Co-fluxes and growth rates in ferromanganese deposits from central Pacific seamount areas

Recent geochemical studies on ferromanganese deep-sea nodules have shown that the relationship between Mn and Fe allows us to differentiate between a diagenetic type (Mn/Fe ≥ 2.5) and a hydrogenetic type (Mn/Fe ≤ 2.5)1–7. Diagenetic and the lower parts of mixed type nodules are mainly supplied by an early diagenetic pore water source; hydrogenetic type nodules grow by very slow accumulation of inorganic colloidal particles of hydrated metal oxides from near-bottom seawater. Growth rates of hydrogenetic nodules generally do not exceed 5 mm Myr−1 (refs 8–11), whereas diagenetic nodules were found to grow much faster: an extreme example is the rate of 168 mm M yr−1 that was recently reported for a Mn-rich diagenetic type of nodule from the Peru Basin6. A different inter-element relationship of Co to the main metals Mn and Fe in different type of nodules has also been observed. We show here that the flux of Co into ferromanganese seamount crusts from the central Pacific is similar to the flux into pelagic nodules: the amount of Co supplied per unit of time and area is nearly constant in the oceanic water column. The most important factor governing the Co-enrichment is the growth rate; no local source is needed to explain the observed Co concentrations up to 2% in ore samples from summit areas.