Large 210Po deficiency in the northern South China Sea

Po-210 and Pb-210 were measured on the suspended particulate matter (SPM) and filtered water samples collected in profile from seven stations in the northern South China Sea (SCS), including the Luzon Strait, during three cruises aboard R/V Ocean Researcher I in 2000. The dissolved Pb-210 profiles generally display a maximum around 15-23 dpm/100 kg in the surface layer due to atmospheric input. This maximum as an excess Pb-210 may penetrate to a few hundred meters depth, and a minimum varying from 5 to 15 dpm/100 kg is observed around 500-1000 m. Below 1000 m, the dissolved Pb-210 either remains unchanged or increases to a maximum of about 19dpm/100kg around 2500-3000m depth. The particulate Pb-210 is nearly constant around 3-5 dpm, 100 kg for the entire water column. The dissolved Po-210 profiles are somewhat parallel to the dissolved Pb-210 profiles generally with much lower activity. The particulate Po-210 is quite comparable to the particulate Pb-210 at all the stations. Both the dissolved and the total (dissolved + particulate) Po-210 to Pb-210 activity ratios vary between 0.1 and I with a mean of 0.6, indicating a large and variable deficiency of Po-210 relative to Pb-201 in the northern SCS. The mean ratio corresponds to a Po-210 scavenging mean residence time of about 10 months. Since the SPM values are generally 0.2 mg/kg or less, the high and comparable particulate Po-210 and Pb-210 activities obtained from the SPM samples yield very high and variable specific activities for these nuclides, ranging mostly from about 100 to 500 dpm/g. The time-averaged particulate flux in the deep water (3000 m) at a southern and a northern mooring station in the study area is about 250 and 550 mg/m(2)/d, respectively. The Po-210 flux in the deep water may vary at least from about 75 to 165 dpm/m(2)/d if the mean Po-210 specific activity of the SPM at comparable depth (about 300dpm/g) is assumed to be the mean Po-210 activity of the trapped particulates. However, even the upper value of 165 dpm/m(2)/d can only account for about 20% of the total Po-210 removal rate estimated from the total water column Po-210 deficiency at about the same depth. Even greater imbalance has been observed in the Sargasso Sea where Po-210 flux has been measured from sediment traps. Our observation supports a recent hypothesis that Po-210 may be absorbed or utilized by plankton biomass through bacteria and cyanobacteria assimilation in an oligotrophic ocean. Po-210 in the deep layer is deficient due to removal by preferential scavenging of organic particulates which may be transported laterally in addition to vertical sinking. (c) 2005 Elsevier Ltd. All rights reserved.