1. Particulate and Dissolved 210Po and 210Pb in Water Columns from the
OS11A-05
Particulate and Dissolved 210Po and 210Pb in Water Columns from the
East and West of the Luzon Strait
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 2200m) 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 210Po and 210Pb 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 226Ra and 228Ra. This paper is to focus on the 210Po and 210Pb 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 HNO3 (pH < 1) and then brought back to laboratory together with the particulate sample for further analyses.
Po analysis
Each sample was spiked with a known amount of 209Po and Pb(NO3)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 (pH=2) for spontaneous deposition of polonium isotopes onto a silver disc for 3 hours at 60oC. 210Po activities on the disc were counted with a silicon surface-barrier detector attached to an alpha spectrometer (EG & G Ortec 576).
Fig. 1 Location map for sites where water samples were collected.
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 HNO3. After repeating the step twice, the sample was dissolved in 100 ml D.D. water and Na2CrO4 was added to precipitate PbCrO4 that was filtered through 0.45 μm filter. The precipitate was dissolved in 100 ml 1.5N HCl and loaded onto a preconditioned 1.5 N HCl anion exchange column (AG 1-X8 resin, mesh). The Pb retained on the column was eluted with hot D.D. water (70-85oC). Concentrated H2SO4 was then added to precipitate PbSO4 which was dried, ground and evenly deposited onto an aluminum disc for counting of the energetic beta particles emitted by 210Bi, the daughter of 210Pb, using a low background gas-flowing anti-coincidence beta counter (Tennelec α/β counting system , LB5100).
3.Results and discussion
Figure 2 shows the 210Pb and 210Po 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 210Pb is higher at St. 5 than at St. F, probably due to advective transport from the Equatorial Current to the Kuroshio. The dissolved 210Po is only slightly higher at St. 5 than at St. F, but particulate 210Po and 210Pb at both stations are comparable although quite scattered.
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 total 210Po is higher at St. F than at St.5 above 1500m depth, but it is reversed at 1800m depth. The total 210Pb is higher at St. 5 than at St. F (Fig. 3).
The specific 210Po 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 210Po activity in surface water (2m) 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).
The specific 210Pb activity in surface water (2m) 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 210Po and lower 210Pb are observed in the planktons, and lower 210Po and higher 210Pb 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 210Po 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 1800m ( ), suggesting a 210Po 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.