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

摘要

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.