| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 98 === Four vertical profiles of dissolved and particulate of 210Pb and 210Po were measured at the SouthEast Asian Time-series Study (SEATS) station in the northern South China Sea (18°N and 116°E) from October 2007 to June 2008. In this study, floating traps and moored traps were also deployed to directly measure the fluxes of 210Pb and 210Po at the site.
All the profiles show a large 210Pb deficiency relative to 226Ra in the deep water of the South China Sea. According to this study, the 210Pb/226Ra ratio range between 0.3 and 0.5, similar to the ratio to the findings in the Arabian Sea. Compared with the 210Pb profiles, 210Po profiles display a small deficiency with respect to 210Pb. Significant variation of 210Po profiles were found among the four cruises. The data of this study is more consistent with Obata et al. (2004) than with Chung and Wu (2005).
The residence time of 210Pb is ~50 years in the open ocean. Since the removal is more intense in the marginal seas, i.e., East China Sea and Japan Sea, in which more particulate matters was found, the residence time of 210Pb in these regions is shorter than 15 years. By using the irreversible scavenging model, we calculated the residence time of 210Pb in the deep water of the South China Sea, which range between 7 and 29 years. Shorter residence of 2 years for 210Po in the deep layer of the South China Sea was found. The scavenging rate of 210Po in the South China Sea is similar to the Arabian Sea.
Based on the deficiencies of 210Pb and 210Po in the water column, the removal fluxes of 210Pb and 210Po at 3500 m are 39~46 dpm/m2/day and 430~1033 dpm/m2/day, respectively. Direct measurements of limited samples collected by the moored traps (KK6 and KK7) at same depth show that the 210Pb and 210Po fluxes are between 31.5~51.1 dpm/m2/day and between 25.8~52.2 dpm/m2/day, respectively. The 210Pb removal flux estimated from the scavenging model is comparable with the directly measured flux, whereas 210Po removal flux is much higher than that directly measured by the moored sediment traps. Large temporal variability of particle flux at the site is proposed to be the cause of large discrepancy between the modeled 210Po flux and the measured flux.
We also deployed the floating traps to collect sinking particle at 30 m, 100 m and 160 m during the four cruises. The 210Pb fluxes range between 9 and 36, 8 and 34, 14 and 37 dpm/m2/day, respectively. The 210Po fluxes range from 31 to 127, from 11 to 108, from 12 to 42 dpm/m2/day at 30 m, 100 m and 160 m, respectively. The removal fluxes estimated from the 210Pb/226Ra and 210Po/210Pb disequilibrium at 160 m, are 11~27 dpm/m2/day and 5.8~31.2 dpm/m2/day, for 210Pb and 210Po, respectively.
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