ウラン・トリウム系列核種と海洋での物質循環 (1988年度日本地球化学会奨励賞受賞記念) [in Japanese] Uranium and thorium series radioisotopes and the cycle of chemical materials in the ocean [in Japanese]
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This article summarizes the author's work on the distributions of U, <sup>234</sup>Th, <sup>226</sup>Ra, <sup>210</sup>Pb and <sup>210</sup>Po in seawater. Some recent studies of other investigators relevant to them are also reviewed. Removal of uranium from seawater occur in anoxic sediments where soluble U(VI) is reduced to insoluble U(IV). Radium isotopes are useful tracers for the study of water movement althouth distribution of <sup>226</sup>Ra is largely governed by the biogeochemical cycle. Based on the <sup>226</sup>Ra distribution and a box-model of the Japan Sea, the residence times of the Japan Sea Proper Water with respect to exchange with the surface water by vertical mixing and with the open ocean water were estimated to be 300〜400 years and 700〜1000 years, respectively. Measurements of <sup>234</sup>Th/<sup>238</sup>U in the surface waters indicates that the degree of the radioactive disequilibrium is large in the northern North and the equatorial Pacific suggesting that the biological particles are responsible for <sup>234</sup>Th removal. In the deep sea, the removal of <sup>210</sup>Pb and <sup>231</sup>Pa occurs intensively in the high productivity areas. The increase of <sup>210</sup>Pb concentration was observed in the bottom water of Funka Bay in summer when the sediment became a weakly reducing condition. The concentrtion of <sup>210</sup>Po, the daughter nuclide of <sup>210</sup>Pb, was also extremely high in the bottom water of the anoxic marine lake in Palau. These observations suggest that lead and polonium are naturally mobilized in anoxic surface sediments.
Chikyukagaku 23(1), 1-11, 1989
The Geochemical Society of Japan