Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

M (Multidisciplinary and Interdisciplinary) » M-TT Technology & Techniques

[M-TT48] Frontiers in Geochemistry

Sun. May 26, 2019 1:45 PM - 3:15 PM 201A (2F)

convener:Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Tetsuya Yokoyama(Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology), Hajime Obata(Marine inorganic chemistry division, Atmosphere and Ocean Research Institute, University of Tokyo), Chairperson:Hirochika Sumino, Hajime Obata(Atmosphere and Ocean Research Institute, University of Tokyo), Tetsuya Yokoyama(Tokyo Institute of Technology)

3:00 PM - 3:15 PM

[MTT48-06] Magnetic mapping of a speleothem from the southern Pacific with a scanning SQUID microscopy and its magnetic mineralogy

*Naoto Fukuyo1,2, Hirokuni Oda3, Yusuke Yokoyama1,2, Geoffrey Clark4, Yuhji Yamamoto5 (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Department of Earth and Planetary Science, The University of Tokyo, 3.Institute of Geology and Geoinformation, 4.College of Asia and the Pacific,The Australian National University, 5.Center for Advanced Marine Core Research, Kochi University)

Keywords:Scanning SQUID microscopy, paleomagnetism, speleothem

Speleothems can be an ideal record of paleomagnetism since they retain continuous geomagnetic records in stable condition as well as having applicability of high-precision radiometric dating via U-series and radiocarbon techniques. Although their weak magnetic signals hinder this archive to be widely used in the field, Lascu et al. (2016) successfully reconstructed paleomagnetic signatures during the Laschamp excursion from a stalagmite. A scanning SQUID Microscope (SSM) can image very weak magnetic fields at high spatial resolution, and hence could potentially solve this obstacle. However, only a primitive paleomagnetic mapping without interpretation has been made on a speleothem using SSM (Myre et al., 2019). In this study, we have conducted magnetic imagings at submillimeter scale with an SSM at Geological Survey of Japan, AIST on a speleothem collected at Anahulu cave in Tongatapu Island, the Kingdom of Tonga. The 14C age of the surface part of the stalagmite is around 10 ka. The stalagmite block samples were cut perpendicular to the growing direction and shaped to a thickness of ca.0.2 mm before measurements with the SSM. The measured magnetic field is ~±1.5 nT at a senor-to-sample distance of ~200 µm. In association with the laminated structures of the speleothem, we observe stronger magnetic field for the surface layer compared with that of the inner layers. Furthermore, we conducted several magnetic experiments; i.e. first-order reversal curve (FORC) measurements, and low temperature magnetometry. We also conducted decomposition of Isothermal Remanent Magnetization (IRM) acquisition curves at a room temperature and a temperature below Verwey transition. These results show that the speleothem contains a mixture of magnetite and maghemite with different magnetic domain states. The proportion of magnetite and maghemite are shown to be different between the surface layer and the inner layers. These observations are consistent with the magnetic image of a rock magnetic parameter representing coercivity (S-ratio) calculated by inversion of a series of IRM measurements with the SSM.