210Po Enrichment Relative to 210Pb in the Planktons Collected from the

1
210Po 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
OS11A-03
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 in-grown 210Po or counting its in-grown
energetic beta 210Bi activity.
Results and Discussion The measured 210Po
and 210Pb 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 non-planktons
(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. Conclusion
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.
Figure 2. The 210Po and 210Pb activities in the
planktons of different
sampling stations in different seasons.
Figure 3. 210Po /210Pb activity ratio or
enrichment factor (EF) in the
planktons, mentioned in Fig. 2.