Distribution and budget of 137Cs in the China Seas

The Kuroshio radiocesium stream

The Fukushima accident released short-lived Cs-134 and longer-lived Cs-137 to the ocean. The amount, although substantial, is much less than that produced during the atomic bomb tests 60 years ago. But, the latter has received little attention. Here we found only Cs-137 in waters near the origin of the Kuroshio. The speed of the Kuroshio current generally decreases with water depth, yet, the Cs-137 activity increases with depth to reach a subsurface maximum of 2.4 Bq m^-3. As a result, a core of high Cs-137 flux, or a radiocesium stream, exists at approximately 200-400 m in depth. In total, the Kuroshio transports about 1 PBq year^-1 Cs-137 northward between 121 and 123°E, 1000 times more than the 0.73-1.0 TBq year^-1 discharged to the ocean at Fukushima in 2016-2018.

Establishing historical 90Sr activity in seawater of the China seas from 1963 to 2018

Historical ⁹⁰Sr activity in seawater was established in the China seas from 1963 to 2018. Based on the exponential decrease in ⁹⁰Sr activity in seawater, the effective half-life (EHL) of ⁹⁰Sr was quantified to be 11.5 ± 1.6 a, 16.5 ± 2.4 a, 27.2 ± 6.2 a, and 26.7 ± 4.3 a in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea, respectively. We found contrasting patterns in the EHL of ⁹⁰Sr and ¹³⁷Cs in the marginal seas and open oceans that were closely related to the subtly different pathways of ⁹⁰Sr and ¹³⁷Cs in marine environment. Additionally, we demonstrated that Fukushima-derived ⁹⁰Sr (<0.01 Bq/m³) would be difficult to identify in the China seas. Our study not only provided the key parameter of the EHL in marine models for predicting the ⁹⁰Sr activity in the China seas in the post-Fukushima era but also enhanced our understanding of ⁹⁰Sr behavior and its fate in marine environments.

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

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.