Pu isotope ratio analysis by sector-field ICP-MS reveals contamination sources and transport mechanism in teh western northwest Pacific

Plutonium was introduced into the environment mainly from the fallout of atmospheric nuclear weapons tests. There has been an increasing interest to study the distribution of Pu and its isotopes in the western North Pacific Ocean due to the application potential for tracing oceanographic processes occurring in the region and the need for radiological assessment. Over the years, we developed highly sensitive analytical methods for ultra-trace analysis of Pu isotopes using sector-field ICP-MS combined with a high-efficiency sample introduction system (APEX-Q), which allowed the determination of Pu isotope ratio at femtogram levels in various marine samples (sediments, settling particles and seawater). We made a systematic investigation on the distribution of Pu isotopes in sediments in the NW Pacific Ocean and its adjacent seas. Based on the obtained 240Pu/239Pu ratio signature, we identified the wide presence of Bikini close-in fallout Pu in the studied regions. We proposed that the oceanic process is responsible for the wide presence of Bikini close-in fallout Pu. The contribution of Bikini close-in fallout Pu ranged from 10 % to 60 % in the studied regions. To verify our hypothesis on the oceanic process for the transport of Pu in the Pacific Ocean, we further investigated the Pu isotopic signature in seawater and settling particles in the western Northwest Pacific. The water column vertical profile of 240Pu/239Pu atom ratios in Sagami Bay showed no notable variation from the surface to the bottom with an average value of 0.234. This atom ratio was significantly higher than the mean global fallout ratio of 0.18, and was in good agreement with that observed in the underlying sediment column, proving again the existence of close-in fallout Pu originating from the Pacific Proving Grounds (PPG). It was found that Pu isotopes from the two sources of global fallout and close-in fallout have been homogenized in the water column in the western Northwest Pacific margin during the past four decades. Pu scavenging process was studied by measuring Pu isotopes in settling particles because settling particles have been regarded as the primary means by which surface-introduced Pu is transported to the sediments. To the best of our knowledge, this is the first time that both Pu activity and Pu isotope ratio data have been obtained for settling particles in the Pacific Ocean. The 240Pu/239Pu atom ratios are quite constant; the change of total mass flux in the different seasons did not result in a variation of Pu atom ratios in settling particles. The high 240Pu/239Pu atom ratios (0.23-0.27) in settling particles confirmed that Pu from the PPG source in the central Pacific is transported toward the western Northwest Pacific. Three transport processes were identified to be responsible for the distribution and fate of Pu isotopes in the NW Pacific margin: (1) advective lateral transport of dissolved Pu from open ocean to ocean margin; (2) vertical transport of Pu isotopes via particle scavenging; and (3) the bottom layer lateral transport and redistribution of Pu isotopes.2008 Third Asia-Pacific Winter Conference on Plasma Spectrochemistr