Abstract

Brown Fe and black Fe-Mn coated grains are present at a water depth of 25-30 m in the subsurface reworked deposits of the Sanaga Pleistocene delta (Cameroon); most are former fecal pellets with concentric layering composed of one to four layers. Brown grains are composed of a medium to well crystallized goethite. Black grains commonly have a metallic luster and show todorokite microcrusts on the surfaces of the goethite rims. Cyanobacterial microstructures that developed in the voids between the nuclei and the cortices of the grains as well as between successive layers within the cortices suggest a weakly microlaminated oncolite and biomediation of the oxidation. These grains were generated between c. 9600 and 8000 yr BP by successive centripetal coatings. The most likely environment is thought to be oxygenated seawater over offshore bars near the mouth of the Sanaga river. Successive steps are involved in the buildup of Fe-Mn coatings: (1) upward flux of Mn (super 2+) and Fe (super 2+) from the sediment column to the sediment-water interface, (2) Fe (super 3+) precipitation as Fe oxyhydroxide and catalyzed deposition of Mn (super 4+) on the oxyhydroxide iron surface, and (3) bacterial biomediation of the Fe-Mn oxidation. Modern nearshore sediments on the Cameroonian shelf and modern and Quaternary sediments on other West African shelves contain only goethite-coated grains; the todorokite formation off the Sanaga mouth appears to have no modern analog. On the basis of the present-day factors--high seawater temperature (>30-31 degrees C), low salinity (24-28 per mil), high bacterial activity, and high concentration of MnO (0.27 per mil) in the suspended matter of the upstream Sanaga waters--we consider the formation of the analyzed coated grains to have resulted from the same enhanced factors associated with the early Holocene transgressive environment.

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