深海堆積物中のラジウムの挙動

抄録

Two deep-sea sediments taken from the wastern North Pacific Ocean were analyzed for ²³²Th, ²³⁰Th and ²²⁶Ra. The observed vertical distributions of ²²⁶Ra fit pretty well with those having diffusion coefficients of 1-3x10^-9 cm²/sec. The flux of ²²⁶Ra out of the sediment are calculated to be 0.022 dpm/cm²/yr to 0.037 dpm/cm²/yr from the deficiency of ²²⁶Ra relative to ²³⁰Th in the sediments. This indicates that ²²⁶Ra in sea water is mainly supplied from bottom sediments. The concentration of ²²⁶Ra in the layer extremely enriched in Mn was also in large excess relative to its parent ²³⁰Th. This fact may be due to its post depositional migration and adsorption on Mn oxide. The deposition rate of ²²⁶Ra in the Mn rich layer is estimated to be 0.0065 dpm/cm²/yr and 0.010 dpm/cm²/yr from the excess of ²²⁶Ra. The rates of migration of heavy metals (e.g. Ni and Co) in the sediments are also discussed by applying the above results on ²²⁶Ra.