Temporal Variation of Sinking Fluxes of 210Pb and 210Po in the Deep Basin of the South China Sea

Vertical fluxes of total mass (Fmass), 210Pb (FPb-210) and 210Po (FPo-210) were determined by sediment traps deployed at two depths, 2000 m and 3500 m, at SEATS (South East Asian Time-series Study,116º00 º E,18 º 00 º N) in the northern South China Sea (SCS) during September 2013 - May 2015. The Fmass varied seasonally, ranging from 27.0 to 447.2 mg m-2d-1 and from 30.8 to 528.4 mg m-2d-1 at 2000 m and 3500 m, respectively. The FPb-210 ranged from 7.7 to 48.4 dpm m-2d-1 and from 7.0 to 98.5 dpm m-2d-1 at the two depths, respectively. Noticeable correlation between the Fmass and the FPb-210 was found, indicating the dominant role of particle settling in the 210Pb removal from the water column of the SCS. The FPo-210 ranged from 3.5 to 233.1 dpm m-2d-1 and from 24.8 to 427.9 dpm m-2d-1 at 2000 m and 3500 m, respectively. Similar to the findings of literature data in 2008 and 2009, both the FPb-210 and the FPo-210 tend to increase with depth. Concerns were raised regarding the effect of particle decomposition on the solubilization of elements into dissolved phase in sediment traps during sample storage. The effect of 210Po regeneration and its influence on the accuracy of the determination of the FPo-210 were evaluated. Based on the 210Po concentration in the supernatant of the sampling cups, it is found that the 210Po was less than 1% of the total activity in the particles, implying that the effect of re-solubilization is minimal and no correction was needed for the FPo-210 determination. Based on the literature data of vertical profiles and the fluxes determined by this study, the budgets of 210Pb and 210Po in South China Sea basin were summarized. Due to the evident difference of the 210Pb activity in the Western Philippine Sea and the South China Sea, boundary scavenging plays an important role on 210Pb cycling in the SCS basin. On the contrary, less effect of water exchange between the two oceans was found for 210Po cycling in the SCS.