Seasonal distributions and fluxes of ²¹⁰Pb and ²¹⁰Po in the northern South China Sea

Abstract. Vertical distributions of dissolved and particulate ²¹⁰Pb and ²¹⁰Po in the water column at the SouthEast Asian Time-series Study (SEATS, 18°00´ N and 116°00´ E) station in the northern South China Sea were determined from four cruises between January 2007 and June 2008. A large deficiency of ²¹⁰Pb, 379 ± 43 × 10³ dpm m⁻², from the secular equilibrium was found within the 3500 m water column. On the other hand, a smaller deficiency of ²¹⁰Po, 100 ± 21 × 10³ dpm m⁻², relative to ²¹⁰Pb was found in the water column. Time-series data showed insignificant temporal variability of the ²¹⁰Pb and ²¹⁰Po profiles. To balance these deficiencies, the removal fluxes for ²¹⁰Pb and ²¹⁰Po via particle settling ranging from 45 to 51 dpm m⁻²d⁻¹ and from 481 to 567 dpm m⁻²d⁻¹, respectively, are expected at 3500 m. The ²¹⁰Pb removal flux is comparable with, whereas the ²¹⁰Po removal flux is much higher than, the flux directly measured by moored sediment traps. The discrepancy between the modeled ²¹⁰Po flux and the measured flux suggests that sporadic events that enhance ²¹⁰Po removal via sinking ballast may occur in the water column at the site.