Abstract
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.
Highlights
Coral skeletons, mainly composed of aragonite, have been used for the reconstruction of past sea-level and paleoclimate in the tropics
Many studies on the rock magnetism of shallow-water carbonates have been conducted, recently ceased coral skeletons are a fairly new material applied to rock magnetism
This study focuses on four recently ceased coral tsunamigenic boulders collected from Ishigaki Island (Figure 1)
Summary
Mainly composed of aragonite, have been used for the reconstruction of past sea-level and paleoclimate in the tropics. Most coral skeletons have shown extremely weak magnetization, and their magnetic origin is not well understood. Sato et al (2014) [2] succeeded in measuring sufficient magnetization of ceased coral tsunamigenic boulders washed up on the beach. Many studies on the rock magnetism of shallow-water carbonates have been conducted, recently ceased coral skeletons are a fairly new material applied to rock magnetism. It is necessary to determine the characterization of magnetic assemblages in these coral skeletons to utilize them as new paleomagnetic recorders because paleomagnetic records are affected by past geomagnetic field variations and by the lithologic factors of the samples, such as the mineralogy, grain size, and concentration of the magnetic phases [3]
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