Sinking fluxes of 210Pb and 210Po in the deep basin of the northern South China Sea

Vertical fluxes of total mass (Fmass), particulate organic carbon (FPOC), particulate inorganic carbon (FPIC), 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, 116°00°E, 18°00°N) in the northern South China Sea during June 2008-June 2009. The Fmass ranges from 12.2 to 55.1 mg m−2 d−1 and from 89.3 to 250.8 mg m−2 d−1, at 2000 m and 3500 m, respectively, and shows seasonal and inter-annul variation. The temporal variation of FPOC, FPIC, and FPb-210 were in phase with the Fmass, which was coupled with the seasonal cycles of primary production in the euphotic layer. The FPb-210 ranges from 5 to 48 dpm m−2d−1 and from 38 to 105 dpm m−2d−1, at 2000 m and 3500 m, respectively. Contrasting with 210Pb, the FPo-210 shows poor correlation with Fmass. The FPo-210 ranges from 3 to 146 dpm m−2d−1 and from 50 to 309 dpm m−2d−1, at 2000 m and 3500 m, respectively. Episodic events of the settling of biological particles from the surface layer and the regeneration processes the deep layer control the 210Po removal in the water column of the South China Sea. Strong correlations of the flux and source ratio of 210Pb, (F/P)Pb-210, and the particulate carbon fluxes were found, which give relationships of FPOC (μg cm−2y−1) = 26.8 + 371.0 (F/P)Pb-210 and FPIC (μg cm−2y−1) = −1.4 + 533.1 (F/P)Pb-210.