Stratigraphic Geochemical Variations in Western Pacific Fe–Mn Crusts: Controls of Phosphatization on Element Redistribution

Ferromanganese (Fe–Mn) crusts are important archives of long-term oceanographic and redox conditions, but post-depositional phosphatization can alter their primary hydrogenetic geochemical signatures. This study investigates mineralogical and geochemical variations among representative stratigraphic layers in Fe–Mn crust cores recovered from western Pacific seamounts to better understand the effects of phosphatization on crust evolution. The crusts were divided into four layers (L1–L4) to compare non-phosphatized (L1–L2) and phosphatized (L3–L4) intervals and evaluate the effects of phosphatization on primary geochemical records. The crusts show predominantly hydrogenetic characteristics, including vernadite-dominated mineralogy, low Mn/Fe ratios, and seawater-like REY signatures. Phosphatized layers contain carbonate fluorapatite (CFA) and are enriched in REY, P, and Ca. Bulk REY concentrations show strong positive correlations with P and Ca, indicating incorporation into CFA during the phosphatization process. However, several individual REEs, including La, Nd, Sm, Gd, and Lu, are relatively enriched in the non-phosphatized layers, suggesting decoupling between bulk REY enrichment and primary hydrogenetic REE signatures. Variations in Ce anomalies are consistent with REY systematics being influenced not only by ambient redox conditions but also by post-depositional phosphatization. Within the context of previously published regional chronostratigraphic models, these results suggest a two-stage evolutionary history involving hydrogenetic precipitation under oxic seawater conditions followed by phosphatization under relatively suboxic diagenetic conditions. These findings provide new insights into the layer-specific mineralogical and geochemical evolution of hydrogenetic Fe–Mn crusts in the western Pacific and emphasize that REY-based paleo-redox interpretations should explicitly consider the effects of phosphatization and associated local redox variability.