Marine Geology 183 (2002) 163^178
www.elsevier.com/locate/margeo
210
Pb, 137 Cs and 239;240 Pu in East China Sea sediments:
sources, pathways and budgets of sediments and radionuclides
Chih-Chieh Su, Chih-An Huh Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, Taiwan
Received 5 February 2001; accepted 12 December 2001
Abstract
Profiles of 210 Pb, 137 Cs and 239;240 Pu measured in 83 sediment cores collected from various sedimentary regimes in
the East China Sea were analyzed to elucidate the sources, routes and budgets of sediments as well as these
radionuclides. Distributions of sedimentation rates and nuclide inventories reveal alongshore transport of sediments,
137
Cs and 239;240 Pu from the mouth of the Yangtze River toward the south, largely confined to the inner-shelf area
(water depth 6 70 m). Mass balance calculations suggest that the East China Sea is a sink for the particle-reactive
210
Pb and 239;240 Pu, with about one-sixth of their sedimentary budgets supplied via boundary scavenging. In contrast,
due to lower affinity of 137 Cs for particles and rapid turnover of the shelf water, the East China Sea serves as a source
for 137 Cs. About two-thirds of the cumulative input of 137 Cs have been transported out of the East China Sea, leaving
the remaining one-third stored in the bottom sediments and the overlying water column. As for the sediment budget,
mass balance cannot be established due to a shortfall in sediment supply of more than 30% based on a comparison
between input terms documented thus far and the sedimentation flux derived from this study. It is very likely that we
have overestimated the sediment burial flux or that long-distance transport from the Yellow River’s dispersal system
to the East China Sea is underestimated. Alternatively, the imbalance could be explained by the discrepancy between
sediment input and output on decadal to centennial timescales. 7 2002 Elsevier Science B.V. All rights reserved.
Keywords: sedimentation; East China Sea;
210
Pb;
137
Cs; plutonium; Changjiang River
1. Introduction
The East China Sea (ECS) is a marginal sea
over a broad continental shelf located between
the largest continent (Asia) and the largest ocean
(Paci¢c) in the world (Fig. 1). In the west and
north, it receives riverine inputs from two of the
* Corresponding author. Tel.: +886-2-2783-9910 x607;
Fax: +886-2-2783-9871.
E-mail address: [email protected] (C.-A. Huh).
largest rivers in the world, namely the Yangtze
River (Changjiang) and the Yellow River
(Huanghe). In the east, it is separated from the
open ocean by the Kuroshio (KC), which is a
western boundary current £owing northward
along the edge of the continental shelf. In the
middle of the ECS shelf and parallel to the Kuroshio is the Taiwan Warm Current (TWC),
which £ows toward the Tsushima Strait and is
joined by a branch of the Kuroshio to become
the Tsushima Current (TC). Through the circulation of currents and mixing of water masses, the
0025-3227 / 02 / $ ^ see front matter 7 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 5 - 3 2 2 7 ( 0 2 ) 0 0 1 6 5 - 2
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C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
ECS shelf water is turned over approximately
every two years (Nozaki et al., 1989; Chen and
Huh, 1999), causing fairly rapid exchange of
chemical materials between this marginal sea
and the open ocean. Such a setting makes the
ECS an interesting and important area for studying the role of marginal seas in global biogeochemistry. Therefore, it was identi¢ed as a test
ground to examine the hypotheses of the Kuroshio Edge Exchange Processes (KEEP) program
(Wong et al., 2000). Furthermore, it has also
been realized during the course of the KEEP program that the human dimension should be included in the scope of this study as well. Because
the ECS is adjacent to one of the most heavily
populated region in the world currently under intense development, the impact of human activities
to this marginal sea has aroused contemporaneous issues such as coastal pollution (e.g., Huh and
Chen, 1999) and changes of sediment yield (e.g.,
Lu and Higgitt, 1998). Of special concern is the
construction of the Three Gorges Dam in the
upper reaches of Changjiang, which would be by
far the largest dam in the world. Upon its completion and operation, scheduled for 2009, the
supply of sediments to the ECS will most likely
be reduced substantially. The out£ow of water
and various chemical materials will conceivably
be changed as well. Consequently, the coastal environment may be altered irreversibly. With an
uncertain future ahead, the environmental baseline data in this marginal sea remains inadequate.
It is an urgent task to establish the database in a
more comprehensive and systematic manner, so
that the would-be changes in the future can be
monitored and studied.
As a component of the KEEP program, our
objective is to elucidate sedimentation processes
in the ECS using a multitracer approach. For
this study, we have since 1996 made ¢ve cruises
and collected a large number of sediment cores
throughout the ECS shelf and sometimes down
the slope. The cores were analyzed for 210 Pb,
137
Cs and 239;240 Pu, among other radionuclides
and chemical materials. This paper is built
upon an earlier one (Huh and Su, 1999). Now,
by integrating a substantially expanded database, we intend to delineate a more detailed
picture of the sources, routes and budgets of
sediments and the studied radionuclides in the
ECS.
2. Materials and methods
2.1. Characteristics and provenance of sediments in
the East China Sea
Based on sediment grain size together with the
overlying water masses, the ECS can be divided
into three regimes : the inner-shelf mud area, the
outer-shelf sand area, and the slope plus Okinawa
Trough mud area (Qin et al., 1987; Saito and
Yang, 1994). The inner-shelf area, with water
depths within V70 m, extends southward from
the Changjiang’s mouth to 26‡N along the coast
of the Mainland China. The alongshore distribution of ¢ne-grained sediments in this area manifests the dispersal of Changjiang’s plume (McKee
et al., 1983). The outer-shelf area is in£uenced by
the Taiwan Warm Current and covered primarily
by relict sand and shell fragments. The slope and
Okinawa Trough area is underlain by silty clay
along the track of Kuroshio.
Based on distinct mineral assemblages, the ECS
was also divided into three regimes by Chen et al.
(1984a,b): muscovite, chlorite and calcite in the
western province; orthoclase, glauconite, hornblende and epidote in the middle province; and
volcanic glass, pyroxene and magnetite in the
eastern province. It is informative to note that
these three provinces coincide approximately
with the three regimes de¢ned by grain size and
the associated water masses. Thus, it is quite obvious that the distributions of grain size, mineralogy, and other characteristics in ECS sediments
re£ect the combined e¡ects of source region, topography and hydrodynamic conditions.
Although we do not have any mineralogical
data from this work and our grain size data are
limited, we found that the activity 214 Pb (which is
available for most of the samples) correlated positively with water content in the bulk sediment but
negatively with grain size. This is obviously due
to the enrichment of this 238 U-series nuclide in
¢ne-grained clay minerals. Thus, we believe that
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165
Fig. 1. Map showing the sampling sites, bathymetry and general circulation in the East China Sea and adjacent areas. The circulation pattern is modi¢ed from Beardsley et al. (1985) and described in Su (2000).
spatial and down-core distribution of this nuclide
in conjunction with water content (as summarized
in http://www.earth.sinica.edu.tw/~huh/keep.htm)
can be used to index the variations of sediment
grain size and lithological changes in the ECS.
2.2. Sampling and analytical methods
The sediment cores were collected onboard R/V
Ocean Researcher-I on cruises OR-460 (August
1996), OR-493 (July 1997), OR-499 (August
1997), OR-525 (August 1998) and OR-551 (June
1999). The sampling locations are shown in Fig.
1. Near the mouth of Changjiang, where steep
physical and chemical gradients are expected,
the coring sites were spaced more closely. In contrast, virtually no sediment cores can be collected
in the mid-shelf area under water depths of 100^
200 m. This is because sea£oor in that area is
primarily covered by relict sand and shell fragments, which are old materials deposited in littoral zones during low sea stands in the last glacial
stage. Such material is too hard to be penetrated
and collected by the corer.
In order to calculate sedimentation rates and
nuclide inventories more rigorously, it is necessary
to recover sedimentation records of at least ¢ve
decades. Thus, in addition to the collection of box
cores at all sites, gravity cores were also collected
in the estuarine area and at some inner-shelf sites
with high sedimentation rates. Upon collection of
the gravity cores or subcores from box cores, sediments were extruded vertically with a hydraulic
jack and sampled at 1^2 cm intervals. The outer
rim (V0.5 cm) of each sediment slab was
trimmed o¡ to minimize contamination between
layers. The sectioned samples were sealed in 8-oz
plastic jars and kept frozen until further process-
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C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
ing in the home laboratory. Based on weight loss
after freeze-drying, the water content of the wet
sediments was calculated. A correction for the
amount of salt retained in the dry sediments was
made based on the water content and the salinity
of the bottom water. Activities and inventories of
nuclides and mass accumulation rates reported in
this paper are calculated on salt-free basis.
210
Pb (via 210 Po) and 239;240 Pu were determined
by K-spectrometry. 209 Po and 242 Pu obtained from
ORNL were added as the yield determinants prior
to total digestion of samples. The 209 Po and 242 Pu
spikes have been calibrated versus NIST-certi¢ed
208
Po (SRM-4327) and 244 Pu (SRM-996), respectively. Polonium isotopes were plated onto a silver
disc from the sample solution (in 1.5 N HCl, in the
presence of ascorbic acid) at 80^90‡C for V2 h.
Isotopes of Pu were electroplated onto stainless
steel discs. The counting results were corrected
for the decay of 210 Po (from the time of plating
to counting) and 210 Pb (from sample collection to
Po plating). More detailed description of the radiochemical procedures can be found elsewhere
(Huh et al., 1987, 1990, 1996).
214
Pb (a precursor of 210 Pb, used as an index of
supported 210 Pb) and 137 Cs were measured by Qspectrometry based on photon energies at 351.99
keV and 661.62 keV, respectively. The counting
system is equipped with a 150% e⁄ciency (relative
to 3U3 NaI) HPGe detector (EG and G ORTEC
GEM-150230) interfaced to a digital Q-ray spectrometer (DSPec0 ). The detector was calibrated
using NIST SRM 4353 (Rocky Flats soil) and
4350B (Columbia River sediments), and IAEA
standard SD-N-1 (Irish Sea sediments). At
661.62 keV, for example, the absolute counting
e⁄ciency for our samples (in plastic jars of 8.5cm diameter, 7-cm height) varied from 6.15% for
10-g samples to 4.66% for 100-g samples, and the
peak resolution was 1.21 keV (FWHM) with a
peak-to-Compton ratio higher than 90.
3. Results and discussion
Space limitation does not allow us to list the
very massive nuclide activity data here. Interested
readers can browse the complete data set (plotted
as downcore pro¢les) at the web site http://
www.earth.sinica.edu.tw/~huh/keep.htm. In what
Fig. 2. Pro¢les of 210 Pb, 137 Cs and 239;240 Pu in core OR499-16 showing good agreement between the derived sedimentation rates.
The horizontal dashed line mark the mixed layer depth, while the hatched lines on the Pu and 137 Cs plots represent the time horizon circa 1963 deduced from 210 Pb-based sedimentation rate.
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167
follows, we choose to present some representative
pro¢les and show salient features. The overall results on sedimentation rates and nuclide inventories are plotted in ¢gures and contour maps to
facilitate the discussion.
3.1. Spatial variation of sedimentation rate
A primary objective of this study was to derive
sedimentation rates over the ECS, so that the history of deposition and burial £uxes of radionuclides and various chemical materials can be
studied by us as well as other components of
the KEEP program. In calculating sedimentation
rates, all nuclides analyzed (namely 210 Pb, 137 Cs
and 239;240 Pu) are employed as sediment chronometers and the results are compared. Given in Fig.
2 is an example from OR499-16, a core located at
a depocenter in the estuary area. It shows that
sedimentation rates calculated from the downward decrease of excess 210 Pb and the subsurface
peaks of 239;240 Pu and 137 Cs (taken to be the fallout maximum circa 1963; Huh and Su, 1999)
compare favorably with one another. Fig. 3 shows
that, for cores collected from Changjiang’s estuary and the inner shelf areas to the south, good
agreement between sedimentation rates derived
from di¡erent tracers is generally observed. This
is primarily due to the fact that sedimentation
rates in these near shore areas are fast enough
to dominate over other terms in controlling pro¢les of nuclides. Away from the near shore areas,
the much lower sedimentation rates render postdepositional processes (i.e., mixing and di¡usion)
important in modifying the distribution of nuclides. Consequently, sedimentation rates based
on di¡erent tracers become less consistent.
Despite the varied degree of consistency between sedimentation rates derived using di¡erent
methods for di¡erent sedimentary regimes in the
ECS, the spatial distribution and variation can
still be gleaned based on any of the nuclides
(please see http://www.earth.sinica.edu.tw/~huh/
keep.htm). Because the data are more complete
for 137 Cs, we choose to use 137 Cs pro¢les to
show spatial trends. Shown on Fig. 4 are 137 Cs
pro¢les at sites along ¢ve lines (A, B, C, D, E
and F) in the cross-shelf direction and one line
Fig. 3. Comparison of sedimentation rates derived from different nuclides.
(G) alongshore in the inner shelf. Along lines D,
E and F, the depth of the subsurface maximum
and/or the penetration depth of 137 Cs deepen toward the shore and pinch out toward the east,
indicating o¡shore decrease of sedimentation
rates. The same trend can also be seen on line
G, but the deepening is alongshore toward the
mouth of Changjiang. Also following the systematic trend is the downcore inventory of 137 Cs (Fig.
6a), which decreases o¡shore toward the east and
alongshore toward the south. All these point to
the fact that Changjiang is a major source of sediments and 137 Cs in the ECS shelf.
Along lines A, B and C, the picture is complicated by localized e¡ect of topography and water
£ow, as explained below. There is a belt of abnormally low sedimentation rate trending in the NW^
SE direction and crossing the middle of lines A
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168
Fig. 4. Downcore pro¢les of
penetration depth of 137 Cs.
C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
137
Cs in the East China Sea showing spatial variation in the depth of the subsurface peak and the
and B. This belt coincides with a submarine canyon, which is a drowned relict river valley o¡ the
mouth of the old Changjiang. It is important to
note that, up the canyon toward the northwest
£ows a branch of the Taiwan Warm Current,
which causes year-round upwelling o¡ the Changjiang estuary (Chao, 1993; Jacobs et al., 2000).
Because the £ow of the Taiwan Warm Current
and the topographically induced upwelling is
much stronger than the river plume and alongshore current, it impedes o¡shore transport of
riverine sediments across the valley (Zhang,
1999). Besides low sedimentation rates, inventories of all fallout nuclides also show minimum
values along the axis of the valley (to be further
discussed later).
Fig. 5 provides an overview of the spatial distribution of sedimentation rates in the indicated
area. It was derived from 137 Cs-based sedimenta-
tion rates taking the subsurface 137 Cs maximum
as the time horizon of 1963. The contour lines are
plotted by the SURFER software package. Aside
from the general decrease of sedimentation rates
o¡shore from the largest sediment source in the
ECS (i.e., Changjiang), the zone of minimum sedimentation mentioned above emerges as a rather
pronounced feature.
3.2. Sources, pathways and budgets of nuclides
3.2.1. Distribution of 137 Cs inventory
Shown in Fig. 6 are sediment inventories of
137
Cs, excess 210 Pb and 239;240 Pu in the ECS. The
distribution of 137 Cs inventory (Fig. 6a) mimics
the pattern of sedimentation rate (Fig. 5), indicating again that riverine input is a major source for
this anthropogenic nuclide in the ECS. Considering that the area of Changjiang’s drainage basin
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Fig. 5. Contour map showing spatial variation of
(1 800 000 km2 ; Milliman and Jin, 1985) far exceeds that of the ECS (353 000 km2 ; used for this
study), it is quite conceivable that delayed wash-in
input of 137 Cs from the drainage basin may be
more important than direct fallout from the atmosphere above the ECS. This point can be further argued by comparing observed sediment inventories of 137 Cs with expected cumulative
fallout as follows. Inventories of 137 Cs in outer
shelf sediments ( 6 1 dpm cm32 ) are much lower
than that can be expected from global fallout,
which is 7.1 dpm cm32 as of January 1998 at
137
169
Cs-derived sedimentation rates in the East China Sea.
this latitudinal band (Huh and Su, 1999), indicating that much of the fallout 137 Cs still resides in
the seawater reservoir. On the other hand, the
sediment inventory of 137 Cs near Changjiang’s
mouth (e.g., 53 dpm cm32 in core OR499-16) is
seven to eight times higher than expected from
cumulative fallout, which is primarily due to the
deposition of 137 Cs associated with river-borne
particles. In other words, scavenging from the
water column alone cannot account for the observed 137 Cs inventories. More on this will be discussed by mass balance calculations later.
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C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
Fig. 6. Contour maps showing spatial distribution of sediment inventories of (a)
East China Sea.
3.2.2. Distribution of excess 210 Pb inventory and
an evaluation of riverine input
The distribution of excess 210 Pb inventory in
sediments is quite di¡erent from that of 137 Cs in
showing high values at the shelf edge and in the
southern part of the inner shelf (Fig. 6b). Riverine
input does not seem to be as pronounced for
210
Pb as it is for 137 Cs, and sediment inventories
of excess 210 Pb in much of the estuary area is even
lower than expected from global fallout (Huh and
Su, 1999). Its cause can be revealed from mass
balance calculations below.
We are not aware of any published data for
210
Pb in the Changjiang drainage basin or the
river water, thus it is not possible to get an accurate estimate of the riverine input of this radionuclide. Although several previous studies (e.g.,
Benninger, 1978; Olsen et al., 1986; Baskaran
and Santschi, 1993) have indicated the insigni¢cance of riverine input relative to the atmospheric
input of 210 Pb at some estuaries, it has also been
shown that, for estuaries with large drainage basin to estuarine area ratios, riverine input could be
an important 210 Pb source (Ravichandran et al.,
1995; Baskaran et al., 1997). With a drainage basin area of 1 800 000 km2 , which is about 50 times
137
Cs, (b) excess
210
Pb and (c)
239;240
Pu in the
that of the estuary area (35 000 km2 ), the Changjiang estuary certainly falls in the latter category.
Based on this area ratio and by taking 3000 yr as
the residence time of 210 Pb in the drainage basin
(Smith and Ellis, 1982; Dominik et al., 1987), the
riverine input of 210 Pb is roughly estimated using
the same equation applied in previous studies
(Ravichandran et al., 1995; Baskaran et al.,
1997), i.e.,
Id ¼
Ad
UI f UI e
Aes
ð1Þ
where Ad is the area of the drainage basin, Aes is
the area of the estuary, If is the inventory of 210 Pb
(58 dpm cm32 ; DeMaster et al., 1986) and fe
is the fraction of the inventory of 210 Pb eroded
each year from the watershed (fe = ln 2/3000
yr = 2.3U1034 yr31 ). The calculation results in
a riverine £ux of about 0.69 dpm cm32 yr31 , corresponding to 38% of the atmospheric £ux.
It should be noted that, the residence time of
210
Pb in the Changjiang drainage basin is a highly
uncertain factor. Although we used 3000 yr as the
drainage basin residence time of 210 Pb in the calculation above, values from 800 to 10 000 yr have
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C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
been estimated for various drainage basins
around the world (Benninger et al., 1975; Lewis,
1977; Smith and Ellis, 1982; Dominik et al.,
1987; Porcelli et al., 2001). Considering that the
drainage area of the Changjiang River is among
the largest in the world, it may be reasonable to
take 10 000 yr as the residence time and hence
7U1035 yr31 for fe in Eq. 1. If so, the riverine
£ux of 210 Pb is reduced to 0.21 dpm cm32 yr31 ,
corresponding to V12% of the atmospheric £ux.
3.2.3. Pu distribution and inventory in sediments
Because of the time-consuming and laborious
procedures for the separation and puri¢cation of
Pu, 239;240 Pu analysis was performed only on selected cores. Thus, the inventory of 239;240 Pu in
sediments (Fig. 6c) is depicted less clearly.
Although 239;240 Pu has the same source as 137 Cs
(both come from nuclear tests), it behaves more
like 210 Pb in terms of a⁄nity for particles. As with
137
Cs, sediment inventory of 239;240 Pu decreases
o¡shore, indicating that riverine input is an important source. Unlike 137 Cs, however, is that
sediment inventory of 239;240 Pu in the eastern
side of the ECS is close to or higher than that
(0.21 dpm cm32 ; Hardy et al., 1973) expected
from global fallout, indicating that 239;240 Pu is
more e¡ectively scavenged from the water column
by settling particles.
3.3. Sedimentary budgets of nuclides
In what follows, an attempt is made to calculate the mass balance of these nuclides. To facilitate the calculation, the area covered in this study
is divided into four boxes: estuary, inner shelf,
outer shelf, and slope (Fig. 7). Although the compartmentalization is a bit arbitrary in some places,
it is basically based on the circulation pattern,
topography and grain size distribution, as described earlier. The estuary area is bound between
30‡N and 32‡N, with its eastern border set at 60 m
isobath from 30‡N to 31‡N and along the axis of
the old Changjiang relict valley from 31‡N to
32‡N. Sediments in this region are mainly silt in
the north and clay in the south. The inner shelf
area is de¢ned as the coastal zone at the south of
the estuary, between 26‡N and 30‡N and at water
171
depth within 70 m. This area is along the path of
the Changjiang Coastal Water and is covered
mainly (70% on average) by clay-sized ( 6 63
Wm) sediments. The outer shelf area is the rest
of the continental shelf extending to the shelf
break at 200-m water depth. The Taiwan Warm
Current is the dominant £ow in this area where
most of the sea£oor is underlain by sand, silty
sand and shell fragments. The slope area is a relatively narrow band between the 200-m and 1500m isobaths, stretching from 26‡N to 31‡N. It coincides with the path of the Kuroshio where the
bottom sediments are clay or silty clay. The surface areas of the four boxes are 35 000, 65 000,
230 000 and 23 000 km2 , respectively. The corresponding mean depths estimated using the SURFER program based on NOAA’s ETOPO5 bathymetry database are approximately 50 m,
50 m, 80 m and 1000 m, respectively. Thus, the
seawater volumes are 1750, 3250, 18 400 and
23 000 km3 , respectively.
First, it is more straightforward to begin the
calculation for 210 Pb, because riverine input is
less important for this nuclide (Huh and Su,
1999; Lin et al., 1996), which can be calculated
by: 0.69 dpm cm32 yr31 60.031 yr31 U36 000
km2 = 7.7U1015 dpm. If the 210 Pb £ux of 1.8
dpm cm32 yr31 in the nearby Hangzhou City (DeMaster et al., 1985) is applied to the estuary box,
atmospheric input would account for 20.3U1015
dpm of excess 210 Pb in sediments of the estuary
box at steady state. Production of 210 Pb from the
decay of 226 Ra (taking the mean 226 Ra concentration to be 0.18 dpm l31 ; Lin et al., 1996) in the
water column would contribute to 0.3U1015 dpm.
The actual amount of excess 210 Pb buried in the
estuary box is estimated to be 11.6U1015 dpm. To
balance the budget, V59% of the total excess
210
Pb input must be exported from the estuary
box. Indeed, summer deposition followed by winter resuspension is a perennial process in the
Changjiang’s estuary (McKee et al., 1983; Liu et
al., 2000), which may very well be responsible for
the observed de¢ciency in the sedimentary budget
of 210 Pb over there.
Sediments removed from the estuary are transported by the alongshore current and deposited in
the inner shelf area in the south (Huh and Su,
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C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
Fig. 7. The division of the study area into four boxes for mass balance calculations. The four boxes representing di¡erent sedimentation regimes are: (a) estuary; (b) inner shelf; (c) outer shelf; and (d) slope.
1999). In the inner shelf box, the import of excess
210
Pb from the estuary (16.7U1015 dpm) combined with atmospheric input (37.7U1015 dpm)
and water column production (0.4U1015 dpm)
contribute to 82% of the total burial in sediments
(66.9U1015 dpm). The remaining 18% (or
19.8U1015 dpm) must be from the o¡shore area,
most likely via the transport of the Taiwan Warm
Current. Performing the same calculation for the
outer shelf and the slope areas in sequence, we
found that lateral input from o¡shore, or the
so-called ‘boundary scavenging’ (Bacon et al.,
1976; Spencer et al., 1981), is required to balance
the budget for excess 210 Pb. This is conceivably
promoted by the £ow of the Kuroshio along the
shelf edge and its intrusion onto the shelf at some
places such as the northeast of Taiwan (Hsueh et
al., 1992; Tang et al., 2000) and at 29‡N (Nitani,
1972). The above calculation is summarized schematically in Fig. 8. If all four sedimentation regimes are combined into a single domain, it shows
that atmospheric fallout is responsible for 76% of
the excess 210 Pb budget in the ECS as a whole,
with the remaining 24% derived from boundary
scavenging (17%), riverine input (4%) and in situ
production from 226 Ra (3%).
In calculating the budget for 239;240 Pu and
137
Cs, the four boxes discussed above are merged
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173
Fig. 8. Schematic diagram showing the mass balance of excess 210 Pb in the East China Sea based on the four-box model. The
unit (1015 dpm) for the numbers results from £ux (dpm cm32 yr31 )6V (yr31 )Uarea (cm2 )61015 , where V ( = 0.031 yr31 ) is the
decay constant of 210 Pb.
into one to avoid over-interpretation, which is
necessary in light of the uncertainty in riverine
inputs for these two anthropogenic nuclides. First,
in view of the similarity in geochemical behavior
between Pu and 210 Pb, it may be reasonable to
assume that, as with 210 Pb, boundary scavenging
also accounts for 17% of the sedimentary budget
of 239;240 Pu. Thus, of the total 239;240 Pu stored in
sediments (2.5U1015 dpm), 0.4U1015 dpm is derived from boundary scavenging. Then, with total
atmospheric input ¢xed at 0.7U1015 dpm, the
riverine input of 239;240 Pu is estimated to be
1.4U1015 dpm, accounting for 56% of the sedimentary budget of 239;240 Pu in the ECS. This
is summarized in Fig. 9. The distribution of
239;240
Pu inventory in the estuary area (Fig. 6c)
is consistent with large riverine input. One may
question why riverine input is so much more important for Pu than for 210 Pb. A very possible
cause is the release of Pu (as well as 137 Cs) from
Fig. 9. Mass balance of 239;240 Pu in the East China Sea. The unit (1015 dpm) for the numbers results from cumulative fallout or
sedimentary inventory (dpm cm32 )Uarea (cm2 )61015 .
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174
C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
Fig. 10. Mass balance of
137
Cs in the East China Sea. See text for explanation.
nuclear facilities along Changjiang and its tributaries (Norris et al., 1993), whose discharges are
unknown, and hence not considered in our budget
calculation.
The mass balance calculation for 137 Cs (shown
in Fig. 10) is complicated by its lower a⁄nity for
particles and, therefore, partition between dissolved and particulate forms. Based on our previous estimate, direct precipitation from the atmosphere over the ECS would lead to a cumulative
137
Cs fallout of 7.1 dpm cm32 (Huh and Su,
1999), thus a total standing stock of 25U1015
dpm (assuming no loss from the ECS). If 137 Cs
bears the same riverine input to atmospheric input
ratio as Pu, then riverine input of 137 Cs would
result in a cumulative input of 50U1015 dpm,
which could be an underestimate in view of the
fact that 137 Cs has lower a⁄nity for particles,
hence is expected to be washed o¡ the drainage
basin more easily than Pu. So, the sum of total
137
Cs input (riverine plus atmospheric) would be
at least 75U1015 dpm, about four times that
buried in sediments (i.e., 19U1016 dpm). To account for the di¡erence, we still need to estimate
the standing stock of 137 Cs in the seawater reservoir, as follows. Multiplying the mean seawater
concentration of 137 Cs in the ECS (taken to be
0.19 dpm l31 ; Su, 2000) by the total volume of
seawater (46 400 km3 ), we obtain a total 137 Cs
standing stock of 9U1015 dpm in the seawater
reservoir, which is about one-half of that buried
in sediments. With the combined standing stock
(28U1015 dpm) subtracted from the total input,
we still have at least 47U1015 dpm unaccounted
for, which can only be explained by transport of
137
Cs out of the ECS in dissolved and suspended
forms. Export of 137 Cs can be facilitated by rapid
turnover of the ECS shelf water (V2 yr residence
time; Nozaki et al., 1989; Chen and Huh, 1999).
In summary, the amount of 137 Cs exported out of
the ECS is V2^3 times that buried in sediments,
suggesting that this marginal sea is an e¡ective
source of 137 Cs to other ocean basins.
3.4. Sediment budget
With the updated database, a re-evaluation of
the sediment budget in the ECS may be in order.
Based on the spatial distribution of sedimentation
rates and using the SURFER program, an annual
deposition of approximately 13U108 tons in the
covered area is calculated. This deposition £ux is
more than the reported discharge from Changjiang (5U108 tons yr31 ; Milliman and Meade,
1983) plus the erosion from Taiwan (2.6U108
tons yr31 ; Water Resources Planning Commission, 1996). As discussed earlier, much of the sediments discharged from Changjiang are deposited
in the estuary and the inner shelf area to the
south, which can be largely substantiated by the
MARGO 3064 2-5-02 Cyaan Magenta Geel Zwart
C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
distribution of sedimentation rates in these two
areas. So, the problem may rest on the o¡shore
areas. The circulation pattern of currents suggests
that the Taiwan Strait in the south and the Yellow Sea in the north are the possible source regions for sediments in the vast mid-shelf and outer shelf regions. However, even if all sediments
eroded from Taiwan can be transported to the
ECS shelf (by the Taiwan Warm Current), we still
need an annual input of 5.4U108 tons from the
Yellow Sea to balance the budget. This additional
input is about one-half of the reported annual
discharge from the Yellow River (Milliman and
Meade, 1983; Schubel et al., 1984). Previous studies (Zhang, 1996; Lee and Chough, 1989) suggested that only a small portion (V15%) of the
sediment load discharged by the Yellow River is
transported southward into the ECS by the Yellow Sea Coastal Current (YSCC). Could it be that
long-distance transport from the Yellow River
and Yellow Sea to the ECS, perhaps via reworking of sediments, is more than previously
thought ? From a di¡erent standpoint, it is also
highly likely that sedimentation rates we obtained
for the middle and outer shelf sediments may be
overestimated due to sediment mixing, reworking,
or improper interpolation of their spatial distribution (e.g., over the mid-shelf area covered by relict
sediments where there is probably no net sedimentation).
Although it is not yet possible to establish
mass balance for sediments in the ECS, we
have obtained a mean sediment burial £ux in
the ECS shelf that is fairly comparable with
the £ux of sediment derived from the adjacent
basins and seas. It should be pointed out that
sedimentation rates deduced here are on time
scales of a few decades (based on 137 Cs and
239;240
Pu) or up to V100 yr (based on 210 Pb).
Even if the documented values of river discharge
or land erosion are reasonably correct, it is unknown how variable they could be on decadal to
centennial time scales in response to environmental changes, either natural or induced by human
activities. In other words, insofar as the covered
time is concerned, we are probably only looking
at a transient state rather than a steady state
condition.
175
4. Conclusions
Based on pro¢les of 210 Pb, 137 Cs and 239;240 Pu
measured in 83 sediment cores collected from various parts of the ECS, spatial distributions of sedimentation rates and nuclide inventories in this
marginal sea are delineated. The results lead us
to the following conclusions:
(1) The highest sedimentation rates ( s 2 g
cm32 yr31 ) are found in the estuary of Changjiang, the largest source of sediments in the
ECS. There exists a general decrease of sedimentation rates from the mouth of Changjiang alongshore toward the south and o¡shore toward the
east. Superimposed on the overall trend is a small
but pronounced belt of abnormally low sedimentation along the valley of a submarine canyon,
which is the drowned relict river valley o¡ the
mouth of the old Changjiang. The strong £ow
up the canyon (i.e., a branch of the Taiwan
Warm Current) serves as a barrier preventing
Changjiang’s sediment plume from crossing the
valley.
(2) Spatial distributions of sediment inventories
of 137 Cs and 239;240 Pu in general follow the distribution of sedimentation rates, suggesting that
out£ow from Changjiang is a dominant source
of these two anthropogenic nuclides. For 210 Pb,
riverine input is much less than direct precipitation from the atmosphere over the ECS.
(3) The e¡ect of boundary scavenging for the
particle-reactive 210 Pb and 239;240 Pu is enhanced
by the transit of the Kuroshio and the Taiwan
Warm Currents. This process is responsible for
about one-sixth of the sedimentary inventories
of these two nuclides in the ECS. By contrast,
the ECS as a whole serves a source for the somewhat conservative 137 Cs. Of the total 137 Cs delivered into the ECS (from the drainage basin and
the atmosphere), only about one-third resides in
the ECS (sediments plus water column); the majority of the remainder is transported out.
(4) From the distribution of sedimentation
rates, a sedimentation £ux of 13U108 tons yr31
in the ECS is calculated. This is much more than
the sum of Changjiang’s sediment discharge
(5U108 tons yr31 ) and the sediment yield of Taiwan (2.6U108 tons yr31 ). It could be that sedi-
MARGO 3064 2-5-02 Cyaan Magenta Geel Zwart
176
C.-C. Su, C.-A. Huh / Marine Geology 183 (2002) 163^178
Table 1
Comparisons of some attributes between: (a) the Changjiang and the Amazon Rivers and (b) the East China Sea and the Amazon continental shelves
(a) Comparison of river length, drainage basin area, water and sediment discharge, and sediment yield
River length (km)
Drainage area (km2 )
Water discharge (m3 yr31 )
Sediment discharge (tons yr31 )
Drainage basin sediment yield (tons km32 yr31 )
(b) Comparison of the area and
210
Changjiang River
Amazon River
6300
1.8U106
9U1012
0.5U109
278
6400
6.9U106
6U1012
1.2U109
174
Pb sources in the continental shelf
East China Sea
2
Shelf area (km )
Atmospheric £ux (dpm cm32 yr31 )
Atmospheric fallout (%)
Riverine input (%)
In situ production (%)
Open ocean input (%)
a
b
c
d
Amazon Shelf
4
90U10
Nearshore 1.8a
76.4
3.9
2.9
16.8
Nearshore 1.15b
8.9U104
0.33c
2.3d
31d
0.1d
67d
DeMaster et al. (1985)
Huh (unpublished data)
DeMaster et al. (1986)
Smoak et al. (1996)
ment input versus output in the ECS is out of
equilibrium on decadal time scales, or that longdistance transport from the Yellow River’s dispersal system to the ECS is more than previously
thought.
Finally, in order to put what we have learned
from the ECS into a global perspective, it should
be informative to compare our results with corresponding data for the Amazon continental shelf.
As summarized in Table 1, the Changjiang River
and the Amazon River are fairly comparable in
some attributes; both are among the largest rivers
in the world. However, there is a contrasting difference in their drainage basin to shelf area ratios:
V2 for the ECS shelf and V78 for the Amazon
shelf. Consequently, the impact of riverine input
as a source for 210 Pb (and presumably for other
fallout nuclides as well) is much greater for the
latter, whereas atmospheric input is more important for the former. As for the e¡ect of boundary
scavenging (i.e., input from the open ocean), it
also appears to be more pronounced at the narrow Amazon shelf than at the broad ECS shelf on
per unit area basis.
Acknowledgements
We are thankful to Y.-C. Huang, K.-J. Chang,
S.-F. Lu, and S.-Y. Lee for sampling at sea and/or
radiochemical analysis in the laboratory. The
constructive reviews by the editor and Drs. S.A.
Kuehl, M. Baskaran and M.P. Bacon greatly
improved the manuscript. This work is supported
by National Science Council Grants NSC 87-2611M-001-002, 88-2611-M-001-002, and 89-2611-M001-002.
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