The influence of suboxic diagenesis on the formation of manganese nodules in the Clarion Clipperton nodule belt of the Pacific Ocean

Abstract

Abstract Polymetallic nodules and a single Fe-Mn crust from the Clarion and Clipperton Zone (CCZ) of the equatorial eastern Pacific have been analyzed to determine their chemical and mineralogical compositions. ICP-OES analyses of bulk nodules from the CCZ indicate that the nodules formed under oxic-diagenetic conditions with typical Mn/Fe ratios of 4–6, Ni + Cu values of 2–3 wt.%, Co contents of 0.12–0.17 wt.%. However, detail analyses of individual growth layers by electron microprobe analyses (EMPA) reveal much larger chemical heterogeneity of individual layers with Mn/Fe ratios ranging from 800. Two different genetic types of layers can be distinguished: Layer type 1: dense layer with average Mn/Fe ratios of 1.80, average Ni + Cu of 0.81 wt.%, Co contents of 0.30 wt.% (hydrogenetic metal precipitation from oxic waters). These layers consist of Fe-vernadite (δ-MnO2) epitaxially intergrown with feroxyhyte (δ-FeOOH) nanoparticles. Layer type 2: growth structures (dendritic growth structures and dense layers) with average Mn/Fe ratios of 96, Ni + Cu of 3.9 wt.%, Co contents of 0.08 wt.%, and a distinct Ni + Cu maximum of 6.51 wt.% at a Mn/Fe ratio of 56 (suboxic-diagenetic, metal precipitation from suboxic pore waters). Type 2 layers mainly consist of turbostratic phyllomanganates such as 10 A vernadite, 7 A vernadite and todorokite are minor compounds. Mixed layer type 3: These layers can occur as zones of low reflectivity in the transition from layer type 1 to 2 or build up inhomogeneous growth structures of nm-thin layers of low or high reflectivity. The Mn/Fe ratios of this material range between 3 and 11, Ni + Cu of 1–4.6 wt.%, and Co contents are between 0.02 and 0.77 wt.%. In most cases these growth structures represent a mixture of type 1 and 2 layers. X-ray photoelectron spectroscopy (XPS) of the recent outermost nm layers of nodules (top, rim and bottom side) indicates hydrogenetic accretion processes under recent oxic conditions both from seawater and pore water. The mixed-type CCZ nodules are interpreted to consist of an oxic-hydrogenetic and a suboxic-diagenetic end member. Calculations of the proportions of the individual layer end members show that suboxic layers make up about 50–60% of the chemical inventory of the CCZ nodules whereas oxic-hydrogenetic layers comprise about 35–40%. The remaining part (5–10%) consists of incorporated sediment particles occurring along cracks and pores. These results demonstrate that suboxic conditions alternate with oxic conditions during the growth of nodules in the eastern CCZ. This is probably due to fluctuating bioproductivity in the equatorial Pacific surface waters during glacial–interglacial periods which led to changing organic carbon flux to the sediment and changing oxygen consumption in near surface sediments. Furthermore, reduced ventilation of the deep ocean during glacial periods may have led to suboxic conditions in near-surface sediments. Our investigations prove that bulk chemistry data of whole nodules only provides an average of individual layers that build up the nodules and is inappropriate for genetic interpretation.