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

ABSTRACT

The release of anthropogenic radiocesium to the North Pacific Ocean (NPO) has occurred in the past 60 years. Factors controlling 137Cs (half-life, 30.2 year) and 134Cs (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 134Cs and 137Cs activity concentrations between 2018 and 2019. A principal component analysis (PCA) was performed to analyze the controlling factors of radiocesium. Results of all 134Cs activity concentrations were below the detection limit (0.5 Bq m-3). Analyses of water column 137Cs profiles revealed a primary concentration peak (2.1-2.2 Bq m-3) at a depth range of 200-400 m (potential density σθ 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 137Cs peak (1.90 Bq m-3) was observed in the near-surface waters (σθ = 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, 137Cs 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 137Cs at the shelf break of the western NPO.