Liu Guangshan

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

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 average 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 consistently 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 fractionation 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 relating to microbial activities and the cycling of sulfur group elements (S, Se, Te and Po).

AMS Measurement of 53 Mn at CIAE

The long-lived isotope 53Mn cosmogenically produced in situ is a very useful indicator for geochronological dating. A new 53Mn accelerator-mass-spectrometry (AMS) measurement method is developed based on the HI-13 Tandem Accelerator and ΔE-Q3D detection system at China Institute of Atomic Energy. The performance of 53Cr isobar separation and suppression is tested by a series of 53Mn standards and commercial MnO samples. The results show that the ΔE-Q3D detection system brought about an overall suppression factor of more than 107 for the interfering isobar 53Cr.