Regional and fine-scale variability in composition and structure of hydrogenetic ferromanganese crusts: Geological characterization of 25 drill cores from the Marcus-Wake seamounts

Hydrogenetic ferromanganese crusts in the northwestern (NW) Pacific seamounts are considered as potential future sources of strategic metals. The details of their regional and fine-scale variations in composition and structure are currently unknown. We attempted to characterize the variation patterns in a selected area of the NW Pacific seamounts, based on geological and geochemical analyses, which were performed with a TV-monitored drill machine and remotely operated vehicles, after full bathymetric mapping and an acoustic survey. Subsequent chemical, mineralogical, and structural analyses of the well-preserved crust cores indicated that the hydrogenetic crusts in the seamount area have similar distinct fine-scale variations throughout the growth layers from the substrate to the surface. The crust cores show almost continuous stratigraphic growth since the middle Miocene (approximately 15 Ma ago) or earlier, with significant variations in metal concentration, growth structure, and mineralogy. The fine-scale stratigraphic variations are surprisingly well-correlated across the seamount areas of approximately 1000 km apart; thus, the bulk compositional variation becomes much smaller than a secular variation when compared within the seamount area. Stratigraphic analysis is crucial for the compositional characterization of ferromanganese crusts on various scales.