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High spatial resolution magnetic mapping using ultra-high sensitivity scanning SQUID microscopy on a speleothem from the Kingdom of Tonga, southern Pacific
Earth, Planets and Space ( IF 3.362 ) Pub Date : 2021-03-23 , DOI: 10.1186/s40623-021-01401-8
Naoto Fukuyo , Hirokuni Oda , Yusuke Yokoyama , Geoffrey Clark , Yuhji Yamamoto

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.



中文翻译:

使用超高灵敏度扫描SQUID显微镜对南太平洋汤加王国的鞘脂进行高空间分辨率磁性制图

鞘粉菌是环境磁性和古磁性的理想档案,因为它们在稳定的条件下保持连续的磁信号,并且可以用于使用U系列和放射性碳方法进行可靠的放射性测年。但是,它们的弱磁信号阻碍了该档案馆在地球科学领域的广泛使用。尽管先前的研究成功地重建了古磁特征和古环境变化,但所提供的时间分辨率还是不够的。最近在该领域出现的扫描SQUID显微镜(SSM)可以对非常弱的磁场成像,同时保持可以克服这一障碍的高空间分辨率。在这项研究中,我们采用SSM对汤加王国汤加塔普岛Anahulu洞穴收集的石笋进行了高空间分辨率的磁性测绘。5 mT交变磁场消磁后的平均测得磁场约为。0.27 nT,传感器到样品的距离约为200 µm。在亚毫米尺度上,与灰鞘层的层压结构相关的白色内部部分(平均:大约0.09 nT)相比,在浅灰色表层上方观察到了更强的磁场(平均:大约0.62 nT)。这项研究中SSM的分辨率可与Speleothem的年增长率相媲美。通过等温剩余磁化强度(IRM)的反演计算出的蛇麻草样品的磁化强度还表明,表层中的磁性矿物含量高于内部。通过低温磁力分析进一步研究了该特征。2 O 3。一阶反转曲线(FORC)的测量结果和IRM曲线的分解表明,该蛇麻草包含具有不同矫顽力和畴态的磁性矿物混合物。磁赤铁矿对浅灰色表层的总磁化强度的贡献远高于白色内部。浅灰色表层的磁性矿物学与内部的磁性矿物学的这种差异表明,沉积环境发生了变化,并且可能由于氧化环境而发生了变化。

更新日期:2021-03-24
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