Historical changes of 236U/238U and 235U/238U isotopic ratios in Tokyo Bay from the 1960s to the 2000s

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.