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

Abstract

Abstract Deep-ocean sediments, similarly as seawater, are important reservoirs of 137Cs, an anthropogenic radionuclide with a relatively long half-live found in the Earth system. To better understand the geochemical behavior of 137Cs in the ocean, we examined the temporal changes of 137Cs 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 1998 - 2021. The 137Cs activity levels showed exponential changes during the observed periods. The decay-corrected change rates of 137Cs in SOJ exhibited a slow increase during the observed period, whereas 137Cs 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. To elucidate the transfer processes of 137Cs from seawater to sediment, we discussed the temporal changes of the partition coefficients (Kd) of 137Cs between the overlying water and the surface sediment. In shallow areas (< 1500 m depth), Kd values were almost constant within the sampling periods, although the temporal changes in Kd values occurred in deeper waters (> 2500 m depth). The Kd values increased with increasing depth, which may reflect a pressure effect as a possible mechanism. These findings suggest that the chemical process may be important as factors controlling the transport of 137Cs between seawater and sediment, although more complicated phenomena occurred in deep waters and sediments of the SOJ (> 3000m depth).