Estimating growth rates of ferromanganese nodules from chemical compositions: implications for nodule formation processes

Using available information on the accretion rates and the bulk chemical compositions of oceanic ferromanganese nodules, it can be shown that the accumulation rate of manganese is proportional to the square of the accumulation rate of iron. This relationship has been used to derive an equation that predicts nodule growth rates from their chemical compositions. The equation accurately predicts growth rates up to about 50 mm/10⁶ yr, but yields incorrect rates for faster-growing concretions. From this relationship nodules underneath water of high primary productivity grow most rapidly and accumulate transition metals most rapidly; however, nodules from regions of highest primary productivity do not have the highest concentrations of nickel and copper. Reduction of manganese within the sediment column and remobilization of a fraction to the surface sediments adds manganese to nodules without substantially augmenting the supply of other transition elements. The addition of remobilized manganese to the nodule dilutes the concentrations of nickel, copper and iron, even though their rates of accumulation are also high.