[S-EM22] Geomagnetism, paleomagnetism and rock magnetism

[SEM22-P06] Magnetic minerals in a speleothem from Tonga, the southern Pacific: Observation from scanning SQUID microscopy and rock magnetism

*Naoto Fukuyo^1,3, Hirokuni Oda², Geoffrey Clark⁴, Yusuke Yokoyama^1,3 (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Institute of Geology and Geoinformation, Geological Survey of Japan, AIST, 3. Department of Earth and Planetary Science, The University of Tokyo, 4.College of Asia and the Pacific,The Australian National University)

Keywords:Scanning SQUID microscopy, environmental magnetism, speleothem

Speleothems could be an ideal archive of paleomagnetism since they retain continuous geomagnetic records in stable conditions as well as their applicability of reliable radiometric datings such as U-series and radiocarbon techniques. Furthermore, speleothems can be useful tools in studies of environmental magnetism. The magnetic minerals in the speleothems can provide records of environmental variability by rock magnetic properties to changes of detrital input reflecting regional and global environmental changes such as paleofloods, precipitation, and anthropogenic influence. However, their weak magnetic signals hinder this archive from being widely used in the field. A scanning SQUID Microscope (SSM) can image very weak magnetic fields with high spatial resolution and hence could potentially solve this obstacle. In this study, we have conducted paleomagnetic measurements based on magnetic mapping with an SSM and rock magnetic measurements on a stalagmite collected at Anahulu cave in Tongatapu Island, the Kingdom of Tonga. We observe a stronger magnetic field above the surface layer compared with that of the inner layers associated with the laminated structures of a speleothem at the submillimeter scale with SSM. Further, rock magnetic measurements show that the concentration of magnetic minerals increased, and maghemite and goethite contents had changed from the inner white layers to the surface black layer of this speleothem. These differences could occur because the depositional environment for the speleothem has been changed to oxidative.