In recent years, numerical simulation of the three-dimensional geodynamo has advanced considerably, and some fundamental properties of convection and magnetic field generation in the Earth's core have been understood. In this paper, we introduce the latest results of threedimensional geodynamo simulation, and show some bases of convection and magnetic field generation processes. With decrease in the Ekman number, the length scale of convection becomes much smaller, while the magnetic field tends to show large-scale structure as the dipole field. At a high Rayleigh number, polarity reversals of the dipole field occur irregularly. Furthermore, it turns out that the reversal frequency depends strongly on the Rayleigh number. Although nondimensional parameters used in simulations are still far from the real Earth, some characteristic features such as a power spectrum similar to the geodynamo are found in numerical dynamo models. Using numerical dynamo simulations, we would infer dynamics deep in planets.
The Earth possesses its intrinsic magnetic field generated by fluid motion in the electrically conducting core, known as the geodynamo. Provided the spatial distributions of the Earth's magnetic field and its temporal variations are known at the core surface, it seems to be possible to estimate the fluid motion there by solving an inverse problem. The magnetic diffusion term is much smaller than the advection term in the magnetic induction equation, and therefore it can be neglected for the time scale much shorter than the magnetic diffusion time. This is called as the frozen-flux approximation, and magnetic lines of force behave as if they are frozen-in fluid elements. Because of its fundamental non-uniqueness, additional constraints, for example, toroidal flow, steady flow, geostrophic flow, or a combination there of, are imposed in estimating the flow at the core surface. Estimated fluid motions have common features : large vortices at midlatitudes and westward flow near the equatorial region. It is now possible to carry out numerical simulations of three-dimensional magnetohydrodynamic dynamos in rotating spherical shells. Inversion methods are then tested for synthetic data of numerical models. It is concluded that some meaningful information on the core flow can be recovered, but that the resolution of the magnetic field at the core surface has a crucial effect on the flow structure. Hence, to construct a realistic geodynamo model, it is important to determine the magnetic field precisely with smaller length scales at the core surface. It is required to monitor the geomagnetic field by launching satellites for magnetic field measurements periodically, for example every five years.
Recent progress of numerical simulations on the dynamo process in the core and the mantle convection provides a clue to understanding the origin of global variations of the Earth system during the last 150 Ma, in which long-term variations of geomagnetic reversal frequency and mantle activity are closely related in time. Recent MHD dynamo simulations suggest that the increase of the total heat flow through the CMB changes the geodynamo from stable dipolar dynamos to unstable multipolar dynamos, and on axially symmetric and equatorial symmetric pattern of heat flux produces stable dipolar dynamos. Numerical modelings of mantle convection indicate 3 convection regimes, whole-mantle, intermittent, and 2-layer convections, in a parameter space of the Rayleigh number (Ra) and the Clapeyron slope (dP/dT) of the phase transition at a depth of 660 km. In the intermittent convection regime, the convection vacillates between wholelayer and the 2-layer regimes, and the surface and CMB heat flows fluctuate with time. The global variation of the Earth system might be attributed to this intermittent convection mode of the present Earth. However, the apparent out of phase variation of the total heat flow through the CMB inferred from the reversal frequency and the mantle activity requires some mechanism for the phase shift of the variations..
Geomagnetic paleointensity variations can continuously be estimated using marine sediment cores having relatively uniform magnetic properties. The occurrence of large paleointensity fluctuations with periods on the order of 103 to 104 years has been recently established. The ages of paleointensity lows often correspond to those of geomagnetic excursions that have been reported so far. These results have changed our view of the geomagnetic field : it is rather unstable even within periods of constant polarity. Arguments have begun based on paleointensity records as to whether the Earth's orbital parameters and/or paleoclimate modulates the geomagnetic field. Discoveries of the Milankovitch orbital frequencies in paleomagnetic records have been reported, whereas some researchers have postulated that such frequencies are artifact caused by magnetic-property changes in sediments induced by paleoclimatic changes. It is necessary to evaluate the effect of magnetic property changes with a detailed comparison between paleointensity and magnetic properties using sediments that responded differently to paleoclimate changes : for example, between sediments with a magnetic grain-size increase in glacial periods and those with a grain-size decrease during the same period of time. The geomagnetic field produces a shielding effect in the influx of high-energy galactic cosmic rays, and a change in field intensity causes a corresponding change in the production rate of cosmogenic radionuclides. The major part of the variations in the atmospheric 14C/12C ratio during the past 50 kyrs is explained by the paleointensity changes. Studies on high-resolution paleointensity estimation are hence useful for calibration of radiocarbon timescales and a better understanding of global carbon-cycle changes. Recently, a relationship among Solar activity, influx of cosmic-rays, and climatic changes in the order of 101 to 102 years has been discussed intensively. The geomagnetic field might have modulated climatic changes over a longer timescale because its large fluctuation in the order of 103 to 104 years should have caused significant variations in the influx of cosmic rays.
The geomagnetic field generated by the geodynamo is one of the most important properties of the Earth. Since the intrinsic field can be approximated by a dipolar one with time variation, it is essential to know time-averaged value of past geomagnetic dipole moments for evaluating the current status of the geodynamo. From archeological materials and volcanic rocks, we can measure absolute paleointensities and estimate past geomagnetic dipole moments as virtual axial dipole moments (VADMs). The Thellier method with the pTRM check is so far regarded as the most reliable paleointensity determination technique. Many efforts have been made to increase its reliability. Based on the Thellier paleointensities reported so far, the time-averaged VADM for the last five million years is believed to be almost the same as the present geomagnetic dipole moment (8 × 1022 Am2) However, it has recently been revealed that Thelliertype methods have some problems resulting in the overestimation of paleointensities. Instead, the LTD-DHT Shaw method, a lately developed technique in Japan, shows a higher degree of reliability for paleointensity determination of historical lava flows. Applying this method, our study of volcanic rocks from the Society Islands, French Polynesia, gives a new mean VADM of 3.64 ± 2.10 × 1022 Am2, though number of the data is still small. This average is significantly lower than the previous one and is nearly half of the present dipole moment, suggesting that the current status of the geomagnetic field may not be typical of the geodynamo.
Until now, a large number of records of geomagnetic excursions have been reported both for volcanic rocks and sedimentary sequences. By straightening up the, extensive excursion records, 23 geomagnetic excursions were identified in total for the Brunhes. Eighteen out of 23 excursions are considered to be reliably determined and dated. Above all, Mono Lake (32-34 ka), Laschamp (39-41 ka), Blake (115-122 ka) and Iceland Basin (180-190 ka) are well-documented worldwide excursions. Others include Hilina Pali (18-23 ka), Norwegian-Greenland Sea (60-80 ka), Albuquerque (140-160 ka), Jamaica/Pringle Falls (210-220 ka), Fram Strait (255-265 ka), Kolbeinsey Ridge (280-290 ka), Calabrian Ridge 1 (310-320 ka), Agulhas Ridge (330-340 ka), Weinen (400-420 ka), West Eifel (500-510 ka), Calabrian Ridge 2 (535-545 ka), Emperor/Big Lost (550-570 ka), Calabrian Ridge 3 (580-605 ka), and Delta (665-690 ka). Most of these excursions can be correlated with minima in relative paleointensity records of Sint-800 and ODP Site 983, suggesting that excursions occur due to dominance of a non-dipole field at minima of the main dipole field intensity.
We report the paleomagnetic behaviors at the beginning of the latest geomagnetic polarity reversal (Brunhes-Matuyama reversal of 0.78 Ma age) recorded in Tahitian lava flows. We applied an improved paleointensity determination method (double heating technique of the Shaw method with low-temperature demagnetization, LTD-DHT Shaw method), and detected oscillation-like changes in virtual dipole moments (VDMs) with a large amplitude ranging from 1.5 × 1022 to 10.9 × 1022 AM2 just prior to the directional reversal. The VDM changes are almost linearly correlated with the virtual geomagnetic pole (VGP) latitudes (60-90°S). We call this VDM-VGP latitude trend “reversal line”. Although the Thellier data reported for the Brunhes-Matuyama reversal are more scattered than our data, they also show a weak trend in the VDM-VGP latitude diagram, supporting the reversal line from a different locality. The scatter in the Thellier data may be caused by overestimated paleointensities due to methodological problems which are indicated in the recent studies of historical lavas. Therefore it appears that the reversal line from the Tahitian lavas in this study does not show a local change but represents a global-scale phenomenon in the geomagnetic reversal process.
Paleomagnetism has been applied to tectonics studies on the basis of the geocentric axial dipole (GAD) field hypothesis. However, the applicability of the GAD hypothesis is still under investigation in the paleomagnetic community. It is directly connected to the precision of the tectonic movements derived from paleomagnetism, so that the the morphology of the geomagnetic field should draw wide attention among Earth scientists. The morphology of the long term geomagnetic features is often represented by the time-averaged field (TAF) and the paleosecular variation (PSV). They are extracted from the paleodirections of lavas erupted during the past 5 My. The recent TAF and PSV models indicate following; (1) the GAD hypothesis is not strictly accurate, (2) the geocentric axial quadupole (g02) field exists in the TAF and it cannot be neglected, and (3) the inclination anomaly calculated from the averaged paleomagnetic data consists also of the apparent effect of the PSV, which additionally cannot be ignored. These components should be paid close attentions to specially in the case of detailed tectonics such as highresolution plate motion. The inclination anomaly due to g02 in the TAF and the effect of the PSV would produce a mistake of the paleolatitude, which becomes totally up to several degrees.
Knowledge of the Earth's magnetic field during Archean and Proterozoic times can provide important sources of information for understanding the internal and environmental evolution of the Earth. The long-term variation in field intensity and reversal rate is considered to reflect mode changes in powering the geodynamo. Several recent efforts to reconstruct the magnetic field of the early Earth have reported relatively low to moderate field accompanied by occasional polarity reversals. The volume of reliable paleomagnetic data, however, is still insufficient to characterize its long-term nature. Here, recent paleointensity and paleodirectional studies on Archean and Proterozoic rocks including our new findings are reviewed, and their problems and further perspectives are argued.
Pseudotachylytes and chondrules appear to be reliable materials for paleomagnetic study because of their melt-quenched regimes in terrestrial and extraterrestrial environments. However, they show different magnetic stabilities due to protolith variation, cooling history and an ambient chemical environment. Here, we present rock magnetic and microtextural examinations of Sudbury pseudotachylytes and artificial chondrules, suggesting that the presence of finegrained ferromagnetic crystals embedded within silicate crystals is responsible for the thermal and chemical stability of magnetic remanence information.
Paleomagnetic intensity variation through marine magnetic anomalies could provide the finest time-depended information on plate tectonics. Rapid progress in the study of marine magnetic anomalies at the mid-ocean ridge area and developments in observation techniques have revealed a link between magnetic anomalies and paleomagnetic intensity variation. Combining recent high-resolution magnetic anomaly data from sea-bottom surveys and sediment-derived paleomagnetic intensity variations has played a key role in providing hope to the hypothesis attributing “tiny wiggle” in marine magnetic anomalies to the paleomagnetic field. However, this does not mean that the other competing hypothesis based on the variation of magnetic layer thickness instead of paleomagnetism, is ruled out. More data and higher analytic techniques would narrow down these possible sources. We introduce some significant steps to the evidences of the paleomagnetic field in magnetic anomalies along with history that sea-going geophysicists have piled up, including the development of instrument and magnetic structure models, and a shift in interpreting magnetic signals as geochemistry of the extrusive rocks, bathymetry expression, low-temperature alteration of basalt by fissuring and cracking, weathering by oxidation with age and the variation of magnetic layer thickness.
Since three-dimensional micromagnetic modeling was introduced to rock magnetism in the late 1980s, we have gained clearer perspectives beyond the classical Neel's theory. at present, we can predict the magnetic properties of even non-uniformly magnetized grains, and directly compare simulated and experimental results, which makes interpretations of measurement data much easier and more appropriate. The longstanding issue of the pseudo-single-domain can be regarded as arising from the vortex structure of magnetization based on micromagnetic modeling results. Recently, micromagnetic modelings have been applied to simulate hysteresis measurements, low-temperature magnetometry, and First-Order Reversal Curves (FORC). Calculations of demagnetizing energy are still demanding in terms of computation load, but we can obtain magnetization structures of micron-size magnetites whose Bitter patterns are observabl.e under an optical microscope. In conjunction with micromagnetic modelings, FORC methods should be useful for illustrating how seriously magnetic interaction affects paleointensity data from sediments.
We present a new method of paleointensity determination based on comparing the thermal demagnetization of natural remanent magnetization (NRM) with that of an artificial total thermoremanent magnetization (TRM). Igneous rocks often contain pseudo-single domain (PSD), multidomain (MD), and/or single domain (SD) particles as magnetic remanence carriers under strong magnetic grain (domain) interactions. The magnetic grain interactions have particular disastrous effects on paleointensity experiments, which make determination of paleointensity unreliable. We have critically examined how magnetic grain interactions affect the Thellier experiment, and have developed a new technique for correcting grain-interaction effects in the experiment of paleointensity estimation. The essential point of our experimental method is that by comparing the thermal demagnetization of natural remanent magnetization (δ NRM_loss) with that of an artificial total TRM (δ TRM_loss) for estimating its paleointensity, rather than that by comparing the remaining of NRM during thermal demagnetization (NRM_remaining) with a progressive TRM_gain in the traditional Thellier-Coe method, which essentially requires the additivity of partial TRM and independence of pTRMs. Using our new method, a mild alternating field (AF) demagnetization pre-treatment is applied to destroy most of the low coercivity remanence, which makes the samples behave more suitebly for a paleointensity study. We also make an apparent paleointensity estimation with pTRM, which is acquired in the perpendicular direction of NRM in a narrow non-overlapping temperature interval and cooled slowly in air. In this way, the non-ideal behavior of samples is detected most sensitively by the discrepancy between NRM loss and pTRM gain. Finally, we employ an artificial total TRM test to elucidate the relation between TRM_loss and pTRM_gain, and to correct interference caused by the non-ideal behavior. We have applied our new method to several representative suites of historical lava flows of known geomagnetic field intensity, and successfully extracted reliable paleointensity with a precision higher than 95% from samples even containing PSD and MD grains.
Geomagnetic polarity time scales for the late Cretaceous and Cenozoic were evaluated, mainly from a viewpoint of their applications to geomagnetochronology. A composite geomagnetic polarity sequence, which was derived primarily on data from the South Atlantic with fine-scale information from the Pacific and Indian Oceans (Cande and Kent, 1992), is nearly complete with high quality and precision. However, a time scale generated by using a spline function to fit calibration points to the composite polarity sequence is not stout, but actually highly vulnerable to the calibration points. Three time scales with different calibration points were compared, resulting in a difference of 1.3 Myr (8 %) at maximum during the Miocene time. Such a large difference signals a warning to geomagnetochronology. The tiny wiggles corresponding to polarity events of less than 30 kyr (defined as a cryptochron) take an important role in examining the long-term nature of the dynamo. We need to clarify whether they are reversal phenomena. A time scale covering the last 5.23 Myr constructed using polarity sequences from marine sediments dated by astronomical tuning is nearly complete. A fairly constant spreading rate in the South Atlantic since 5.23 Ma, that has been verified by the time scale, may suggest a principle for future revisions of the time scale. Further improvements can be achieved by more detailed dating with astronomical tuning, and construction of high-resolution magnetostratigraphy incorporating excursions.
The present status of environmental magnetism is briefly reviewed as it might not be widely know in Japan. The principal purpose of environmental magnetism is to elucidate the process of material transportation on Earth's surface through various magnetic methods either in the past (paleoenvironmental studies) or present day (pollution studies). The reasons why the magnetic method is capable of to carrying out these kinds of missions reside in its universal applicability, quick and non-destructive analytical procedure, and the capability to indicate mean grain-size of magnetic grains in a sediment sample. Several topics, such as the studies on the Chinese loess, lake and deep-sea sediments, and a pollution mapping, are quickly reviewed to show the excellent performance and flexibility of the environmental magnetism.
Magnetic petrology is the science of magnetic minerals, mainly iron-titanium oxide minerals. It aims to identify the magnetic carriers and to determine the factors that control the occurrence and abundance of these phases by using two traditional fields of study, rock magnetism and petrology. By rock magnetic analyses, we can find the magnetic properties, that give us information about magnetic minerals. By petrological analyses, we can find the occurrence of oxide minerals. By integrating both types of information, the origin of magnetization and the evolution of oxide minerals can be considered. In volcanic materials, there are abundant iron-titanium oxide minerals and they show a wide variation in a single lava dome, a single lava flow and one sequence of tephra. Therefore, by magnetic petrological analyses, we can estimate the genesis and evolution of magma. In this paper, I describe three studies about magnetic petrology. 1) Oxides can be frequently oxidized during initial cooling between the magma reservoir and a deposit. According to the difference of temperature, cooling rate and oxygen fugacity, various oxide minerals are produced in lava dome eruptions. 2) In many volcanic materials, like lava flows or tephra, fine-grained oxides can be crystallized in a matrix during the cooling process. This process is important in considering the genesis of the magma or the materials. 3) Heating experiments indicate that exsolution titanohematite lamellas in titanomagnetite can be an indicator of heating temperature and duration.
Paleomagnetism and rock magnetism have played roles of great importance in various aspects of the ocean drilling projects like IPOD, DSDP, and ODP for over 40 years. This article reviews the major contributions of paleomagnetism, and introduces what is aniticipated for paleo-and rock magnetism in IODP, a new phase of international ocean drilling program begun in 2003.