Transcript 投影片 1 - National Sun Yat

210Po
OS11A-03
Enrichment Relative to 210Pb in the Planktons Collected from the
Northern South China Sea and the Luzon Strait
P. Wang and Y. Chung
Institute of Marine Geology and Chemistry
National Sun Yat-sen University, Kaohsiung, Taiwan
Introduction
210Pb and 210Po, members of the 238U decay chain, are particle-reactive parent-daughter pair
radionuclides. 210Pb (half-life =22.3 yrs) tends to be associated with inorganic particles while 210Po
(half-life =138 days) with organic particles. Being preferentially scavenged by various sinking particles,
both nuclides tend to be in disequilibrium with their respective parents.
The 210Po/210Pb activity ratio in surface seawater is much less than 1 due to atmospheric 210Pb input
and 210Po consumption by planktons. The ratio in particulates collected from sediment traps in open
oceans is about 2, indicating enrichment of 210Po over 210Pb. The high ratio in the planktons around
Taiwan (about 10 to 40) indicates that these planktons are highly enriched in 210Po while depleted in
210Pb. This poster presents our measurements of 210Po and 210Pb on the plankton samples collected
by net-towing in the northern South China Sea (SCS) and the Luzon Strait in order to evaluate the
enrichment of 210Po relative to 210Pb as a means to interpret the large 210Po deficiency as observed in
the water column in the SCS.
The study area mainly coves the northern SCS and the Luzon Strait where a dozen or so stations
were chosen for several plankton net-towings in different seasons (Fig. 1). The SCS, a semi-enclosed
marginal sea west of the tropical western Pacific, covers a wide continental shelf that is receiving fresh
water from several large rivers, and the central deep basin that is over 4000m depth. It is bordering the
landmass of southern China and the Indo-China Peninsula on the north and west, and the Philippine
Island chain, Borneo and Sumatra on the east and south. Its water mass may exchange with the East
China Sea through Taiwan Strait, and with the West Philippine Sea through the Luzon Strait where the
Kuroshio water may intrude into the SCS with variable intensity in different seasons.
Figure 1. Location map for sites where plankton samples were collected.
Sampling and analysis
Plankton samples were collected by towing of plankton nets within the upper 100m layer. Plankton nets of 300 mesh size were chemically cleaned before sample collections. The
collected samples were then preserved in 5% formalin-seawater solution awaiting laboratory analyses.
In laboratory, each of the plankton samples was totally digested after a known 209Po spike and a known amount of stable Pb had been added as yield tracers. The Po isotopes in
the sample were auto-plated onto a silver disc for alpha assay of its 210Po activity. The sample was then purified for 210Pb determination by either a second alpha assay of its ingrown 210Po or counting its in-grown energetic beta 210Bi activity.
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Results and Discussion
The measured
and
activities in the planktons are displayed for the
stations arranged from west to east according to their longitudinal positions (Fig.2).
These data show significant temporal and spatial variations for both nuclides,
especially for 210Po. In summer, 2003, 210Po in the planktons varies from about 16
to 350dpm/g, while 210Pb varies generally between 7 and 28 dpm/g, indicating a
large excess or enrichment of 210Po relative to 210Pb (Fig. 2). The 210Po/210Pb ratio
(enrichment factor, EF) also varies greatly (about 6 to 38, see Fig. 3). In winter,
2004, 210Po in the planktons ranges from about 83 to 481 dpm/g, while 210Pb varies
between 2.4 and 20.3 dpm/g, indicating even a larger enrichment of 210Po relative
to 210Pb. The resulting 210Po/210Pb ratio varies from about 6 to 45. In fall, 2004,
both nuclides in the planktons of the Luzon Strait area are significantly higher than
those of the northern SCS in the other reasons (Fig. 2.). 210Po activities in the
Luzon Strait area are generally over 600 dpm/g with highest values approaching
1200 dpm/g. Most 210Pb values are over 50 dpm/g, much higher than those
observed in the northern SCS. These high values extend westward to St. F and St.
M1 in the northern SCS, implying intrusion of the Kuroshio. However, in terms of
the activity ratio, fairly similar values (~10-15) are obtained in all seasons in the two
areas (Fig. 3). It is not yet clear why much higher values of the ratio are obtained in
summer, 2003. Some particulate 210Po and 210Pb measurements on the filtered
samples whose organic matter (POM) contents were estimated (25 to 35 percent)
would allow the partition of 210Po and 210Pb between the planktons and nonplanktons (inorganic or aluminosilicate materials) to be evaluated. The 210Pb in the
planktons stays fairly low in the northern SCS. In this area, 210Po in the planktons is
about 5 to 8 times higher than in the non-organic particulates, but 210Pb in the
planktons remains at about 4 percent of the non-organic particulates. Large
variation of 210Po and 210Pb activities observed on the planktons collected from
different areas in different seasons may be due to different types and amounts of
the planktons as well as varying particulate matter properties, such as the
abundance and size distribution.
Po-210(Jul-2003)
Pb-210(Jul-2003)
Po-210(Feb-2004)
Pb-210(Feb-2004)
Po-210(Oct-2004)
Pb-210(Oct-2004)
1200
210Pb
1000
Plankton Activity(dpm/g)
210Po
800
600
200
0
H
G
A
1
I
C
J
M1
F
D
S1
S3
S4
S5
Station
Figure 2. The 210Po and 210Pb activities in the planktons of different
sampling stations in different seasons.
40
Jul-2003
Feb-2004
Oct-2004
The 210Po/210Pb activity ratio in surface water is less than unity, showing 210Po
deficiency due to 210Pb input from the atmosphere and 210Po removal by organisms
and sinking particulates. The 210Po/210Pb ratio in settling particles is about 2,
indicating a 210Po excess or enrichment over 210Pb. The 210Po/210Pb ratio is much
higher in the planktons than in the settling particles. 210Po is highly enriched while
210Pb is deficient in the planktons. The 210Pb and 210Po activities in the planktons
are clearly higher in the Luzon Strait than in the northern SCS, although there must
be a seasonal effect. The stations close to the Luzon Strait in the northern SCS
(M1 and F) also have higher activities, suggesting an intrusion of the Kuroshio
water into the northeastern corner of the SCS. This is consistent with hydrography
and other tracer distribution, such as 228Ra. The enrichment factor of 210Po relative
to 210Pb is quite similar (about 10 to 40) between the study area and the area
around Taiwan where earlier data are available. However, the activities of both
nuclides in the planktons are higher in the study area than in the Taiwan
surrounding area. The enrichment of 210Po supports the observation that 210Po is in
large deficit in the SCS water column.
Activity Ratio (EF)
30
Conclusion
20
10
0
H
G
A
1
I
C
J
M1
F
D
S1
S3
S4
Station
Figure 3.
210Po /210Pb activity ratio or enrichment factor (EF) in
the planktons, mentioned in Fig. 2.
S5