Disequilibria between 210Po and 210Pb in surface waters of the southern South China Sea and their implications

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Activities of the naturally occurring radionuclides, 210 Pb and 210 Po, were measured in both dissolved ( 0.45 mm) phases from surface waters of the southern South China Sea. The average activity of particulate 210 Pb, 0.23 Bq/m 3 ( n =23), accounted for about 12% of the total 210 Pb, which corresponds with values of open oceans. Particulate 210 Po, with an average activity of 0.43 Bq/m 3 , accounted for about 40% of the total 210 Po, which was much higher than those of open and eutrophic oceans. The residence times of total 210 Po and 210 Pb in surface waters estimated from an irreversible steady-state model were 0.82 a and 1.16 a, respectively. The consistently high fractionation factor calculated either by scavenging rate constants (5.42) or K d values (6.69) suggested that a significant fractionation occurred between 210 Po and 210 Pb during their removal from solution to particles and that the two radionuclides had different biogeochemical cycling pathways in the oligotrophic South China Sea. Furthermore, our re-sults indicated that there exist different fractionation mechanisms between 210 Po and 210 Pb in different marine environments: in eutrophic ocean, plankton detritus and fecal pellets are the main carrier of 210 Po and 210 Pb, by which 210 Po and 210 Pb have been scavenged and removed; while in oligotro-phic ocean, microbes could become the main carrier of 210 Po and fractionate 210 Po and 210 Pb significantly as a result of scarce plankton detritus and fecal pellets. These results suggest the use of 210 Po to trace marine biogeochemical processes relating to microbial activities and the cycling of sulfur group elements (S, Se, Te and Po).