210Po and 210Pb disequilibrium at the PN section in the East China Sea

Publication date: August 2017
Source:Journal of Environmental Radioactivity, Volume 174
Author(s): Kaijun Su, Jinzhou Du, Mark Baskaran, Jing Zhang
Lead-210 and ²¹⁰Po 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 ²¹⁰Po/²¹⁰Pb 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 ²¹⁰Po was observed with respect to its parent nuclide ²¹⁰Pb (i.e., a ²¹⁰Po/²¹⁰Pb activity ratio < 1.0). The (²¹⁰Po/²¹⁰Pb)dissolved, (²¹⁰Po/²¹⁰Pb)particulate and (²¹⁰Po/²¹⁰Pb)total 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 (Kd) of ²¹⁰Po and ²¹⁰Pb were 12.1× 10⁴ ml g⁻¹ and 8.8× 10⁴ ml g⁻¹, with an average (²¹⁰Po/²¹⁰Pb) total 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 ²¹⁰Po, which preferentially scavenges ²¹⁰Po and produces a lower (²¹⁰Po/²¹⁰Pb) total activity ratio (0.49 ± 0.12, n = 22) with a Kd for ²¹⁰Po of 13.8× 10⁴ ml g⁻¹ in the water column. The variations in the fractionation factor (1.48 ± 0.66) of ²¹⁰Po/²¹⁰Pb reveal distinct differences between the distribution and scavenging of ²¹⁰Po and ²¹⁰Pb by particulate matter in different marine environments: in the estuarine zone (a high turbidity area), terrigenous suspended particulate matter scavenges ²¹⁰Pb from the water column, while in areas dominated by biogenic particular matter, ²¹⁰Po is preferentially scavenged from the water column. Using the ²¹⁰Po/²¹⁰Pb disequilibrium in the water column, we estimated the removal fluxes of POC from the upper waters downward to be 25.0 mg C m⁻² d⁻¹, comparable to those in other marginal seas. Moreover, a decreasing trend of POC removal fluxes was observed with increasing distance offshore.



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