[A-HW22] 水循環・水環境

[AHW22-19] Anthropogenic iodine-129 in the Japan Sea Bottom Water and Dense Shelf Water of Okhotsk Sea

*松中 哲也¹、長尾 誠也¹、井上 睦夫¹、落合 伸也¹、森田 貴己²、三木 志津帆²、荒巻 能史³、工藤 勲⁴、本多 直人²、滝川 哲太郎⁵、笹 公和⁶、本多 真紀⁶、末木 啓介⁶ (1.金沢大学環日本海域環境研究センター、2.水産研究・教育機構、3.国立環境研究所、4.北海道大学、5.長崎大学、6.筑波大学)

キーワード：日本海、オホーツク海、底層水、大陸棚高密度水、ヨウ素129

Long-term oceanographic observation revealed that warming and oxygen decrease of the Japan Sea Bottom Water (JSBW) and Dense Shelf Water (DSW) of Okhotsk Sea are responding to air temperature raise in winter [1–2]. The investigation of water dynamics in the Japan Sea and Okhotsk Sea using radioactive tracer are essential for elucidating the interaction between climate change and these convection systems. Anthropogenic ¹²⁹I (T_1/2 = 15.7 million years) produced from the thermal neutron fission, is dominated by release from nuclear fuel reprocessing plant in the Europe. To illuminate the availability of ¹²⁹I as a tracer of surface circulations and vertical convections in the Japan Sea and Okhotsk Sea, we investigated the horizontal and vertical distributions of the ¹²⁹I at the large area of these seas in 2017. The dissolved ¹²⁹I in surface water varied from 17.3±0.7 to 23.1±1.0 nBq L^–1 at the area of 38–46°N and 135–141°E in the Japan Sea, and was negatively correlated with salinity (R² = 0.82, n = 9). This salinity-dependent distribution revealed that the dissolved ¹²⁹I in the area was controlled by mixing of water mass from the Liman Current to the Tsushima Current. Meanwhile, the dissolved ¹²⁹I in the JSBW observed at the layer of 2480–3500 m of the Japan Basin in 2017 was 4.1±0.5 nBq L^–1, which increased by 1.1 nBq L^–1 in comparison to that in 2007 [4]. The estimated turnover time for the JSBW of 205±25 years using the ¹²⁹I was within the range of 75–380 years evaluated by bomb-derived ¹⁴C [5–6]. The ¹²⁹I is considered to be a tracer for the surface subarctic front and deep JSBW in the Japan Sea.

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[2] T. Nakanowatari et al., Geophys. Res. Lett., 34, 1955–2004 (2007)
[3] T. Suzuki et al., Nucl. Instr. Meth. B, 294, 563–567 (2013)
[4] T. Suzuki et al., Nucl. Instr. Meth. B, 268, 1229–1331 (2010)
[5] T. Gamo and Y. Horibe, J. Oceanogr. Soc. Japan, 39, 220–230 (1983)
[6] Y. Kumamoto et al., J. Oceanogr., 64, 429–441 (2008)