239Pu and 240Pu inventories and 240Pu/239Pu atom ratios in the equatorial Pacific Ocean water column

The 239+240Pu concentrations and 240Pu/239Pu atom ratios were determined by alpha spectrometry and inductively coupled plasma mass spectrometry for seawater samples from two stations, one at the equator and the other in the equatorial South Pacific. To better understand the fate of Pu isotopes, this study dealt with the contribution of the close-in fallout Pu from the Pacific Proving Grounds (PPG) in water columns of the Pacific Ocean. The 239Pu, 240Pu and 239+240Pu inventories over the depth interval 0–3000 m at the equator station were 10.4, 8.9 and 19.3 Bq m−2, respectively. Further, no noticeable difference was observed in 239Pu, 240Pu and 239+240Pu inventories over the depth interval 0–3000 m between the two stations. The total 239+240Pu inventories were significantly higher than the expected cumulative deposition density of global fallout. Water column 239+240Pu inventories measured in this study were lower than those reported for comparable stations in the Geochemical Ocean Sections Study, indicating that these inventories have been decreasing at average rates of 0.89 ± 0.07 and 0.16 ± 0.07 Bq m−2 yr−1 at the equator and equatorial South Pacific stations, respectively, from 1973 to 1990. The obtained 240Pu/239Pu atom ratios were higher than the mean global fallout ratio of 0.18. These high atom ratios proved the existence of close-in tropospheric fallout Pu from the PPG in the Marshall Islands. The 239+240Pu inventories originating from the close-in fallout in the entire water column were estimated to be 11.1 Bq m−2 at the equator station and 7.1 Bq m−2 at the equatorial South Pacific Ocean station, and the relative percentages of close-in fallout Pu were 40% at the former and 34% at the latter. A significant amount of close-in fallout Pu originating from the PPG has been transported to deep layers below the 1000 m depth in the equatorial Pacific Ocean. - Highlights: ►239+240Pu inventories were higher than the deposition density of global fallout. ►240Pu/239Pu atom ratios were higher than the mean global fallout ratio of 0.18. ► High atom ratios proved the existence of close-in tropospheric fallout Pu. ► A significant amount of close-in fallout Pu has been transported to deep layers. ► The close-in fallout is a dominant source of Pu in the equatorial Pacific Ocean.