Continental Shelf Research 19 (1999) 1049}1064
Lead-210 and polonium-210 in the winter
well-mixed turbid waters in the mouth
of the Yellow Sea
Gi-Hoon Hong *, Sun-Kyu Park , M. Baskaran,
Suk-Huyn Kim , Chang-Soo Chung , Sang-Han Lee
Korea Ocean Research and Development Institute, Ansan P.O. Box 29, Seoul 425-600, South Korea
Department of Marine Science, Texas A and M University, Galveston, TX 77551, USA
Received 2 February 1998; accepted 16 December 1998
Abstract
Concentration pro"les of Pb and Po were measured along a traverse of the mouth of
the Yellow Sea in February 1993. Winter time suspended particulate matter concentration was
more than 10}100 mg l\ in the coastal domain and less than 10 mg l\ in the central domain.
Concentrations of dissolved Pb over the area were low ((5 dpm kg\) due to the e$cient
removal of Pb from the water column over the shelf. Evidence for release of Po is seen in
a sub-surface layer, close to the sediment}water interface, where Po is enriched in the
dissolved form and depleted in the particles. The high concentration of SPM in the mouth of the
Yellow Sea appears to determine dissolved and particulate Pb and Po activities. The
atmospheric input of Pb is the major source of Pb in the region with minor contribution
from the Pb rich Kuroshio Water. The K values of Po varied by a factor of 50 while the
corresponding values of Pb varied only by a factor of 4. It appears that in waters where
particle concentrations are high ('10 mg l\), the K appears to be independent of particle
concentration. 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Pb; Po; Turbid waters; Yellow sea
1. Introduction
Po (t "0.378 y) has a potential utility as tracer for biogeochemical processes,
such as primary production, zooplankton grazing, degradation of particles, etc. on
time scales less than a few years because of its half-life and highly mobile nature
* Corresponding author.
0278-4343/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 8 - 4 3 4 3 ( 9 9 ) 0 0 0 1 1 - 4
1050
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
(Shannon et al., 1970; Nozaki et al., 1990). Earlier studies have concluded that
polonium behaves more like the nutrient elements (Kharkar et al., 1976; Fisher et al.,
1983). Several studies of oceanic Pb (t "22.3y) distribution have shown that
scavenging from the water column is enhanced at the continental margins relative to
the open ocean (e.g. Bacon et al., 1976). Extensive measurements on Po and its
parent Pb in open - ocean surface waters have shown that the Po/Pb activity
ratio, although geographically variable, averages about 0.5 (Tsunogai and Nozaki,
1971; Nozaki et al., 1976; Bacon et al., 1976). The mean ratio corresponds to the
box-model mean residence time of 0.6 y, which is a factor of 2}3 shorter than that of
atmospherically derived Pb, suggesting the preferential removal of Po relative
to Pb from the surface water. However, the Po/Pb activity ratio below 100 m
are close to unity, since the deep-sea Pb residence times are generally of the order
of a few decades (Craig et al., 1973; Broecker and Peng, 1982). It was observed that
Po is enriched relative to Pb in soft tissues of marine organisms (Shannon et al.,
1970; Kharkar et al., 1976). In oxygenated seawater the biological uptake may be
more important than inorganic adsorption for Po scavenging while the opposite is
true for Pb (Fisher et al., 1983; Kadko, 1993; Wei and Murray, 1994). The
Fig. 1. Sampling locations of YS9302 expedition.
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
1051
preferential scavenging of Po relative to Pb in photic zone caused primarily by
biological uptake and such a mechanism would not cause any signi"cant radioactive
disequilibrium between Po and Pb in the deep water as a whole (Nozaki et al.,
1990; Bacon et al., 1988). In areas where hydrothermal activity is strong (East Paci"c
Rise), enhanced scavenging of Po in association with hydrothermal activity was
observed (Kadko et al., 1987).
The Yellow Sea (including Bohai Sea), bounded by contiguous landmass of China
and Korean Peninsula, has a total area of 0.42;10 km with average depth of 44 m
(Fig. 1). The sea is also characterized as extremely turbid water caused by the ample
supply of riverine particulate matter and resuspension from the bottom. The Yellow
Sea is also experiencing seasonal strati"cation during summer. Nozaki et al. (1991)
reported that during water column strati"cation period in summer, Po/Pb
activity ratio averages about 0.3 and the residence times of Pb and Po in the
waters were about 2 months in the mouth of the Yellow Sea. Total Po and Pb
concentrations could also undergo seasonal change in the temperate regions. Tanaka
et al. (1983) reported that total Po and Pb concentrations were high in winter
and low in summer in the seasonally strati"ed Funka Bay, Japan. However, relatively
few studies have been undertaken simultaneously to determine Pb and Po
concentrations in the dissolved and particulate phases in seawater (Chung and Finkel,
1988; Bacon et al., 1988; Nozaki et al., 1990; Wei and Murray, 1994). We have
measured Pb and Po in the dissolved and particulate phases during winter in
order to (1) establish the detailed Po } Pb disequilibrium relationship in the
water column and (2) elucidate the fractionation of Po and Pb between dissolved and
particulate phases as a function of suspended particulate loading.
2. Sampling and methods
In the framework of Yellow Sea Coastal Ocean Flux Program which was aimed to
address the origin and biogeochemical provenance of water masses using temperature, salinity and dO, primary productivity using C incubation and heavy metal
distribution in the sea for the period of 1991 to 1993, winter cruise using RV Eardo
was carried out from 4 February to 18 February 1993 in the mouth of the Yellow Sea.
Fig. 1 shows the station locations at which Po and Pb pro"les were measured.
Stations D7 and D9 are in the relatively less turbid central water domain, stations
D1}D5 are in the western shallow water domain, and stations D10}D11 are in the
eastern shallow water domain (Fig. 2c). Continuous temperature and salinity (conductivity) pro"les and discrete water samples were obtained by a Sea-Bird CTD 25
assembly "tted with twelve 10-l Niskin bottles. Discrete salinity samples were taken to
check the accuracy of the CTD data. Samples for suspended particulate matter (SPM)
were "ltered through the pre-combusted at 5503C for 3 h and pre-weighed 0.7 lm
GF/F Glass "ber "lters, and washed 5}6 times with distilled water to remove salt in
the "lter and kept in frozen on board. The frozen "lters were freeze-dried in the
laboratory. The dried "lters were weighed and SPM concentrations were determined
by subtracting original "lter weight. The precision was estimated to be less than 4%.
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G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
Fig. 2. Depth distribution of temperature (a), salinity (b), and SPM (c) concentration in the mouth of the
Yellow Sea.
Two aliquots of 3 l samples were passed through a 0.7 lm pore size, 47 mm
diameter glass "ber "lter for collection of particulate phase of Pb and Po, and
the "ltered water was stored in a polyethylene bottle. Shortly after collection, the
"ltered water samples were acidi"ed to pH 2 with concentrated HCl. The Po spike
(0.18 dpm) and iron carrier (50 mg as Fe) were added to one of the aliquots and Po
(together with Po) was coprecipitated with ferric chloride and isolated from Pb
by plating Po onto a silver disk. This was done in the laboratory within a week after
sampling. The electroplated disk was counted for polonium in a high-resolution alpha
spectrometer coupled to a multichannel analyzer. The Po activity was decay
corrected from the time of collection to mid-counting date. The other aliquot was
stored for more than 2 y, during which time, the in situ Po decayed away
completely and Po produced from Pb reached an almost equilibrium value with
Pb in the water and then Po was electroplated onto silver disks and alpha counted.
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
1053
For particulate Po activity determination, one of the two "lters used was digested
with Conc. HF, HNO , HCl successively after adding Po spike. Polonium was
electroplated from the "nal solution and alpha counted. The other "lter was also
stored for more than 2 y, during which time, Po reached an almost equilibrium
value with Pb in SPM on the "lter. The Pb activity was calculated from the
Po value (Nozaki et al., 1991). The Pb decay correction for the time allowed for
the ingrowth of Po was applied.
3. Results and discussion
The hydrographic data are shown in Fig. 2. The water column is well mixed at this
time of the year (Fig. 2b). The distribution and variability of water properties in the
Yellow Sea are in#uenced by strong seasonal variations of river discharge, air}sea
interaction, tidal mixing and the Kuroshio Current. The seasonal cycle of freshwater
discharge from these rivers dominates the surface distribution of water properties in
the sea, especially during summer when the river input is the largest. Air}sea interaction is responsible for the vertical strati"cation of water masses in the sea where the
water is mixed vertically by strong surface cooling and wind mixing during winter and
restrati"ed by strong surface heating during summer. Non-linear interaction between
strong semi-diurnal tidal currents over the bottom in the shallower regions can cause
both strong tidal mixing and generate subtidal #ow (Lie, 1989; Suk et al., 1996).
During 4}10 February 1993 in the mouth of the Yellow Sea, water temperature was
the highest (123C) in the central region of the Yellow Sea trough, and lowest in the
coastal regions of China and Korea. Salinity was the highest (34.20 psu) in the Yellow
Sea trough, and got lower approaching coasts of China and Korea. Temperature and
salinity are higher in the Korean coast than in the Chinese coast. Both temperature
and salinity were vertically homogenous. Suspended particulate matter (SPM) concentrations were very high (SPM '30 mg l\) in the shallow region (less than 40 m
depth), and relatively low ((10 mg l\) in the central region. Winter condition in the
mouth of the Yellow Sea may be characterized by the warm saline water that intrude
into the Yellow Sea from the southeast to the northwest. The Yangtz River-derived
water was reported to extend only up to 1243E 32.53N (Hong et al., 1995a). The results
of Po and Pb are given in Table 1 and plotted against depth in Fig. 3.
3.1. Pb proxles
In general, dissolved Pb activities were less than 3 dpm/100 l in the western
turbid shallow domain and less than 2 dpm/100 l in the central clear water region (Fig.
3). Particulate Pb activities increased with depth and high in the shallow domains
(Fig. 4). Speci"c Pb activities in SPM ranged between 4 and 23 dpm (g SPM)\
with values low in the western shallow domain and high values in the eastern shallow
domain. These values are considerably higher than the 0}1 cm section of the bottom
sediment core. Pb activity in the sur"cial sediment varies from 2.8 to 8.0 dpm
g\ (Hong et al. unpublished, DeMaster et al., 1985). The Yellow Sea water is
1054
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
Fig. 3. Depth distributions of dissolved Po (a), particulate Po (b), dissolved Pb (c), and particulate
Pb (d) in the mouth of the Yellow Sea.
characterized by Pb de"ciency (dissolved phase as well as total) relative to Ra
value (Elsinger and Moore, 1984; Nozaki et al., 1991), which is indicative of removal
by scavenging. Most of the Pb in the upper part of the water column is supplied by
deposition from the atmosphere, which is presumed to be relatively uniform over the
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
1055
Fig. 4. Speci"c activities of Po (a) and Pb (b) in the suspended particulate matter in the mouth of the
Yellow Sea.
entire area. The strong concentration gradient across the front is due to variations in
the e$ciency of removal by particles relative to regeneration in the water column.
This horizontal gradient should give rise to a net #ux of Pb across the frontal zone
onto the central region.
3.2. Po proxles
Dissolved Po activities peak at the sub-surface at the western turbid waters
(4}8 dpm/100 l) and are low ((2 dpm/100 l) and vertically homogenous in the saline
clear central waters (Fig. 3). Particulate Po activities gradually increase with depth
(Fig. 4). Speci"c Po activities on suspended particles ranged between 4 and 13 dpm
(g SPM)\ and particles in the western shallow domain is more enriched with Po
than those in the eastern shallow domain. The behavior of Po is best observed by
taking its activity di!erence relative to the parent nuclide Pb (excess Po). Thus
a value of zero will occur when Po is in radioactive equilibrium with Pb. The
dissolved excess Po was nearly zero in the central domain and enriched in the
western and eastern shallow domains (Fig. 5). The particulate excess Po was nearly
zero in the central domain and enriched in the surface waters of the eastern shallow
domain, while particles in the bottom layer show a depletion of Po with respect to
Pb indicating regeneration of Po from the particulate (Fig. 5). The desorption of
Po from the particulate matter only partly balances the excess in the dissolved
form. This indicates a net transport of dissolved Po out of the mouth of the Yellow
Sea.
1056
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
Fig. 5. Excess dissolved Po (a) and particulate Po (b) in the mouth of the Yellow Sea. The excess
amount was obtained by subtracting Pb activity from Po activity.
It has been suggested that zooplankton grazing and fecal pellet production may be
important for the vertical transport of Po (Beasley et al., 1978). For Po,
biological uptake is more important than inorganic adsorption on to suspended
particles (Fisher et al., 1983; Kadko, 1993) while the opposite is true for Pb. This
contrasting behavior of Pb and Po for particles can be seen from the empirical
distribution coe$cients and fractionation factor for the two nuclides. The distribution
coe$cient K is given by
K "C /(C ;TSPM),
where C and C are concentrations of the nuclides in particulate and dissolved forms,
respectively, and TSPM is the total suspended particulate matter concentration. The
K values for Po varied between 3.9;10 cmg\ and 331;10 cmg\. The
K values for Po were high in the central domain and an order of magnitude lower
in the shallow domains due to di!erences in the concentration of SPM. The K values
for Pb were more uniform, varying between 1.7;10 cm g\ and 7.2;10 cm
g\ (Table 1). This relatively constant values of K for Pb could suggest that the
distribution of Pb is less dependent on the concentration of particles. The fractionation factor, F
, is simply the ratio of the distribution coe$cients, K./K.
..
(Bacon et al., 1988). The fractionation factor varied between 0.19 and 6.4. For Po
the highest distribution coe$cients are generally found in the surface layer, and high
fractionation factors 1 indicate the preferential uptake of Po by phytoplankton
relative to Pb by inorganic particles (Shannon et al., 1970). Minimum values of
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
1057
K. and F
are generally found in the near sediment-water interface zone where
..
most of the particles are inorganic; biogenic particles are likely regenerated before
reaching the sediment-water interface (Fig. 6). Wei and Murray (1994) also observed
that the K for Po in the euphotic zone is much greater than that of Pb. In
terrigenous particles, such as aluminosilicates, the Po is expected to be radioactive
equilibrium with its parent, Pb while in biogenic particles, gross disequilibrium is
possible. The fractionation factor F
could be an important parameter to charac..
terize biotic versus abiotic oceanic particles.
3.3. Mass balance for Pb in the mouth of the Yellow Sea
The major source of Pb to the continental shelf is the atmospheric deposition.
The measured atmospheric depositional #ux of Pb for Ansan, mid-western coast of
Korean Peninsula (for a period of 18 months) is 2.0 dpm cm\ y\ (Nozaki et al. 1991;
Park, 1993). Supply of Pb by rivers is generally assumed to be insigni"cant due to
rapid scavenging of Pb by riverine suspended sediment particles and e$cient
trapping of sediments in estuaries (Benninger et al., 1975). An additional supply of
Pb to the shelf is from the Kuroshio Current, due to higher concentrations of
dissolved Pb o!shore and exchange across the shelf-Kuroshio front (Nozaki et al.,
1991). This input can be estimated from a knowledge of the average residence time of
water in the mouth of the Yellow Sea (3}4 years, Nozaki et al., 1991), the in#ux of
Kuroshio Water (&6;10 m y\, Hong et al., 1995b), and concentration of Pb
in the Kuroshio Water (26.6 dpm /100 l, Nozaki et al., 1991). Using these values, the
oceanic input of Pb is estimated to be about one-"fth of the atmospheric input.
In order to identify if the scavenging of particle-reactive nuclide in this area is
controlled by diapycnal (vertical scavenging) processes or isopcynal transport (lateral
transport), the steady state #ux of Pb to the bottom sediment can be compared to
the inputs (atmospheric fallout # input from Kuroshio Current) for shallow waters.
The Pb #ux to the bottom sediment along the mouth of the Yellow Sea have been
determined using excess Pb down core distribution and the values range between
0.1 and 2.9 dpm cm\ yr\ in the central (0.14, 0.74, 0.80 dpm cm\ yr\) and the
western shallow (2.9 dpm cm\ yr\) domains (Hong GH, unpublished data). Sediment inventories of excess Pb vary from 4 to 94 dpm cm\ (4.3, 23.84, 25.55, 94.51
dpm cm\) depending upon the sediment accumulation rate (Hong, unpublished
data). Total Pb inventory in the water column is about 1 dpm cm\ (Table 1)
which is much less than the sediment inventory. Although the sediment inventories in
general are comparable to the input #ux, isopycnal transport is stronger than
diapycnal transport in the sea. Strong tidal current could potentially redistribute the
particles in shallow waters. There are other shelf areas where the sediment inventories
are comparable to the atmospheric fallout. Bacon and Belastock (1991) observed that
the sediment inventories of Pb are similar to the inventory predicted from the
atmospheric supply in the shelf sediments of mid-Atlantic Bight since the Pb laden
silt- and clay- particles are e$ciently trapped on the shelf because deposition is
followed by downward mixing into the sediment column by benthic organisms. The
sediment inventories of Pb in shelf and slope regions of the Gulf of Mexico are
1058
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
Table 1
Pb/Po disequilibria and oceanographic variables in the mouth of the Yellow Sea during winter 1993
(YS9302).
Stn.
Depth
(m)
Pb (dpm/100 l)
Dissolved
Particulate
Po(dp m/100 l)
Dissolved
Particulate
Po/Pb
Dissolved
D1
0
10
30
0
10
30
0
10
30
50
0
10
30
50
75
0
10
30
50
100
0
30
50
65
0
10
30
55
1.99$0.25
1.90$0.27
1.75$0.29
1.84$0.31
3.33$0.37
1.31$0.25
1.99$0.27
1.79$0.26
2.23$0.26
2.46$0.44
1.84$0.28
1.48$0.23
1.52$0.18
1.64$0.19
1.49$0.21
1.81$0.25
2.30$0.39
2.69$0.25
1.93$0.24
1.95$0.20
3.25$0.33
3.11$0.28
3.01$0.27
2.50$0.33
ND
5.28$0.54
4.06$0.60
4.11$0.78
2.8$0.8
1.5$0.5
9.3$3.0
1.2$0.6
11.1$1.5
1.4$0.4
0.3$0.2
2.6$0.8
1.0$0.2
2.2$0.7
1.6$0.4
1.4$0.3
0.5$0.2
1.6$0.4
1.4$0.4
5.6$1.5
2.8$0.6
1.2$0.3
1.6$0.6
2.3$0.7
4.7$1.0
3.8$0.9
3.7$0.8
4.1$1.1
ND
5.7$1.0
6.4$1.1
7.6$1.5
1.41$0.18
0.79$0.11
5.31$0.90
0.65$0.11
3.33$0.38
1.07$0.21
0.15$0.02
1.45$0.22
0.45$0.05
0.89$0.16
0.87$0.14
0.95$0.15
0.33$0.04
0.98$0.12
0.94$0.14
3.09$0.44
1.22$0.21
0.45$0.04
0.83$0.11
1.18$0.13
1.45$0.15
1.22$0.12
1.23$0.12
1.64$0.22
ND
1.08$0.12
1.58$0.24
1.85$0.36
D3
D5
D7
D9
D 10
D11
29.8$0.8
31.9$0.7
41.3$0.9
42.8$0.9
55.7$1.1
55.0$1.3
14.7$0.6
13.9$0.5
19.2$0.7
15.0$0.7
5.7$0.3
6.2$0.4
9.5$0.4
8.1$0.4
7.3$0.6
5.8$0.4
6.4$0.5
5.0$0.3
4.6$0.3
29.9$0.8
19.1$0.7
26.4$0.8
38.4$1.0
59.1$1.2
ND
21.4$0.8
31.3$1.0
33.6$1.0
29.4$0.9
45.7$1.4
42.6$1.3
$1.3
41.2$1.2
64.3$1.9
14.1$0.4
12.0$0.4
16.8$0.5
12.5$0.4
1.2$0.0
6.6$0.2
10.0$0.3
7.8$0.2
8.5$0.3
6.9$0.2
5.4$0.2
4.5$0.1
3.9$0.2
33.3$1.1
16.0$0.5
23.5$0.7
40.6$1.2
20.5$0.6
08.5$0.3
61.4$1.8
29.9$0.9
32.0$1.0
comparable to the expected inventory based on atmospheric fallout (Baskaran and
Santschi, 1999). Similar observation was also reported for the shelf regions of the East
Chukchi Sea, Alaskan Arctic (Baskaran and Naidu, 1995).
3.4. Water column stratixcation and seasonal variations of the concentrations of SPM,
Po, and Pb
The surface water concentrations of Pb and Po were much lower in summer
than in winter. Nozaki et al. (1991) sampling was carried out during May}June 1987
and thus may represent summer conditions and our data were obtained during
February 1993 and could represent winter conditions (Hong et al. 1995a). May}June
1987 sampling period appears to be strati"ed as evidenced by the depletion of
dissolved silica concentration (Nozaki et al., 1991). Total (dissolved and particulate)
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
Pb R¹
(Month)
0.41
0.52
0.77
0.84
1.54
1.43
1.60
1059
K (cmg\)
Po
K (cmg\)
Pb
K./K.
Temp.
(3C)
Sal
(psu)
SPM
(mg/l)
2.67E#05
3.96E#05
8.32E#04
6.63E#05
3.85E#04
6.22E#05
3.31E#06
2.05E#04
5.68E#05
1.93E#05
1.37E#05
5.55E#05
1.84E#06
4.38E#05
5.34E#05
2.78E#05
3.03E#05
5.78E#05
2.34E#05
3.97E#05
2.16E#05
2.19E#05
2.17E#05
9.35E#04
ND
4.96E#05
1.38E#05
ND
3.80E#05
2.19E#05
4.29E#05
4.17E#05
1.74E#05
5.68E#05
5.19E#05
3.42E#05
2.92E#05
2.08E#05
5.48E#05
4.96E#05
5.76E#05
4.42E#05
4.32E#05
7.22E#05
4.40E#05
2.86E#05
2.28E#05
4.20E#05
3.72E#05
3.00E#5
2.52E#05
4.43E#05
ND
1.87E#05
2.27E#05
ND
0.70
1.81
0.19
1.59
0.22
1.09
6.38
0.60
1.95
0.93
0.25
1.12
3.20
0.99
1.24
0.39
0.69
2.02
1.03
0.94
0.58
0.73
0.86
0.21
ND
2.65
0.61
ND
8.72
8.76
8.79
8.54
8.47
8.46
11.45
11.46
11.47
11.46
12.51
12.51
12.51
12.51
12.49
ND
10.27
10.33
10.94
11.87
7.70
7.71
7.72
7.48
8.53
8.53
8.64
32.1
32.1
32.1
31.6
31.7
31.7
34.0
33.9
33.9
33.9
34.2
34.3
34.3
34.3
34.3
ND
33.6
33.7
33.8
34.0
33.1
33.1
33.1
33.1
33.2
33.2
33.2
39.3
76.8
55.0
55.8
96.3
73.8
14.2
22.6
29.5
29.3
5.6
8.5
10.9
11.1
11.4
4.4
6.3
6.5
10.5
36.5
15.8
28.3
50.6
53.4
6.6
21.7
34.0
activities of Pb and Po varied between 4 and 10 dpm/100 l and between 0.8 and
3.4 dpm/100 l, respectively, in summer (Nozaki et al., 1991), and 8 and 44 dpm/100
l and 3 and 46 dpm/100 l (Table 1), respectively, in winter. Total Po and Pb
concentrations were also observed to be high in winter and low in summer in the
northern temperate seasonally strati"ed Funka Bay, Japan (Tanaka et al., 1983). SPM
loading in the water column undergoes seasonal changes due to seasonality of the
forcing functions. In general, SPM concentrations are high during winter and low in
summer (Milliman et al., 1986; Wells, 1988).
3.5. Residence time of Po and Pb in the water column
As discussed before Pb and Po concentrations are considerably lower in the
shelf waters than in the Kuroshio Current (Nozaki et al., 1991), and Pb in the
surface water is predominantly of atmospheric origin. The atmospheric #ux of Pb
1060
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
Fig. 6. Percentage of particulate Po and Pb in the mouth of the Yellow Sea.
in this region is about 2 dpm cm\ yr\ (Nozaki et al., 1991), which is much higher
than in situ production from Ra in the water (about 0.03 dpm cm\ yr\ in
a 100 m water column). The input #ux from Kuroshio Current was estimated to be
&0.4 dpm cm\ yr\. Since the decay of Pb in seawater is also small, the mean
residence time of Pb in the entire water column with respect to particle scavenging
is obtained simply by dividing the amount of Pb (dpm cm\) in the water column
by the combined input from atmosphere and Kuroshio Current (dpm cm\ yr\).
The calculated Pb residence times are given in Table 1. The Pb residence time
was less than 1 month in the turbid western shallow water domain and about
2 months in the clear water domain as observed during summer by Nozaki et al.
(1991).
Nozaki et al. also suggested that Po residence times (RT) could be calculated by
the following equation:
RT "(R/(1!R));1/j ,
.
.
where R is the (Po/Pb) ratio in the water and j is decay constant of Po. In
.
deriving the above equation, the atmospheric #ux of Po is ignored because it is
small compared to in situ production from Pb in the water (Bacon et al., 1976;
Nozaki and Tsunogai, 1976). However, the data in the mouth of the Yellow Sea do not
allow us to make a simple box-model calculation, yielding unreasonably long Po
residence times. This is presumably due to the high suspended load with the
(Po/Pb) ratio of &1 (an equilibrium value expected for terrestrial particles
older than 2 yr, Table 1). As noted by Nozaki et al. (1991), the residence times of Pb
in the shelf water are short compared to the residence times of the waters of 2}3 y
(Nozaki et al., 1991). Thus, it is likely that this reactive nuclide is largely deposited on
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
Fig. 7. Plot of Log K versus Log SPM for (a) Po and (b) Pb versus Log SPM.
1061
1062
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
the shelf sediments prior to transport from the shelf to the open ocean. This conclusion probably applies to other land-derived heavy metals and pollutants that have
chemical reactivities similar to Pb, as suggested by Nozaki et al. (1991).
3.6. Control of trace metal concentrations in coastal seawater through partition onto
suspended particulate matter
It has been shown that the concentration of particle-reactive nuclides in areas where
the suspended particle concentrations are high ('10 mg l\), a major fraction of the
nuclide exists in the particulate form (for example, Baskaran and Santschi, 1993). The
distribution coe$cient for particle-reactive nuclides, such as Th, has been observed to
be correlated with SPM concentration (Honeyman and Santschi, 1989). Balls (1988)
had shown that the distribution of metal concentration between dissolved and
particulate phases in coastal seawater depends on the amount of SPM loading. Due to
the high loading of SPM, Pb and Po are mostly distributed in the particulate
phase (Fig. 6). Percentage of total Pb concentration in particulate phase in the
mouth of the Yellow Sea is similar to the stable Pb in the coastal waters in the North
Sea (Balls, 1988). However, percentage of total Pb concentration in particulate
phase is more strongly dependent upon SPM loading than that of total Po
concentration in particulate phase.
The distribution coe$cient, K , of Po and Pb, is plotted against the particle
concentration in Fig. 7. For other particle-reactive nuclides such as Be and Th,
negative log K versus log SPM correlations have been previously observed and this
observation is known as the particle concentration e!ect (McKee, 1986; Honeyman
et al., 1988). Several hypotheses have been put forward to explain this observation,
including kinetics, irreversible adsorption, "ltration artifacts, particle}particle interaction and presence of colloid-bound metals which are often included in the "ltrate
fraction. Of these, the explanation due to the arti"cial separation of dissolved and
particulate phases (due to presence of colloid-bound metals) is widely accepted
(Honeyman et al., 1988). The lack of inverse correlation between log K of Po
(or Pb) with log SPM clearly indicates that there is no particle concentration e!ect
in areas where particle concentrations are greater than 10 mg l\ and the variations in
particle-concentration is less than two orders of magnitude.
4. Conclusions
We have measured the dissolved and particulate concentrations of Po, Pb
and SPM concentrations in the mouth of the Yellow Sea. From the present investigation, the following conclusions can be drawn: (i) The high concentration of SPM in the
mouth of the Yellow Sea appears to determine dissolved and particulate Pb and
Po activities. (ii) The residence time of Pb was less than 1 month in the turbid
western shallow water domain and about 2 months in the clear water domain. (iii) The
atmospheric input of Pb is the major source of Pb in this region with minor
contribution from the Pb rich Kuroshio Water. (iv) The K values of Po varied
G.-H. Hong et al. / Continental Shelf Research 19 (1999) 1049}1064
1063
almost by a factor of 85 while the K for Pb varied only by a factor of 4. Thus, the
K values of Po as well as the ratio of K./K. can be utilized to infer the nature of
particles, viz., biogenic versus abiogenic particles. (v) In coastal waters where particle
concentrations are greater than 10 mg l\ and variations on particle concentration is
rather narrow, the K s appear to be independent of SPM (no particle concentration
e!ect).
Acknowledgements
We thank the crew of the R/V Eardo for their assistance in sample collection. We
also thank two anonymous reviewers for their considerate and thorough reviews. This
work was supported by Ministry of Science and Technology and Ministry of Environment, Korea, under the Grants in Aid BSPN 203 and BSPN 218.
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