the Frontal Prodelta, has an
extremely high sedimentation rate. Values of between 33
and 94 cm yr¡1 are obtained when extrapolating excess
activities downward beyond the length of the core and
assuming supported activities of between 36.4 and
41.3 Bq kg¡1 as measured in other cores. The extremely high
sedimentation rate at station 3 reXects direct inXuence of
settling from the river plume.
Cores from the Northern Prodelta (10 and 11) have
intermediate sedimentation rates of between 1.5 and
2.1 cm yr¡1. The low accumulation rate (1.0 cm yr¡1) is comparable to stations along the distal edge of the prodelta.
4.3. 234Th analysis
Downcore analysis of excess 234Th is commonly used for

the assessment of short-term and seasonal diVerences in
deposition, re-suspension, and mixing-processes in various
marine environments (Aller et al., 1980; Aller and DeMaster, 1984; Fuller et al., 1999). 234Th activities were analyzed
on 14 boxcore samples from the Ba Lat prodelta (Fig. 5).
Surface 234Th excess activities Xuctuated between negligible
values and 200 Bq kg¡1. Maximum penetration depths
ranged between 1 and 2.5 cm. During the dry season in
March, highest inventories were recorded at stations 14–16,
located in the zone of maximum deposition of the Southern
Prodelta (Fig. 6). Unfortunately, no wet season measurements are available for these stations.
The highest inventory during the wet season was
recorded at station 18, followed by stations 7 and 26. Stations 13 and 27, located on the oVshore margin of the
Southern Prodelta, both have intermediate wet season


G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

A

B

551

210Pb(tot)
210Pb(tot) - fraction < 25 µm
m/m% > 25 µm

25 µm

Activity (mBq/g)

0

50

100

150

0

Volume% in range

10

core depth (cm)

20

30

40

50

60

70

80

0.1

1

10
100
Particle size (µm)

1000

0

20
40
60
80
mass/mass % (fraction >25 µm)

100

Fig. 3. Graphs showing the grain-size eVect on 210Pb activity in core 10. (A) Grain-size distribution plots at 5 cm intervals. (B) Mass percentages of the
>25 m fraction of the bulk samples (black dots), 210Pb activity of the bulk samples (closed squares) and 210Pb activity of the <25 m fraction (open
squares).

inventories. Notably, station 26, located along the oVshore
edge of the Southern Prodelta, has a much higher wet season inventory than stations 13 and 27, which are closer to
the Ba Lat. In the Northern Prodelta inventories for the dry
and wet season are close to zero at station 10, reXecting
consistent non-sedimentation or erosion during both seasons. At station 11 inventories during both seasons are of
intermediate magnitude, but the dry season inventory is

slightly higher than that of the wet season.
4.4. Organic carbon and nitrogen
Color banding is well-developed in cores from the
Frontal Prodelta (cores 1–3) and to a lesser degree in the
more distal cores from the Southern Prodelta (cores 6–7,
15–16 and 23). In core 2 the dark bands are 1–3 cm thick,
show Munsell colors 7.5YR3/2 or ¡3/3 (brownish black
or dark brown). The dark bands tend to have a sharp base
with the dark colors gradually fading upwards (Fig. 7).
The lighter colored bands show Munsell color 5YR4/3
(dull reddish brown) and may be thicker or thinner than
the dark colored bands. In the cores from the Southern
Prodelta dark/light bands are weaker developed, show
more mottling and are on average thicker (10–15 cm). In

order to investigate weather these color bands were generated by possible seasonal variability in the supply of
organic material, cores 2 and 15 were selected for analysis
of organic carbon (Corg) and nitrogen (Ntotal). Both cores
have high 210Pb accumulation rates and therefore potentially a good resolution.
Core 2 is composed of clayey silts with a few pronounced coarser-grained, silty levels, containing benthic
foraminifera and ostracods besides siliciclastic grains. The
Corg in core 2 Xuctuates around an average of 0.7%, with a
single peak of over 3% coinciding with the coarse-grained
layer at 143 cm core depth, which contains macroscopically
visible plant remains (Fig. 7). The C/N ratio shows a single
peak of 19.2 at this sandy layer, in accordance with landderived plant material (Hedges et al., 1999). For the remaining part of the core the Corg and Ntotal contents are positively linked to each other, the C/N ratio Xuctuating
between 4.2 and 9.1, indicating a common marine source.
These values are comparable to bottom sediments from the
Mekong prodelta (Landmann et al., 1998). They do not
correlate with volume percentages of the size fractions

smaller than 8 m, 25–63 m or larger than 63 m. Neither
is there a strong correlation between darker color and
higher Corg.


552

G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557
Northern prodelta
Core 10
0

Core 11

50

100

0

Frontal prodelta
Core 18

Core 12

50

100

0

50

100

0

50

100

Core 1
150

0

Core 2

50

100

0

50

C ore 3
100

150

0

50

100

0

20

40

Core depth (cm)

60

80

100

120

140

210Pb (bulk
fraction)
Csupp. set at value
core 10
Csupp. set at value

core 12
Csupp. core 10

210Pb (<25mu fraction)

160
fit incl. lower point

180

fit excl. lower points
226Ra

200

Southern prodelta
Core 15
0

best fit
Csupp. best fit

Csupp. cores 12 &
15

Core 7

50

100

0

Core 6

5
0

100

0

50

Csupp set at value
core 6
Csupp core 6

210Pb (bulk
fraction)
Csupp. set at value
core 10
Csupp. Set at value
core 6
Csupp. core 10

210Pb (bulk
fraction)
Csupp. set at value
core 10

Csupp. set at value
core 6
Csupp. core 10

Csupp. best fit

Csupp. core 6

Csupp. core 6

210Pb (<25 mu
fraction)
best fit

210Pb (<25 mu
fraction)

Core 25
100

150

0

50

100

210Pb (bulk
fraction)

Csupp. set at
value core 10
Csupp. set at
value core 6
Csupp. core 6
Csupp. core 10

Core 26
150

0

50

100

150

0

20

40

core depth (cm)

60

80

100

120

140
210Pb (bulk fraction)

160

fit with SML
fit without SML

180

210Pb (bulk
fraction)
Csupp. Set at value
core 6
Csupp. Set at value
core 10
Csupp. core 10

210Pb (bulk
fraction)
fit with SML

210Pb (bulk
fraction)
best fit

fit without SML

Csupp. core 6

226Ra (average)

Csupp. set at value
cores 12 & 15
Csupp. cores 12 &
15
Csupp. best fit

226Ra

226Ra
226Ra (average)

210Pb (bulk
fraction)
Csupp. set at value
core 6
best fit
Csupp. core 6
Csupp. best fit

200

Fig. 4. 210Pb total activity proWles of core sediments from the Ba Lat prodelta. Open squares represent data points that were omitted during Wtting, because
they were coarser-grained than the remaining part of the core. For many cores multiple Wtting solutions are plotted, depending on the model choice (with
or without SML) or on the choice of the supported 210Pb activity (based on minimum and maximum values obtained in other cores).

The granulometric curve of core 15 (Fig. 2C) shows similar Xuctuations as in gravity core 2. Organic carbon percentages are Xuctuating around an average of 0.9%, slightly

higher than in core 2. The organic carbon and nitrogen
curves show two large peaks at 10.5 and at 116.5 cm core
depth, both levels with macroscopically visible plant

Table 2
Parameters derived from model Wtting of 210Pb activity proWles
Station C0 (Bq kg¡1) Csuppa (Bq kg¡1) Zmix (cm)
1
2c
3
6
7
10c
11
12c
15
18c
25
26

80.3
82.4
74.4
92.9
84.2
87.7
107.3

82.6
78.8
119.5
116.9
104.8

41.3
41.3
36.4
36.4
41.3
41.3
41.3
37.3
37.6
36.4
47.2
39.6

0
0
0
37.8
50
0
0
20
30
0
30.9

26.9

min.b (cm yr¡1) C0 (Bq kg¡1) Csupp (Bq kg¡1) Zmix (cm)
3.5
3.1
33.5
1.4
1.9
1.8
1.5
0.2
3.2
0.9
0.7
0.3

80.0
87.7
75.2
99.1
90.5
86.2
106.1
82.6
82.9
118.3
131.8
105.1

36.4

36.4
41.3
36.4
36.4
41.3
37.6
37.0
37.6
34.3
37.0
36.4

0
0
0
0
0
0
0
20
0
0
0
25.3

max.b (cm yr¡1)
4.1
3.4
93.8
2.0

3.6
2.1
1.7
0.2
4.5
1.0
1.56
0.4

average (cm yr¡1)
3.8
3.3
63.7
1.7
2.8
1.9
1.6
0.2
3.9
1.0
1.1
0.4

a
For most proWles supported activity was not reached at depth. Csupp values obtained by measuring 226Ra activities are printed bold. For cores where
Csupp was not directly measured and was not reached at the core base, values from other cores were substituted.
b
Depending on the choice of the supported activity, various best Wt solutions were found. To the left values leading to a minimum accumulation rate (
min), to the right values leading to a maximum accumulation rate ( max) are given.
c

In these stations the model choice (with or without SML) lead to slightly varying solutions. Only the sieved fraction <25 m was used for 210Pb analysis.


G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

core depth (cm)

Station 10, waterdepth 22 m

0.0

0

0.5

0.5

0.5

1

1.0

1

1.5

1.5

1.5

2

2.0

2

2.5

2.5

2.5

3.0
0

core depth (cm)

3
0

50 100 150 200 250 300
Bq/kg

Station 14, water depth 21 m:

50 100 150 200 250 300
Bq/kg

0

0

0

0.5

0.5

0.5

1

1

1

1.5

1.5

1.5

2

2

2

2.5

2.5

2.5

3
0

50 100 150 200 250 300
Bq/kg

Station 7, waterdepth 22 m:

50 100 150 200 250 300
Bq/kg

Station 16, waterdepth 16.5 m Station 15, waterdepth 23 m:

0

3

core depth (cm)

Station 11, waterdepth 27.5 m Station 18, water depth 30.5 m

0

3

553

Dry season: diamonds
Wet season: triangles
238

U-based supported
activities indicated by
dashed line

3
0

50 100 150 200 250 300
Bq/kg

0

50 100 150 200 250 300
Bq/kg

Station 13, water depth 26.5 m Station 27, waterdepth 27.5 m: Station 26, waterdepth 29 m

0

0

0

0

0.5

0.5

0.5

0.5

1

1

1

1

1.5

1.5

1.5

1.5

2

2

2

2

2.5

2.5

2.5

2.5

3

3
0

50 100 150 200 250 300
Bq/kg

3
0

50 100 150 200 250 300
Bq/kg

3
0

50 100 150 200 250 300
Bq/kg

0

50 100 150 200 250 300
Bq/kg

Fig. 5. 234Th activity proWles of boxcores from the Ba Lat prodelta.

remains. The C/N ratios are considerable higher at these
level (17.3 and 16.1, respectively) than in the remaining part
of the core.
5. Discussion
5.1. Spatial accumulation patterns
The diVerence in color and mineral content between the
gray shelf sands and the reddish brown delta deposits
reXects a diVerence in depositional conditions. Iron minerals are concentrated in the Wne-grained sediments that settle
out from the modern river plume, whereas the sandy shelf
sediments were most likely deposited under high energy
conditions. The gray shelf sands are generally well consolidated and the echosounder proWles did not show active
bedforms, so they are assumed to be relicts from the postglacial transgression.

Maximum grain-size of suspended sediments from the
Ba Lat plume is up to 60 m, with a median grain-size varying between 4 and 8 m (van Maren and Hoekstra, 2004).
This corresponds well with the median grain-sizes in the
muddy prodelta deposits, indicating that the major sediment source is provided by suspended sediment from the
river. The turbid plume extends some 5 km oVshore from
the river mouth during the wet season. The steepest part of
the Frontal Prodelta down to a depth of 22 m lies directly
under the inXuence of this wet-season turbid plume. Further oVshore surface waters are still characterized by relatively low salinities (<12 ppt at least 10 km in oVshore
direction), but these older generations of river plumes

appear to have lost most of their suspended sediment load
due to Xocculation processes (van Maren and Hoekstra,
2004). Direct settling from the river plume leads to an
extremely high 210Pb accumulation rate on the Frontal
Prodelta slope at station 3 (30–90 cm yr¡1). The rapid


554

G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

dry season appeared larger than during the wet season, suggesting that southward bottom transport and focusing of
re-suspended sediments occurs mostly during the dry season. Cores 10, 11, 21 and 29, all from the Northern prodelta, are capped by a 5 to 10 cm thick interval that is
markedly coarser-grained than the sediment underneath
(Fig. 2A). These coarse-grained bottom sediments seem to
represent lag-deposits that result from winnowing of the
silt-clay fraction by bottom currents.
The observed long-term accumulation at station 10, as
indicated by the 210Pb activity proWle, could be explained by
attributing the accumulation at this station as dating back
to the period before 1971, when the main river outlet
entered the sea opposite station 10.
Fig. 6. Comparison of 234Th inventories for the various stations during
March (dry season) and August (wet season).

accumulation explains the lowest DBD values at this station (average DBD D 0.66 g cm¡3). The high accumulation
by means of Xocculation and settling maintains the steep
slope of the Frontal Prodelta slope. At stations 1 and 2,
located oVshore from the slope break at 22 m water depth
(in between stations 2 and 3), the accumulation rate has

decreased drastically down to values of between 3 and
4 cm yr¡1. The distal slope break of the prodelta marks the
point beyond which direct settlement from the turbid
plume rapidly decreases.
The large thickness of the youngest acoustic unit (Unit
Z1) in the area southwest of the river mouth (see Fig. 6 in
van den Bergh et al., 2006) suggests that after settling, bottom transport redirects a large amount of sediment in
southwestern direction. This is again conWrmed by the relatively high accumulation rate at station 15 (3.2–4.5 cm yr¡1).
Towards the eastern margin of the Southern Prodelta and
further south the 210Pb accumulation rates decrease with
decreasing thickness of the youngest acoustic unit. In the
Northern Prodelta on the other hand (stations 10 and 11)
accumulation rates are low, varying between 1.5 and
2 cm yr¡1. These observations are in line with the prevailing
current patterns. Southward directed residual Xow dominates near the Red River Delta coastline during the NE
Monsoon (dry season). During the wet season residual Xow
is probably weaker and more variable, though primarily
directed southward. The Coriolis force seems to be a dominant factor in the southward Xow of surface currents,
strengthened by the prevailing wind patterns during the dry
season (van Maren and Hoekstra, 2005). However, of
greater importance for sediment bed-load transport is the
tidal asymmetry, which leads to a net southwards transport
throughout the year (van Maren et al., 2004). It is likely
that the stations in the Northern Prodelta receive most of
their sediment by means of longshore transport from the
more northern river mouths. The 234Th measurements indicate that non-deposition or erosion during both seasons
occurs at station 10. At station 11 the 234Th inventory of the

5.2. Tempestites
The occurrence of coarser-grained layers in the prodelta

deposits indicates that occasionally high energetic conditions occur near the sea-bed, that are able to transport
sand-sized particles as bed-load. The sandy silt layers and
silt layers developed in most cores except at the most distal
deeper water stations, are likely formed during stormy conditions. In the Ba Lat area, landward-directed storms are
generated due to the intense summer lows and winter highs
that characterize the local climate. Table 3 presents a list of
typhoons that made landfall within 100 km from the Ba Lat
between 1964 and 2000. On average the delta is aVected by
a heavy storm once every 4 years. Heavy storms directly
hitting the Ba Lat area occurred in 1973 (Kate) and 1983
(Georgia).
Storms frequently produce sheet-like sands of considerable lateral extent, which may extend to depths several tens
of meters below the fair-weather wave base. These tempestites are individual graded sandy beds produced in waters
where sandy transport is not an important factor during
fair weather conditions. Tempestites show a sharp erosional
base, followed by a coarse-grained (lag) deposit overlain by
Wning upward sediments. They are typically 5–10 cm thick
and may exhibit cross-bedding structures at the base or
intermediate levels (Einsele, 1992). Parallel lamination
replaces the cross-stratiWcation at greater water depths.
When a storm is accompanied by heavy Xooding in the
near-by land areas, large volumes of sediment and landderived organic material may be supplied in addition to
sediment eroded from the shoreface.
The laminated sandy layers in cores 2 and 15 meet these
criteria in most cases. An exception is the layer at 113–
120 cm in core 15, which shows a coarsening upward trend
(Fig. 2C). However, the larger particles in this layer represent mostly plant remains. Typhoons in North Vietnam
occur mostly towards the end of the wet season and thus do
not necessarily co-occur with the major supply of sediment
rich in organic material. On the other hand, storm surges

Xooding the coastal areas may transport large amounts of
terrestrial organic material seawards. Events where large
amounts of terrestrial organic material was transported


1994
1995

x

Only storms are listed that made landfall within 100 km from the Ba Lat (Data compilation by the Vietnam University of Science, Hanoi).
a

1994
1996
25
15

71
1984
64

x
x
x

1979
81

25

15

1981

1960
143
1971
113

x
x
x
x
x

980
965
965
985
970
982
985
998
982
30
43
33
>40
35
32

27
23
30
NE
SE
NE
SSE
SE
N
NE
NE
SE
1973 July 26
1973 September 15
1975 September 20
1980 August 23
1983 July 18
1983 October 1
1994 August 28
1994 September 7
1996 August 23
Kate
Magre
Alice
Joe
Vera
Georgia
Harry
Joel
Niki

Thaibinh-Namha
Thanhhoa
Thanhhoa
Haiphong
Haiphong-Quangninh
Thaibinh-Namha
Haiphong-Quangninh
Haiphong-Quangninh
Ninhbinh-Thanhhoa

Pb age
of top

210

Depth top sand
layer (cm)
Pb age
of top
Direct

Indirect Depth top sand
layer (cm)

210

Correlation with sand layers in core 15
InXuence

Air pressure
(mb)
Wind speed
(m/s)
Direction
Area
Date
Namea

Table 3
Major storms in the period between 1964 and 2000, and possible correlation with tempestites in gravity cores 15 and 2

Correlation with sand layers in core 2

G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

555

seawards are documented by the coarse-grained layers at
143–144 cm depth in core 2 (Fig. 7), and at 15–18 cm and at
113–120 cm in core 15. In order to determine if sandy layers
could coincide with known storm events, the top levels of
sandy layers in cores 15 and 2 were dated with the 210Pb
accumulation rates of 3.87 cm yr¡1 and 3.3 cm yr¡1, respectively. Apart from the silty layer at 40 cm core depth, all
other coarse-grained layers in core 15 correspond with
storm years within 1–3 years accuracy (Table 3). The sandy
layer at 119–113 cm core depth likely represents the
typhoon ‘Kate’, which should have had a strong impact as
it directly hit the Ba Lat area in 1973. In the foregoing year
the sand barrier encroaching the delta platform had broken

through by the highest recorded Xooding ever, Wxing the
main outlet at its present position. The combination of this
event with typhoon Kate may account for the coarsening
upward trend and the presence of abundant plant remains
in the layer. In core 2 the correlation of the upper 3 sandy
layers with typhoons is also good, but the layer which is
presumably associated with typhoon ‘Kate’ (at 143 cm core
depth, Fig. 7) has an 210Pb age that is apparently too old
(1960). As argued below, the short-term accumulation rate
may be higher than the time-averaged 210Pb accumulation
rate, which could account for this discrepancy.
5.3. Color banding
The clearly developed color banding observed in the
cores located close to the river mouth (Fig. 7) likely reXects
seasonal cycles. The relatively low C/N ratios indicate a
mostly non-terrestrial source for Corg. SEM photographs of
Wltered water samples showed the presence of abundant
string-shaped diatoms in the water column during the wet
season, but not during the dry season. This suggests that
seasonal diatom blooms in combination with high accumulation rates may account for formation of the colored
bands, though there is no well developed correlation
between total organic carbon content or C/N ratios on the
one hand and darker color or grain-size parameters on the
other hand. As mentioned above, storm generated coarsegrained layers may or may not contain elevated concentrations in organic material of predominantly terrestrial
organic carbon, and may produce dark colors as well. The
distinct carbon sources in layers of distinct energetic conditions may explain the poor correlation between carbon
content and the other parameters measured.
If the alternation of dark and light colored bands reXect
dominantly seasonal diVerences in marine production of
organic carbon, the average thickness of combined dark/

light couplets should match the annual average accumulation rate. The upper 110 cm of core 2 contain 20 dark-colored bands. In core 15 banding is less well developed,
presumably due to more intense bioturbation, and will not
be considered here. In core 2 the interval between 110 and
125 cm core depth does not show clear banding either, and
the organic carbon content of this interval show a rather
constant value. An average thickness of 5.5 cm for the dark/


556

G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

does not indicate any notable changes invoked by the construction of the Hoa Binh Dam so far. Considering the relatively short time that has elapsed since its construction, it
could be still too early to Wnd clear imprints of a decrease in
sedimentation rate on the Ba Lat prodelta. The eVect of the
southward shift of the main Ba Lat outlet between 1971
and 1973 (Thanh et al., 1997) seems to have had a more
pronounced impact on the depositional patterns. As was
mentioned earlier, the Northern Prodelta is presently being
reworked and redistributed by southward running longshore currents, resulting in zero deposition at station 10.
Other factors, such as the closure of the Ngo Dong and
Ha Lan mouths in the 1970s further blurs the recognition
of possible impacts of the Hoa Binh Dam construction on
the prodelta accumulation. Erosion along the Hai Hau
coast southwest of the Ba Lat is visible on the acoustic proWles, and within the last 30–40 years the shoreline in this
region was forced back over a distance of about 1000 m
(Nhuan, pers. comm.). The erosion seems to be related to
the progressive closure of these outlets rather than to a
decrease in sediment supply resulting from the Hoa Binh
Dam. The Southern Prodelta is detached from the Hai Hau

coastline, and the sediment supplied by the Ba Lat is transported southward and bypasses the shoreface along this
strip of coast. Along the shoreface in water of less than 8 m
deep, wave action, in particular during severe storms, is
thought to be responsible for the observed bottom erosion.

light colored couplets is obtained for the upper part of the
core. The 210Pb sedimentation rate obtained at station 2 is
around 3.3 cm yr¡1. The irregular jumps in the upper part of
the activity proWle (Fig. 4) make this outcome rather unreliable. The 210Pb sedimentation rate may be lowered because
of erosion that occurred during typhoon events recorded in
the upper part of the core (Fig. 7) by the silty intervals
between 15 and 25 cm core depth. Using a sedimentation
rate of 3.3 cm yr¡1 for the upper interval and a rate of
5.5 cm yr¡1 (5.5 cm is the average thickness of the dark/light
colored mud couplets) for the interval below 25 cm depth,
the tempestite at 143 cm core depth is dated at 1973, exactly
matching typhoon ‘Kate’. This adds support to the hypothesis that the dark/light colored couplets developed in core 2
indeed represent annual accumulation bands.
5.4. EVects of damming
One of the aims of this study was to see if the construction of the Hoa Binh dam has aVected depositional patterns
in the Ba Lat Delta. In the Nile Delta for example, the construction of the Aswan Dam has greatly reduced annual
discharges. Less than 10% of the former Nile river sediment
load is now delivered to the Mediterranean coast, leading
to accelerated coastal erosion, increased landward incursion of saline groundwater and subsidence (Stanley and
Warne, 1998). Our data set from the Red River, however,

Core 2
Lithology

DBD & magsus

CGS units
0

50

%C org

Grainsize

C/N ratio

(µm)
100 1

10

100

0

1

2

3

4 0

10

20

0

20
Tempestite

40

Depth (cm)

60

80

100

120

140

160
0.0

0.5

1.0

1.5

DBD (g.cm-3 )

0

0.1

0.2

%N tot

Fig. 7. Corg contents and C/N ratios and other analyses for gravity core 2 (for legend see Fig. 2A). To the left a detailed photograph of the interval 60–
110 cm core depth is shown with the alternation of dark and light colored couplets, thought to represent annual layers. The gray bands in the 2 columns to
the right correspond with tempestites. Only the lowest tempestite is characterized by the abundance of land-derived plant material.


G.D. van den Bergh et al. / Journal of Asian Earth Sciences 29 (2007) 545–557

6. Conclusions
The modern prodelta of the Ba Lat progrades over transgressive sands of presumably early Holocene age, that cover
the shelf of the Gulf of Tonkin. The prodelta shows a
strongly asymmetrical depositional pattern, related to dominantly southward Xow directions. Accumulation on the
Frontal Prodelta is dominated by rapid settling of Wnegrained sediment from the river plume within 5–7 km from
the river mouth. Little if no sediment from the Ba Lat moves
towards the north. Tidal bottom currents erode the steep
Frontal Prodelta and move the muddy sediments predominantly in southwestern direction. These processes result in a
zone of maximum accumulation in the area southwest of the
Ba Lat outlet, where an accumulation rate of 3.9 cm yr¡1 was
recorded at 16 km from the river mouth. Accumulation rates
decrease further southward and in oVshore direction. In the
northern part of the prodelta accumulation seems to have

been greatly reduced since a southward shift of the main
channel between 1971 and 1973. Here at present accumulation is either zero or restricted to the dry season, when some
resuspended sediments are transported from the North by
longshore and tidal currents.
The muddy deposition on the prodelta is occasionally
interrupted by the formation of coarse-grained sandy and
silty layers with erosional bases. These coarser-grained layers are attributed to typhoon events that aVect the Ba Lat
area on average once every four years. These storm layers
tend to show concentrations of reworked carbonate, and
may contain high levels of terrestrial organic carbon. The
inXuence of storms decreases with increasing water depth.
Acknowledgements
Financial support for this study was provided by the
Netherlands Organization for Research in the Tropics
(WOTRO; Grant WT75-364). We thank other members of
the WOTRO Red River Delta Research Programme for
their assistance, support, and constructive discussions in the
course of this study. Bas van Maren is thanked for the tidal
corrections and construction of the bathymetric map, as
well as the conversion of the raw DGPS data to UTM coordinates.
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