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

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.

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 2

10Pb 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.

Figure 1. Location map for sites where plankton samples were collected.

Station

H G A 1 I C J M1 F D S1 S3 S4 S5

Pla

nkto

n A

ctiv

ity(d

pm/g

)

0

200

600

800

1000

1200

1400Po-210(Jul-2003)Pb-210(Jul-2003)Po-210(Feb-2004)Pb-210(Feb-2004)Po-210(Oct-2004)Pb-210(Oct-2004)

Station

H G A 1 I C J M1 F D S1 S3 S4 S5

Act

ivity

Rat

io (E

F)

0

10

20

30

40Jul-2003Feb-2004Oct-2004

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.

OS11A-03