Geochemistry of Ferromanganese Coatings and Muddy Sediments Taken by the Research Dive 134 of Submersible “Shinkai 6500” from the Oceanward Slope of the Japan Trench

Abstract

The chemistry of ferromanganese oxides coating pumice gravels and muddy sediment cores taken by the Research Dive 134 of the submersible “Shinkai 6500” from the oceanward slope of the Japan Trench off Miyako, Iwate Prefecture, are described. The ferromanganese coatings are quite thin and are poor in heavy metal elements including Fe and Mn, compared with previously reported ferromanganese nodules and crusts, from the seamounts and ridges in the Japan and Ogasawara Trenches. In the Fe-Mn- (Cu+Ni+Zn) × 10 ternary plot, the published ferromanganese nodules and crusts fall on a typical hydrogenous region, but the ferromanganese coatings in this study lie between hydrogenous and oxic diagenesis regions. This suggests that the former were formed by direct precipitation of colloidal metal oxides from seawater, while the latter grew by elemental supply both from pore waters in the underlying sediments and by the direct precipitation of colloidal particles. The differences of the accretionary process in the ferromanganese oxides from some of the trench areas, are caused by local environmental variations, related to the topography and sedimentary conditions including bottom currents.