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Uranium-series and AMS C studies of modern phosphatic pellets from Peru shelf muds |
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| Authors: | William C Burnett KBryan Baker Philip A Chin William McCabe Robert Ditchburn |
| Institution: | 1 Department of Oceanography, Florida State University, Tallahassee, FL 32306-3048, U.S.A. 2 Institute of Nuclear Sciences, Department of Scientific and Industrial Research, Lower Hutt, New Zealand |
| Abstract: | Recent sediments and separated phosphate pellets ( 125–500 μm in diameter) from the Peru shelf have been analyzed for uranium decay-series isotopes and 14C in order to determine age relationships and mineralization rates. Uranium-series ages of pellets separated from one box core are significantly higher than AMS radiocarbon ages determined for the same pellets. These differences appear to be a consequence of mixing of an older generation of pellets with ones which are more recently formed. Postdepositional adsorption of reactive elements such as thorium and protactinium onto pellet surfaces may also contribute to the observed discordancy with radiocarbon ages.Sediment radiocarbon and 210Pb sediment results, as well as some trends in the uranium-series data, suggest that high concentrations of phosphate pellets have accumulated in some Peru shelf sediments without extensive reworking. Individual pellets apparently form very quickly, on time scales of a few years. Estimated authigenic uptake rates of phosphorus into pellets ranges from 0.5 to 9.40 μmol-P cm? 2 yr? 1, somewhat higher than rates measured for nodules from the same area. This is consistent with observations that pelletal morphologies predominate over nodular forms within ancient phosphorite deposits. |
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