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Pb and Cs in sediments from Sagami Bay, Japan: sedimentation rates and inventories |
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| Authors: | Y Kato H KitazatoM Shimanaga T NakatsukaY Shirayama T Masuzawa |
| Institution: | a Tokai University, School of Marine Science and Technology, Shimizu 424-8610, Japan b Department of Life and Earth Sciences, Shizuoka University, Shizuoka 434-8529, Japan c Ocean Research Institute, The University of Tokyo, Tokyo 164-8639, Japan d Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan e Seto Marine Biological Laboratory, Kyoto University, Shirahama 649-2211, Japan f Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan |
| Abstract: | Profiles of the radioisotopes 210Pb and 137Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of 210Pbex (unsupported) in core top sediments increased with water depth from 25 dpm g−1 on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g−1 at the deep-sea station SB. The high 210Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective 210Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, 137Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced 137Cs fallout.The mean sedimentation rates estimated by the 210Pbex inventory method, rather than using 210Pbex profiles, ranged from 0.06 g cm−2 y−1 to 0.14 g cm−2 y−1. The average value of the rates in SB cores was calculated to be 0.11 g cm−2 y−1, which was similar to that calculated under the assumption that the age of the 137Cs peak corresponds to its maximum fallout year in 1963.Although 137Cs inventories represented one tenth of the anthropogenic fallout of 137Cs until 1997, they correlated with the increase in 210Pbex inventory. This suggests that the scavenging of refractory 137Cs as well as 210Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site. |
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