Kiho Yang

Variations in the geochemistry and mineralogical compositions of ferromanganese (Fe‐Mn) nodules are closely related to variations in environmental parameters. Therefore, analysis of Fe‐Mn nodules can reconstruct the paleo‐ocean environment. Here, three differently shaped Fe‐Mn nodules were collected from the open seamount 9‐1 of the western Pacific Magellan Seamount. Geochemical and mineralogical analyses were conducted using micro X‐ray fluorescence (μ‐XRF) and Synchrotron X‐ray powder diffraction to reconstruct the paleo‐ocean environment. We found that the shape of the three nodules was different; however, the variations in their geochemical and mineralogical properties were similar. For all three nodules, the μ‐XRF elemental distribution revealed a distinct alternation between the Mn‐rich and Fe‐rich layers. The Mn‐rich and Fe‐rich layers surrounded the nuclei and appeared as concentric circles. Fe‐rich and Mn‐rich layers exhibited low Mn/Fe ratios (<2.5) and were dominated by vernadite, whereas layers with high Mn/Fe ratios (>2.5) were dominated by todorokite, respectively. These findings suggest an early diagenesis process at tabletop of the Magellan Seamount. The high contents of Ca and P and the existence of carbonate fluorapatite in the center of Fe‐Mn nodules indicate that the formation of these nodules began during the Miocene phosphatization event (19–16 Ma). In the early stages of nodule formation, a diagenetic process drove Fe‐Mn nodule formation. Because of global cooling from 9 Ma, early diagenesis was weakened, after which the formation of hydrogenetic Fe‐Mn nodules became dominant. As a result, variations in oxygen minimum zones and oxic‐suboxic fronts were recorded in Fe‐Mn nodules from tabletop of the seamount.