Distribution of Iodine-129 in the Japan Sea and the Bering Sea: Oceanographic Insights
Takashi Suzuki¹, Shigeyoshi Otosaka², Jun Kuwabara¹, Jun Nishioka^3
1Japan Atomic Energy Agency, ²The University of Tokyo, ³Hokkaido University

Introduction
Iodine-129 (half-life: 1.6 × 10⁷ years) is a cosmogenic radionuclide. Anthropogenic ¹²⁹I has been released primarily through atmospheric nuclear weapons testing and the operation of nuclear fuel reprocessing facilities. Smaller amounts were also released during the Chernobyl and Fukushima nuclear accidents. The largest source of ¹²⁹I emissions is nuclear fuel reprocessing plants operating in the United Kingdom and France, from which released ¹²⁹I has dispersed globally.
Atmospheric ¹²⁹I emitted from European reprocessing plants has been transported globally and deposited onto the Earth's surface. The ocean acts as a major reservoir for this radionuclide. The concentration of ¹²⁹I in seawater provides insight into atmospheric transport pathways as well as its behavior in the ocean interior, making it a useful oceanographic tracer ¹.

Experiment
Seawater samples for ¹²⁹I analysis were collected during the OS07 cruise by T/V Oshoro-maru and SY09-5 by R/V Soyo-maru in the Japan Sea, and during the Mu-18 cruise by R/V Multanovskiy in the Bering Sea. Samples were collected from the surface to deep layers using Niskin bottles, while temperature and salinity were measured with a conductivity–temperature–depth (CTD) profiler.
The collected seawater samples were transported to the laboratory. Iodine was extracted using solvent extraction and precipitated as silver iodide (AgI). The iodine isotopic ratio (¹²⁹I/¹²⁷I) was measured by accelerator mass spectrometry (AMS) at the Aomori Research and Development Center of the Japan Atomic Energy Agency ².

Results and Discussion
¹²⁹I in Surface Seawater
The concentrations of ¹²⁹I in surface seawater ranged from 1.3 to 2.1 × 10⁷ atoms/L in the Japan Sea and from 1.2 to 1.4 × 10⁷ atoms/L in the Bering Sea. In the Japan Sea, concentrations tended to increase northward, whereas they were nearly constant with latitude in the Bering Sea.
The sampling areas were located at 38.5–42.0ºN in the Japan Sea and 55.9–64.0ºN in the Bering Sea. Compared with the Pacific Ocean at similar latitudes (36.8–42.1ºN), where ¹²⁹I concentrations range from 1.3 to 1.8 × 10⁷ atoms/L, the Japan Sea shows comparable levels, whereas lower concentrations were observed in the higher-latitude Bering Sea.
This difference can be attributed to the major source of ¹²⁹I—European nuclear fuel reprocessing plants located around 50ºN. ¹²⁹I released from these facilities is transported by the prevailing westerly, resulting in elevated concentrations along transport pathways and lower influence outside these regions.
Inventory of ¹²⁹I
The vertical profiles of ¹²⁹I in both the Japan Sea and the Bering Sea show higher concentrations at the surface, decreasing with depth. At the same depth, concentrations in the Japan Sea were consistently higher than those in the Bering Sea.
The total inventory from the surface to the seafloor was estimated to be 13.2–18.0 × 10¹² atoms/m² in the Japan Sea and 4.2–6.7 × 10¹² atoms/m² in the Bering Sea. Compared to reported values in the Pacific Ocean (6.3–7.4 × 10¹² atoms/m²), the Japan Sea exhibits approximately twice the inventory, indicating significant accumulation of ¹²⁹I.
This is likely due to the transport of ¹²⁹I deposited in the surface layer to the deep ocean via water mass subduction, as well as the semi-enclosed nature of the Japan Sea, which limits exchange with the open ocean.
Estimation of Turnover Time of Japan Sea Bottom Water
The ¹²⁹I concentrations in the Japan Sea Bottom Water (JSBW) below 2000 m depth ranged from 0.18 to 0.42 × 10⁷ atoms/L, significantly higher than the natural background level in seawater (0.03 × 10⁷ atoms/L). This indicates that anthropogenic ¹²⁹I has reached the bottom water.
Based on the fact that anthropogenic ¹²⁹I has been released since 1945, the turnover time of the JSBW was estimated using an approach analogous to that employed for radiocarbon (¹⁴C) ^{3, 4}. The estimated turnover time was 200–520 years, comparable to previously reported values based on ¹⁴C (75–380 years).
These results suggest that ¹²⁹I can serve as an effective tracer for water mass circulation in the Japan Sea.

Acknowledgement
This work was supported by JSPS KAKENHI Grant Number JP25K03281

References

1. Suzuki et al., Marine Chemistry 204 (2018): 163-71.
2. Suzuki et al., Journal of Nuclear Science and Technology 43 (2006): 1431-35.
3. Gamo et al., Journal of Oceanographical Society of Japan 39 (1983): 220-30.
4. Kumamoto et al., Geophysical Research Letters 25 (1998): 651-54.

 

Iodine-129, the Japan Sea, the Bering Sea