Temporal variability of plutonium in surface waters of the Sea of Japan

Long-term temporal variations of plutonium in Sea of Japan (SOJ) surface waters have been examined with the aim to better understand its behavior during several decades. The first observation is that ^239,240Pu activity concentrations in surface waters of the SOJ during 1977-2019 were 6.5 ± 4.7 mBq m^-3 in average, and 5.1 mBq m^-3 as the median, whereas ¹³⁷Cs and ⁹⁰Sr activity concentrations decreased with time, except of the perturbation due to the 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Another observation is that sporadic high ^239,240Pu activity concentrations occurred in the east Japan Basin, ranging from 1 to 39 mBq m^-3. The spatial distribution of ^239,240Pu activity concentrations in surface waters revealed that high ^239,240Pu levels (>20 mBq m^-3) occurred in 1994 in the northern SOJ, which was considered to be due to winter convection. To elucidate factors controlling the temporal variability of surface ^239,240Pu levels in the SOJ, a relationship between surface ^239,240Pu activity concentrations and vertical diffusion coefficients was examined. The results revealed that this relationship could be classified into two groups: one group did not show a change with increasing diffusion coefficient, while the other group showed a positive correlation. The vertical ^239,240Pu distribution in SOJ waters suggests that the high surface ^239,240Pu levels occurred due to the upwelling of cyclonic eddy. The rapid recycling of deeper plutonium occurred in the SOJ due to deep winter convection and upwelling associated with cyclonic eddy. The plutonium levels in the SOJ have been found to be sensitive to climate changes. Warming of the SOJ may cause a reduction of winter convection and eddy activity as a result of increasing sea surface temperature. This leads to a decline of recirculation of plutonium and other bioavailable elements from Japan Sea Proper Water (JSPW) to surface water layers. Plutonium would be, therefore, an important indicator of biogeochemical processes in the marine environment, helping to assess climate change impacts on marine ecological systems.