Particulate and Dissolved 210 Po and 210 Pb
Particulate and Dissolved 210 Po and 210 Pb in Water Columns from the East and West of the Luzon Strait OS 11 A-05 Yu-Tien Su and Y. Chung Institute Marine Geology and Chemistry National Sun Yat-sen University Kaohsiung, Taiwan 1. Intoduction The Luzon Strait between Taiwan and the Luzon Island is where the Kuroshio intrudes into the South China Sea (SCS). Its deep sill depth (about 2200 m) also allows transport and exchanges of the deep water and its properties between the SCS and the West Philippine Sea (WPS). During a recent cruise (October, 2004) conducted for biogeochemical studies in the Luzon Strait area, water samples were collected and filtered on board for measurements of 210 Po and 210 Pb in both the particulate and dissolved phases in attempt to decipher the transport and exchange processes via the distribution of these and other radionuclides, including 226 Ra and 228 Ra. This paper is to focus on the 210 Po and 210 Pb distributions at a location east of the Strait (St. 5) along the Kuroshio path and a location west of the Strait (St. F) in the northeast SCS (Fig. 1). 2. Experimental methods 2. 1. Sampling and on-board procedures Seawater samples of 20 liters size were collected from these two stations using a rosette sampler. Each water sample was filtered through 0. 45 µm filter to separate the particulate and dissolved phases. The filtered water was acidified with 50 ml of concentrated HNO 3 (p. H < 1) and then brought back to laboratory together with the particulate sample for further analyses. 2. 2. 210 Po analysis Each sample was spiked with a known amount of 209 Po and Pb(NO 3)2, and then proceeded for chemical digestion (particulate sample) and co-precipitation (dissolved sample). The purified sample was dissolved in diluted HCl as the plating solution (p. H=2) for spontaneous deposition of polonium isotopes onto a silver Fig. 1 Location map for sites where water samples were collected. disc for 3 hours at 60 o. C. 210 Po activities on the disc were counted with a silicon surface-barrier detector attached to an alpha spectrometer (EG & G Ortec 576). 2. 3. 210 Pb analysis After spontaneous deposition of polonium, the remaining solution was evaporated and then the oxidized residue was dissolved in 1 ml of 9 N HNO 3. After repeating the step twice, the sample was dissolved in 100 ml D. D. water and Na 2 Cr. O 4 was added to precipitate Pb. Cr. O 4 that was filtered through 0. 45 μm filter. The precipitate was dissolved in 100 ml 1. 5 N HCl and loaded onto a preconditioned 1. 5 N HCl anion exchange column (AG 1 -X 8 resin, 100 -200 mesh). The Pb retained on the column was eluted with hot D. D. water (70 -85 o. C). Concentrated H 2 SO 4 was then added to precipitate Pb. SO 4 which was dried, ground and evenly deposited onto an aluminum disc for counting of the energetic beta particles emitted by 210 Bi, the daughter of 210 Pb, using a low background gasflowing anti-coincidence beta counter (Tennelec α/β counting system , LB 5100). 3. Results and discussion Figure 2 shows the 210 Pb and 210 Po profiles in both the dissolved and particulate phases for Station F (St. F) west of the Luzon Strait and Station 5 (St. 5) east of the strait. It is quite clear that the dissolved 210 Pb is higher at St. 5 than at St. F, probably due to advective transport from the Equatorial Current to the Kuroshio. The dissolved 210 Po is only slightly higher at St. 5 than at St. F, but particulate 210 Po and 210 Pb at both stations are comparable although quite scattered. The total 210 Po is higher at St. F than at St. 5 above 1500 m depth, but it is reversed at 1800 m depth. The total 210 Pb is higher at St. 5 than at St. F (Fig. 3). The specific 210 Po activity in the particulate phase ranges from 140 to 1120 dpm/g at St. F; it ranges from 65 to 490 dpm/g at St. 5, much lower than at St. F. The specific 210 Po activity in surface water (2 m) particulates is lower (257 dpm/g at St. F; 278 dpm/g at St. 5) than that in the plankton samples (623 dpm/g at St. F; 1019 dpm/g at St. 5). Fig. 2 Particulate and dissolved Pb-210 and Po-210 at St. F and St. 5. Fig. 3 Total Pb-210, total Po-210 and Po/Pb from St. F and St. 5. The specific 210 Pb activity in surface water (2 m) particulates is much higher (293 dpm/g at St. F; 820 dpm/g at St. 5) than that in the plankton samples (65 dpm/g at St. F; 85 dpm/g at St. 5). As the particulate samples contain both organic and inorganic particles while the plankton samples are almost entirely organic, higher 210 Po and lower 210 Pb are observed in the planktons, and lower 210 Po and higher 210 Pb are seen in the filtered particulate matter. The Po/Pb ratio is higher at St. F than at St. 5 (Fig. 3), but both stations show large 210 Po deficiency. The ratio falls between 0. 3 and 0. 7, corresponding to a scavenging mean residence time of about 3 to 15 months with the lower end at St. 5. The ratio at St. F tends to increase somewhat with depth to 1800 m (0. 6 -0. 7), suggesting a 210 Po contribution from dissolution or decomposition of sinking organic particulates. Clearly, the distribution and variation of both nuclides are different between the east and west of the Luzon Strait, reflecting the Kuroshio effect.
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