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

Abstract

Green grains concentrations were studied through the 22 m Pleistocene sedimentary column recovered from Ocean Drilling Project Site 959 (in 2100 m water depth) located on a small plateau on the northern flank of the Côte d'Ivoire–Ghana marginal ridge. The green clay material usually fills the chambers of pelagic foraminifers. Taking into account the variance in color and mineralogic composition of the bulk green grains fraction observed in each level, 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 lastly, 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/white ratio and increasing abundance of cracked dark green grains. These irregular concentrations are not consistent directly with a general climatic forcing as evidenced by δ 18O isotopic curve, but are the result of recurrent bottom-current activity. Thus, winnowing process is the cause of the long lasting ionic exchange and mineralogical evolution at the water sediment 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 smectites were observed both to the sequential concentration scale (bulk samples) 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, Fe 3+-rich montmorillonite displays octahedral charge, tetrahedral charge appears only when Fe content increased >1.2 Fe per formulae. With the maturation process the growing of the closed layers is observed (mixed layer smectite–glauconite 80/20). Nearly similar 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 previously 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 (∼3°C). It is suggested that in deep-water environment, bottom-current controlled iron deposition and disponibility, strongly influencing the mineralogic evolution. A too lengthy dispersal of iron microparticles played an ultimate role in the development of the montmorillonite way rather than the nontronite way. This would be the most commonly found in shallow-water environment.