Analysis of ferromanganese deposits from the West African margin and the Mediterranean Sea

The microstructures observed in the hydrogenetic ferromanganese crusts of neogene age sampled on the West African margin of the Central and Equatorial Atlantic are laminated. Three types of laminae were recognized. Several arguments, in particular TEM observations of ultra-thin sections, indicate that the laminae probably correspond to mineralized microbial films. The study of quaternary to recent ferromanganese coatings sampled in the Atlantic and in the Mediterranean Sea leads to the same conclusion. The mineralization of the biofilm is a step process. The sequestration of Fe and of variable amounts of Si preceeds the Mn scavenging. The type of lamina depends on the general hydrologic conditions of the bottom waters (currents activity, nutriments and Mn availability) but, also, on the microtopography of the sea water-lamina interface, probably because it introduces significant differences about Eh and Mn fluxes. Considering the low accretion rate of this type of deposit, it appears that the biofilm growth may be controled by a self-regulating process: the initial period of active production is followed by a long time interval of low production or of equilibrium between accumulation and removal of slime.