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The geochemistry of uranium and thorium in coastal marine sediments and sediment pore waters

1986; Elsevier BV; Volume: 50; Issue: 5 Linguagem: Inglês

10.1016/0016-7037(86)90344-3

1872-9533

J. Kirk Cochran, Anne E. Carey, Edward R. Sholkovitz, Lolita D. Surprenant,

Radioactivity and Radon Measurements

Pore water profiles of uranium and thorium isotopes in the muddy sediments of Buzzards Bay, MA permit an assessment of the effect of diagenetic redox reactions on the geochemical behavior of these elements. Uranium shows a pronounced minimum pore water concentration (~ 1.2 dpm/kg) near the sediment water interface (0–3 cm) which coincides with the pore water Fe maximum. U concentrations increase with depth to a broad maximum which is greater than the overlying seawater value, then decrease. Laboratory sediment tank experiments maintained without macrofauna also show the near-interface minimum and increase, and sediment incubation experiments show removal of U from the pore water to a constant value by about 40 days. The 234U238U activity ratio is equal to the seawater value in all samples. Oxidation state separations of pore water U in HCl solution (using anion exchange) demonstrate that U is present in the pore water as U(VI). The coincidence of the pore water uranium and Fe maximum is consistent with the hypothesis of reduction of U(VI) to U(IV) and removal from solution at about the same depth (or pe) as Fe is reduced. Increases in pore water U with depth appear to be related to release of authigenic (seawater) U(VI) to solution, possibly as alkalinity and pH increase and organic matter with which the U is associated is oxidized. Lower concentrations at depth in the sediment column may be linked to increases of effective U reducing agents like hydrogen sulfide in the pore water or to the activities of macrofauna. In contrast, pore water profiles of the long-lived Th isotopes 232Th and 230Th show low activities (≲ .02 dpm/kg) and relatively little change with depth, although values are greater than those in the overlying water. Solid phase activities are similarly constant with depth, and the distribution may be controlled by a sorption equilibrium between pore water and the solid phase. The short-lived isotope 234Th shows greatest pore water activities in the upper 5 cm and is present at virtually zero activities elsewhere in the sediment column. The upper few centimeters of the sediment generally contain excess 234Th scavenged from the overlying water column and distributed by bioturbation, as well as pore water Fe and Mn profiles which show maxima indicating reduction and dissolution of Fe and Mn oxyhydroxides. The pore water 234Th profile can be explained by release of 234Th from the solid phase as FeMn oxide surface coatings are reduced and dissolved.