Po and Pb disequilibrium at the PN section in the East China Sea.

Lead-210 and Po have been widely used as tracers for quantifying particulate scavenging in the upper layer of the oceanic water column. In this study, we investigated the Po/Pb disequilibrium in the water column of the PN section in the East China Sea (ECS) during autumn 2013. In most of the water column, a deficiency of Po was observed with respect to its parent nuclide Pb (i.e., a Po/Pb activity ratio < 1.0). The (Po/Pb), (Po/Pb) and (Po/Pb) activity ratios ranged from 0.29 to 0.71 (average: 0.53 ± 0.13, n = 27), 0.31 to 1.42 (average: 0.70 ± 0.27, n = 27) and 0.22 to 0.62 (average: 0.50 ± 0.12, n = 27), respectively. The distribution coefficients (K) of Po and Pb were 12.1× 10 ml g and 8.8× 10 ml g, with an average (Po/Pb) activity ratio of (0.50 ± 0.12, n = 27). However, over the continental shelf, planktonic detritus and fecal pellets appear to be the main carriers for Po, which preferentially scavenges Po and produces a lower (Po/Pb) activity ratio (0.49 ± 0.12, n = 22) with a K for Po of 13.8× 10 ml g in the water column. The variations in the fractionation factor (1.48 ± 0.66) of Po/Pb reveal distinct differences between the distribution and scavenging of Po and Pb by particulate matter in different marine environments: in the estuarine zone (a high turbidity area), terrigenous suspended particulate matter scavenges Pb from the water column, while in areas dominated by biogenic particular matter, Po is preferentially scavenged from the water column. Using the Po/Pb disequilibrium in the water column, we estimated the removal fluxes of POC from the upper waters downward to be 25.0 mg C m d, comparable to those in other marginal seas. Moreover, a decreasing trend of POC removal fluxes was observed with increasing distance offshore.