Geochemistry of ferromanganese micronodules recovered from sediment-core in the western Nigeria continental margin, Eastern Equatorial Atlantic: Implications on the genesis and depositional history.

Abstract

This study examines the geochemical compositions, origin and depositional characteristics of ferromanganese (Fe–Mn) micro-nodules recovered from 350 cm long sediment core DY26III-Nig-S60-GC2 at 2946 m water depth from the slope of the Eastern Equatorial Atlantic (EEA), offshore Nigeria (4°28′56′′.388′E, 3°33′10′′.764′N) within the 200 nautical miles of the Exclusive Economic Zone (EEZ). The hydrographic regime of the area of study is dominated by westward flowing North and South Equatorial Currents and the eastward flowing shallow water mass referred to as Guinea current. Observations made on the morphology of the Fe–Mn micro-nodules recovered allowed us to distinguish two distinct varieties (type-1 and type-2). The chemical composition of the type-1 Fe–Mn micro-nodules contain enriched Fe (8.55–29.05 wt%), Pb (0.03–0.35 wt%) and Co (0.04–0.68 wt%) relative to type-2 Fe–Mn micronodule that contains enriched Mn (23.85–48.75 wt%), Ni (0.07–0.75 wt%), Cu (0.25–1.55 wt%), Zn (0.05–0,56 wt%) and Ca (3.95–5.56 wt%). The geochemical investigations suggest hydrogenetic origin for the formation of the Fe–Mn micro-nodules from the EEZ, offshore, Nigeria. The sediment's depositional conditions of type-1 Fe–Mn micronodule correspond to a dry period of reduced sea-level rise (Marine isotope stage; MIS-2) during the late Pleistocene with the relative age of 16.96–12kyr. However, type-2 Fe–Mn micro-nodule corresponds to a warm period of sea-level rise and increased fluvial discharges (MIS-3) with sediment's depositional age of 47.22–41.93kyr. Based on chemical composition, discrimination plots and statistical analysis, the type-1 and 2 Fe–Mn micronodules can be classified as hydrogenetic, with additional remobilization of metals through the underlying sediments on the type-2 Fe–Mn micronodule.