Reconstruction of the Paleo-Ocean Environment Using Mineralogical and Geochemical Analyses of Mixed-Type Ferromanganese Nodules From the Tabletop of Western Pacific Magellan Seamount

Abstract

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.