Spatial Distribution of Critical Metals and Chemostratigraphy in Co-Rich Ferromanganese Nodules in the Northwestern Pacific Ocean

Ferromanganese nodules are important marine storehouses for critical metals and windows for changing oceans. Although advanced in situ analytical techniques have been applied to visualize the elemental distribution in the nodule cross-sections, their spatial distribution remains largely uncertain. This study addresses this gap by employing micro X-ray fluorescence mapping of parallel nodule cross-sections to delineate the spatial distributions of critical metals (Mn, Fe, Co, Ti, Ni, and Cu) in three Co-rich ferromanganese nodules from the northwestern Pacific Ocean. The 10-layer Os isotopic compositions of one nodule closely align with the well-documented marine Os isotope evolution of seawater, providing a chronological framework and a maximum age of ∼36 Ma for these nodules. Three concentric chemostratigraphic layers, labeled L1, L2, and L3, were identified from the inside out, based on microscopic structures and the distributions of critical metals. The early growth stage was marked by Mn-rich, Si-rich, and high Mn/Fe ratios, suggesting a diagenetic-driven process attributed to high paleoproductivity conditions because of the low latitude of the study area at that time. The subsequent growth stages are all hydrogenetic in origin to be rich in Fe, Co, and Ti with low Mn/Fe ratios. The apparent detritus present during the second growth stage of the nodules may correspond to the stronger bottom currents in the early Miocene. The final mineralization stage indicates a more stable environment with diminished bottom current activity, leading to the formation of a dense, laminar hydrogenetic layer.