||The purposes of this study are to understand and compare: the distributions of the particulate and dissolved 210Po and 210Pb in the northern South China Sea and in the east and west sides of the Luzon Strait; the associated biogeochemical processes, including their mechanisms and rates involved in the area; and the disequilibrium between 226Ra and the 210Pb as observed in profiles. Seawater samples were collected from three stations in the northern South China Sea (station I, J, and F) and one station to the east of the Luzon Strait (station S5) using a Rosette sampling system during three Ocean Researcher I cruises (ORI-688 in July, 2003, ORI-707 in February, 2004, and ORI-734 in October, 2004).|
The dissolved 210Pb in surface water generally displays higher activities (around 14-28 dpm/100kg) relative to deep water, reflecting atmospheric input. The dissolved 210Pb profile as seen at station F displays a maximum of 26 dpm/100kg at 200 m and lower values at about 10 dpm/100kg below 1500 m. Highest value at 28 dpm/100kg was observed at S5 surface water. It is probably due to being on the path of the Kuroshio which has accumulated atmospheric 210Pb continually from the northern Equatorial Current on its way.
The mean residence time with respect to particle scavenging is about 0.62 years for total 210Po within a 50 m mixed layer, and ranges between 0.34 to 1.13 years for total 210Pb in the mixed layer. 210Po is deficient relative to 210Pb in most profiles, suggesting that 210Po is more readily scavenged.
The mean scavenging residence time is about 87 years for total 210Pb in the deep water at station F, and 60 years at station J. This residence time is longer than that in both the Pacific and East China Sea deep waters, i.e. a lower scavenging rate in the South China Sea. As the upwelling rate is much higher in the South China Sea than in the Pacific, it may effectively reduce the sinking rate of the sinking particulates, resulting in higher 210Pb residence time in the water columns of the South China Sea.