Abstract
Recent studies on marine ferromanganese nodules have suggested a relationship between their growth rate and their chemical composition1–3. The suggestion is based on the hypothesis that the metals in nodules have two sources of supply: bottom seawater (hydrogenous) and sediment pore water (diagenetic)4–7. The diagenetic source calls for upward diffusion of Mn mobilized in a deeper, low Eh zone of the sediment column and its subsequent precipitation in an oxidized surface zone8. Addition of the remobilized Mn to surface nodules would augment their accretion rate. It would also render the oxide layers to contain higher Mn/Fe ratios than layers resulting from hydrogenous precipitation. We report here a radiometrically determined accretion rate of 168±24 mm Myr−1 for a deep-sea manganese nodule from the Peru Basin in the Pacific. This rate is about two orders of magnitude higher than that frequently found in nodules of deep-sea origin, and emphasizes the importance of sediment diagenetic processes on the growth of manganese concretions.
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