Rotation of a Ferromanganese Nodule in the Penrhyn Basin, South Pacific, Tracked by the Earth's Magnetic Field

Ferromanganese nodules are cm‐sized, authigenic, abyssal manganese‐iron‐hydroxide concretions. They grow very slowly at rates of a few mm per million years. Although their ages are older than millions of years, they are often found half buried on the modern sediment surface. The mechanisms for the nodules' persistence at the surface without complete burial could be linked to their occasional motion or agitation. Here, we report evidence for the rotation of a nodule from the Penrhyn Basin, South Pacific detected by paleomagnetism. The paleomagnetic inclinations of specimens from the nodules' surface are consistent with the recent geomagnetic fields. The paleomagnetic directions from the surface to the core show successive changes and form a great circle with a pole at (azimuth = 53.9°, dip = 32.1°). This suggests 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 due to bottom current underwashing. Rock magnetic analyses of the nodule suggest the presence of magnetite in single domain and vortex states. Low temperature magnetometry revealed that magnetite grains were heavily oxidized to maghemite, especially close to the core of the nodule. The rotation may have exposed the rising part of the nodule to oxidative pore water. Oxygenated Antarctic Bottom Water might have caused remagnetization due to low temperature oxidation of magnetite. The rotation would also facilitate the omnidirectional growth of the nodules' mixed layer of diagenetic buserite and hydrogenetic vernadite. Plain Language Summary Ferromanganese nodules are deep marine mineral resources that grow very slowly at a speed of few mm per million years on the seabed. They are very old, but often remain on the sediment surface. It remains unclear, why they were not buried by deep‐sea sedimentation. A nodule recovered from the Penrhyn Basin in the South Pacific was measured to reveal its fossil magnetization by the ancient Earth's magnetic field. Each specimen from the nodule showed a stable magnetization that is linearly decreasing toward the origin during alternating field demagnetization. The dip of the magnetization for the surface is consistent with the recent Earth's magnetic field, while the magnetization directions toward the nodule's center form a great circle. This suggests that the nodule rotated along a pole while successively recording magnetizations. T