Sources of Plutonium in the Pacific Ocean and Its Isotopic Ratio 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 240Pu/239Pu atom ratios
in the surface and deep seawater ranged from 0.172 to 0.304, averaging
0.233 ± 0.023, a value consistently higher than that of the global
fallout at ∼0.180. The distribution of 240Pu/239Pu atom ratios exhibited a decreasing trend along the North
Equatorial Current Kuroshio to their extension areas. The activity
levels of 239+240Pu in the Pacific Ocean surface seawater
showed an increase with latitude. We determined 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,
plutonium from the PPG was estimated to average 70.7 ± 12.9%
and 29.0 ± 4.2% of surface seawater in the North Pacific and
South Pacific, respectively. The apparent residence time of surface 239+240Pu was calculated to be ∼12.8 years in the Pacific
Ocean. The depth range of maximal 239+240Pu activity in
the Pacific Ocean was well defined, averaging 608 ± 137 m. The
vertical distribution of the 239+240Pu inventory showed
that most Pu (75.1 ± 12.0%) was retained in the upper 3000 m,
suggesting that there is a huge Pu reservoir with higher accumulation
compared to the adjacent marginal seas. We also identified the transport
pathway of Pu-PPG in the Pacific Ocean and demonstrated that the 240Pu/239Pu atom ratio holds great promise as a
tracer of ocean circulation.