Ferromanganese and Phosphatic Hardgrounds on the Western Pacific Guyots Drilled during Legs 143 and 144

Abstract

Ferromanganese and phosphatic hardgrounds were recovered during Legs 143 and 144 on the Mid-Pacific Mountains and Marshall Islands guyots within the condensed section between the shallow-water carbonate sequence and the pelagic cap on the guyot summits. Ferromanganese oxyhydroxides and phosphates are coupled because of their formation under the same long-term nondepositional environments. Phosphatization was traced downhole to at least 62 mbsf in Hole 867B (Resolution Guyot) within the Albian shallow-water sequence. Mineralogy and geochemistry indicate a hydrogenetic origin of the ferromanganese oxyhydroxides. Phosphates are formed by metasomatic replacement of pelagic and shallow-water biogenic calcite by carbonate-fluorapatite. Uniform composition and lattice structure of the apatite are evidence of equilibrium with normal seawater. Phosphorus for the hardground formation can be supplied from the deep-water reservoir or from the intermediate oxygen-minimum zone. An alternative endo-upwelling model is also discussed.