Temporal changes of 239,240Pu activity concentrations in overlying waters and sediments in the Far Eastern Seas: Partitioning of 239,240Pu between bottom waters and sediments.

Temporal changes of 239,240Pu activity concentrations in the overlying waters and sediments of the Far Eastern Seas (Sea of Japan: SOJ, and Okhotsk Sea: OS) were examined during the period 1994 to 2021 with the aim to better understand the biogeochemical behavior of plutonium in the ocean. The 239,240Pu activity concentrations in the overlying waters and sediments did not show clear trends during the observed periods, in contrast to the 137Cs decline trends, although a gradual decrease of 239,240Pu occurred in the shallow layer waters of the SOJ, while slow increases appeared in deep waters. To elucidate the transfer processes of 239,240Pu from seawater to sediment, we examined the temporal changes of the operational distribution coefficients (Kd,o) of 239,240Pu between the overlying water and the surface sediment. The Kd,o values of 239,240Pu in the Far Eastern Seas showed large spatial-temporal variability (<0.06 × 103 to 280 × 103 L kg-1). The Kd,o values at eight SOJ sites were almost constant within the sampling periods, although large temporal changes occurred in two deep water sites. The Kd,o values of 239,240Pu decreased with increasing depth, in contrast to 137Cs, which may reflect a pressure effect as a possible mechanism. This finding suggests that the return of 239,240Pu to deep water from sinking and sediment particles may occur, instead of the accumulation of 239,240Pu in deep sediment.