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.
Disequilibria between 210Po and 210Pb in the upper water and their potential applications as a proxy of particle export and remineralization were examined in the Southern Ocean (station IV3) and the South China Sea (NS44). 210Po was deficit in surface waters but excessive relative to 210Pb in subsurface waters. Good positive correlation between 210Po and particulate organic carbon (POC) indicated deficits and excess of 210Po resulted from particulate organic matter (POM) export and remineralization respectively, which was also supported by the decreased δ 13C and increased δ 15N downwards as a result of particle remineralization. On the basis of 210Po/210Pb box-model, POC export flux out of the surface waters were 1.2 mmol C·m-2·d-1 and 2.3 mmol C·m-2·d-1 for station NS44 and IV3, respectively. In the subsurface waters, remineralization fluxes of 210Po were 0.062 Bq·m-2·d-1 and 0.566 Bq·m-2·d-1 for station NS44 and IV3 along with the recycle efficiency of 52±26% and 119±52%, respectively. Remineralized fluxes of POM derived from 210Po and exported POC were 0.6 mmol C·m-2·d-1 and 2.7 mmol C·m-2·d-1 for NS44 and IV3. This study suggested that 210Po was a powerful tracer of particle export and remineralization.
YANG WeiFeng HUANG YiPu CHEN Min QIU YuSheng PENG AnGuo ZHANG Lei
Activities of the naturally occurring radionuclides, 210Pb and 210Po, 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 210Pb, 0.23 Bq/m3 (n=23), accounted for about 12% of the total 210Pb, which corresponds with values of open oceans. Particulate 210Po, with an av-erage activity of 0.43 Bq/m3, accounted for about 40% of the total 210Po, which was much higher than those of open and eutrophic oceans. The residence times of total 210Po and 210Pb in surface waters estimated from an irreversible steady-state model were 0.82 a and 1.16 a, respectively. The consis-tently high fractionation factor calculated either by scavenging rate constants (5.42) or Kd values (6.69) suggested that a significant fractionation occurred between 210Po and 210Pb 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 frac-tionation mechanisms between 210Po and 210Pb in different marine environments: in eutrophic ocean, plankton detritus and fecal pellets are the main carrier of 210Po and 210Pb, by which 210Po and 210Pb have been scavenged and removed; while in oligotrophic ocean, microbes could become the main carrier of 210Po and fractionate 210Po and 210Pb significantly as a result of scarce plankton detritus and fecal pellets. These results suggest the use of 210Po to trace marine biogeochemical processes relat-ing to microbial activities and the cycling of sulfur group elements (S, Se, Te and Po).
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
