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 Cs-137 (half-life, 30.2 year) and Cs-134 (half-life, 2.06 year) activity concentrations in the Kuroshio east of Taiwan and the Taiwan Strait (latitude 20 degrees N-27 degrees N, longitude 116 degrees E-123 degrees E) remain unclear. This study collected seawater samples throughout this region and analyzed Cs-134 and Cs-137 activity concentrations between 2018 and 2019. A principal component analysis (PCA) was performed to analyze the controlling factors of radiocesium. Results of all Cs-134 activity concentrations were below the detection limit (0.5 Bq m(-3)). Analyses of water column Cs-137 profiles revealed a primary concentration peak (2.1-2.2 Bq m(-3)) at a depth range of 200-400 m (potential density sigma(theta:) 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 Cs-137 peak (1.90 Bq m(-3)) was observed in the near-surface waters (sigma(theta) = 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, Cs-137 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-137 at the shelf break of the western NPO.