Accumulation of uranium in sediments and phosphorites on the South West African shelf |
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| Authors: | HH Veeh SE Calvert NB Price |
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| Institution: | 1. School of Physical Sciences, Flinders University of South Australia, Bedford Park, S.A. Australia;2. Institute of Oceanographic Sciences, Wormley, Godalming Great Britain;3. Grant Institute of Geology, University of Edinburgh, Edinburgh Great Britain |
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| Abstract: | Organic-rich sediments and coexisting phosphorites from the continental shelf off South West Africa have been analysed for uranium and thiorum by alpha-spectrometry. The uranium concentrations in the sediments range from 10 to 55 ppm, with an isotopic composition close to that of sea-water, indicating that uranium is passing into the sediments at the present time. The phosphorites occur in the sediments as thin unconsolidated laminae and as lithified nodules and pellets, with uranium contents ranging from 79 to 158 ppm. Based on the uranium isotopic composition, only the unconsolidated phosphorite laminae are recent, while the lithified nodules and pellets, with 234U/238U and 230Th/234U ratios close to radio-active equilibrium, appear to be inherited from a previous period of phosphorite deposition. Deposition of uranium appears to take place predominantly by incorporation into carbonate fluorapatite growing authigenically within the sediment. Uranium accumulation rates, computed from 14C-dated sections of the sediment cores, and using only uranium values with modern isotopic composition, range from 232 to 765 μg/cm2 per 1,000 years. These results stress the importance of organic-rich sediments containing authigenic phosphorite beneath areas of high organic productivity as a major sink for uranium in the ocean. |
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