Biosedimentary process and the genesis of ferromanganese deposits. The example of hydrogenetic crusts and coatings from the West African margin and from the Mediterranean Sea.

The microstructures observed in the hydrogenetic ferromanganese crusts of n eogene age sampled on the West African margin of the Central and Equatorial Atlantic are laminated. Three types of laminae were recognized. Several ar guments, in particular TEM observations of ultra-thin sections, indicate th at the laminae probably correspond to mineralized microbial films. The stud y of quaternary to recent ferromanganese coatings sampled in the Atlantic a nd in the Mediterranean Sea leads to the same conclusion. The mineralizatio n 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 th e general hydrologic conditions of the bottom waters (currents activity, nu triments and Mn availability) but, also, on the microtopography of the sea water-lamina interface, probably because it introduces significant differen ces 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 accumulati on and removal of slime.