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

25 This study examined plutonium (Pu) sources and distribution in the Pacific Ocean 26 based on extensive field datasets over the past 50 years. The basin wide 240 Pu/ 239 Pu atom 27 ratios in surface and deep seawater ranged from 0.192 to 0.279 averaging 0.235 ± 0.019, a 28 value consistently higher than that of global fallout at ~0.180. The distribution of 29 240 Pu/ 239 Pu atom ratios exhibited a decreasing trend along the North Equatorial Current- 30 Kuroshio to their extension areas. The activity levels of 239+240 Pu in Pacific Ocean surface 31 seawater ranged widely from 0.2 to 43.5 mBq m -3 , and increased with latitude. We 32 determined the Pu sourced from the Pacific Proving Grounds (PPG) and global nuclear 33 fallout in the Pacific Ocean based on the Pu isotopic composition. Using a mixing model, 34 we found that the PPG made the dominant Pu contribution (average=69.6 ± 14.4%) to 35 Pacific Ocean surface seawater. The depth range of maximal 239+240 Pu activity in the Pacific 36 Ocean was well defined, averaging 608 ± 137 m. The vertical distribution of the 239+240 Pu 37 inventory showed most of Pu retained in the upper 3000 m, namely, the contributions of 38 239+240 Pu inventories at 0-1000 m and 0-3000 m depths accounted for 43.5±9.0% and 39 75.1±12.0% of the total, respectively. We identified the transport pathway of Pu-PPG in 40 the Pacific Ocean and demonstrated that Pu isotopes hold great promise as tracers of ocean 41 circulation. Finally, via this extensive compilation of Pu isotopic compositions in the 42 Pacific Ocean water, we established a Pu baseline in the region. 43

the limited available Pu data usually cover a local area, and therefore cannot yield unbiased 85 interpretation for the Pacific Ocean as a whole. Large Pu dataset in both surface and deep 86 water are rarely available simultaneously for the Pacific Ocean, thus limiting our ability to 87 fully understand the chemical behavior of Pu in the open ocean. 88 The present study examined the source functions and aimed to gain a better 89 understanding of the fundamental processes that control the transport and fate of Pu by  The NEC is divided into the Kuroshio (northern branch) and the Mindanao Currents substantially higher (by 34.4%) than the values from global fallout, indicating that this area 146 has received non-global Pu fallout. Potential non-global fallout sources of Pu include low-147 latitude above-ground nuclear weapons test sites located within the basin (i.e., the PPG), 148 Fukushima derived from nuclear contamination, the Chernobyl nuclear accident and Asian Semipalatinsk nuclear test sites relative to the China Seas, the Pu contribution from these 162 sites appears to be negligible. Plutonium from the PPG was confirmed to be the only non-163 global source in the Pacific Ocean. Therefore, we constrained the Pu sources in the Pacific 164 Ocean to include the PPG and global fallout, the former being continuously supplied to the 165 Pacific Ocean via ocean circulation after the banning of nuclear tests (e.g., NEC and 166 Kuroshio, North Pacific Current) and resulting in the differential distribution of Pu activity 167 between North Pacific and South Pacific 1 .  Ocean, the 239+240 Pu activity of surface seawater varied from 0.6 to 43.5 mBq m -3 , averaging 173 5.9±7.7 mBq m -3 (n=50). In the South Pacific Ocean, they varied from 0.2 to 17.0 mBq 174 m -3 , with a mean value of 2.7±5.8 mBq m -3 (n=8). In the present study, we highlight the 175 analysis and discussion of 239+240 Pu activity in the North Pacific in light of the additional 7 field data included.

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In the northwest Pacific, 239+240 Pu activities in Kuroshio water were in the range of 178 leading to a decline in the supply of Pu.

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The bulk of Pu in Kuroshio water should originate mainly from the PPG. The

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The vertical profile of Pu isotopes in the water column at 25 sites in the Pacific Ocean 247 was summarized 1,12,14,29,31,33,41,55 and plotted in Figure 3.  i.e., the depth in the high latitude zone is greater than that at low latitudes. The relationship 285 between the depth of maximal Pu activity and longitude is also plotted in Figure 4c, 286 showing that this depth in the eastern hemisphere is more variable than in the western 287 hemisphere. Indeed, it was determined that particle concentration was higher in the upper 288 100 m (DCM layer), which would enhance Pu scavenging in this layer. Beyond 100 m, 289 particle concentrations remained low, and regeneration due to decomposition of particles 290 by microorganisms during downward transport is expected to be dominant 4,58-59 .  (2) as: where R represents the 240 Pu/ 239 Pu atom ratio, and the coefficient 3.674 is a conversion 367 factor between the activity ratio and atom ratio of 240 Pu/ 239 Pu.

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while the Pu-PPG contribution to the Pacific sample can be expressed as: In the calculation,