Abstract
The radioisotopes 210Pb and 137 Cswere measured in twenty sediment cores obtained from Sagami Bay, Japan. Total 210Pb concentrations of settling particles, collected by time-series sediment traps moored at 350m and 20m above the sea-floor (the water depth of 1550m) near the deep-sea station SB in the central region of the bay, were also determined in order to estimate their settling fluxes.
Mean sedimentation rates calculated from the inventories of 210Pbex.(unsupported) in sediment cores ranged from 0.05 to 0.19g cm-2y-1 At SB site where 6 cores were collected, the average rate was 0.11±0.02g cm-2y-1, which was similar to that calculated under the assumption that the age of the 137 peak corresponds to its maximum fallout year of 1963.
From the results of t he time series sediment trap experiment, the average 210Pb concentration of settling particles was 264 dpm g-1 in the upper trap and 248 dpm g-1 in the lower one, while the annual mean 210Pb fluxes were calculated to be 388 dpm m-2d-1 at the upper layer and 1392 dpm m-2d-1 at the lower, respectively. The mass balance calculation of 210Pb in the water-sediment column aptly demonstrates that the increase in 210Pb flux at the lower layer results from the lateral transport of particulate 210Pb near the bottom, and suggests that the amount of particulate 210Pb corresponding to ca. 38% of the sinking flux to the bottom is not deposited into sediments but transported seaward through the benthic boundary layer.
