Distribution of plutonium in the Pacific Ocean and implications for tracing of ocean circulation

This study examined plutonium (Pu) sources and distribution in the Pacific Ocean based on extensive field datasets over the past 50 years. The basin wide 240 Pu/ 239 Pu atom ratios in surface and deep seawater ranged from 0.192 to 0.279 averaging 0.235 ± 0.019, a value consistently higher than that of global fallout at ~ 0.180. The distribution of 240 Pu/ 239 Pu atom ratios exhibited a decreasing trend along the North Equatorial Current-Kuroshio to their extension areas. The activity levels of 239+240 Pu in Pacific Ocean surface seawater ranged widely from 0.2 to 43.5 mBq m − 3 , and increased with latitude. We determined the Pu sourced from the Pacific Proving Grounds (PPG) and global nuclear fallout in the Pacific Ocean based on the Pu isotopic composition. Using a mixing model, we found that the PPG made the dominant Pu contribution (average = 69.6 ± 14.4%) to Pacific Ocean surface seawater. The depth range of maximal 239+240 Pu activity in the Pacific Ocean was well defined, averaging 608 ± 137 m. The vertical distribution of the 239+240 Pu inventory showed most of Pu retained in the upper 3000 m, namely, the contributions of 239+240 Pu inventories at 0-1000 m and 0-3000 m depths accounted for 43.5 ± 9.0% and 75.1 ± 12.0% of the total, respectively. We identified the transport pathway of Pu-PPG in the Pacific Ocean and demonstrated that Pu isotopes hold great promise as tracers of ocean circulation. Finally, via this extensive compilation of Pu isotopic compositions in the Pacific Ocean water, we established a Pu baseline in the region.