5:15 PM - 6:30 PM
[MZZ46-P05] Reconstruction of rotation by paleomagnetic analysis of manganese nodules collected in the South Pacific
Keywords:manganese nodule, paleomagnetism, geomagnetic inclination, growth process
Marine manganese nodules are believed to grow at or within the deep-sea sediments, and assumed uplifting and overturn during the growth without burial. We try to reconstruct the rotation and overturn during the growth process based on a paleomagnetic perspective using the manganese nodules collected with deep sea clay sediments.The Earth's magnetic field averaged over thousands of years can be approximated by a geocentric axial dipole, and the remanent magnetization direction recorded on the surface of a manganese nodule growing in a magnetic field is expected to correspond to the magnetic field direction at that time. If the manganese nodule has not been rotated during the growth process, the paleolatitude can be determined by determining inclination from the remanent magnetization direction. On the other hand, if the manganese nodule has rotated, the paleolatitude calculated from inclination will be different from the actual latitude, and the amount of rotation can be detected from this discrepancy.
The nodules were collected by a box corer (158°30'64" W, 12°00'03" S) from a depth of 5248 m in the GH83-3 Hakurei-maru cruise conducted by the Geological Survey in 1983 in the Penrhyn Basin. This is a spherical sample (B92-1-1; 74 mm long, 60 mm wide, and 66 mm high). The nodules were marked at the apex (center) with a white dot on board as a reference for the vertical direction.
The sample was split in half lengthwise on the plane including the white dot and the center of the nodule, and the vertical slice sample A was taken from the section as well as another vertical slice. Both slices were divided into five rows of strips, and then further divided in the growth direction into five subsamples. The samples were further divided into five or ten subsamples in the direction of growth to allow confirmation of changes in paleomagnetic and rock magnetic parameters with growth. The natural remanent magnetization of the subsamples was measured using a pass-through superconducting rock magnetometer (2G Enterprises Model 760R) at the National Institute of Advanced Industrial Science and Technology (AIST), and the primary remanent magnetization component was determined by stepwise alternating current demagnetization experiments and analysis of the demagnetization curve. Furthermore, rock magnetic properties such as magnetic hysteresis, isothermal remanent magnetization acquisition curve, and First Order Reversal Curve (FORC) were measured using a vibrating sample magnetometer (Lakeshore VSM 8604).
The paleomagnetic data confirmed that the paleomagnetic inclination of the samples near the surface near the white marker for both samples A and B are in good agreement with the expected geomagnetic inclination value (-23 degrees) at the sample collection point. This confirms that the surface layer of the sample records the current geomagnetic field. In addition, the paleomagnetic inclination tends to become deeper as we go back in time, indicating the sample was rotating. We discuss the possibility of rotation of manganese nodule during growth on the sediments, and the types and origin of magnetic minerals based on rock magnetic data.
The nodules were collected by a box corer (158°30'64" W, 12°00'03" S) from a depth of 5248 m in the GH83-3 Hakurei-maru cruise conducted by the Geological Survey in 1983 in the Penrhyn Basin. This is a spherical sample (B92-1-1; 74 mm long, 60 mm wide, and 66 mm high). The nodules were marked at the apex (center) with a white dot on board as a reference for the vertical direction.
The sample was split in half lengthwise on the plane including the white dot and the center of the nodule, and the vertical slice sample A was taken from the section as well as another vertical slice. Both slices were divided into five rows of strips, and then further divided in the growth direction into five subsamples. The samples were further divided into five or ten subsamples in the direction of growth to allow confirmation of changes in paleomagnetic and rock magnetic parameters with growth. The natural remanent magnetization of the subsamples was measured using a pass-through superconducting rock magnetometer (2G Enterprises Model 760R) at the National Institute of Advanced Industrial Science and Technology (AIST), and the primary remanent magnetization component was determined by stepwise alternating current demagnetization experiments and analysis of the demagnetization curve. Furthermore, rock magnetic properties such as magnetic hysteresis, isothermal remanent magnetization acquisition curve, and First Order Reversal Curve (FORC) were measured using a vibrating sample magnetometer (Lakeshore VSM 8604).
The paleomagnetic data confirmed that the paleomagnetic inclination of the samples near the surface near the white marker for both samples A and B are in good agreement with the expected geomagnetic inclination value (-23 degrees) at the sample collection point. This confirms that the surface layer of the sample records the current geomagnetic field. In addition, the paleomagnetic inclination tends to become deeper as we go back in time, indicating the sample was rotating. We discuss the possibility of rotation of manganese nodule during growth on the sediments, and the types and origin of magnetic minerals based on rock magnetic data.