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Authigenic uranium in Atlantic sediments of the last glacial stage — a diagenetic phenomenon

Earth and Planetary Science Letters
Publication Date
DOI: 10.1016/0012-821x(90)90061-2


Abstract Cores from three different Atlantic localities (equatorial, Cape Verde Rise and Porcupine Abyssal Plain) are shown to have anomalous high U contents (5–8 ppm total) in sediments laid down during the last glacial stage (12–24 ky BP). Radiocarbon data demonstrate that the sediments hosting the peak U levels were accumulated at rates similar to those immediately above and below. All the cores exhibit maximum Mn levels, characteristic colour changes, and maximum U levels in the same sequence with increasing depth in core. On the evidence of the similarities between the cores, and pore water U data from a Porcupine Abyssal Plain site, it is proposed that the authigenic U enrichments are syndiagenetic and possibly active. No correlation is observed between sediment authigenic U and C org contents. The source of enrichment is bottom water U which has diffused downwards into the sediments to be sorbed at a particular redox level, located 10–30 cm below the oxic/post-oxic boundary marked by the colour change. The magnitude of the enrichments is caused by the persistence of this boundary at a particular level as a result of the decrease in mean sediment accumulation rate between the last glacial stage (5.2 up to at least 19.1 cm ky −1) and the Holocene (2.2–4.1 cm ky −1). Similar accumulation rate contrasts are expected to be widespread in the Atlantic, and the implications for previous reported work, particularly from the Mid-Atlantic Ridge, are discussed.

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