Ronald T. Merrill

The Magnetic Field of the Earth: Paleomagnetism, the Core, and the Deep Mantle

History of Geomagnetism and Paleomagnetism: Discovery of the Main Magnetic Elements. Fossil Magnetism and the Magnetic Field in the Past. Investigations of the External Magnetic Field. Origin of the Earths Magnetic Field. The Present Geomagnetic Field: Analysis and Description from Historical Observations: Magnetic Elements and Charts. Spherical Harmonic Description of the Earths Magnetic Field. Uniqueness and Other Mathematical Problems. Geomagnetic Secular Variation. The External Magnetic Field. Foundations of Paleomagnetism: Rock Magnetism. Magnetic Mineralogy. Paleomagnetic Directions and Poles. Paleointensity Methods. Age Determinations. The Recent Geomagnetic Field: Paleomagnetic Observations: Archeomagnetic Results. Analysis of Recent Lake Sediments. Geomagnetic Excursions. The Geomagnetic Power Spectrum. Reversals of the Earths Magnetic Field: Evidence for Field Reversal. Marine Magnetic Anomalies. Analysis of Reversal Sequences. Polarity Transitions. The Time-Averaged Paleomagnetic Field: Geocentric Axial Dipole Hypothesis. Second-Order Terms. Variation in the Earths Dipole Moment. Paleosecular Variation from Lavas (PSVL). Processes and Properties of the Earths Deep Interior: Basic Principles: Seismic Properties of the Earths Interior. Chemical and Physical Properties. Thermodynamic Properties of the Earths Deep Interior. Thermal History Models. Non-dynamo Models for the Earths Magnetic Field. Fluid Mechanics Fundamentals. Energy Sources. Introduction to Dynamo Theory: The Dynamo Problem. The Magnetic Induction Equation. The a and w Effects of Dynamo Theory. Waves in Dynamo Theory. Symmetries in Dynamo Theory. Theories for Geomagnetic Secular Variations and magnetic Field Reversals. Dynamo Theory: Vector Spherical Harmonics. Kinematic Dynamos. Cowlings Theorem and Other Constraints. Turbulence in the Core. Dynamo Waves. Dynamics of the Geodynamo. The Magnetic Fields of the Sun, Moon, and Planets: Origin of the Solar System. The Sun. The Moon. Meteorites. Magnetic Fields of the Planets. Geomagnetic Relevance. Examples of Synthesis: Fluid Velocities in the Core. Core-Mantle Coupling: Length of Day. Paleomagnetism and Dynamo Theory. Variations at the Core-Mantle Boundary and the Earths Surface. Appendices. References. Subject Index.

Geomagnetic polarity transitions

The top of Earths liquid outer core is nearly 2900 km beneath Earths surface, so we will never be able to observe it directly. This hot, dense, molten iron-rich body is continuously in motion and is the source of Earths magnetic field. One of the most dynamic manifestations at Earths surface of this fluid body is, perhaps, a reversal of the geomagnetic field. Unfortunately, the most recent polarity transition occurred at about 780 ka, so we have never observed a transition directly. It seems that a polarity transition spans many human lifetimes, so no human will ever witness the phenomenon in its entirety. Thus we are left with the tantalizing prospect that paleomagnetic records of polarity transitions may betray some of the secrets of the deep Earth. Certainly, if there are systematics in the reversal process and they can be documented, then this will reveal substantial information about the nature of the lowermost mantle and of the outer core. Despite their slowness on a human timescale, polarity transitions occur almost instantaneously on a geological timescale. This rapidity, together with limitations in the paleomagnetic recording process, prohibits a comprehensive description of any reversal transition both now and into the foreseeable future, which limits the questions that may at this stage be sensibly asked. The natural model for the geomagnetic field is a set of spherical harmonic components, and we are not able to obtain a reliable model for even the first few harmonic terms during a transition. Nevertheless, it is possible, in principle, to make statements about the harmonic character of a geomagnetic polarity transition without having a rigorous spherical harmonic description of one. For example, harmonic descriptions of recent geomagnetic polarity transitions that are purely zonal can be ruled out (a zonal harmonic does not change along a line of latitude). Gleaning information about transitions has proven to be difficult, but it does seem reasonable to draw the following conclusions with varying degrees of confidence. There appears to be a substantial decrease in the mean intensity of the dipole field during a transition to ∼25% of its usual value. The duration of an average geomagnetic polarity transition is not well known but probably lies between 1000 and 8000 years. Values outside these bounds have been reported, but we give reasons as to why such outliers are likely to be artifacts. The reversal process is probably longer than the manifestation of the reversal at Earths surface as recorded in paleomagnetic directional data. Convection hiatus during a geomagnetic polarity transition seems unlikely, and free-decay models for reversals appear to be generally incompatible with the data. This implies that certain theorems in dynamo theory, such as Cowlings theorem, should not be invoked to explain the origin of reversals. Unfortunately, the detailed description of directional changes during transitions remains controversial. Contrary to common belief, certain low-degree nondipole fields can produce significant longitudinal confinement of virtual geomagnetic poles (VGP) during a transition. The data are currently inadequate to refute or verify claims of longitudinal dipole confinement, VGP clustering, or other systematics during polarity transitions.

The time‐averaged paleomagnetic field 0–5 Ma

Persistent departures from the geocentric axial dipole field model of the time-averaged paleomagnetic field over the past 5 Myr have been analyzed using oceanic data from deep-sea cores and continental data from igneous rocks and sediments. The data set comprises the equivalent of 9490 spot readings of the field (5831 normal and 3659 reverse) from 930 groups of data. Variations in declination anomalies between groups of data strongly suggest the existence of second-order tectonic effects (small rotations) that make it difficult, if not impossible, to identify any nonzonal terms that might exist. Inclination anomalies have therefore been modeled in terms of low-degree zonal harmonics. The calculated inclination anomalies within 10° latitude bands are widely scattered (for both oceanic and continental data) and do not appear to have been drawn from distributions with common means. We attribute this large scatter to previously undetected or uncorrected second-order tectonic effects in the continental data or, in the oceanic data, to small departures from the vertical in deep-sea cores. If it is assumed that these effects are random between groups of data and without a systematic bias, they can be averaged out to a large extent if there are sufficient numbers of groups within each latitude band. This method also has the major advantage that it maximizes time averaging of the field. When this is done, an acceptable fit to the inclination anomaly data is found using a zonal harmonic model. Although the point estimates for the reverse polarity zonal quadrupole term are consistently larger than those for the normal polarity zonal quadrupole term, the difference is not significant and is likely due to contamination effects. This finding is true for a combination of all the data or for separate combinations of continental igneous rocks and ocean sediments. This conclusion differs from all previous analyses. The major differences occur in model estimates of the zonal octupole term, the estimates varying widely depending on the particular data combination. We show that false octupole terms can be introduced by several factors, including inclination errors in sediments and lavas, the use of unit vectors in the analyses, and incomplete magnetic cleaning, particularly of reversely magnetized rocks. Thus we cannot impute a geomagnetic significance to the estimates of a zonal octupole term. We suggest that the best estimates for the zonal quadrupole G2 = g20 / g10 are obtained from the combined Brunhes age igneous rocks and oceanic normal data (0.033 ± 0.019) and for the combined Matuyama age igneous rocks and oceanic reverse data (0.042 ± 0.022). When these two sets are combined, the overall best estimate of G2 is 0.038 ± 0.012.

Geomagnetic field stability: Reversal events and excursions

Abstract It has been suggested that there are several reverse subchrons in the Brunhes, and this suggestion is gaining some acceptance. However, such a suggestion has fundanmental implications for the reversal process, because it implies that the reverse polarity state has been substantially less stable than the normal polarity state during the Brunhes Chron. Analysis of the latest polarity reversal timescale confirms that there is no reason to reject the hypothesis of a common stability for the two polarity states. The Cretaceous Superchron appears to be a time when the reversal rate slowed to the point where the reversal process actually halted, so does not indicate any difference in the stability of the two polarity states. After examining the data we found no convincing evidence for even a single reversal event during the Brunhes, and we conclude that the only reversal excursions that are sufficiently well documented for the Brunhes are the Laschamp and the Blake. Further, we note that the question of whether of large excursions represent aborted reversals is unresolved, so excursion data should not at this stage be grouped with transition polarity data. Finally, we note that, most of the time, the nearly axially symmetric normal and reverse polarity states appear to be remarkably stable.

A comparison of ARM and TRM in magnetite

Abstract Experiments comparing anhysteretic remanence (ARM) and thermoremanence (TRM) in samples containing natural and synthetic magnetite, whose mean particle sizes range from single domain to multidomain, show that ARM and TRM are very similar (but not identical) in their stabilities with respect to alternating field (AF) demagnetization, temperature cycles in zero field to below magnetites isotropic temperature near 130°K, and stability with respect to spontaneous decay in zero field. Therefore, for magnetites, ARM can be used to model (with reasonable success) these stability properties of TRM. The field dependence of the acquisition of ARM and TRM shows that the low field susceptibility ratio, χ ARM /χ TRM , has a particle size dependence, increasing from 0.1 for certain submicron particles to 2.0 for large multidomain crystals. Even for samples whose remanence is predominantly carried by submicron particles χ ARM /χ TRM is highly variable, 0.11 ≤ χ ARM /χ TRM ≤ 0.50 . Therefore, ARM paleointensity methods which do not take into account the large variability in and the particle size dependence of χ ARM /χ TRM are subject to order-of-magnitude uncertainties.

Nucleation theory and domain states in multidomain magnetic material

Abstract Calculations are made to determine the number of domains that can exist in a given size grain. The nucleation of domain walls at grain boundaries is investigated for materials free of crystalline defects. Owing to the energy required to nucleate domain walls a much larger range of grain sizes can have the same number of domains than indicated by considering lowest energy domain states alone.

The variation of intensity of earth’s magnetic field with time

Abstract A new and larger database is established to assess the variation of paleointensity with time. It is shown that submarine basaltic glass probably contains a grain-growth chemical remanent magnetization in magnetite. Intensity estimates from these samples are used to establish a lower bound estimate on intensities. Statistical tests on various subsets of the database are used to establish a more reliable database for finding long-term trends of the paleointensity. We can obtain a reasonable estimate for intensity versus time for the Cenozoic but not for the Mesozoic or late Paleozoic. The distributions of data as a function of time also provide valuable information. In particular, the two well-documented superchrons exhibit different intensity properties, which suggests that there is not a simple correlation between reversal rate and intensity.

Rock Magnetism and Paleomagnetism of Some North Pacific Deep-Sea Sediments

Detailed paleomagnetic and rock magnetic studies have been conducted on eight deep-sea cores from the North Pacific. Magnetic studies include alternating field demagnetization, thermal demagnetization, anhysteretic remanent magnetization studies, magnetic hysteresis measurements over a variety of different temperatures, viscous and drying effects, strong field versus temperature measurements, x-ray diffraction, and x-ray fluorescence analyses. Six of the eight cores studied contain an abundance of fossils, particularly silicoflagellates, and appear to have acquired their remanent magnetization sufficiently close to the surface to reliably record the Earth9s paleomagnetic field. The remaining two cores do not contain fossils and do not appear to accurately record the Earth9s paleomagnetic field. Low-temperature oxidation appears to have occurred in situ in these cores. A gamma phase (cation-deficient spinel) iron-titanium oxide with lattice parameter of 8.38 A and Curie temperature of 545°C near the top of the cores changes with depth to a gamma phase with lattice parameter of 8.33 A and Curie temperature near 600°C close to the bottom of the cores. These chemical changes appear to be associated with the production of a chemical remanent magnetization that makes it impossible to use these cores for paleomagnetic studies. This work summarizes many of the problems in obtaining reliable paleomagnetic results from deep-sea cores, including possible spurious magnetic directions resulting from chemical changes, drying, and coring effects.

The magnetic moments of non-uniformly magnetized grains

Abstract Variation techniques are used to determine simultaneously the configurations of magnetic moments in a multidomain material. It is shown that there are several minimum energy states available to grains. Usually the lowest energy state does not appear to be occupied, based on comparisons between theory and observations of domains. The consequences of this can be severe with regard to estimations of the net amount of remanence in domains and in domain walls.

The myth of the Pacific dipole window

The angular dispersion of virtual geomagnetic poles (VGP) from lava flows is often cited as having been anomalously low in the Pacific during the Brunhes epoch because the dispersion from Hawaiian lavas is said to be much lower than measured elsewhere. This led to the concept of the Pacific dipole window or Pacific non-dipole low. Because lavas tend to be erupted in bursts of activity, many of the Hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations. Geochronological controls, as are available for Hawaii, have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field. Unfortunately, in that analysis the angular dispersion of VGPs was calculated about the mean VGP instead of about the spin axis, and thus underestimated the dispersion. We have therefore reanalyzed the relevant data using the following criteria. (1) Only those lavas with α95 < 10° have been considered. (2) A latitude-dependent cut-off angle is used to eliminate those vectors that are not part of the normal secular variation. (3) We have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector, because suitable geochronological controls are rarely available. Application of this new method to the Hawaiian data shows excellent agreement with the results from using geochronological controls. Where possible we have therefore included this method in our overall analysis. As expected, the resulting global data are compatible with a Fisher distribution about the spin axis. Brunhes age data from Hawaii (N = 96 independent measurements) give a VGP angular dispersion about the spin axis of SF = 12.4°, and for the Pacific region as a whole SF = 12.5° (N = 190) between latitudes 15° and 30° (north or south). These values are the same as SF = 12.4° (N = 160) calculated for the rest of the world in the same latitude range. This clearly demonstrates that the hypothesis of the Pacific dipole window may confidently be rejected.