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
Size-fractionated 210Po and 210Pb, in the size fractions >0.4 μm, >2 μm and >10 μm, were examined to determine the seasonal variability of particulate fluxes in Xiamen Bay. Good correlations between 210Po and particulate organic carbon (POC) or non-Particulate Organic Matter (nPOM) suggested that 210Po can be used to trace the export fluxes of POC and nPOM. Both steady-state (SS) model and nSS model were used to evaluate fluxes of size-fractionated 210Po, results showed that nSS model was better than the SS model in coastal areas. Based on the nSS model, size-fractionated POC fluxes decreased with increasing particle size. For the particle size studied, maximum POC fluxes occurred in autumn, followed by spring, winter, and summer. Fluxes of nPOM were an order of magnitude higher than the corresponding size-fractionated POC fluxes. Differences between size-fractionated nPOM fluxes indicated that hydrodynamic conditions were the main factor regulating transportation of particulate out of the inner Bay. In winter most particulates, including >10 μm particles, were transported under the strongest hydrodynamic conditions. In contrast, only a fraction of the <2 μm particulates were transported from the inner Bay in spring. This study suggested that 210Po is a powerful tracer of seasonal particulate export in coastal seas.
Surface seawater was collected for ^(226)Ra measurement in the North Pacific Subtropical Gyre from July to October, 1999 and October to December, 2003. Combined with the historical data reported for this sea area, a declined trend of surface ^(226)Ra concentrations was observed since 1960s, indicating the ecosystem shift in response to global warming. On one side, the enhanced stratification of the upper water column resulting from global warming reduced the ^(226)Ra input from the depth, on the other, the temporal increase of biological production resulting from the climate-related ecosystem structure change strengthened the ^(226)Ra removal from the surface ocean. Both the physical and biological processes resulted in the decrease of surface ^(226)Ra concentrations in the North Pacific Subtropical Gyre. The temporal trend of surface ^(226)Ra concentrations was consistent with the trends of chlorophyll a, silicate, phosphate and primary production previously reported. This study provided ^(226)Ra evidence for the ecosystem shift under global change.
YANG JunHongCHEN MinQIU YuShengLI YanPingMA QiangLUE EZHANG RunHUANG YiPu
