Some geochemical controls on lead and barium concentrations in ferromanganese deposits

The Pb content of natural deep-sea ferromanganese deposits decreases with depth. This is commonly explained by assuming oxidation of Pb²⁺ and uptake as (Mn, Pb)O₂ in normally oxygenated shallow water, a process that is less likely to occur in deep water which is somewhat depleted in oxygen. This concept is discussed and rejected, both on theoretical grounds and based on experimental results of Pb²⁺ sorption by Mn oxides. Comparative sorption experiments on Mn oxides were carried out with Pb and Ba. It is shown that Pb is preferentially adsorbed by birnessite at $\text{pH}\text{? 4}$. The uptake ratio is, however, much lower than the relative enrichment factor in natural nodule material. Suggested explanations for this discrepancy are: influence of hydrostatic pressure and extent of hydrolysis of the cations at the pH of ocean water.