MANGANESE DIAGENESIS IN DIFFERENT GEOCHEMICAL ENVIRONMENTS OF THE RIA DE VIGO (GALICIA, NW IBERIAN PENINSULA)

Two geochemical scenarios with sharp differences have been identified in the Ria de Vigo (NW Spain). One of them corresponds to San Simon Bay, located in the innermost section of the Ria de Vigo, where the high organic matter content present in the sediment promoted high production of methane, which diffuses throughout the sediment column until it reaches the sediment-water interface. This diffusion process promotes anaerobic oxidation of sulfate with subsequent generation of H2S and intense Mn pyritization throughout the sediment column, a process similar to those observed in euxinic environments.
The second scenario corresponds to the rest of the Ria de Vigo (middle and outermost sections), where the dominant Mn fraction was Mn-carbonate, even at depth in the sediment and in the presence of methane, where the concentration of this fraction is comparable to that of Mn-pyrite.