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 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 ¹³⁷Cs 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, 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.