Occurrence of thin, metal-rich layers in deep-sea sediments: a geochemical characterization of copper remobilization

B.J.H. Van Os, H.J. Visser, J.J. Middelburg, G.J. De Lange

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    Distinctly coloured trace metal enriched fronts with a six-fold Cu-enrichement occur in the interbedded pelagic-turbiditic sediments from the Madeira Abyssal Plain. To decipher their mode of formation, two sequential extraction techniques, an acid-gradient and a “Tessier-type” scheme, were applied. Two different front types could be distinguished: Type-1 fronts are mostly black and enriched in Co, Zn, Ni and Cu, and associated with the oxidation fronts that developed after deposition of turbidites. Type-2 fronts are purple, extremely enriched in Cu, and associated with sulphur. These fronts are sometimes found in relation with a relict oxidation front, but often show no clear relation with lithology. Two different modes of formation are postulated. (1) The transition-metal enrichment in the type-1 fronts is typical for a manganese-oxide association often found in marine sediments. After the dissolution of manganese-oxide during sub-oxic diagenesis, the transition metals were liberated and subsequently relocated and precipitated in response to a redox-gradient. (2) The Cu---S association in type-2 fronts is also formed during sub-oxic diagenesis. The main process leading to the formation of this Cu---S enrichment is the mobilization of inherited Cu---S compounds from organic-rich turbidites. The accumulation of these compounds is possibly related to redox changes at relict oxidation fronts and to gradual, probablly slowly evolving, redox changes that occur at present.
    Original languageEnglish
    Pages (from-to)1713-1730
    JournalDeep Sea Research Part I-Oceanographic Research Papers
    Issue number9
    Publication statusPublished - 1993

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