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The effect of environmental controls on the metal content in ferromanganese crusts and nodules from the Mozambique Ridge and in the Mozambique Basin, southwestern Indian Ocean

Applied Geoscience Research Group, CSIR, P.O. Box 91230, Auckland Park, 2006, South Africa, e-mail: sperritt{at}csir.co.za

Mike K. Watkeys

School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: watkeys{at}ukzn.ac.za

Major and minor element compositions of a suite of ferromanganese nodules and crusts dredged from the Mozambique Ridge on the RV Sonne cruise in 2005 are combined with previously published data from both the Mozambique Ridge and adjacent Mozambique Basin, in order to examine the relationship between regional variations in chemical composition and environmental controls on ferromanganese mineralisation. Three principal environmental controls on ferromanganese nodule and crust composition have previously been recognised: primary productivity in the surface waters, the oxygen minimum layer, and the calcium carbonate compensation depth. Of these three, the oxygen minimum layer is identified as having the greatest controlling influence on the composition of the ferromanganese deposits of the Mozambique Ridge and Basin, with primary productivity and the calcium carbonate compensation depth being secondary in importance. As a result, three different mineralisation zones can be defined in the study area: deepwater basin deposits, ridge flank deposits formed below the base of the oxygen minimum layer, and ridge crest deposits formed within the oxygen minimum layer. Nodules and crusts from these different zones display distinct variations in chemistry and mineralisation style.