Fukuyo et al. Earth, Planets and Space (2021) 73:77 https://doi.org/10.1186/s40623-021-01401-8 EXPRESS LETTER Open Access High spatial resolution magnetic mapping using ultra-high sensitivity scanning SQUID microscopy on a speleothem from the Kingdom of Tonga, southern Pacifc Naoto Fukuyo1,2,3* , Hirokuni Oda3, Yusuke Yokoyama1,2, Geofrey Clark4 and Yuhji Yamamoto5 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 feld of geoscience. While previous studies successfully reconstructed paleomagnetic signatures and paleoenvironmental changes, the time resolutions presented were insufcient. Recently emerging scanning SQUID microscopy (SSM) in this feld can image very weak magnetic felds 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 feld after 5 mT alternating feld demag- netization is ca. 0.27 nT with a sensor-to-sample distance of ~ 200 µm. A stronger magnetic feld (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) associ- ated 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-temper- ature 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 frst-order reversal curve (FORC) measurements and the decomposition of IRM curves show that this speleothem contains a mixture of magnetic minerals with diferent coer- civities 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 diferences 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. Keywords: Scanning SQUID microscopy, Paleomagnetism, Environmental magnetism, Speleothem, Magnetite, Maghemite, Hematite, Epsilon iron oxide, South pacifc Introduction Recently, the magnetism of speleothems has been used to reconstruct the geomagnetism and paleoenvironment *Correspondence: [email protected] because they capture the magnetic signals synchronously 1 Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan with the formation of carbonate layers incorporating Full list of author information is available at the end of the article magnetic minerals (Lascu and Feinberg 2011). Magnetic © The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Fukuyo et al. Earth, Planets and Space (2021) 73:77 Page 2 of 13 minerals deposited on speleothems are sourced from Polynesia (e.g., Clark et al. 2015). However, few quanti- food waters, which fow into caves and/or drip action tative paleoenvironmental studies using Earth-scientifc fltering in overlying soils. Te magnetic minerals in spe- methods have been reported (e.g., Roy 1997; Fukuyo et al. leothems can, therefore, record regional and global envi- 2020). Te speleothem magnetism is expected to contrib- ronmental changes such as paleofoods, precipitation, ute to the studies of archaeology, such as the Polynesian and anthropogenic infuences as variations in rock mag- expansion over the South Pacifc islands during the Hol- netic properties (e.g., Lascu and Feinberg 2011; Font et al. ocene, by providing information on magnetic minerals 2014; Jaqueto et al. 2016; Chen et al. 2019; Feinberg et al. refecting the change in depositional environment such 2020). For example, Jaqueto et al. (2016) used stalagmites as oxidation states, which might originate from human from Brazil and suggested that more negative (positive) activity and/or environment. values of oxygen and carbon isotopes correspond to lower (higher) values of magnetic mineral content. Tey Sample and methods suggest that higher isotopic values are interpreted as A stalagmite sample, AAC, was retrieved from the Ana- drier periods, suggesting that vegetation cover controls hulu cave in Tongatapu island (21° 12′ 26″ S, 175° 05′ the magnetic input in the cave (and vice versa). Drier 59″ W, Additional fle 1: Figure S1, Worthy et al. 1991). periods (higher δ18O and δ13C) are associated with less It consists mostly of the white part, with gray layers vegetation cover and more erosion and more transport only in the surface portion of the stalagmite (ca. 1 mm of Fe-rich sediments into the cave, resulting in higher depth). Moreover, the thin reddish-brown layer exists at magnetization. Similarly, Zhu et al. (2017) used mag- the boundary between the gray and white part (Fig. 1a). netic mineral concentration in Chinese speleothems to Stalagmite AAC was frst cut perpendicular to its growth reconstruct paleo-precipitation. Tey suggest that a large direction. For SSM measurements, the topmost part of number of storms and results in extreme rainfall events the speleothem was sliced and polished to a thickness increase the fux of pedogenic magnetite from soils to of approximately 200 μm (Fig. 1b, c). For rock magnetic the cave, enhancing magnetic mineral quantities. Since measurements using a magnetic property measurement the frequency of storms in central China is correlated system (MPMS) and an alternating gradient force mag- with El Niño Southern Oscillation (ENSO), they used netometer (AGM), a small quantity of material was col- speleothem magnetic minerals to examine the variability lected from the gray (AAC-B3) and white (inner part, of ENSO over the last 8.6 ka. High peaks in IRM of the AAC-W3) sections of the speleothem using a ceramic soft component are coeval with stronger ENSO periods scalpel, and diamond wire saw. (El Niño) and higher values of carbon isotopes, which is Te reddish-brown layer, however, could not be sepa- interpreted here as wetter periods. rated from the gray layer and white part because of its Despite the advantages mentioned above, the mag- thinness (< 0.1 mm) and proximity to the gray layer (ca. netic signals of speleothems are too weak to reconstruct 0.7 mm). Tus, a sample of a mixture of gray layers and high-resolution paleomagnetic records by conventional white part, including the reddish-brown layer (AAC-M), magnetometers (Lascu et al. 2016). Tus, previous stud- was also collected for MPMS and AGM measurements. ies had to sacrifce the temporal resolution that speleo- For radiocarbon measurements, approximately 5 mg of thems could potentially ofer. However, scanning SQUID calcites were drilled from the stalagmite growth layers microscopy (SSM) can image very weak magnetic felds (Fig. 1b). Te powdered sample was graphitized using above natural geological samples with a high spatial reso- the method described by Yamane et al. (2019) and then lution of ~ 100 µm (e.g., Lima and Weiss 2016; Oda et al. measured by a single-stage accelerator mass spectrome- 2016), which could solve the problem of weak magnetism try at the Atmosphere and Ocean Research Institute, Te associated with paleo- and environmental magnetic stud- University of Tokyo (Yokoyama et al. 2019). Te 14C ages ies using speleothems. To date, only a few preliminary were calibrated to calendar ages using OxCal 4.4 (Ram- relevant studies have been reported using SSM for spe- sey 2009a) with SHCal20 (Hogg et al. 2020). Moreover, leothems (e.g., Myre et al. 2019; Feinberg et al. 2020). a portion of the dating results was constrained using a Here, we report for the frst time a high-resolution and P_sequence model (Ramsey 2008; Ramsey and Lee 2013) high-sensitivity magnetic mapping using an SSM for a and a general temporal outlier model (Ramsey 2009b). speleothem collected from the South Pacifc together Te SSM measurements were conducted using the with dedicated rock magnetic measurements to unveil SSM at the Geological Survey of Japan (GSJ), National the magnetic minerals and their origins. Te speleo- Institute of Advanced Industrial Science and Tech-