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

Deep-ocean sediments, similarly to seawater, are important reservoirs of Cs-137, an anthropogenic radionuclide with a relatively long half-live found in the Earth system. To better understand the geochemical behaviour of Cs-137 in the ocean, we examined the temporal changes of Cs-137 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 Cs-137 activity levels showed exponential changes during the observed period. The decay-corrected change rates of Cs-137 in deep waters of SOJ exhibited a slow increase, while Cs-137 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 Cs-137 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 Cs-137 from seawater to sediment, we discussed the temporal changes of the partition coefficients (K-d) of Cs-137 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 Cs-137 between seawater and sediment, although more complicated phenomena occurred in deep waters and sediments of the SOJ (> 3000 m depth).