[S-EM22] 地磁気・古地磁気・岩石磁気

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

*福與 直人^1,3、小田 啓邦²、Geoffrey Clark⁴、横山 祐典^1,3 (1.東京大学大気海洋研究所、2.産業技術総合研究所地質情報研究部門、3.東京大学理学系研究科地球惑星科学専攻、4.オーストラリア国立大学アジア太平洋学部)

キーワード：走査型SQUID顕微鏡、環境磁気学、鍾乳石

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.