E-H, pH diagrams have been calculated using the PHREEQC programme in order to establish the predominance fields of Mn, Fe, Co, Ni, Cu and As in bottom waters from the Angola Basin. Predominance fields are presented separately for both aquatic species and solid mineral phases in order to simplify interpretation of the data. The diagrams show significant differences from standard E-H, pH diagrams for these elements calculated for freshwater at 25 degrees C and 1 bar which assume an element concentration of 10(-6) M. In particular, our diagrams show that Mn2+ and NiCO30 are the predominant aquatic species for Mn and Ni in bottom seawater and FeOOH, Fe2O3, Fe3O4, CoFe2O4, CuFe2O4, CuFeO2, and Ba-3(AsO4)(2) the predominant solid phases for Fe, Co, Cu and As, respectively. Mn and Ni are therefore undersaturated and Fe, Co, Cu and As supersaturated in bottom seawater from the Angola Basin. Neither rhodochrosite (MnCO3) nor siderite (FeCO3) can form in this marine environment in equilibrium with seawater. A mixed Mn-Ca carbonate is therefore formed within the pore waters of reducing sediments. The high Ni/Cu ratios in cobalt-rich manganese crusts formed adjacent to the oxygen minimum zone may be explained by the change from Cu2+ to CuCl32- as the dominant aquatic species of Cu in seawater at an E-H of +0.48 V.
