第七届青年地学论坛

In order to examine the sources of plutonium (Pu) and elaborate its scavenging and accumulation processes, ²⁴⁰Pu/²³⁹Pu atom ratios and ^239+240Pu activities 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), indicative of non-global fallout Pu sources were observed both in the surface water and at depth during 2012-2014. 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 compared to the northern SCS shelf.
 

Plutonium, South China Sea, Pacific Proving Grounds, scavenging, accumulation, Kuroshio, North Equatorial Current