Low Level Radioactivity Laboratory, Kanazawa University, O-24, Nomi, Ishikawa, 923-1224, Japan.
Fisheries Resources Institute, 116 Katsurakoi, Kushiro, Hokkaido, 085-0805, Japan.
Sci Rep. 2023 May 9;13(1):7524. doi: 10.1038/s41598-023-34775-8.
We studied the spatiotemporal variations in Cs, Cs, and Ra concentrations at the sea surface off southeastern Hokkaido, Japan (off-Doto region) from 2018 to 2022 using low-background γ-spectrometry. The Cs concentrations in the off-Doto region, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, exhibited wide lateral variation each year (e.g., 0.7-1.1 mBq/L in 2020). By studying the Ra concentrations and salinity, this variation was explained based on the current mixing patterns. Furthermore, the Cs concentrations in the waters highly affected by the Oyashio Current (OYC) gradually increased from 2018 to 2020, and subsequently decreased in 2022. This implies that the water mass maximally contaminated with Cs was transported back to the side of the Japanese islands 10 years after the FDNPP accident along with counter-clockwise currents (e.g., the OYC) in the northern North Pacific Ocean. The Cs concentrations in the OYC-affected waters in the off-Doto region in 2020 were ~ 1/6 times those in the Cs-enriched core of waters off the western American Coast in 2015, which can be ascribed to dilution via spatial dispersion during subarctic current circulation. Overall, we elucidated the ocean-scale subarctic current systems in the northwestern North Pacific Ocean, including water circulation timespans.
我们使用低本底γ谱仪研究了 2018 年至 2022 年期间日本北海道东南部（远藤地区）海域海面 Cs、Cs 和 Ra 浓度的时空变化。远藤地区的 Cs 浓度（校正到福岛第一核电站事故发生的日期）每年都有很大的横向变化（例如，2020 年为 0.7-1.1 mBq/L）。通过研究 Ra 浓度和盐度，根据当前的混合模式解释了这种变化。此外，受亲潮（OYC）影响较大的水域中的 Cs 浓度从 2018 年到 2020 年逐渐增加，随后在 2022 年下降。这意味着，在福岛第一核电站事故发生 10 年后，Cs 污染最严重的水团沿着北太平洋的逆时针环流（如亲潮）被输送回日本群岛一侧。2020 年远藤地区受亲潮影响水域中的 Cs 浓度约为 2015 年美国西海岸富 Cs 水域核心区的 1/6，这可归因于亚北极海流循环过程中的空间扩散稀释。总的来说，我们阐明了北太平洋西北部的海洋尺度亚北极海流系统，包括水的循环时间跨度。
