Stratigraphic condensed deposition and diagenetic evolution of green clay minerals in deep water sediments on the Ivory Coast-Ghana Ridge

Green grains concentrations were studied through the 22 m Pleistocene sedim entary column recovered from Ocean Drilling Project Site 959 (in 2100 m wat er depth) located on a small plateau on the northern flank of the Cote d'Iv oire-Ghana marginal ridge. The green clay material usually fills the chambe rs of pelagic foraminifers. Taking into account the variance in color and m ineralogic composition of the bulk green grains fraction observed in each l evel, a methodological approach was especially performed: mineralogical and chemical analyses Were done successively on the bulk green material, then on hand-picked white, pale green, medium green and dark green grains, and l astly, surface and spot analysis of nanno-structures were obtained using an energy dispersive microprobe coupled with a scanning electron microscope. Green grains concentrations are evidenced showing both rising up of green/w hite ratio and increasing abundance of cracked dark green grains. These irr egular concentrations are not consistent directly with a general climatic f orcing as evidenced by delta O-18 isotopic curve, but are the result of rec urrent bottom-current activity. Thus, winnowing process is the cause of the long lasting ionic exchange and mineralogical evolution at the water sedim ent interface. Clay infilling composed of smectite (35-40%), kaolinite (45- 50%), traces of mica, calcite and quartz was a precursor material. Along with dissolution of kaolinite, crystallochemical modifications of sme ctites were observed both to the sequential concentration scale (bulk sampl es) and to the individual grain scale (from white to dark green): increment of Fe content associated decrease of Al and increase of layer charge and K content. In the layers with the highest green grain concentrations and in agreement, with crystallochemical formulae, Fe3+-rich montmorillonite displ ays octahedral charge, tetrahedral charge appears only when Fe content incr eased >1.2 Fe per formulae. With the maturation process the growing of the closed layers is observed (mixed layer smectite-glauconite 80/20). Nearly s imilar evolution patterns were recorded at various levels (0.01, 0.85, 2.2 Ma) of the Pliocene-Pleistocene accumulation, however the nontronite way is evidenced in the lowest green grains concentrations and remind the previou sly described shelf processes. Independently of the morphologically induced winnowing, in these marine tropical deposits, the glauconitization process was especially favored by abundant iron supply and was not affected by low temperature (similar to3 degreesC). It is suggested that in deep-water env ironment, bottom-current controlled iron deposition and disponibility, stro ngly influencing the mineralogic evolution. A too lengthy dispersal of iron microparticles played an ultimate role in the development of the montmoril lonite way rather than the nontronite way. This would be the most commonly found in shallow-water environment. (C) 2001 Elsevier Science B.V. All righ ts reserved.