Formation age of the dual structure and environmental change recorded in hydrogenetic ferromanganese crusts from Northwest and Central Pacific seamounts

Abstract Thick hydrogenetic ferromanganese (Fe Mn) crusts from the northwest and central Pacific seamounts often show a distinct dual structure composed of a typical hydrogenetic porous, friable upper part of Fe Mn oxides (Layer 1) and the underlying dense, hard phosphatized growth generation of Fe Mn oxides (Layer 2 in this study). Layer 2 always appears above the substrate rock and composes the lower part of the crust; it is never found as the upper crust layer in contact with seawater. The chemical composition of Layer 2 clearly differs from the younger Layer 1 hydrogenetic Fe Mn oxides, and is depleted in Fe, Al, Ti, and Co, and enriched in Ni, Cu, and Zn relative to Layer 1. The Be isotope age models of the crusts were refined with paleomagnetic and paleontological information, and applied to selected crust samples. The age model indicates fairly continuous growth from the substrate to the surface and fairly constant growth rates during the past 17 Ma. The growth rate from the Miocene to the present has varied by a factor of two, about 2–4 mm/Myr in Layer 1, while Layer 2 has similar but more variable growth rates than Layer 1. The calculated age for the base of Layer 1, and possibly the age of termination of phosphatization, is never younger than the late Miocene. The age seems to vary with water depth, shallower-water crusts (between 991 and 1575 m) showing a younger age of about 10 Ma whereas the deeper-water (2262 m) crusts have extrapolated ages for the base of Layer 1 of be 17.1 ± 2.5 Ma. This trend indicates that phosphatization took place in a less-oxidizing environment during growth of Layer 2, followed by a weakened oxygen-minimum zone or intensified AABW during growth of Layer 1.