Uranium contents and 234U/238U activity ratios of modern and fossil marine bivalle molluscan shells

Uranium contents and 234U/238U activity ratios in modern and fossil marine bivalle molluscan shells were measured by alpha-spectrometry. Uranium contents and 234U/238U activity ratios in modern shells were averaged to be 0.266 (dpm/g), and 1.18, respectively and those in fossil shells were averaged to be 0.747 (dpm/g), and 1.19, respectivily. Uranium contents in fossil shells were obviously higher than those in modern shells. It can be explained by the addition of uranium to shell during the deposition. In fossil shells, 234U/238U activity ratio decreases as 238U content increases the same tendency is not found in modern shells. The author proposed a mechanism of selective loss of 238U from the fossil shells for the explanation of this tendency. The height activity ratio of 234U/238U measured on the fossil shells than that measured on the modern shells, also support the selective loss of 238U from the fossil shells. (author)