ABSTRACT

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.