Uranium in coastal sediments of Tokyo Bay and Funka Bay

Abstract

The sediment cores from Tokyo Bay and Funka Bay were analyzed for U and its isotopic ratio, ²³⁴T U/²³⁸U, after dissolving them in 0.1M HC1, and 30% H₂0₂ in 0.05M HC1. A small fraction of U in the anoxic sediments was dissolved in 0.1M HC1 and even the added yield tracer, ²³²U, was lost. The isotopic ratio of H₂O₂ soluble U in the sediments was equal to that of seawater, suggesting that the H₂O₂ soluble U in the sediments is authigenic. The 6M HC1 solution dissolved part of the lithogenic U besides the authigenic U. The depth profiles of U from the two bays resembled each other. The authigenic U comprised more than half of the total U even at the surface and increased with depth down to 70cm, showing small maxima at about 20cm. The concentration of refractory U was nearly constant with depth and similar to that of the pelagic sediments. The highest U concentration, 6 μg g^-1 which was about 5 times that of the pelagic sediments, was observed in the layer between 70 and 160cm depth in Tokyo Bay. The annual sedimentation rates of U in the Tokyo Bay sediments were 2.6 tons at the surface and 7.0 tons at the 70-160cm depth. The increase in U with depth should be due to the deposition of interstitial U either diffusing downward from the surface indicating the trapping of seawater U, or otherwise diffusing upward from the deeper layer indicating the internal cycling of U within the sediments.