Some geochemical observations on manganese-iron oxide nodules from different depth environments

Abstract

The chemical composition of recent manganese-iron bearing nodules in lacustrine and shelf environments shows low Mn Fe , O Mn ratios and only small concentrations of minor elements Co, Ni, Cu, Zn and Pb, in contrast to open oceanic nodules which have significantly higher values. Surface sedimentary oxides of littoral and shelf environments have higher Zn and Pb than pelagic sedimentary oxides, while the latter oxides display much higher levels of Ni, Co and Cu. Burial of shallow-water oxides causes reaction with organic matter, mobilising much of the Mn and minor elements into interstitial waters. Resultant precipitation or accretion of these elements occurs at or near the sediment-water interface depending on the physicochemical conditions within the sediment and bottom waters. The chemistry of metal organic complexes is thought to play an important part in the uptake of minor elements in shallow water nodules. Marginal oceanic nodules from various depths are chemically similar to shelf nodules; again the organic matter in sediments, not the depth of location, governs their composition. Work on some ancient nodules from Timor has been reviewed.