Sources and accumulation of plutonium in a large Western Pacific marginal sea: the South China Sea

In order to examine the sources of plutonium (Pu) and elaborate its scavenging and accumulation processes, 239+240Pu activities and 240Pu/239Pu atom ratios in the water column of the South China Sea (SCS) were determined and compared with our previously reported data for the sediments. Consistently high 240Pu/239Pu atom ratios that ranged from 0.184-0.250 (average=0.228±0.015) in both the surface water and deep water (0-1000 m) were observed in the SCS during 2012-2014, indicating non-global fallout Pu sources into the SCS. The spatial distribution of the 240Pu/239Pu atom ratio in the SCS showed a decreasing trend away from the Luzon Strait, which was very consistent with the introduction pathway of the Kuroshio Current. The Kuroshio had an even heavier Pu isotopic ratio ranging from 0.250-0.263 (average=0.255±0.006), traceable to the non-global fallout Pu signature from the Pacific Proving Grounds (PPG). Using a simple two end-member mixing model, we further revealed that this PPG source contributed 41±17% of the Pu in the SCS water column. The 239+240Pu activities in the SCS surface seawater varied from 1.59 to 2.94 mBq m-3, with an average of 2.34±0.38 mBq m-3. Such an activity level was ~ 40% higher than that in the Kuroshio. The distribution of 239+240Pu in the surface seawater further showed a general trend of increase from the Kuroshio to the SCS basin, suggesting significant accumulation of Pu within the SCS. The 239+240Pu inventory of the water column in the SCS basin at the SEATS station with a total depth of ~ 3840 m was estimated to be ~29 Bq m-2, which was substantially higher than the sediment core estimates made for the SCS basin (3.75 Bq m-2) but much lower than the sediment core estimates made for the shelf of the northern SCS (365.6 Bq m-2). Such differences were determined by the lower scavenging efficiency of Pu in the SCS basin (11.6%) compared to the northern SCS shelf (69.6%)