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We examine the historical changes of 236 U/ 238 U and 235 U/ 238 U 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 236 U/ 238 U ratio of the sediment core shows peak values in the 1970s. The 235 U/ 238 U isotopic ratios in samples deposited in the early 1960s are almost identical to that of natural uranium, implying that the 236 U might have originated from global fallout. A decrease in 235 U/ 238 U was observed in the late 1960s-2000s, suggesting that depleted uranium from nuclear fuel reprocessing increased the 236 U/ 238 U ratios in the sediment. The 236 U/ 238 U 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

We examine the temporal changes of 236 U/ 238 U and 235 U/ 238 U in atmospheric deposition from samples collected in Tokyo and Akita from 1963 to 1979 and elucidate the spatial distribution and historical changes of the anthropogenic sources of uranium in Japan. The 236 U/ 238 U ratio of atmospheric deposition in Tokyo peaked in 1963 and again during the 1970s, while the corresponding 235 U/ 238 U ratios of atmospheric deposition during the second peak period were lower than that of natural uranium. The 236 U/ 238 U ratios of atmospheric deposition in Akita samples peaked in 1963. The 235 U/ 238 U ratios in Akita samples were almost identical to that of the natural uranium ratios. These results suggest that the peak of 236 U/ 238 U in 1963 corresponds to what is recognized as representative for global fallout. The increase of 236 U/ 238 U and the decrease of 235 U/ 238 U observed simultaneously in the 1970s indicate that depleted uranium has subsequently been released into the environment around Tokyo. The cumulative deposition density of 236 U for atmospheric fallout samples collected in Tokyo from 1968 to 1979 is an order of magnitude larger than that of the global fallout, suggesting that the depleted uranium in the 1970s is a major component of 236 U in Tokyo and should be considered as an end-member when using 236 U as an environmental tracer in the industrial city. This knowledge can facilitate future research using 236 U as an effective environmental tracer., Competing Interests: Declaration of competing interest The authors have no conflicts of interest directly relevant to the content of this article., (Copyright © 2021 Elsevier B.V. All rights reserved.)