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Open AccessArticle

Ferromagnetic Resonance Spectroscopy and Rock Magnetic Characterization of Fossil Coral Skeletons in Ishigaki Islands, Japan

1
Department of Earth Science, Tohoku University, 6-3, Aramaki-aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan
2
Institute for Excellence in Higher Education, Tohoku University, 41, Kawauchi, Aoba-ku, Sendai, Miyagi 980-8576, Japan
3
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba central 7, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8567, Japan
*
Author to whom correspondence should be addressed.
Geosciences 2018, 8(11), 400; https://doi.org/10.3390/geosciences8110400
Received: 9 October 2018 / Revised: 27 October 2018 / Accepted: 31 October 2018 / Published: 2 November 2018
(This article belongs to the Section Biogeosciences)
Skeletons of hermatypic corals (e.g., Porites) might have enormous potential as a high-resolution paleomagnetic recorder owing to their rapid and continuous growth over hundreds of years at a rate of up to 2 cm/year, although typical corals show an extremely weak intensity of remanence and low stability. We found that coral tsunami boulders with negligible amounts of calcite on Ishigaki Island show a measurable intensity of remanence; thus, we attempted to characterize the magnetic assemblages in this coral skeleton to determine whether it is of biogenic or detrital magnetite using first-order reversal curve (FORC) measurements, ferromagnetic resonance (FMR) spectroscopy, and petrological observations through field-emission type scanning electron microscope (FE-SEM) with an acid treatment. The FMR derivative spectra of coral skeleton samples represent multiple derivative maxima and extended low-field absorption, indicating the presence of intact biogenic magnetite chains. FORC diagrams represent a “central ridge” signature with a vertical spread. These FMR and FORC features indicate the magnetization of these coral skeletons that are mainly created using intact biogenic magnetites and mixtures of grains from collapsed biogenic magnetites, pseudo-single domain grains, and multi-domain grains such as detrital magnetite. FE-SEM observations confirm the presence of a chain-like structure of iron oxides corresponding to the features of biogenic magnetite. Therefore, the magnetic mineral assemblage in coralline boulders from Ishigaki Island consists of dominant biogenic-origin single-domain magnetite and a trace amount of detrital component, indicating that fossil coral skeletons in Ishigaki Island have potential for utilization in paleomagnetic studies. View Full-Text
Keywords: ferromagnetic resonance; rock magnetism; coral skeletons; biogenic magnetite ferromagnetic resonance; rock magnetism; coral skeletons; biogenic magnetite
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Kumagai, Y.; Nakamura, N.; Sato, T.; Oka, T.; Oda, H. Ferromagnetic Resonance Spectroscopy and Rock Magnetic Characterization of Fossil Coral Skeletons in Ishigaki Islands, Japan. Geosciences 2018, 8, 400.

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