10:45 AM - 12:15 PM
[MZZ44-P04] Rotation of a Ferromanganese Nodule in the Penrhyn Basin, South Pacific, Tracked by the Earth’s Magnetic Field
Keywords:ferromanganese nodule, paleomagnetism, maghemite
Ferromanganese nodules are cm-sized, authigenic, abyssal manganese-iron-hydroxide concretions. They grow very slowly at rates of a few mm per million years (e.g. Verlaan and Cronan, 2022). Although their ages are older than millions of years, they are often found half buried on the modern sediment surface (e.g. Usui and Ito, 2004). The mechanisms for the nodules’ persistence at the surface without complete burial could be linked to their occasional motion or agitation. Here, we report on evidence for the rotation of a nodule from the Penrhyn Basin, South Pacific detected by paleomagnetism (Oda et al., 2023).
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 (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 Antactic 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 rich in sediments and hydrogenetic vernadite.
Acknowledgement
Kochi Core Center Grant No. 21A034/21B032 supported MPMS measurements. HO was supported by JSPS KAKENHI Grant No. 20KK0082/21H04523.
References
Oda, H. et al. (2023) G-cubed, doi: 10.1029/2022GC010789
Usui, A. & Ito, T. (1994). Marine Geology, 119, 111-136.
Verlaan, P.A. & Cronan, D.S. (2022) Geochemistry, 82, 125741.
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 (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 Antactic 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 rich in sediments and hydrogenetic vernadite.
Acknowledgement
Kochi Core Center Grant No. 21A034/21B032 supported MPMS measurements. HO was supported by JSPS KAKENHI Grant No. 20KK0082/21H04523.
References
Oda, H. et al. (2023) G-cubed, doi: 10.1029/2022GC010789
Usui, A. & Ito, T. (1994). Marine Geology, 119, 111-136.
Verlaan, P.A. & Cronan, D.S. (2022) Geochemistry, 82, 125741.