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

Activities of the naturally occurring radionuclides, Pb-210 and Po-210, were measured in both dissolved (< 0.45 mu m) and particulate (>0.45 gm) phases from surface waters of the southern South China Sea. The average activity of particulate Pb-210, 0.23 Bq/m(3) (n=23), accounted for about 12% of the total Pb-210, 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 Po-210 and Pb-210 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 Po-210 and Pb-210 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 results indicated that there exist different fractionation mechanisms between Po-210 and Pb-210 in different marine environments: in eutrophic ocean, plankton detritus and fecal pellets are the main carrier of Po-210 and Pb-210, by which Po-210 and Pb-210 have been scavenged and removed; while in oligotrophic ocean, microbes could become the main carrier of Po-210 and fractionate Po-210 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).