Radiocesium in the Taiwan Strait and the Kuroshio east of Taiwan from 2018 to 2019.

The release of anthropogenic radiocesium to the North Pacific Ocean (NPO) has occurred in the past 60 years. Factors controlling Cs (half-life, 30.2 year) and Cs (half-life, 2.06 year) activity concentrations in the Kuroshio east of Taiwan and the Taiwan Strait (latitude 20° N-27° N, longitude 116° E-123° E) remain unclear. This study collected seawater samples throughout this region and analyzed Cs and Cs activity concentrations between 2018 and 2019. A principal component analysis (PCA) was performed to analyze the controlling factors of radiocesium. Results of all Cs activity concentrations were below the detection limit (0.5 Bq m). Analyses of water column Cs profiles revealed a primary concentration peak (2.1-2.2 Bq m) at a depth range of 200-400 m (potential density σ 25.3 to 26.1 kg m). The PCA result suggests that this primary peak was related to density layers in the water column. A secondary Cs peak (1.90 Bq m) was observed in the near-surface waters (σ = 18.8 to 21.4 kg m) and was possibly related to upwelling and river-to-sea mixing on the shelf. In the Taiwan Strait, Cs activity concentrations in the near-surface waters were higher in the summer than in the winter. We suggest that upwelling facilitates the vertical transport of Cs at the shelf break of the western NPO.