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

Abstract

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.

A Review of Speleothems as Archives for Paleofire Proxies, With Australian Case Studies

“…Recent advances in microscopy through the advent of Quantum Diamond Microscopy (QDM) and scanning superconducting quantum interference device (SQUID) microscopy has allowed for speleothems, which typically do not have sufficient concentrations of magnetic minerals for high‐resolution paleomagnetic analyses (Lascu et al., 2016), to be analyzed at spatial resolutions of 200 μm or better (Feinberg et al., 2020; Fu et al., 2021; Naoto et al., 2021). QDM achieved a spatial resolution of 4.7 μm for the analysis of an annually laminated speleothem from Brazil, finding that changes in magnetism reflected changes in hydroclimate (Fu et al., 2021).…”

“…Considering that changes in soil and ash mineral magnetism have been observed following fires (Clement et al., 2011; Jordanova et al., 2019; Lu et al., 2000), the potential for speleothem paleomagnetism to record past fire events or regimes should be further investigated. Changes in the magnetic properties of a Tongan speleothem have been tentatively linked to anthropogenic fire use, although the authors offer an alternative hypothesis that volcanic ash may have been the cause, inducing the oxidation of soil magnetite to maghemite (Naoto et al., 2021). Further research effort is required to determine whether speleothem mineral magnetism is a reliable indicator of past fire activity.…”