Ferromagnetic resonance spectroscopy and rock magnetic characterization of fossil coral skeletons in Ishigaki Islands, Japan

Yuho Kumagai*, Norihiro Nakamura, Tetsuro Sato, Toshitaka Oka, Hirokuni Oda

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)


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.

Original languageEnglish
Article number400
JournalGeosciences (Switzerland)
Issue number11
Publication statusPublished - 2018 Nov 1
Externally publishedYes


  • Biogenic magnetite
  • Coral skeletons
  • Ferromagnetic resonance
  • Rock magnetism

ASJC Scopus subject areas

  • General Earth and Planetary Sciences


Dive into the research topics of 'Ferromagnetic resonance spectroscopy and rock magnetic characterization of fossil coral skeletons in Ishigaki Islands, Japan'. Together they form a unique fingerprint.

Cite this