Temporal trend of 240Pu/239Pu atom ratios in water columns in the Western North Pacific Ocean and its marginal seas

The Pu-240/Pu-239 atom ratios were determined by inductively coupled plasma mass spectrometry for seawater samples from the Okinawa Trough in the western North Pacific Ocean margin to identify their Pu sources and to elucidate the temporal variability in atom ratios in the water columns in the western North Pacific Ocean and its marginal seas. The Pu-239, Pu-240 and Pu239+240 water column inventories were 12.2 +/- 0.5, 10.5 +/- 0.5 and 22.7 +/- 0.7 Bq m(-2), respectively. The observed Pu-240/Pu-239 ratios were clearly greater than the mean global fallout ratio of-0.18 with a mean value of 0.236 +/- 0.014. The observed greater atom ratios confirmed the presence of close in fallout Pu from nuclear testing at the Pacific Proving Grounds (PPG). The relative percentage of the PPG derived Pu to global fallout Pu was estimated as 41 +/- 10%. The Pu239+240 water column inventory originating from the PPG close-in fallout was also calculated as 9.2 +/- 0.4 Bq m(-2). The temporal variability in the mean Pu-240/Pu-239 atom ratios in the water columns in the western North Pacific Ocean and its marginal seas was obtained through comparison of the present study values and the previously reported values, and it had no significant temporal variation with a mean value of 0.237 +/- 0.004 during the observation period of 1984-2015. The Pu-239 and Pu-240 derived from the two different sources of global fallout and the close-in fallout at the PPG were homogenized in the water masses in the studied areas.

Automated summary

The study determined Pu-240/Pu-239 atom ratios in seawater samples from the Okinawa Trough, showing greater ratios than global fallout due to close-in fallout Pu from nuclear testing at the Pacific Proving Grounds (PPG). Temporal variability in atom ratios was negligible from 1984 to 2015. Pu sources in the water columns mainly comprised of global fallout and PPG close-in fallout.