Characterization of Submicron-Thick Layered Structure in Hydrogenetic Ferromanganese Nodule Suggests Short-Term Redox Fluctuation of Paleo-Ocean

Ferromanganese (Fe-Mn) nodules are widely considered significant economic potential and paleoenvironmentally significant archives. Generally, submicron-thick layers (SMLs) are basic units of the Fe-Mn nodules and record valuable information about the environment of Fe-Mn nodule formation, but their structural, mineralogical, and chemical features are not well resolved. Here, we investigated a typical hydrogenetic Fe-Mn nodule from the Penrhyn Basin in the South Pacific Ocean, using various microanalytical techniques. The nodule mainly consists of a micronodule-dominant region around the core, a crust near the surface with Mn- and Fe-rich layers, and stromatolite-like textures between them. These textures are composed of distinct and indistinct SMLs parallel to their growth front. Although the distinct and indistinct SMLs have different mineral phases, the transmission electron microscopy revealed that these layered structures are actually interstratification of SMLs with different crystal sizes of foliated phyllomanganates. Large crystals contain higher Mn oxidation states than small crystals. Such variations of Mn oxidation state with different crystal sizes along the growth direction imply the short-term regular fluctuation in the redox condition of the Lower Circumpolar Deep Water. Ferromanganese (Fe-Mn) nodules are the precipitation of Fe and Mn (oxyhydr)oxides, generally occurring as submicron-thick layers (SMLs). Due to the slow growth rates (millimeters per million years) of Fe-Mn nodules, these Fe and Mn (oxyhydr)oxides can adequately adsorb various ions from seawater. Besides, as Fe and Mn are highly redox active, their oxides are very sensitive to the redox condition of the environment. Therefore, the physicochemical characteristics of SMLs are the critical key to decipher the metallogenetic environment of the nodules. In this study, we systematically studied the SMLs in a typical hydrogenetic Fe-Mn nodule collected from the South Pacific Ocean. We discovered that the SMLs are composed of alternating porous and dense layers caused by the crystal size of minerals. The adjacent porous (large crystal size) and dense (small crystal size) layers have the same mineral phases and similar chemical compositions, while the Mn oxidation states are different, revealing the short-term change of redox conditions of the Lower Circumpolar Deep Water. Our findings not only reveal the characteristics of SMLs, but also provide a new insight into the redox environments of the formation of Fe-Mn nodules. Submicron-thick layered structures in ferromanganese nodule correspond to the alternation of large and fine phyllomangante crystallitesMn oxidation state in the alternating layers is variable depending on the crystal size of phyllomanaganteThe variation in Mn oxidation state across the layers may reflect short-term changes in redox environment of the ocean