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 (K_d,o) of ^239,240Pu between the overlying water and the surface sediment. The K_d,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 K_d,o values at eight SOJ sites were almost constant within the sampling periods, although large temporal changes occurred in two deep water sites. The K_d,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.