深海 雄介

We examine the historical changes of 236U/238U and 235U/238U in a sediment core collected in Tokyo Bay and elucidate the anthropogenic sources of uranium in the 1960s-2000s. Uranium-236 was detected in samples deposited in the 1960s-2000s, and the 236U/238U ratio of the sediment core shows peak values in the 1970s. The 235U/238U isotopic ratios in samples deposited in the early 1960s are almost identical to that of natural uranium, implying that the 236U might have originated from global fallout. A decrease in 235U/238U was observed in the late 1960s-2000s, suggesting that depleted uranium from nuclear fuel reprocessing increased the 236U/238U ratios in the sediment. The 236U/238U values in sediments from the 1980s-2000s were lower than those in the 1970s but considerably higher than those in the 1960s, suggesting that the main source of depleted uranium still remains around Tokyo Bay. Our results demonstrated that the depleted uranium released in the 1970s should be considered as an important end-member when using uranium isotopic ratios as environmental tracers in closed aquatic environments around industrial cities.

<p>The extinct, relatively short-lived nuclide ¹⁸²Hf produced ¹⁸²W as a decay product. Fractionation of Hf-W in the very early Earth led to variations in the ¹⁸²W/¹⁸⁴W ratios of terrestrial rocks; however, because these variations are very small, quantifying ¹⁸²W/¹⁸⁴W ratios requires an extremely precise method. Here, we propose an improved method for highly precise and accurate method for measuring the ¹⁸²W/¹⁸⁴W ratios of terrestrial rocks. Samples were extracted with 4-methyl-2-pentanone and purified by cation and anion exchange chromatography prior to determination of the W isotope ratio by multiple collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) system coupled with a desolvating nebulizer. Sample preparation removed matrix elements (e.g., Hf, Ta, Os, and dimers of Nb and Mo) with masses similar to those of W isotopes, resulting in these elements having a negligible influence on the measured ¹⁸²W/¹⁸⁴W ratios. A W standard solution processed by ion exchange chromatography and/or solvent extraction showed a ¹⁸³W deficiency, even after mass fractionation correction of the measured isotope data. As reported previously, mass-independent fractionation increases the ¹⁸²W/¹⁸⁴W ratio if the ¹⁸³W/¹⁸⁴W ratio is used to correct for mass fractionation to for better precision in natural samples. However, accurate ¹⁸²W/¹⁸⁴W ratios for a basalt reference material (JB-2) were obtained, even if ¹⁸³W was used for mass fractionation correction. Our results show that it is also possible to correct for the effects of mass-independent fractionation on the ¹⁸³W/¹⁸⁴W ratio by sample-standard bracketing using a W standard solution subjected to the same preparation procedure used for the samples. A major advantage of the newly developed method is that it requires a smaller amount of sample (0.2–0.3 g; 50–80 ng W for JB-2) compared with that needed for other reported methods (typically 0.7–15 g; 500–1000 ng W). This decrease in sample amount was possible by removing matrix elements from the sample solutions, and cleaning the membrane of the desolvating nebulizer between analyses also contribute to enhancing the W ion beam intensity and to high precision. Analysis of different basalts from the Loihi, Kilauea islands and Ontong Java Plateau with various W isotopic compositions consistent with the previous studies demonstrated the reliability of the method.</p>