Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-EM Earth's Electromagnetism

[S-EM34] Geomagnetism, paleomagnetism and rock magnetism

Sun. May 24, 2015 4:15 PM - 6:00 PM 102A (1F)

Convener:*Nobutatsu Mochizuki(Priority Organization for Innovation and Excellence, Kumamoto University), Masaki Matsushima(Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology), Chair:Yuhji Yamamoto(Center for Advanced Marine Core Research, Kochi University), Koji Fukuma(Department of Environmental System Science, Faculty of Science and Engineering, Doshisha University)

5:30 PM - 5:33 PM

[SEM34-P07] Remanent magnetization of a sediment core from Haneji-naikai, Okinawa : Diagenetic modification of magnetic mineral

3-min talk in an oral session

*Yutaro TAKANASHI1, Akira HAYASHIDA2, Kazuyoshi YAMADA3, Katsuya GOTANDA4, Hitoshi YONENOBU5 (1.Doshisha Univ., Grad. School Sci. & Engineer., 2.Department of Environmental Systems Science, Doshisha University, 3.Museum of Natural and Environmental history, Shizuoka, 4.Faculty of Polycy Informatics, Chiba University of Commerce, 5.Graduate School of Education, Naruto University of Education)

Keywords:paleomagnetism, natural remanent magnetization, diagenesis, sediment, red soil, Okinawa

We have studied magnetic properties of sediment core samples from the Haneji Inner Bay and the Shioya Bay on the northwest coast of Okinawa Island in order to investigate runoff of red soils associated with environmental changes in the watershed. Here we present results of measurements of natural remanent magnetization (NRM) of the core sample from Haneji-naikai Bay.
Pass-through measurement of the u-channel samples revealed that NRM above 120 cmbsf (cm below sea floor) has a stable component, which shows linear decay toward the origin (MAD < 10 degree). By contrast, NRM intensity below 130 cmbsf is only 2% of the upper interval and no characteristic magnetic component was isolated (MAD ≥ 10 degree). While low-field magnetic susceptibility shows gradual down-core decrease of at 100-150 cmbsf, anhysteretic remanent magnetization (ARM) decreases sharply at 140-160 cmbsf. Isothermal remanent magnetization (IRM), which was measured for discrete cubic specimens of 1 cm3 subsampled from the u-channels, showed consistent variation with the ARM. It was also found that proportion of low-coercivity (< 0.3 T) magnetic minerals (S-ratio) decreases at 140-160 cmbsf. Thermal demagnetizations of three-component IRM made for selected specimens suggest abundance of titanomagnetite and magnetite at the upper interval, but such medium to low coercivity minerals were not observed in the lower part. It is suggested that a loss of fine-grained magnetite have occurred due to reductive diagenesis, resulting destruction of stable NRM signals below 130 cmbsf.