Temporal changes of (137)Cs concentrations in the Far Eastern Seas: partitioning of (137)Cs between overlying waters and sediments

• Published in: Scientific Reports
• Main Authors: Hirose, Katsumi, Povinec, Pavel P.
• Format: Text
• Language: English
• Published: Nature Publishing Group UK 2023
• Subjects: Article; Okhotsk; Fukushima; okhotsk sea; Subarctic
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC10752905/; http://www.ncbi.nlm.nih.gov/pubmed/38151506; https://doi.org/10.1038/s41598-023-49083-4

Deep-ocean sediments, similarly to seawater, are important reservoirs of (137)Cs, an anthropogenic radionuclide with a relatively long half-live found in the Earth system. To better understand the geochemical behaviour of (137)Cs in the ocean, we examined the temporal changes of (137)Cs activity concentrations in the overlying waters and in sediments from the Far Eastern Seas (Sea of Japan, SOJ, and Okhotsk Sea, OS) during the period of 1998–2021. The (137)Cs activity levels showed exponential changes during the observed period. The decay-corrected change rates of (137)Cs in deep waters of SOJ exhibited a slow increase, while (137)Cs levels in seawater and sediment in OS decreased gradually. This reflects a topographical difference, as SOJ is a semi-closed sea, whereas OS receives continuously inflow of subarctic waters. It was confirmed that (137)Cs released after the Fukushima Dai-ichi Nuclear Power Plant accident was rapidly transported into the deep waters of the SOJ. To elucidate the transfer processes of (137)Cs from seawater to sediment, we discussed the temporal changes of the partition coefficients (K(d)) of (137)Cs between the overlying water and the surface sediment. In shallow areas (< 1500 m water depth), K(d) values were almost constant within the sampling periods, although the temporal changes in the K(d) values occurred in deeper waters (> 2500 m depth). The K(d) values increased with increasing depth, which may reflect a pressure effect as a possible mechanism. These findings suggest that chemical processes may be important factors controlling the transport of (137)Cs between seawater and sediment, although more complicated phenomena occurred in deep waters and sediments of the SOJ (> 3000 m depth).