Direction Of Geomagnetic Field Research Articles

First report of coupled Early Permian paleomagnetic and geochronologic data from the Dunhuang block (NW China), and implications for the tectonic evolution of the Paleo-Asian ocean

In order to better understand the tectonic relationship between the Dunhuang block (DHB) and its adjacent blocks, and to constrain the timing of the closure of the Paleo-Asian ocean, a combined geochronologic and paleomagnetic investigation has been undertaken on Early Permian tuff, basalt flows and sandstones from the Shuangbaotang Formation in the northwestern part of the DHB. U-Pb zircon dating indicates that the age of the strata is between 280.6 ± 2.9 Ma and 291.4 ± 2.6 Ma. Thermal demagnetization of a three-component isothermal remanent magnetization (IRM), and Curie point experiment suggest that magnetite dominates in the rock samples analyzed. In addition, there is a minor amount of hematite in some sandstones. Stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization (ChRM) from 11 tuff layers, two lava flows and nine sandstone beds. Two components were isolated from all samples: a lower temperature component (LTC) and a higher temperature component (HTC). The LTC is near the direction of the present-day geomagnetic field and produced a negative fold test, indicating it is a viscous remanent magnetization in the present-day geomagnetic field. Most of the HTC are reverse polarity (Normal = 4, reverse = 158), which is in accordance with the Kiaman Reversed Superchron that spans the Late Carboniferous-Permian interval. In addition, the HTC of all studied sites passed the fold and reverse tests, suggesting that they likely represent primary remanent magnetization. The tilt-corrected mean direction from all sites (tuff, basalts and sandstones) is Ds = 1.7°, Is = 43.1°, ks = 403, α95 = 1.5°, N = 22. The mean paleopole of the site-mean direction-corresponded VGPs lies at 74.5°N, 268.5°E with A95 = 1.6°. Considering the consistent inclination values recorded among the studied tuff, basalts and sandstones, and the low degrees of anisotropy within all samples, we suggest that there is no significant inclination shallowing caused by depositional compaction in the sedimentary layers of the studied section. Taking into account the results from this study as well as previous Late Paleozoic paleomagentic studies from adjacent tectonic blocks, we conclude that the DHB formed part of the amalgamated Dunhuang-North China-Alxa-Qaidam mega-block during the Early Permian, but was separated from the Tarim block by a small ocean (here named the Qiemo-Xingxingxia paleo-ocean) at this time. A comparison of the Early Permian paleolatitudes of these and other adjacent blocks suggest that the Paleo-Asian ocean (sensu lato) was still open at this time. Combined with other geological evidence, a paleogeographic reconstruction of the Paleo-Asian ocean has been reconstructed for the Early Permian.

Refining Holocene geochronologies using palaeomagnetic records

Abstract The aperiodic nature of geomagnetic field variations, both in intensity and direction, can aid in dating archaeological artefacts, volcanic rocks, and sediment records that carry a palaeomagnetic signal. The success of palaeomagnetic dating relies upon our knowledge of past field variations at specific locations. Regional archaeo- and palaeomagnetic reference curves and predictions from global geomagnetic field models provide our best description of field variations through the Holocene. State-of-the-art palaeomagnetic laboratory practices and accurate independent age controls are prerequisites for deriving reliable reference curves and models from archaeological, volcanic, and sedimentary palaeomagnetic data. In this review paper we give an overview of these prerequisites and the available reference curves and models, discuss techniques for palaeomagnetic dating, and outline its limitations. In particular, palaeomagnetic dating on its own cannot give unique results, but rather serves to refine or confirm ages obtained by other methods. Owing to the non-uniform character of magnetic field variations in different regions, care is required when choosing a palaeomagnetic dating curve, so that the distance between the dating curve and the record to be dated is not too large. Accurate reporting and incorporation of new, independently dated archaeo- and palaeomagnetic results into databases will help to improve reference curves and global models for all regions on Earth.

A 2000 year geomagnetic field record from the Gulf of Papua

A high resolution Holocene record of geomagnetic field direction and intensity in the Gulf of Papua was constructed using paleomagnetic data from eight piston cores collected in the region. The chronology of the cores was based largely on radiocarbon dates from benthic foraminifera. Correlation of seismic reflectors imaged in subbottom data between nearby cores was consistent with the chronostratigraphic framework based on radiocarbon dates and provided additional confidence of the age model. In other regions, where cores sampled different depositional settings associated with the three-dimensional prograding lobes, reflectors could not be confidently correlated between cores. Furthermore, radiocarbon ages from the bottoms of trigger cores and tops of co-located piston cores revealed varying amounts of overpenetration of the piston core below the seafloor. The relative paleointensity, and a general eastward trend in the declination and steepening of the inclination in our new record are generally compatible with the closest (>4300 km) existing records. Our record does not agree well with global field models CALS3k and pfm9k, likely reflecting the lack of existing data contributing to field models in the region.

Ionospheric Scintillation Simulation on Equatorial GPS Signals for Dynamic Platforms

This paper presents a physics-based, data-consistent, multifrequency GPS scintillation signal simulator for dynamic platforms in the equatorial region. This simulator is based on a two-dimensional two-component power-law phase screen model which is described by a compact set of parameters. The time scale factor is mainly investigated in this paper, which determines the decorrelation time of the simulation signal and is affected by various factors including propagation geometry, irregularity drift velocity, and dynamics of the platform. The comparison between the initialization and simulation data for stationary receivers shows the effectiveness of the simulator. In addition, this paper examines the influence of the platform dynamics on simulation signals with respect to the geomagnetic field direction and the irregularity drift velocity. The simulator presented in this paper will provide a viable tool for dynamic GPS scintillation signal simulation and will aid the design for robust receiver processing algorithms during scintillation for dynamic platforms.

Paleomagnetic evidence for counterclockwise rotation of the Dofan magmatic segment, Main Ethiopian Rift

Twenty-six paleomagnetic sites in basalt and trachyte flows and ignimbrite deposits sampled in the Dofan magmatic segment, Main Ethiopian Rift (MER). From each site, 6 to 8 core samples were collected. The samples were then cut into 200 standard specimens and their Natural Remanent Magnetization (NRM) directions were measured using a JR6A spinner magnetometer. Most specimens were subjected to stepwise alternating field (AF) and at least one specimen per site to thermal (TH) demagnetization. The directional analysis of these individual specimens revealed either one or two components of NRM. Where two components are present, the first is isolated below a temperature of 300 °C or AF field below 20 mT; the second is isolated above those steps and mostly defined straight lines directed towards the origin and are interpreted as the Characteristic Remanent Magnetization (ChRM) acquired during cooling. Rock magnetic experiments on representative specimens indicate that the dominant magnetic minerals are titanium poor titanomagnetite and in few cases titanohematites. The overall mean directions calculated for the 23 sites of Dofan is Dec = 354.1°, Inc. = +11.6° (N = 23, K = 35.1, α95 = 5.2°). When these values are compared with the 1.5 Ma expected mean geomagnetic dipole reference field directions Dec = 1.0°, Inc. = +16.4° (N = 32, K = 105.6, α95 = 2.3°), obtained from African Apparent Polar Wander Path Curve; a difference in declination ΔD = −6.9° ± 4.7° and inclination ΔI = +4.8° ± 5.5° are determined. The declination difference is interpreted as a very slight counterclockwise rotation about vertical axis of the Dofan magmatic segment and the result is consistent with previous paleomagnetic reports and analogue modeling in Fentale magmatic segment.

Fire and collapse: Untangling the formation of destruction layers using archaeomagnetism

AbstractHistorical events are sometimes expressed in destruction layers. We present here a study in which aspects of construction, destruction, and chronostratigraphy of fired mud bricks were explored using archaeomagnetism, infrared spectroscopy, and micromorphology. We measured 88 oriented samples mostly collected from one stratum, dated ca. 1000 B.C.E., representing a destroyed late Canaanite (late Iron Age I) city in Tel Megiddo, Israel. Firing temperatures, evaluated from infrared spectroscopy, micromorphology, and high‐temperature magnetic susceptibility cycles, range between 300°C and 800°C. Samples studied in one archaeomagnetic site yield a single stable magnetization vector in demagnetization experiments. Archaeomagnetic site means of three standing walls are grouped near the expected direction of the ancient geomagnetic field. We propose that walls in the destruction layer were constructed from sun‐dried mud bricks that later burned during the destruction. Collapsed bricks and tilted walls show variable directions, diagnostic for the relative timing of collapse and cooling of bricks, during and following the destruction event. In addition, we attempt to assign stratigraphic affiliation based on archaeomagnetic considerations to standing walls, which are spatially disconnected from the studied destruction layer. Altogether, this study demonstrates the usefulness of archaeomagnetism to understanding site formation processes related to fire and destruction.

Remagnetization mechanisms in Triassic red beds from South China

Paleogeographic reconstructions based on paleomagnetic data rely on the reliability of the natural remanent magnetization (NRM) as a primary geomagnetic signal. Remagnetizations, however, can be common in many rock types, including late Paleozoic and Mesozoic red beds, and they complicate paleogeographic interpretations. Extracting the primary NRM from partially remagnetized rocks, and understanding the remagnetization mechanism are important in these contexts. We carried out a systematic paleomagnetic study of red bed samples from the Triassic Huangmaqing Formation, Nanjing (32.0°N, 118.9°E), South China. Two NRM components carried by secondary and primary hematite are isolated in 47 of the 94 samples studied, where the latter component has a direction in stratigraphic coordinates of D=29.2°, I=34.6° (α95=10.9°, 47 samples from 6 sites) that yields a paleopole of λ=60.8°N, ϕ=228.1°E, dp/dm=12.5/7.2, which is consistent with Triassic pole positions for the South China Block. A secondary chemical remanent magnetization (CRM) (D=227.1°, I=80.8°, α95=7.3°) is documented in all 94 samples from 10 sites and is carried by pigmentary hematite that is inferred to have been generated by magnetite oxidation during orogenic activity. This secondary component has steep inclinations and is interpreted to have been influenced by a combination of the remanence carried by original parent magnetite, the orogenic stress field, and the prevailing geomagnetic field direction during deformation. This CRM direction is recorded commonly by red beds from the South China Block, and is significant for regional tectonic studies in the area.

The Sixth Sense? [Microwave Surfing

For many years, scientists have known that certain biological organisms ranging from bacteria to birds possess the faculty of magnetoreception [3], i.e., the ability to sense the earth’s magnetic field. But do human beings also have this sixth sense? Recent research from the California Institute of Technology (Caltech) offers some tantalizing clues. Let’s start with the bacteria, because their “magnetoreceptors are still the only ones scientists have definitely located and studied.” In 1975, Richard Blakemore at the Woods Hole Oceano graphic Institution in Massachusetts found that certain bacteria from Cape Cod marsh muds “harbored chains of magnetic crystals” and, “as swimming compass needles,” could follow the direction of the local geomagnetic field, which dips down at an angle of 70° toward the north pole. Blakemore’s work inspired Caltech geologist Joe Kirschvink to study magnetic bacteria in the southern hemisphere (actually in a sewage treatment pond near Canberra, Australia), and he found that “sure enough, they swam down toward the south pole."

Theoretical analysis of effects on high frequency vertical sounding by artificial field-aligned irregularities

Ionospheric heating experiments have been conducted widely at high power heating stations, such as Arecibo, Platteville, HAARP, etc. It has been found that once high-power high-frequency (HF) radio wave is injected into the ionosphere, the electron temperature and density in the illuminated region of the ionosphere can be disturbed, and furthermore, a large number of nonlinear phenomena may be triggered because of the complicated instabilities. One of the most interesting heating effects is the generation of the artificial field-aligned irregularities (AFAI), which has profound influences on electromagnetic wave propagation. Many diagnostic methods have been used for studying the characteristics of AFAI, such as the HF vertical/oblique sounding, HF/VHF coherent radar, etc. During the heating experiments, traces spreading on frequency or height are observed from the HF vertical sounding ionograms, which suggests that the propagation of the sounding wave will be affected by AFAI. In the ionosphere F region, the electron diffusion and thermal conductivity rate are greater along the geomagnetic field lines than across the field line, leading to a stretch of AFAI along the geomagnetic field line. For the special structure, the AFAI will scatter the incident wave in a cone with the axis parallel to the geomagnetic field direction, which is called artificial field-aligned scattering (AFAS). Because of the high sensitivity to the geomagnetic field of AFAS, we try to study different effects on the HF vertical sounding of AFAI generated at different latitudes, by constructing a propagation model and performing a simulation, in order to seek the potential applications in HF transmission. Based on the special scattering feature of AFAI and the ray tracing technique, a propagation model for HF vertical sounding scattered by AFAI is proposed. With this model the ray paths of the sounding waves with AFAI are simulated in amid-latitude region, and a new kind of artificial spread trace is found to start from the heating frequency and spread to higher band. Taking account of the strong dependence of the AFAS on the geomagnetic field, the influences of AFAI on the HF vertical sounding at different latitudes are analyzed theoretically. It is indicated that the artificial spread traces will appear only when the following two conditions are satisfied: 1) the sounding wave can reach the AFAI height; 2) the sounding wave is incident perpendicularly to the AFAI. It is also shown that the spread trace becomes shorter with the latitude and the inclination increasing. Furthermore, the simulations from different heating stations suggest that artificial spread traces do not exist when HF vertical sounding is located just below the AFAI, which explains why such phenomena cannot be observed at high latitudes. Nevertheless, if the HF vertical sounding moves outside the heating station toward the south, the spread traces will be apparent for Arecibo, limited for Platteville and still unavailable for HAARP. Finally, if the AFAI is assumed to be present, apparent artificial spread traces of the mid-low latitude are predicted, and the important valuable applications of AFAI in HF transmission are proposed.

Distinguishing and correlating deposits from large ignimbrite eruptions using paleomagnetism: The Cougar Point Tuffs (mid‐Miocene), southern Snake River Plain, Idaho, USA

AbstractIn this paper, we present paleomagnetic, geochemical, mineralogical, and geochronologic evidence for correlation of the mid‐Miocene Cougar Point Tuff (CPT) in southwest Snake River Plain (SRP) of Idaho. The new stratigraphy presented here significantly reduces the frequency and increases the scale of known SRP ignimbrite eruptions. The CPT section exposed at the Black Rock Escarpment along the Bruneau River has been correlated eastward to the Brown's Bench escarpment (six common eruption units) and Cassia Mountains (three common eruption units) regions of southern Idaho. The CPT records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations. Paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries, providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events. To obtain reliable paleomagnetic directions, the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy, and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization.

A theoretical investigation on the parametric instability excited by X‐mode polarized electromagnetic wave at Tromsø

AbstractRecent ionospheric modification experiments performed at Tromsø, Norway, have indicated that X‐mode pump wave is capable of stimulating high‐frequency enhanced plasma lines, which manifests the excitation of parametric instability. This paper investigates theoretically how the observation can be explained by the excitation of parametric instability driven by X‐mode pump wave. The threshold of the parametric instability has been calculated for several recent experimental observations at Tromsø, illustrating that our derived equations for the excitation of parametric instability for X‐mode heating can explain the experimental observations. According to our theoretical calculation, a minimum fraction of pump wave electric field needs to be directed along the geomagnetic field direction in order for the parametric instability threshold to be met. A full‐wave finite difference time domain simulation has been performed to demonstrate that a small parallel component of pump wave electric field can be achieved during X‐mode heating in the presence of inhomogeneous plasma.

Distribution of magnetogravity waves during strong earthquakes (M > 6.5) preparation periods

Magnetogravity activity during the preparation of four strong (M > 6.5) earthquake with epicenters at medium (35–40° N) and low (10° S–10° N) latitudes is detected. For this purpose a comparison of simultaneous disturbances at dynamic spectra of ionospheric layers Es, E, F2 critical frequencies and geomagnetic field X, Y, Z components was performed. It was found that the greatest number of magnetogravity events noted in epicenters vicinity takes place at two days before the considered earthquakes. Evaluation of dynamic spectra intensity level which was obtained by data from magnetic stations located around the epicenter, allows to identify the magnetogravity waves (MGW) propagation direction with least damping. In most cases they coincide with geomagnetic field direction. Attention is paid to ionospheric wind influence for low-frequency MGW mode.

Propagation characteristics of power line harmonic radiation into the stratified anisotropic ionosphere

Power line harmonic radiation (PLHR), which specifically refers to the electromagnetic wave radiation observed in ionosphere or magnetosphere, is radiated by the transmission lines of power systems on the ground. PLHR is shown as a parallel spectrogram between 400 Hz and 5 kHz in frequency-time power spectrogram of electromagnetic field. And the frequency spacing of the parallel spectrogram is 50/100 Hz or 60/120 Hz. As an artificial pollution source in the near earth space, PLHR has attracted more and more attention. However, so far, there have been little proposed quantitative researches on the formation mechanism. This paper studies the propagation model for the electromagnetic waves generated by the electric dipole source above the non-ideal conductive ground in the stratified anisotropic ionosphere. Based on the method by Lehtinen(2008), a new full-wave finite element method is give to solve the problem. By recursively calculating reflection coefficients and mode amplitudes, the method contains no index increasing items. So it can effectively overcome the numerical overflow in programming calculations. In order to verify the correctness of the method, comparison are made between the existing analytical solutions and the solutions obtained from the proposed method, and they are in excellent agreement. Further more, using the present model, the new method and the associated parameters about practical power lines, ground and ionosphere, we have studied the effects of the frequency of dipole source, the bottom boundary height of ionosphere, the earth conductivity, and the geomagnetic field direction on PLHR propagation in the ionosphere. Results show that when the frequency of radiation source equals the cut off frequency of earth-ionosphere waveguide-guided wave modes, the strength of PLHR for penetrating into the ionosphere becomes larger. Keeping the harmonic current constant, a smaller ground conductivity would be accompanied by a larger power of PLHR. PLHR propagates along the direction of the geomagnetic field in the ionosphere. Therefore, it is much easier for a high-order harmonic radiation of transmission lines to penetrate into the ionosphere along the direction of the geomagnetic field in the areas with low ground conductivity under a certain condition. Results obtained in this paper may have important implications to explain the formation mechanism of PLHR.

About this title ‐ The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes

This volume provides a synopsis of current research on volcanic processes, as gained through the use of palaeomagnetic and rock magnetic techniques. Thermoremanent magnetization information provides a powerful means of deciphering thermal processes in volcanic deposits, including estimating the emplacement temperature of pyroclastic deposits, which allows us to understand better the rates of cooling during eruption and transport. Anisotropy of magnetic susceptibility and anisotropy of remanence are used primarily to investigate rock fabrics and to quantify flow dynamics in dykes, lava flows, and pyroclastic deposits, as well as identify vent locations. Rock-magnetic characteristics allow correlation of volcanic deposits, but also provide means to date volcanic deposits and to understand better their cooling history. Because lava flows are typically good recorders of past magnetic fields, data from them allow understanding of changes in geomagnetic field directions and intensity, providing clues on the origin of Earth's magnetic field.

Robust estimators of palaeosecular variation

The Fisher distribution is central to palaeomagnetism but presents several problems when used to characterize geomagnetic field directions as observed in sequences of volcanic rocks. First, it introduces a shallowing effect when used to define the mean of any group of directional unit vectors. This is problematic because it can suggest the presence of persistent non-axial dipole components when none are present. More importantly, it fails to capture the observed ‘long tail’ in distributions of both directions and associated virtual geomagnetic poles in terms of angular distance from a central direction. To achieve a good fit to data, it therefore requires the introduction of a second distribution (and therefore the estimation of additional parameters) or the arbitrary removal of data. Here we present a new distribution to describe palaeomagnetic directions and demonstrate that it overcomes both of these problems, generating robust indicators of both the central direction (or pole position) and the spread of palaeomagnetic data as defined by unit vectors. Starting from the assumption that poles (or directions) have an expected colatitude, rather than a mean location, we derive the spherical exponential distribution. We demonstrate that this new distribution provides a good fit to palaeomagnetic data sets from seven large igneous provinces between 15 and 65 Ma and also those produced by numerical dynamo models. We also use it to derive a new shape parameter which may be used as a diagnostic tool for testing goodness of fit of models to data and use this to argue for a shift in geomagnetic behaviour between 5 and 15 Ma. Furthermore, we point out that this new statistic can be used to determine the most appropriate distribution to be used when constructing confidence limits for poles.

Estimating the Magnetization Direction of Sources through Correlation between Reduced-to-Pole Anomaly and Normalized Source Strength

Under the effects of remanent magnetization, total magnetization direction is different from geomagnetic field direction, which makes magnetic data processing and interpretation complexity. In this paper, we present a new approach for estimating the total magnetization direction of sources via cross-correlation between the reduced-to-pole anomaly and the normalized source strength (who is less sensitive to remanent magnetization). The geomagnetic field direction is used to calculated the normalized source strength, while various assumed total magnetization directions are used to calculated the RTP anomalies. The maximum correlation between the RTP anomalies and the normalized corresponds to the estimated total magnetization direction. Test on synthetic data showed that the new approach is simple and effective.

Instantaneous record of the geomagnetic field direction of various facies from pyroclastic flow deposits: Tests for consistency in paleomagnetic directions

A comparative study of the paleomagnetism of various lithofacies of pyroclastic flow deposits has been undertaken to examine the accuracy of their remanent magnetization direction at the time of deposition. The Aso-2 pyroclastic flow deposit, which erupted at 141ka from Aso caldera, Japan, was chosen as the site of our investigation because the deposit formed over a short time period and the paleomagnetic directions of the deposit are expected to record substantial contemporaneous volcanism. Paleomagnetic samples of densely welded, lava-like rheomorphic, and non-welded pyroclastic deposits were collected at nine sites from the Aso-2 pyroclastic flow deposit. Remanence directions from densely welded pyroclastic deposits display good within- and between-site consistency and are considered to accurately record the ambient geomagnetic field direction at the time of emplacement. Rheomorphic pyroclastic deposits had directions that show good within-site consistency, and it is considered that this material obtains its remanence direction parallel to the ambient field corresponding to the time of its emplacement. Remanence directions from non-welded pyroclastic deposits show large confidence limits or deviations from their expected directions. Such deposits would likely be prone to modification of remanence direction introduced from random rotations of remanence-carrying material during syn- or post-depositional stages. In conclusion, we suggest that among various lithofacies of pyroclastic flow deposits, remanence directions observed from non-welded pyroclastic deposits may need to be interpreted cautiously.

High-frequency polarity swings during the Gauss-Matuyama reversal from Baoji loess sediment

Paleomagnetic records of the Gauss-Matuyama reversal were obtained from two loess sections at Baoji on the Chinese Loess Plateau. Stepwise thermal demagnetization shows two obvious magnetization components. A low-temperature component isolated between 100 and 200–250°C is close to the present geomagnetic field direction, and a high-temperature component isolated above 200–250°C reveals clearly normal, reversed, and transitional polarities. Magnetostratigraphic results of both sections indicated that the Gauss-Matuyama reversal consists of a high-frequency polarity fluctuation zone, but the characteristic remanent magnetization directions during the reversal are clearly inconsistent. Rock magnetic experiments demonstrated that for all the specimens with normal, reversed, and transitional polarities magnetite and hematite are the main magnetic carriers. Anisotropy of magnetic susceptibility indicates that the studied loess sediments have a primary sedimentary fabric. Based on virtual geomagnetic pole latitudes, the Gauss-Matuyama reversal records in the two sections are accompanied by 14 short-lived geomagnetic episodes (15 rapid polarity swings) and 12 short-lived geomagnetic episodes (13 rapid polarity swings), respectively. Our new records, together with previous ones from lacustrine, marine, and aeolian deposits, suggest that high-frequency polarity swings coexist with the Gauss-Matuyama reversal, and that the Gauss-Matuyama reversal may have taken more than 11 kyr to complete. However, we need more detailed analyses of sections across polarity swings during reversals as well as more high-resolution reversal records to understand geomagnetic behavior and inconsistent characteristic remanent magnetization directions during polarity reversals.

Secular variation of the middle and late Miocene geomagnetic field recorded by the Columbia River Basalt Group in Oregon, Idaho and Washington, USA

This study of 118 discrete volcanic flows from the Columbia River Basalt Group is aimed to determine their distribution of geomagnetic field directions and virtual geomagnetic poles (VGPs) and to compare the inherent secular variation parameters with those from other studies. The magnetic signature of these rocks is uniformly carried by primary titanomagnetite, indicating that magnetic changes are due to variations in the magnetic field. Although most flows are flat lying, those that are tilted pass the Tauxe and Watson tilt test. Sequential flows with statistically similar site means were grouped, and directions that were considered outliers were evaluated and removed using the Vandamme cut-off method. Three normal-polarity (N-polarity) and three reversed-polarity (R-polarity) intervals are revealed by the stratigraphically ordered flows and have mean directions of N polarity (dec/inc=6.6°/+61.2°, k=29.3, α 95 =4.2°), and R polarity (dec/inc=178.2°/−59.2°, k=16, α 95 =5.5°). Regression analysis indicates that the secular variation analysis has not been affected by regional rotation, and that apparent polar wander is negligible. The VGP distribution is almost perfectly circular and supports the preference of VGP positions for the dispersion analysis. Dispersion parameters with corrections for within-site scatter (S b ) show a range of 14.3°–25.5°, including error limits, and were consistently higher for R-polarity results than for those of N polarity. Published dispersion parameters for extrusives <5 Ma show S b values slightly lower than ours, yielding values of 16°–19°, although the difference is not statistically significant. In contrast, published dispersion parameters from high quality data from the Cretaceous Normal Superchron are lower than those for the Neogene, which suggests that the noisiness of the magnetic field correlates with the frequency of reversals. Our new results allow us to extend the Plio-Pleistocene palaeosecular variation database to the bottom of the middle Miocene. Many Miocene formations on a variety of continents are suitable targets for future analysis. Furthermore, the significant difference between the reversed and N-polarity dispersion parameters is intriguing and needs substantiation.

A paleointensity technique for multidomain igneous rocks

We developed a paleointensity technique to account for concave‐up Arai diagrams due to multidomain (MD) contributions to determine unbiased paleointensities for 24 trial samples from site GA‐X in Pleistocene lavas from Floreana Island, Galapagos Archipelago. The main magnetization carrier is fine‐grained low‐titanium magnetite of variable grain size. We used a comprehensive back‐zero‐forth (BZF) heating technique by adding an additional zero‐field heating between the Thellier two opposite in‐field heating steps in order to estimate paleointensities in various standard protocols and provide internal self‐consistency checks. After the first BZF experiment, we gave each sample a total thermal remanent magnetization (tTRM) by cooling from the Curie point in the presence of a low (15 µT) laboratory‐applied field. Then we repeated the BZF protocol, with the laboratory‐applied tTRM as a synthetic natural remanent magnetization (NRM), using the same laboratory‐applied field and temperature steps to obtain the synthetic Arai signatures, which should only represent the domain‐state dependent properties of the samples. We corrected the original Arai diagrams from the first BZF experiment by using the Arai signatures from the repeated BZF experiment, which neutralizes the typical MD concave‐up effect. Eleven samples meet the Arai diagram post‐selection criteria and provide qualified paleointensity estimates with a mean value for site GA‐X of 4.23 ± 1.29 µT, consistent with an excursional geomagnetic field direction reported for this site.