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

Wei-Jen Huang,Kai-Jung Kao,Chen-Tung Arthur Chen,Sen Jan,Kuei-Chen Huang,Ming-An Lee,Ming-Ta Lee,Yiing-Jang Yang

doi:10.1038/s41598-021-01895-y

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.

Highlights

The release of anthropogenic radiocesium to the North Pacific Ocean (NPO) has occurred in the past 60 years
Inomata et al.[6] estimated that 5.0% of the total amount of Fukushima Daiichi nuclear power plant (FDNPP)-derived 137Cs in the Subtropical Mode Water (STMW) entered into the Sea of Japan before 2016 through clockwise spreading in the western NPO, East China Sea (ECS), and the Sea of Japan
NTA values associated with low σθ values, i.e., surface waters, deviated significantly from the average value determined in subsurface and deep waters (2309 μmol kg−1) (Fig. 3d), implying that Total alkalinity (TA) values in surface waters were likely to be affected by additional river TA sources

Summary

Introduction

The release of anthropogenic radiocesium to the North Pacific Ocean (NPO) has occurred in the past 60 years. CMW forms north of the Kuroshio Extension (around 36–41° N, 160° E–165° W), is characterised by colder temperatures and a deeper layer (250–500 m)[19] These two water masses (i.e., STMW and CMW) circulate clockwise beneath the surface of the western N PO20. The subsequent injection of FDNPP-derived radionuclides resulted in a contamination plume spreading eastward and merging with the Oyashio Current; the incorporation of radiocesium in the formation of STMW and CMW and its current value as a tracer of ocean c irculation[4] Both water masses (i.e., STMW and CMW) extend clockwise to the western boundary of the NPO, where the Kuroshio is most intense. Seawater 137Cs activity concentrations have hardly been investigated in the Kuroshio east of Taiwan and the Taiwan Strait, after the 2011 event

Methods

Results

Conclusion