Rotation of a manganese nodule proved by the detection of the Earth’s magnetic field - Elucidating the growth process of unburied spheroidal deep-sea resources during their rotation on the deep ocean floor

Summary of the AIST Press Release on April 18, 2023,
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　Ferromanganese nodules are centimeter-sized, authigenic, and abyssal manganese-iron-hydroxide concretions. They grow very slowly at rates of few millimeters per million years. Although their ages are older than millions of years, manganese nodules are often found half buried on the modern sediment surface. The mechanisms for the persistence of nodules at the surface without burial has not yet been elucidated. We report evidence for the rotation of a nodule from the South Pacific detected by paleomagnetism (record of the past geomagnetic field).
　Manganese nodules were collected in the Penrhyn Basin, South Pacific during the GH83-3 cruise and a manganese nodule sample was marked at the top and used for the study (Fig. 1). The paleomagnetic inclinations of specimens from the nodules’ surface are consistent with the recent geomagnetic field. The paleomagnetic directions from the surface to the core show successive changes and form a great circle with a pole at an azimuth of 53.9° and a dip of 32.1°. This indicates that the nodule rotated along its pole while successively recording magnetizations. As the nodule was found on a gentle slope at the foot of an abyssal hill, it may have moved downslope owing to bottom current underwashing. Magnetic analyses of the nodule suggest the presence of magnetite (Fe₃O₄). Low temperature magnetometry revealed that magnetite grains were heavily oxidized to maghemite (γ-Fe₂O₃), especially close to the core of the nodule. The rotation may have exposed the rising part of the nodule to oxidative pore water. Oxygenated Antactic Bottom Water flowing through the Penrhyn Basin might have caused remagnetization owing to the low temperature oxidation of magnetite to maghemite. The rotation would also facilitate the omnidirectional growth of the nodules’ mixed layer that is rich in sediments during burial and hydrogenetic vernadite during exposure to the seawater. This may have significant implications on the chemical compositions of manganese nodules as deep-sea marine resources.
　The result of this study has been published in the journal of Geochemistry, Geophysics, Geosystems on February 28, 2023 (Oda et al., 2023).

Fig. 1 (Left) Seafloor bathymetry and sampling site (red diamond). Yellow dots represent sites where manganese nodules were found.
(Center) Manganese nodules collected by a box corer.
(Right) A manganese nodule used for the study. The white mark represents vertically upward.
This figure has been modified from that in the original publication (Oda et al., 2023; CC BY-NC 4.0).