Recurrent uranium relocations in distal turbidites emplaced in pelagic conditions |
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Authors: | S Colley J Thomson |
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Institution: | Institute of Oceanographic Sciences, Wormley, Godalming, Surrey GU8 5UB, England |
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Abstract: | The sediments of the Madeira Abyssal Plain, east of Great Meteor Seamount, are dominated by distal turbidite deposition. While the turbidites exhibit a wide compositional range (25–80% CaCO3), individual examples can be correlated over a wide area and are relatively homogenous. Organic C oxidation, by bottom water oxygen, proceeds from the turbidite tops downwards after emplacement in pelagic conditions, and the progress of this oxidation front is marked by a sharp colour contrast in the sediments (Wilsonet al., 1985). In turbidites with Corg ? 0.5%, redistribution of authigenic U occurs to form a concentration peak (4–9 ppm U), just below the oxidation front or colour change. Several tens μg U/cm2 may be mobilised, and in all examples studied ?60% of the remobilised U is relocated into the peak. Following burial by subsequent turbidites, such U concentration peaks are persistent as relict indicators of their extinct oxidation fronts for at least 2 × 105 years. In the case of thin turbidites where labile Corg is almost exhausted, the U peaks may be located in underlying sedimentary units because of their relationship to the oxidation front. A redox mechanism for U peak formation is suggested from these data rather than a complexation with organic matter. |
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