Seasonal variation of Pu flux and Pu atom ratio in the west NOrthwestern Pacific

We examined the vertical distributions of 239+240Pu activity and 240Pu/239Pu atom ratio in settling particles and quantified the particulate 239+240Pu fluxes in the water column in the western Northwest Pacific. Time-series settling particle samples were collected using sediment traps to examine the seasonal variation of Pu flux and 240Pu/239Pu atom ratio. Plutonium isotopes were analyzed using a sector field high resolution ICP-MS. 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 high (> 0.18) 240Pu/239Pu atom ratios in settling particles indicate that plutonium from the Pacific Proving Grounds (PPG) source in the central Pacific is transported toward the western Northwest Pacific. Evidence indicates that Pu scavenging onto the settling particles is strongly dependent upon the bulk mass flux. The results suggest that advective lateral transport of dissolved Pu from the open ocean to the ocean margin and removal of Pu into the margin sediments by particle scavenging is a common phenomenon in the Pacific Ocean, and it is likely to be also an important factor in a larger context for the geochemical cycling, i. e. the transport and dispersion of other contaminants that supplied directly from the atmosphere. The Pacific marginal sediments underlying waters with high biological productivity and large suspended sediment loads are major sinks for removal and sequestration of Pu and other contaminants that adsorb strongly to sediments.第8回環境放射能研究