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Diagenetic Enrichment of Manganese and Other Heavy Metals in Hemipelagic Brown Clay of the Palau Trench Floor
A 1089 cm long piston core recovered from the floor of the Palau Trench (8053 m water depth) shows an interbedded sequence of brown clay and carbonate turbidites, with three Mn-enriched layers. The brown clay is recognized as hemipelagic clay by the relation of Fe-Mg in residual clay fractions of the samples and by stratigraphical and mineralogical characteristics. One of the Mn-enriched layers is associated with redox cycles of dark band and brown clay, whereas the others are observed at the boundaries between the uppermost layers of the turbidites and the overlying hemipelagic brown clays. A series of selective leachings with fractionation of Mn and other heavy metals into carbonate and oxyhydroxide fractions indicates that Mn-, Cu-, Ni-, and Co-oxyhydroxides are highly concentrated in the Mn-enriched layers. The enrichment mechanism is interpreted to be a catalytic reaction of Fe-oxyhydroxide in the brown clay with Mn++ in the overlying waters as a result of Mn++ enrichment, the result of either reducing conditions during the formation of redox cycles or to the dissolution of carbonate-bound Mn during the turbiditic sedimentation. Responding to the enrichment mechanism of Mn, other heavy metals such as Cu, Ni, and Co are also enriched in the form of oxyhydroxides. The lack of any Mn-micronodules in the brown clay may be the result of the duration of Mn++ enrichment events in the water column which was not long enough to form them, as well as to the rather high sedimentation rate of the hemipelagic brown clay.
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