HR: 1340h
AN: OS13A-0995    [Abstracts]
TI: Transport processes of Plutonium isotopes in the NW Pacific margin
AU: * Zheng, J
EM: jzheng@nirs.go.jp
AU: Yamada, M
EM: m_yamada@nirs.go.jp
AB: Over the past years, we have made a systematic investigation on the distribution of Pu isotopes in the NW Pacific Ocean and its adjacent seas. More than 30 sediment cores were collected in the western North Pacific and its adjacent seas, namely, the East China Sea, Okinawa Trough, the Japan Sea and the Okhotsk Sea. Based on the obtained 240Pu/239Pu ratio signature, we identified the wide presence of Bikini close-in fallout Pu in the studied regions, which contributed to the excess Pu inventories. We propose that the oceanic process is responsible for the wide presence of Bikini close-in fallout Pu. Using a two fallout end-member model, we resolved the relative contribution of Pu between global fallout and close-in fallout in sediments. It was concluded that 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. It was found that Pu isotopes from the two sources of global fallout and close-in fallout have been homogenized in the water masses in the western Northwest Pacific margin during the past four decades. Settling particles have been regarded as the primary means by which surface-introduced Pu is transported to the sediments. We carried out a sediment-trap experiment in the western Northwest Pacific to investigate Pu isotopes in settling particles. 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.
DE: 4200 OCEANOGRAPHY: GENERAL
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting