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Removal from solution in anoxic environments is one of the major fates of dissolved uranium in the ocean. The mechanism for this phenomenon is thought to be reduction of the carbonate-complex-stabilized UVI found in oxygenated waters to UVI, which is subsequently adsorbed on aluminosilicates or chelated by organic ligands in the solid substrates. Enrichments up to two orders of magnitude have been reported in anoxic sediments relative to sediments deposited in oxygenated environments. However, sediments deposited in Orca Basin, a hypersaline anoxic basin which lies at a depth of from 2,200 to 2,400 m in the northern Gulf of Mexico, show no such uranium enrichment.
Samples analyzed from two piston cores taken in the deepest part of the basin show an average uranium concentration of about 2 ppm, whereas the average uranium concentration in open-marine sediments in the northern Gulf is about 3 ppm on a carbonate-free basis. However, when the samples are corrected for carbonate and salt contents, uranium values approach the average found for sediments deposited in oxic environments.
The lack of uranium enrichment in Orca Basin sediments must be due to the low amounts of uranium in the source of the brines--probably underlying salt domes from which they were dissolved. Diffusion of dissolved uranium from the overlying sea water must be negligible also.
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