The Ryukyu Group in the Ryukyu islands consists of reef limestones of Pleistocene age, and records cycles of marine regression and transgression. Study of the rocks has the potential to constrain precise sea level changes around the middle Pleistocene climate transition from kyr cycles to kyr cycles. To assign a geochronological marker in the age of the transition, we undertook magnetostratigraphic studies of the Ryukyu Group rocks exposed on Miyakojima Island. Paleomagnetic samples were collected at sites from all sequence-stratigraphic units on Miyakojima Island (MY-Units to in ascending stratigraphic order). The conventional thermal demagnetization procedure provided reliable polarity determinations for only seven sites. For the remaining sites, we developed a novel technique of reductive chemical demagnetization (RCD) combined with alternating field demagnetization. This hybrid technique successfully erased the overprinting magnetic components, revealing the primary component. Paleomagnetic directions of sites show that the lower part of the Ryukyu Group (MY-Units-, and the lowest part of MY-Unit) records reversed geomagnetic polarity, whereas the upper part (all but the lowest part of MY-Unit and MY-Unit) records normal polarity. Combining the magnetostratigraphic data with existing calcareous nannofossil data, we conclude that the reversed-normal geomagnetic polarity transition corresponds to the Matuyama-Brunhes boundary (MBB). These magnetostratigraphic data including the MBB improve the geochronology of the Ryukyu Group, which is useful for the temporal correlation between the Ryukyu Group and the other climate records during the middle Pleistocene climate transition.

We investigated temporal changes in the rock-magnetic properties of volcanic ash ejected from the Aso Nakadake volcano during a sequence of ash eruptions from 2019 to 2020. For 39 volcanic ash samples, magnetic hysteresis parameters, including saturation magnetization (Ms), saturation remanent magnetization (Mrs), coercivity (Bc), and coercivity of remanence (Bcr), were obtained. Curie temperature (Tc) of the samples was also estimated using thermomagnetic analyses. Titanium-rich and -poor titanomagnetites were the dominant magnetic minerals in the volcanic ash, of which the titanium-rich phase was dominant. Systematic magnetic measurements of the volcanic ash ejected during the 1-year eruption event indicate that temporal changes in the hysteresis parameters occurred throughout the event. These temporal changes suggest that the Mrs/Ms and Bc values of the volcanic ash increased considerably during several periods. The clear increases in Mrs/Ms and Bc, associated with the central peak in FORC diagrams, indicate that non-interacting single-domain grains increased. For these high Mrs/Ms and Bc samples, thermal demagnetizations of 3-axis IRM show that the low unblocking-temperature component up to 250–300 °C has apparently higher coercivity, suggesting that the above-mentioned, non-interacting single-domain grains are Ti-rich titanomagnetite. Interestingly, the high Mrs/Ms and Bc values were synchronous with observations of volcanic glow. These results suggest that changes in the magnetic properties of titanomagnetite grains in volcanic ash reflect changes in physical conditions from the vent to the conduit of the volcano. Graphical Abstract

We investigated temporal changes in the magnetic properties of volcanic ash ejected from the Aso Nakadake volcano during a sequence of ash eruptions from 2019 to 2020. Titanium-rich titanomagnetite and titanium-poor titanomagnetite were the dominant magnetic minerals in the samples where titanium-rich titanomagnetite was more dominant. From the rock magnetic measurements, parameters such as the saturation remanent magnetization (Mrs), saturation magnetization (Ms), coercivity (Bc), and titanium content estimated from the Curie temperature (Tc) were extracted and checked for their temporal changes. The magnetic behavior of the magnetic minerals was confirmed by the increasing values of Mrs/Ms and Bc at several periods. The samples with higher values of Mrs/Ms and Bc included titanomagnetite with a low Tc (high titanium content). The clear increase in Mrs/Ms and Bc suggests that the ratio of the single-domain volume fraction increased, indicating that the titanomagnetite particles became finer in size. Interestingly, the periods of high Mrs/Ms and Bc were synchronized with observations of the volcanic glow. These results suggest that changes in the magnetic properties of volcanic ash reflect changes in physical and/or thermal conditions from the vent to the conduit.

Chemical demagnetization is not preferred as a demagnetizing method in paleomagnetism because strong acids are cumbersome to handle and require considerable time compared to alternating field and thermal demagnetizations. Particularly, for rocks with carbonate minerals, strong acidic solutions are not applicable. This study presents a new method, termed reductive chemical demagnetization (RCD), using ascorbic acid solution as a reductive etchant. Ascorbic acid is a strong reductive agent and converts Fe 3+ ions of secondary magnetic minerals to water-soluble Fe 2+ ions, which facilitate chemical demagnetization of carbonate rocks. The carbonate frame can remain intact if the pH of the solution is buffered at approximately 7 with sodium bicarbonate. This etchant is more suitable than strong acid in terms of handling in a paleomagnetic laboratory, particularly in a magnetic field free room. To reduce the required time, a technique of dripping the etchant on the sample was also devised. This helps the fresh etchant flow through the voids between the grains of rocks to rapidly remove dissolved Fe 2+ ions. As a case study of RCD, reef limestone samples were examined. The results showed that the dripping experiments with 5% ascorbic acid solution were the most effective. It took 72 h to reach the remaining isothermal remanent magnetization (IRM) constant. Thermal demagnetizations of 3-component IRM indicate that RCD removed the high coercivity remanences carried by hematite and goethite. These magnetic minerals were considered to be precipitated between the grains of the rock, and thus they were dissolved by the RCD treatment. A chemical remanent magnetization (CRM), acquired by secondary magnetic minerals, can easily mask the primary remanence for sedimentary rocks of weak magnetization, and the coercivity or unblocking-temperature spectra of the primary remanence and secondary CRM overlap; however, RCD can effectively remove the secondary CRM. RCD prior to alternating field or thermal demagnetization has the potential to improve paleomagnetic demagnetization of sedimentary rocks.