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

Abstract 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.