Hirokuni Oda

Ultrafine-scale magnetostratigraphy of marine ferromanganese crust

Hydrogenetic ferromanganese crusts are iron-manganese oxide chemical precipitates on the seafloor that grow over periods of tens of millions of years. Their secular records of chemical, mineralogical, and textural variations are archives of deep-sea environmental changes. However, environmental reconstruction requires reliable high-resolution age dating. Earlier chronological methods using radiochemical and stable isotopes provided age models for ferromanganese crusts, but have limitations on the millimeter scale. For example, the reliability of 10 Be/ 9 Be chronometry, commonly considered the most reliable technique, depends on the assumption that the production and preservation of 10 Be are constant, and requires accurate knowledge of the 10 Be half-life. To overcome these limitations, we applied an alternative chronometric technique, magnetostratigraphy, to a 50-mm-thick hydrogenetic ferromanganese crust (D96-m4) from the northwest Pacific. Submillimeter-scale magnetic stripes originating from approximately oppositely magnetized regions oriented parallel to bedding were clearly recognized on thin sections of the crust using a high-resolution magnetometry technique called scanning SQUID (superconducting quantum interference device) microscopy. By correlating the boundaries of the magnetic stripes with known geomagnetic reversals, we determined an average growth rate of 5.1 ± 0.2 mm/m.y., which is within 16% of that deduced from the 10 Be/ 9 Be method (6.0 ± 0.2 mm/m.y.). This is the finest-scale magnetostratigraphic study of a geologic sample to date. Ultrafine-scale magnetostratigraphy using SQUID microscopy is a powerful new chronological tool for estimating ages and growth rates for hydrogenetic ferromanganese crusts. It provides chronological constraints with the accuracy promised by the astronomically calibrated magnetostratigraphic time scale (1–40 k.y.).

Paleomagnetism of Triassic and Jurassic red bedded chert of the Inuyama area, central Japan

A paleomagnetic study was conducted on Triassic and Jurassic red bedded chert from the Inuyama area, central Japan. Paleomagnetic specimens were collected from single chert beds at nine sites within mesoscopic (1∼5 m) folds, whose ages were determined by radiolarian fossils. Progressive thermal demagnetization experiments were conducted, and three of the four magnetization components provided tectonic implications. Component D (unblocking temperature of 560°–680°C), from six sites, passes the fold test with the mesoscopic folds. Paleolatitude was estimated as 12.3°±15.6°N or S, 19.0°±10.5°N or S, and 25.7°±16.7°N or S for 237 Ma, 223.5 Ma, and 210 Ma, respectively. In the case of northern latitude, a northward migration of 5.6± .9 cm/yr is estimated between 237 Ma and 210 Ma, with no significant latitudinal migration from 210 Ma to 160 Ma. Component C (unblocking temperature of 350°–530°C) has normal polarity and is thought to have been acquired as a chemical remanent magnetization carried by magnetite, which was acquired as synfolding magnetization with the Sakahogi Synform. The magnetization is considered to have been acquired through heating in the accretionary wedge during the Cretaceous Normal Superchron (120.5–83.5 Ma). Component B (unblocking temperature of 200°–350°C) has reversed polarity and is thought to have been acquired as a thermoviscous remanent magnetization after the bending of the Sakahogi Synform. The most probable period of the magnetization acquisition is between C27r and C24r (58–63 Ma) after the emplacement of Late Cretaceous granitic batholiths.

Diagenetic effect on magnetic properties of marine core sediments from the southern Okhotsk Sea

It is well known that magnetic records of marine sediments are affected to a certain degree by early diagenesis, the extent and mode of which are controlled by depositional environments. In order to investigate variations in the effects of early diagenesis, we have analyzed 11 gravity cores estimated to be Holocene in age that constitute a depth transect off the southern coast of the Okhotsk Sea. The rock magnetic parameters have peaks produced by the presence of volcanic ashes, basaltic granules and/or relatively coarser-grained layers. The concentration-dependent parameters display characteristic trends of a rapid decrease within the homogeneous silty clay zone in the middle parts of the cores, assuming that the total amount of magnetic minerals decreases through early diagenesis at this interval. This decrease is found in all of the cores from the transect (from both near-shore and offshore sites). In near-shore sites where larger volumes of organic materials are supplied compared to offshore sites, the depth of the decrease in magnetic minerals is relatively shallow. These results imply that the presence of organic materials accelerates the pyritization through the bacterial reduction of ferric ion, thereby causing the decreases in magnetic minerals to become shallower.

A Brunhes-Matuyama polarity transition record from anoxic sediments in the South Atlantic (Ocean Drilling Program Hole 1082C)

A paleomagnetic study was performed on Hole 1082C sediment cores taken during the Ocean Drilling Program (ODP) Leg 175 in the South Atlantic in order to obtain a high-resolution Brunhes-Matuyama (B/M) polarity transition record. An average sedimentation rate was as high as 10 cm/kyr. The cores consist of strongly anoxic sediments, which is common for the areas of large material supply. Anoxic sediments, which are geochemically quite active, were considered to be unsuitable for studies on detailed behavior of the geomagnetic field such as polarity transitions. For global site distribution, however, it is necessary to make efforts to retrieve paleomagnetic records from such sediments. A continuous record of directional changes around the transition was obtained from U-channel samples after cleaning by stepwise alternating-field (AF) demagnetization. Consistency of the record was checked using discrete samples taken from the other half of the cores. The coring-induced magnetic overprint of radial-inward direction, which has often been reported from ODP piston-cores, was negligibly small in our cores. Relative paleointensity variation was estimated from remanent intensities of the discrete samples normalized by artificial remanences. Our record shows following features of the B/M transition similar to those already reported by previous studies. A zone of large directional fluctuations with low paleointensities occurs just before the main transition (788 to 795 ka based on the oxygen-isotope stratigraphy), which would correspond to the “precursor” of Hartl and Tauxe (1996). The virtual geomagnetic poles (VGPs) at the precursor lie along the so-called preferred longitudinal bands over the north-south Americas and Australia-east Asia. After the main transition from the reversed to normal polarity, VGPs stayed in the middle-to-high latitudes over the North America with an intermediate paleointensity for about 5 kyrs, and then moved in the vicinity of the North Pole with full recovery of intensity. Such behavior was reported by Oda et al. (2000). These similarities suggest that the anoxic sediments at Site 1082 could record the behavior of the geomagnetic field rather faithfully, although the remanence may be of chemical origin.

Geomagnetic paleosecular variation for the past 5 Ma in the Society Islands, French Polynesia

We report a paleomagnetic secular variation in the Society Islands, French Polynesia for the past 5 Ma. Paleomagnetic measurements were performed on volcanic rocks applying thermal and alternating field demagnetizations and principal-component analysis. As a result, 82 normal, 48 reversed, and 10 intermediate directions were observed after the correction of the site localities for the absolute motion of the Pacific plate. A mean VGP of the combined data set of normal and reversed polarities is located at 87.5°N and 4.0°W (A95 = 2.3°), which is close to the geographic pole. The angular standard deviation (ASD) around the mean VGP resulted in 14.6° (+1.3°, −1.2°). There is no significant inclination anomaly for the time-averaged field. However, if the plate motion is not taken into account, a significant anomaly of −3.4° would be observed at a 95% confidence level. This suggests that correction for the absolute plate motion is needed for the precise paleodirectional analysis of the Pacific region. These features are also supported by an analysis combining the previous data set with our data.

Geomagnetic 100-ky variation extracted from paleointensity records of the equatorial and North Pacific sediments

We have analyzed three sets of paleointensity series varying in duration from 300 to 1600 ka to confirm the existence of paleosecular variation on a 100-ky time scale. The data series are from sediment cores in the equatorial and North Pacific regions. We transformed the three time series into wavelet space and found characteristic variations in a 100-ky time scale. We also calculated wavelet correlation between the series of the paleointensity and rock magnetic parameters of each core to assess the efffect of rock magnetic properties on relative paleointensity. The wavelet correlation coefficients of the three cores are small, so the change in the paleointensity in each core is concluded to be independent of magnetic properties. In order to confirm the cause of the paleointensity variation, we calculated the wavelet correlation among the three cores. The relative intensity variations of the three cores are significantly correlated, while rock magnetic parameters are not, indicating that there are differences in rock magnetic variations among the cores, although they appear to occur on similar scales. On the other hand, all sets of relative paleointensity have synchronous variations. Such paleointensity variations observed over a wide area are considered to originate from changes in the Earth’s magnetic field.

Anisotropy of Magnetic Susceptibility (AMS) Characteristics of Tide-Influenced Sediments in the Late Pleistocene-Holocene Changjiang Incised-Valley Fill, China

Abstract In order to probe anisotropy of magnetic susceptibility (AMS) characteristics of tide-influenced sediments, AMS analyses and primary sedimentary structure observation and description were conducted on the borehole CM-97 samples from Changjiang delta, China. Primary sedimentary structure (cross-laminations) observation and description were based on a detailed examination of X-ray photographs of samples. Primary cross-laminations were found on 19 of 35 subcores, among which five subcores, A6, A7, B17, B30, and B38, have bidirectional cross-laminations. We found a total of 35 cross-laminations on the subcore sections of tide-influenced sediments, of which 14 were distributed on the five subcores with bidirectional cross-laminations. By their bidirectional dipping foreset laminae, the primary cross-laminations clearly showed bidirectional flow features of the environments in which these sediments formed. Comparing the paleocurrent directions shown by these cross-laminations with those indicated by the in situ AMS data, we found that more than 64% exhibited similar current directions, demonstrating that AMS can supply us with the true paleocurrent directions for such sediments. From the downhole paleocurrent changes inferred from the in situ AMS data, it was also clear that there were bidirectional flows during the deposition of these sediments and that sediments deposited in different environments had different change characteristics with respect to downhole paleocurrents. These differences among the muddy intertidal- to subtidal-flat sediments (unit 5), the Changjiang estuary central basin sediments (unit 6), and the delta front sediments (unit 8) may have resulted from the different hydrodynamic conditions of these sedimentary environments. Furthermore, stratigraphic unit 5 was subdivided into three parts based on downhole AMS characteristics, which may correlate with those subdivided according to downhole paleocurrent changes. Therefore, besides its long recognized role in paleocurrent determination, AMS can also be used to determine stratigraphic divisions and to reconstruct sedimentary paleoenvironments in detail.

Intensity‐Inclination Correlation for Long‐Term Secular Variation of the Geomagnetic Field and Its Relevance to Persistent Non‐Dipole Components

Yamazaki and Oda [2002] reported long-term secular variation in inclination with ∼100 kyr periodicity and coherency between paleointensity and inclination variations using a sediment core from the western equatorial Pacific. In this paper, we first conducted a paleomagnetic study of another core taken nearby (∼120 km away) to examine the reproducibility of the records. The new core covers the last 1.1 Myr. Long-term inclination variations from the new core are almost identical to those of the previous core after conversion to a common age scale by inter-core correlation based on relative geomagnetic paleointensity. Mutually consistent long-term inclination records were also obtained from two cores of Brunhes age in the north central Pacific after inter-core correlation using relative paleointensity. Next, we compared intensity-inclination correlations of cores in the western equatorial Pacific (WEP) with those in the north central Pacific (NCP). A cross-correlation analysis showed that the coherency between intensity and inclination is strong in the WEP but weak in the NCP. Another remarkable difference in inclinations of the two regions is that the cores in the WEP have a large negative inclination anomaly (ΔI), whereas the cores in the NCP have a small ΔI. These observations support the model of Yamazaki and Oda [2002] that long-term secular variations in inclination are controlled by changes in the relative strength of the geocentric axial dipole and persistent non-dipole components. This model predicts that the intensity-inclination correlation should be obvious in regions where ΔI is large, but not where ΔI is small.

Deconvolution of continuous paleomagnetic data from pass-through magnetometer: A new algorithm to restore geomagnetic and environmental information based on realistic optimization

The development of pass-through superconducting rock magnetometers (SRM) has greatly promoted collection of paleomagnetic data from continuous long-core samples. The output of pass-through measurement is smoothed and distorted due to convolution of magnetization with the magnetometer sensor response. Although several studies could restore high-resolution paleomagnetic signal through deconvolution of pass-through measurement, difficulties in accurately measuring the magnetometer sensor response have hindered the application of deconvolution. We acquired reliable sensor response of an SRM at the Oregon State University based on repeated measurements of a precisely fabricated magnetic point source. In addition, we present an improved deconvolution algorithm based on Akaikes Bayesian Information Criterion (ABIC) minimization, incorporating new parameters to account for errors in sample measurement position and length. The new algorithm was tested using synthetic data constructed by convolving “true” paleomagnetic signal containing an “excursion” with the sensor response. Realistic noise was added to the synthetic measurement using Monte Carlo method based on measurement noise distribution acquired from 200 repeated measurements of a u-channel sample. Deconvolution of 1000 synthetic measurements with realistic noise closely resembles the “true” magnetization, and successfully restored fine-scale magnetization variations including the “excursion.” Our analyses show that inaccuracy in sample measurement position and length significantly affects deconvolution estimation, and can be resolved using the new deconvolution algorithm. Optimized deconvolution of 20 repeated measurements of a u-channel sample yielded highly consistent deconvolution results and estimates of error in sample measurement position and length, demonstrating the reliability of the new deconvolution algorithm for real pass-through measurements.

SQUID Microscope With Hollow-Structured Cryostat for Magnetic Field Imaging of Room Temperature Samples

We have developed a high-spatial-resolution superconducting quantum interference device (SQUID) microscope for magnetic field imaging of rock samples at room temperature, using a hollow-structured cryostat. A directly coupled low-temperature SQUID with a 200 μm × 200 μm pickup loop, which is mounted on a sapphire conical rod, is separated from room temperature by a thin sapphire window. Precise and repeatable adjustment of the vacuum gap between the SQUID and the sapphire window is performed by rotating a micrometer spindle connected to the sapphire rod through the hollow portion of the cryostat. A spacing of 230 μm between the SQUID and a sample has been achieved. We demonstrated the imaging of the magnetic field on a zircon containing magnetic grains.