Toward robust deconvolution of pass-through paleomagnetic measurements: new tool to estimate magnetometer sensor response and laser interferometry of sample positioning accuracy

Pass-through superconducting rock magnetometers (SRM) offer rapid and high-precision remanence measurements for continuous samples that are essential for modern paleomagnetism studies. However, continuous SRM measurements are inevitably smoothed and distorted due to the convolution effect of SRM sensor response. Deconvolution is necessary to restore accurate magnetization from pass-through SRM data, and robust deconvolution requires reliable estimate of SRM sensor response as well as understanding of uncertainties associated with the SRM measurement system. In this paper, we use the SRM at Kochi Core Center (KCC), Japan, as an example to introduce new tool and procedure for accurate and efficient estimate of SRM sensor response. To quantify uncertainties associated with the SRM measurement due to track positioning errors and test their effects on deconvolution, we employed laser interferometry for precise monitoring of track positions both with and without placing a u-channel sample on the SRM tray. The acquired KCC SRM sensor response shows significant cross-term of Z-axis magnetization on the X-axis pick-up coil and full widths of ~46–54 mm at half-maximum response for the three pick-up coils, which are significantly narrower than those (~73–80 mm) for the liquid He-free SRM at Oregon State University. Laser interferometry measurements on the KCC SRM tracking system indicate positioning uncertainties of ~0.1–0.2 and ~0.5 mm for tracking with and without u-channel sample on the tray, respectively. Positioning errors appear to have reproducible components of up to ~0.5 mm possibly due to patterns or damages on tray surface or rope used for the tracking system. Deconvolution of 50,000 simulated measurement data with realistic error introduced based on the position uncertainties indicates that although the SRM tracking system has recognizable positioning uncertainties, they do not significantly debilitate the use of deconvolution to accurately restore high-resolution signal. The simulated “excursion” event associated with a significant magnetization intensity drop was clearly recovered in the deconvolved measurements with a maximum error of ~3° in inclination

Three-dimensional stress state above and below the plate boundary fault after the 2011 Mw 9.0 Tohoku earthquake

東北地方太平洋沖地震を引き起こしたプレート境界断層より以深の応力状態を初めて決定 --海洋底掘削で採取されたコア試料の非弾性ひずみ解析からのアプローチ--. 京都大学プレスリリース. 2022-11-11.Finding faults deeply stressful: First investigation of stress state below plate boundary fault of Tohoku earthquake. 京都大学プレスリリース. 2022-12-09.The Integrated Ocean Drilling Program conducted Expedition 343 and 343T, named the Japan Trench Fast Drilling Project (JFAST), to drill through the plate boundary fault that ruptured during the 2011 Mw 9.0 Tohoku earthquake in the area with the largest fault slip displacement near the Japan trench. Analyses of breakouts observed from borehole C0019B produced postearthquake stress states above the plate boundary fault between the subducting Pacific plate and overriding North American plate. To supplement the lack of stress data below the rupture zone of the earthquake, we conducted core-based three-dimensional stress measurements by the anelastic strain recovery (ASR) method using four whole-round core samples of sediments, of which three samples were located above, but one sample was located below the plate boundary fault in borehole C0019E. As a result of the stress measurements, the postearthquake three-dimensional stress magnitudes at ∼802 and ∼828 meters below seafloor (mbsf) across the plate boundary fault at ∼820 mbsf reveal a normal faulting stress regime. The differences between the three-dimensional intermediate principal stress and the minimum principal stress at the two depths are less than 1 MPa, suggesting a complete release of horizontal tectonic stresses that accumulated before the earthquake. In addition, the maximum horizontal stress S[Hmax] azimuth N115°E at ∼828 mbsf below the plate boundary fault from ASR measurements shows consistency with the S[Hmax] azimuth N139 ± 23°E (mean ± standard deviation) at ∼550–810 mbsf from breakout analyses above the fault. Taken together with the similar stress magnitudes at ∼802 and ∼828 mbsf, we interpret that the postearthquake stress states are almost the same in the sediments above and below the plate boundary fault. In other words, the stress state in terms of both orientation and magnitude is continuous across the fault. At a shallower depth of ∼177 mbsf in the slope sediments, the ASR stress data reveal a “stress state at rest”, which is likely free from tectonic effects of plate subduction, suggesting that the stress state was reset by the great coseismic displacement of ∼50 m slipped during the Tohoku earthquake

Bulk magnetic domain stability controls paleointensity fidelity

Nonideal, nonsingle-domain magnetic grains are ubiquitous in rocks; however, they can have a detrimental impact on the fidelity of paleomagnetic records—in particular the determination of ancient magnetic field strength (paleointensity), a key means of understanding the evolution of the earliest geodynamo and the formation of the solar system. As a consequence, great effort has been expended to link rock magnetic behavior to paleointensity results, but with little quantitative success. Using the most comprehensive rock magnetic and paleointensity data compilations, we quantify a stability trend in hysteresis data that characterizes the bulk domain stability (BDS) of the magnetic carriers in a paleomagnetic specimen. This trend is evident in both geological and archeological materials that are typically used to obtain paleointensity data and is therefore pervasive throughout most paleomagnetic studies. Comparing this trend to paleointensity data from both laboratory and historical experiments reveals a quantitative relationship between BDS and paleointensity behavior. Specimens that have lower BDS values display higher curvature on the paleointensity analysis plot, which leads to more inaccurate results. In-field quantification of BDS therefore reflects low-field bulk remanence stability. Rapid hysteresis measurements can be used to provide a powerful quantitative method for preselecting paleointensity specimens and postanalyzing previous studies, further improving our ability to select high-fidelity recordings of ancient magnetic fields. BDS analyses will enhance our ability to understand the evolution of the geodynamo and can help in understanding many fundamental Earth and planetary science questions that remain shrouded in controversy

Low-temperature magnetic properties of pelagic carbonates: Oxidation of biogenic magnetite and identification of magnetosome chains

Pelagic marine carbonates provide important records of past environmental change. We carried out detailed low-temperature magnetic measurements on biogenic magnetite-bearing sediments from the Southern Ocean (Ocean Drilling Program (ODP) Holes 738B, 738C

Paleomagnetic studies on single crystals separated from the middle Cretaceous Iritono granite

Investigations of superchrons are the key to understanding long-term changes of the geodynamo and the mantle’s controlling role. Granitic rocks could be good recorders of deep-time geomagnetic field behavior, but paleomagnetic measurements on whole-rock granitic samples are often disturbed by alterations like weathering, and the presence of multi-domain magnetite. To avoid such difficulties and test the usefulness of single silicate crystal paleomagnetism, here we report rock-magnetic and paleomagnetic properties of single crystals and compare those to the host granitic rock. We studied individual zircon, quartz and plagioclase crystals separated from the middle Cretaceous Iritono granite, for which past studies have provided tight constraints on the paleomagnetism and paleointensity. The occurrence of magnetite was very low in zircon and quartz. On the other hand, the plagioclase crystals contained substantial amounts of fine-grained single-domain to pseudo-single-domain magnetite. Microscopic features and distinctive magnetic behavior of plagioclase crystals indicate that the magnetite inclusions were generated by exsolution. We therefore performed paleointensity experiments by the Tsunakawa–Shaw method on 17 plagioclase crystals. Nine samples passed the standard selection criteria for reliable paleointensity determinations, and the mean value obtained was consistent with the previously reported whole-rock paleointensity value. The virtual dipole moment was estimated to be higher than 8.9 ± 1.8 × 1022 Am2, suggesting that the time-averaged field strength during middle of the Cretaceous normal superchron was several times as large as compared to that of non-superchron periods. Single plagioclase crystals which have exsolved magnetite inclusions can be more suitable for identification of magnetic signals and interpretation of paleomagnetic records than the conventional whole-rock samples or other silicate grains

Asian monsoon modulation of nonsteady state diagenesis in hemipelagic marine sediments offshore of Japan

We have identified millennial-scale variations in magnetic mineral diagenesis from Pacific Ocean sediments offshore of Japan that we correlate with changes in organic carbon burial that were likely driven by Asian monsoon fluctuations. The correlation was determined by identifying offsets between the positions of fossil diagenetic fronts and climatically induced variations in organic carbon burial inferred from magnetic and geochemical analyses. Episodes of intense monsoon activity and attendant sediment magnetic mineral diagenesis also appear to correlate with Heinrich events, which supports the existence of climatic telecommunications between Asia and the North Atlantic region. Several lines of evidence support our conclusions: (1) fluctuations in down-core magnetic properties and diagenetic pyrite precipitation are approximately coeval; (2) localized stratigraphic intervals with relatively stronger magnetic mineral dissolution are linked to enhanced sedimentary organic carbon contents that gave rise to nonsteady state diagenesis; (3) down-core variations in elemental S content provide a proxy for nonsteady state diagenesis that correlate with key records of Asian monsoon variations; and (4) relict titanomagnetite that is preserved as inclusions within silicate particles, rather than secondary authigenic phases (e.g., greigite), dominates the strongly diagenetically altered sediment intervals and are protected against sulfidic dissolution. We suggest that such millennial-scale environmental modulation of nonsteady state diagenesis (that creates a temporal diagenetic filter and relict magnetic mineral signatures) is likely to be common in organic-rich hemipelagic sedimentary settings with rapidly varying depositional conditions. Our work also demonstrates the usefulness of magnetic mineral inclusions for recording important environmental magnetic signal