天然の石灰質物質が有する磁性鉱物の特性決定：サンゴの磁性やIODPのU1490サイトの磁気極性層序への影響

要約のみTohoku University中村教博課

天然の石灰質物質が有する磁性鉱物の特性決定：サンゴの磁性やIODPのU1490サイトの磁気極性層序への影響

要約のみTohoku University中村教博課

Inverse Magnetic Susceptibility Fabrics in Pelagic Sediment: Implications for Magnetofossil Abundance and Alignment

金沢大学理工研究域地球社会基盤学系Single-domain magnetite particles exhibit minimum susceptibility along their elongation, resulting in so-called inverse fabric of the anisotropy of magnetic susceptibility (AMS). We report the discovery of inverse AMS fabrics from pelagic clay recovered by a ∼12 m long piston core from the western North Pacific. A previous study identified fossil single-domain magnetite produced by magnetotactic bacteria (magnetofossils) as the dominant ferrimagnetic mineral in the sediment. The inverse AMS fabrics were found in a ∼2 m zone. The ∼6 and ∼4 m of sediment above and below this zone showed normal, horizontal AMS fabrics. Rock magnetic data and ferromagnetic resonance spectroscopy indicated that magnetofossils account for most of the mean susceptibility regardless of normal or inverse AMS. This was explained by the mixing models where the inverse fabric from magnetofossils is nearly balanced by the normal fabrics of terrigenous minerals. The corrected degree of AMS carried by magnetofossils in the sediment was estimated to be ∼1.01, which is comparable to that of typical pelagic sediment at shallow depth. On the other hand, terrigenous minerals in the sediment were estimated to have higher degree of anisotropy, possibly reflecting burial and subsequent erosion of >80 m of sediment, which was also suggested by a subbottom acoustic stratigraphy. This suggests that inverse AMS fabrics due to magnetofossils may be widespread in pelagic clay without strong compaction. ©2019. American Geophysical Union. All Rights Reserved

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

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

Dating of tsunami boulders from Ishigaki Island, Japan, with a modified viscous remanent magnetization approach

Reworked boulders are expected to acquire a viscous remanent magnetization (VRM) approximately parallel to Earth's modern magnetic field. The magnitude of such a VRM depends on several factors, including the time since reworking and ambient temperature, for which there are well-known theoretical relationships. VRM unblocking temperature can, therefore, be a powerful tool for determining the reworking age of boulders and can be used to assess the timing of geological hazards such as landslides and tsunami events. In this study, VRM unblocking temperatures for twenty-seven samples from four coral tsunami boulders on Ishigaki Island, Japan, are compared with three candidate time–temperature relationships for VRM acquisition. For the Pullaiah nomogram, which is applicable to single-domain magnetite particle assemblages, nineteen samples from four boulders agree well with the expected unblocking temperature derived from previously reported 14C ages. Two samples have low unblocking temperatures and six samples appear to have anomalously high VRM unblocking. The Walton nomogram, which is used for lognormal grain-size distributions, relates acquisition temperatures required to produce equal magnetic (paleo)intensities and cannot explain the high unblocking temperatures; it produces younger predicted ages than the youngest boulder 14C age. We find that an alternative time–temperature relationship, defined by a stretched exponential law, has the potential to yield reworking ages for the anomalous boulders that are consistent with the 14C ages. We suggest that future VRM dating can be undertaken using a combination of the Pullaiah nomogram and the stretched exponential law.This work was supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (C) (No. 18K03755), Grant-in-Aid for Scientific Research (B) (No. 15H02986), Grant-in-Aid for Challenging Exploratory Research (25560173), Grant-in-Aid for Scientific Research (B) (No. 26302007), Grant-in-Aid for JSPS Fellows (No. 15J02522), JSPS Overseas Challenge Program for Young Researchers (20170821), and the Australian Research Council (grant DP160100805). This work was partially supported by the National Institute of Advanced Industrial Science and Technology, Japan

S-Adenosylmethionine Synthesis Is Regulated by Selective N6-Adenosine Methylation and mRNA Degradation Involving METTL16 and YTHDC1

Summary: S-adenosylmethionine (SAM) is an important metabolite as a methyl-group donor in DNA and histone methylation, tuning regulation of gene expression. Appropriate intracellular SAM levels must be maintained, because methyltransferase reaction rates can be limited by SAM availability. In response to SAM depletion, MAT2A, which encodes a ubiquitous mammalian methionine adenosyltransferase isozyme, was upregulated through mRNA stabilization. SAM-depletion reduced N6-methyladenosine (m6A) in the 3′ UTR of MAT2A. In vitro reactions using recombinant METTL16 revealed multiple, conserved methylation targets in the 3′ UTR. Knockdown of METTL16 and the m6A reader YTHDC1 abolished SAM-responsive regulation of MAT2A. Mutations of the target adenine sites of METTL16 within the 3′ UTR revealed that these m6As were redundantly required for regulation. MAT2A mRNA methylation by METTL16 is read by YTHDC1, and we suggest that this allows cells to monitor and maintain intracellular SAM levels. : Shima et al. find that MAT2A mRNA is stabilized upon depletion of intracellular S-adenosylmethionine (SAM). This regulation involves m6A modification in the 3′ UTR, the m6A writer METTL16, and the reader YTHDC1. Additionally, the authors show that multiple specific sites in hairpin regions of the 3′ UTR are targeted by METTL16. Keywords: cycloleucine, MAT2A, methionine adenosyltransferase, METTL16, methyladenosine, RNA, RNA degradation, S-adenosylmethionine, untranslated region, YTHDC