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

Abstract

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.

Keywords: (235)U/(238)U; (236)U/(238)U; Depleted uranium; ICP–MS/MS; Tokyo Bay.