High spatial resolution magnetic mapping using ultra-high sensitivity scanning SQUID microscopy on a speleothem from the Kingdom of Tonga, southern Pacific

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. In this study, we employed SSM for high spatial resolution magnetic mapping on a stalagmite collected at Anahulu cave in Tongatapu Island, the Kingdom of Tonga. The average measured magnetic field after 5 mT alternating field demagnetization is ca. 0.27 nT with a sensor-to-sample distance of ~ 200 µm. A stronger magnetic field (average: ca. 0.62 nT) was observed above the grayish surface layer compared to that of the white inner part (average: ca. 0.09 nT) 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 first-order reversal curve (FORC) measurements and the decomposition of IRM curves show that this speleothem contains a mixture of magnetic minerals with different coercivities and domain states. The contribution from maghemite to the total magnetization of the grayish surface layer was much higher than the white inner part. Such differences in magnetic mineralogy of the grayish surface layer from that of the inner part suggest that the depositional environment shifted and was likely changed due to the oxidative environment.

High spatial resolution magnetic mapping using ultra-high sensitivity scanning SQUID microscopy on a speleothem from the Kingdom of Tonga, southern Pacific

Speleothems are an ideal archive of paleomagnetism since they retain continuous geomagnetic records 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, including geomagnetic excursions, the time resolutions presented were not sufficient. 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. In this study, we employed SSM for paleomagnetic measurements on a stalagmite collected at Anahulu cave in Tongatapu Island, the Kingdom of Tonga. The samples were sliced to a thickness of ca. 0.2 mm and scanned for NRM using SSM, and the signal provides the indications of the influence from viscous remanent magnetization. It was thus removed by alternating field demagnetization (AFD) at 5mT. The average measured magnetic field after 5 mT AFD is ca. 0.27 nT with a sensor-to-sample distance of ~200 µm. A stronger magnetic field (average: ca. 0.62 nT) was observed above the grayish surface layer compared to that of the white inner layer (average: ca. 0.09 nT) associated with the laminated structures of a speleothem at the submillimeter scale with the SSM. The magnetization of the speleothem sample calculated by an inversion of isothermal remanent magnetization (IRM) also suggests that magnetic mineral content in the surface layer is higher than the inner layer. This feature was further investigated by low-temperature magnetometry. The results reveal that it contains magnetite and maghemite. The first-order reversal curve (FORC) measurements and the decomposition of IRM curves show that this speleothem contains a mixture of magnetic minerals with different coercivities and domain states. The contribution from maghemite to the total magnetization of the grayish surface layer is much higher than the white inner layer. The gray and white-colored layers of the speleothem retaining magnetically distinct characters indicates that the depositional environment was shifted when the surface layer was deposited and was likely changed to the oxidative environment.

High spatial resolution magnetic mapping using ultra-high sensitivity scanning SQUID microscopy on a speleothem from the Kingdom of Tonga, southern Pacific

Speleothems can be an ideal archive of paleomagnetism because they retain continuous geomagnetic records 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, including geomagnetic excursions, their time resolutions presented were still not reached to a sufficient level. 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. In this study, we employed SSM to conduct paleomagnetic measurements on a stalagmite collected at Anahulu cave in Tongatapu Island, the Kingdom of Tonga. The sliced sample to a thickness of ca. 0.2 mm was scan for NRM using SSM showed the influence of viscous remanent magnetization. The average measured magnetic field after 5 mT AF demagnetization is ca. 0.27 nT with a sensor-to-sample distance of ~200 µm. A stronger magnetic field (average: ca. 0.62 nT) was observed above the grayish surface layer, as compared to that of the white inner layer (average: ca. 0.09 nT) associated with the laminated structures of a speleothem at the submillimeter scale with the SSM. The magnetization of the speleothem sample calculated by an inversion of isothermal remanent magnetization (IRM) also suggests that magnetic mineral content in the surface layer is higher than the inner layer. This feature was further investigated by low-temperature magnetometry and was suggested that it contains magnetite, maghemite, and goethite. The first-order reversal curve (FORC) measurements and the decomposition of IRM curves show that this speleothem contains a mixture of magnetic minerals with different coercivities and domain states. The contribution from maghemite and goethite to the total magnetization of the grayish surface layer is much higher than the white inner layer. The speleothem retaining magnetically and visually two distinct layers indicates that the depositional environment was shifted when the surface layer was deposited and was likely changed to the oxidative environment.