Continuous growth of hydrogenetic ferromanganese crusts since 17Â Myr ago on Takuyo-Daigo Seamount, NW Pacific, at water depths of 800-5500Â m

On-site geological mapping with ROVs and associated full chemical and mineralogical description for hydrogenetic ferromanganese crusts have shown typical variation patterns of abundance and composition in space and time for the ferromanganese crusts over a seamount “Takuyo-Daigo” NW Pacific. Almost entire surface of rock outcrops is covered with thick ferromanganese crusts up to 100-mm thick hydrogenetic iron and manganese oxide deposits at any water depths from 800 through 5500 m on a slope on the Creataceous giant guyot in the NW Pacific. The crusts have been growing at the rate of 2.3-3.5 mm/m.y. since the Early to Middle Miocene age or earlier without any significant break at all depths even in the modern oxygen minimum zone (OMZ). However, minor redox-sensitive metal elements, cobalt and molybdenum, monoclinally change with water depth in the surface modern precipitates probably controlled by redox condition in seawater.149