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

摘要

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 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 results 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 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 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).