| Popis: |
The sediment-depth distributions of 238Pu, 239,240Pu, 241Am and 137Cs have been measured in a sediment core from the Saguenay Fjord, Quebec, Canada, which has been dated to better than 1-yr. resolution over the past 100 yr. using 210Pb. The historical record of fallout radionuclide fluxes to the sediments has been estimated using a 210Pb constant flux model and variations in the flux distributions have been directly related to changes in the atmospheric input function. Rates of radionuclide transport through the drainage basin have been predicted using a two-component time-dependent model composed of terrestrial (soil) and water phases, the latter including catchment basins upstream of the fjord. The residence times of 137Cs and 239,240Pu in the soil component are 1000 and 3000 yr., respectively, while both radionuclides have equivalent residence times of 1 yr. in the water column component. The atmospheric burn-up of a nuclear-powered SNAP-9A satellite in 1964 has resulted in a well-defined 238 Pu 239,240 Pu horizon appearing in the sediments at the 1967–1968 level. The magnitude and timing of the SNAP-9A-derived 238Pu distribution are in agreement with the drainage basin model predictions and confirm the absence of post-depositional remobilization of Pu in these anoxic sediment regimes. Theoretical estimates of the change in the 241 Am 239,240 Pu activity ratio in sediments deposited since the 1950's, determined using the drainage basin model, are in agreement with the experimental results and indicate that the initial 241 Pu 239,240 Pu activity ratio for the 1950's weapons tests was 15 or more while the later weapons tests were characterized by an activity ratio closer to 12. |
