~(238)U, Th Isotopes, ~(210)Pb and ~(239+240)Pu In settling particles on the continental margin of the East China Sea: Fluxes and particle transport processes

摘要

Settling particles were collected from four locations on the continental margin of the East China Sea using cylindrical and conical time-series taps. Total mass fluxes and activities of ~(238)U, ~(232)Th, ~(230)Th, ~(228)Th, ~(210)Pb and ~(239+240)Pu were determined for the particles to elucidate their transport processes on the margin. Surface sediment samples were also analyzed for ~(238)U, ~(232)Th, ~(230)Th, ~(210)Pb and ~(239+240)Pu. The total mass fluxes from the cylindrical traps were significantly higher than those from the conical traps during nearly the same time interval. There was a tendency for ~(210)Pb concentrations to increase almost linearly with depth from 72 m on the continental shelf edge to 1019 m in the Okinawa Trough; the apparent rate was 17.3 dpm/gper 100 m of the water column with the correlation coefficient of 0.93. In addition, there was a marked increase of ~(210)Pb fluxes with depth, with an especially large increase near-bottom. The estimated scavenging effectiveness ranged from 99 percent to 85 percent for shelf edge and middle continental slope sites, respectively, indicating that ~(210)Pb was effectively removed from the water column at each site. Both total mass fluxes and ~(210)Pb fluxes varied on a very short time scale (approx 1/2 day). The ratios of measured versus expected ~(230)Th fluxes for sediment traps were 183 and 222 in the deeper traps. The ratio of the observed ~(210)Pb fluxes to the ~(210)Pb deficiency fluxes in the near-bottom taps ranged between 8.9 and 46. These results suggest that the episodic lateral transport is a key process for particle transport on this continental margin. Concentrations of ~(238)U, ~(232)Th, ~(230)Th, ~(228)Th, ~(210)Pb and ~(239+240)Pu in the time-series traps at a middle continental slope site had little variations throughout the sampling period. In contrast, the radionuclide concentrations at an upper continental slope site varied inversely with the total mass fluxes. The radionuclide concentrations during high total mass flux events were close to those of the surface sediment. These results suggest that the high total mass flux events observed at an upper continental slope site are mainly due to resuspension of the underlying surface sediments.