Large 210Po deficiency in the northern South China Sea

Abstract

210Po and 210Pb 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 210Pb profiles generally display a maximum around 15–23 dpm/100 kg in the surface layer due to atmospheric input. This maximum as an excess 210Pb 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 210Pb either remains unchanged or increases to a maximum of about 19 dpm/100 kg around 2500–3000 m depth. The particulate 210Pb is nearly constant around 3–5 dpm/100 kg for the entire water column. The dissolved 210Po profiles are somewhat parallel to the dissolved 210Pb profiles generally with much lower activity. The particulate 210Po is quite comparable to the particulate 210Pb at all the stations. Both the dissolved and the total (dissolved+particulate) 210Po to 210Pb activity ratios vary between 0.1 and 1 with a mean of 0.6, indicating a large and variable deficiency of 210Po relative to 210Pb in the northern SCS. The mean ratio corresponds to a 210Po 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 210Po and 210Pb 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 210Po flux in the deep water may vary at least from about 75 to 165 dpm/m 2/d if the mean 210Po specific activity of the SPM at comparable depth (about 300 dpm/g) is assumed to be the mean 210Po 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 210Po removal rate estimated from the total water column 210Po deficiency at about the same depth. Even greater imbalance has been observed in the Sargasso Sea where 210Po flux has been measured from sediment traps. Our observation supports a recent hypothesis that 210Po may be absorbed or utilized by plankton biomass through bacteria and cyanobacteria assimilation in an oligotrophic ocean. 210Po 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.