The geochemistry of pore water uranium in coastal marine sediments from Funka Bay, Japan

抄録

Pore waters in marine sediments from Funka Bay, Japan were squeezed with three squeezing methods and their U concentrations were measured. The pore water U squeezed with a multi-depth core squeezer without exposure to air is one order of magnitude less than the seawater U value. In contrast, the pore water U squeezed with a hydraulic squeezer under air and by a whole-core squeezer shows anomalously high values (maximum 53 μg/kg) even though care was taken to avoid air contact. The wide variation of pore water U from month to month is found for the hydraulic squeezing samples through a year. The difference in pore water U among three squeezing methods is considered to be due to oxidation artifacts during sample handling and squeezing. Oxidation of solid reduced U occurs rapidly on air contact, according to exposure experiments of anoxic sediment to air. The pore water U squeezed with the multi-depth core squeezer without oxidation artifacts decreases from the seawater U value at the surface to about 0.2–0.4 μg/kg at 3–10 cm depth in July, August, September and December 1990. These results suggest that pore water U in the Funka Bay sediments is removed to the sediments close to the sediment–water interface. This indicates removal of dissolved U from bottom seawater to sediment, but this cannot explain the enrichment of U in the solid phase below 40 cm depth. A more likely explanation for the discrepancy in vertical profiles between pore water and solid phase U is considered to be the change of deposition conditions between past and present.