[U-05] Advanced understanding of Quaternary and Anthropocene hydroclimate changes in East Asia

[U05-P07] Can speleothem magnetic minerals record a past volcanic eruption in Tonga?

*福與 直人¹、小田 啓邦²、横山 祐典¹、Geoffrey Clark⁴、山本 裕二³ (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.高知大学 海洋コア総合研究センター、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：岩石磁気学、鍾乳石、噴火履歴、トンガ

Speleothems are ideal archives of environmental magnetism and paleomagnetism since they retain continuous magnetic signals in stable conditions and can be used for reliable radiometric dating using U-series and radiocarbon methods. However, their weak magnetic signals hinder the widespread use of this archive in the field of geoscience. While previous studies successfully reconstructed paleomagnetic signatures and paleoenvironmental changes, the time resolutions presented were insufficient. Recently emerging scanning SQUID microscopy (SSM) in this field can image very weak magnetic fields while maintaining high spatial resolution that could likely overcome this obstacle. This study employed SSM for high spatial resolution magnetic mapping on a stalagmite collected at Anahulu cave in Tongatapu Island, the Kingdom of Tonga. A stronger magnetic field was observed above the grayish surface layer compared to that of the white inner part associated with the laminated structures of the speleothem at the submillimeter scale, which scanning resolution of the SSM in this study is comparable to the annual growth rates of the speleothem. The magnetization of the speleothem sample calculated from an inversion of isothermal remanent magnetization (IRM) also suggests that the magnetic mineral content in the surface layer is higher than the inner part. This feature was further investigated by low-temperature magnetometry. Our results show that the main magnetic carriers of the speleothem under study are magnetite and maghemite, and it can contain hematite or ε-Fe2O3. The contribution of maghemite to the total magnetization of the grayish surface layer was much higher than the white inner part. The distinct gray color on the white calcium carbonate speleothem may have been deposited with influence from non-carbonate minerals. Tongatapu Island consists mainly of limestones overlain by two soil layers of andesitic volcanic ash, which are estimated to be aged at 20,000 and 5,000 years old from active volcanoes in the north, such as Tofua and Kao island. The soils of Tongatapu are high in iron oxide minerals by weight. Furthermore, typical soils containing volcanic ash is acidic; thus, magnetite in the soil could oxidize to maghemite. Therefore, the gray layers may have been formed due to volcanic eruptions at 5,000 years ago that occurred in the northern islands.