Activities of the naturally occurring radionuclides, ^(210)Pb and ^(210)Po,were measured in both dissolved ( < 0.45 μm) and particulate ( > 0.45 μm) phases from surfacewaters 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 oftotal ^(210)Po and ^(210)Pb in surface waters estimated from an irreversible steady-state model were0.82 a and 1.16 a, respectively. The consistently high fractionation factor calculated either byscavenging rate constants (5.42) or K_d values (6.69) suggested that a significant fractionationoccurred between ^(210)Po and ^(210)Pb during their removal from solution to particles and that thetwo radionuclides had different biogeochemical cycling pathways in the oligotrophic South China Sea.Furthermore, our results indicated that there exist different fractionation mechanisms between^(210)Po and ^(210)Pb in different marine environments: in eutrophic ocean, plankton detritus andfecal pellets are the main carrier of ^(210)Po and ^(210)Pb, by which ^(210)Po and ^(210)Pb havebeen scavenged and removed; while in oligotrophic ocean, microbes could become the main carrier of^(210)Po and fractionate ^(210)Po and ^(210)Pb significantly as a result of scarce plankton detritusand fecal pellets. These results suggest the use of ^(210)Po to trace marine biogeochemicalprocesses relating to microbial activities and the cycling of sulfur group elements (S, Se, Te andPo).
YANG Weifeng, HUANG Yipu1,2, CHEN Min1,2, ZHANG Lei, LI Hongbin, LIU Guangshan1,2 & QIU Yusheng1,2 1. Department of Oceanography, Xiamen University, Xiamen 361005, China
