Abstract
The release of anthropogenic radiocesium to the North Pacific Ocean (NPO) has occurred in the past 60 years. Factors controlling (137)Cs (half-life, 30.2 year) and (134)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 (134)Cs and (137)Cs activity concentrations between 2018 and 2019. A principal component analysis (PCA) was performed to analyze the controlling factors of radiocesium. Results of all (134)Cs activity concentrations were below the detection limit (0.5 Bq m(-3)). Analyses of water column (137)Cs profiles revealed a primary concentration peak (2.1-2.2 Bq m(-3)) at a depth range of 200-400 m (potential density σ(θ:) 25.3 to 26.1 kg m(-3)). The PCA result suggests that this primary peak was related to density layers in the water column. A secondary (137)Cs peak (1.90 Bq m(-3)) was observed in the near-surface waters (σ(θ) = 18.8 to 21.4 kg m(-3)) and was possibly related to upwelling and river-to-sea mixing on the shelf. In the Taiwan Strait, (137)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 (137)Cs at the shelf break of the western NPO.