[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-05] The orientation of marine sediment core using by cross-line laser method with SHINKAI 6500 in the Okinawa Trough

*Noriko Kawamura¹, Kuniyuki Furukawa², Tatsuo Kanamaru³, Kenichiro Tani⁴ (1.Japan Coast Guard Academy, 2.Aichi University, 3.Nihon University , 4. National Museum of Nature and Science,Tokyo)

Keywords:Orientation of marine sediment, cross-line laser method

Remanent magnetizations of core sediments are widely used for reconstructing paleomagnetic directions and the determination of geological layers. The declinations of marine core sediments are lost during pulling up the core, and have been overlooked. Instead of declinations, the inclination data has been well used for paleomagnetic studies. We attempt to use a cross-line laser method to collect oriented core samples and to reconstruct the declination in the Okinawa Trough at a site.The core samples on the seafloor were collected using the cross-line laser on the submersible SHINKAI 6500 during the YK23-16S cruise in September 2023. The detail sampling operation is reported by Furukawa et al. (2024, JpGU). The cross-line laser, consisting of horizontal and vertical laser lines, was installed on the pan-tilt unit of SHINKAI 6500. The relative positions data; heading direction, pitch angle, and roll angle of the inertial navigation system are recorded every second. The absolute laser irradiation orientation can be estimated from the pan-tilt unit of SHINKAI 6500. The corer is made by polycarbonate, and is 100 mm in diameter and 500 mm in length. In order to define the sampling angle on seafloor, we drew yellow, red, white, and red lines every 90 degrees on the corer. The sampling operations were recorded by photographs and movies. The corer was pushed on the seafloor using a manipulator directly. When the collecting an oriented sediment core, the center of cross-line laser was focused on the corer. The sediment core samples with a length of 16-20 cm were collected at a site of dive#1730. The color of top sediments was brown, and was gradually olive black indicating the oxic-anoxic layer boundary. The lithology was silty clay. After the operation on seafloor, the plastic cube was subsampled from the sediment cores on the shipboard. Results of stepwise alternating field demagnetization, paleomagnetic directions; the declinations and inclinations indicate from -38.3° to -17.2° and from 26.2° to 48.4° respectively, and are the close values of international geomagnetic reference field. Results of low-temperature magnetometry of the sediment samples indicate that the unclear decreases at 120 K, which represent the clear Verwey transition in magnetite. The oriented sediment cores can be considered to be a good record for paleomagnetic direction. If the method of cross-line laser will be applied to long cores and hard rocks, various paleomagnetic direction data will be measured. The preliminary result of oriented hard rock will be presented by Shibuya et al. (2024, JpGU).