The Earth's Magnetic Interior

Exploration and exploitation of natural resources is one of the main concerns of an earth scientist to benefit humanity. Any country’s economic growth and development can be judged from the availability of natural resources. Electrical and electromagnetic methods are quite useful in the exploration of oil, mineral, water, etc., as they exhibit anomalous electrical conductivity compared to the surrounding media and thus can be identified with ease. In this chapter, the discussion is restricted to four themes – hydrocarbons, geothermal resources, water and mineral. Fair treatment is provided on the global scenario for each item and the use of the methodology with a case study is explained. Detection of hydrocarbons in a most difficult region – sub-basalt – is discussed by considering an example from Gujarat, India. Mapping of geothermal potential is discussed with an example from Korea. A case study of assessment of groundwater potential is provided considering the Parnaiba Basin area in Brazil. Application of 3D modelling is provided for McArthur Basin area, Canada, for exploration of uranium. In the treatment of all these problems, the chapter is mainly focused on the use of natural signals for deep electromagnetic method – magnetotellurics.

Recent advances in application of the artificial neural networks in EM induction studies are discussed. Special attention is paid to 3D reconstruction of the target macroparameters, initial resistivity model construction without prior information about 1D layering, inversion of inhomogeneous magnetotelluric (MT) data, compensation for lack of MT data by estimating the resistivity values using related proxy parameters, joint cluster analysis of the resistivity and other physical properties as well as their indirect estimation from surface EM data.

Current and fossil plate margins offer some of the most rewarding targets for geophysical studies. Particularly, the fluid/melt cycle in subduction zones continues to be of major interest for seismological as well as deep electromagnetic (EM), specifically magnetotelluric investigations. In this contribution we describe a number of experiments which have been conducted in several ocean-continent convergence zones around the world, with a focus on the Andes and Central America, respectively. Zones of potentially high electrical conductivity range from bending-related faulting near the outer rise, the subduction channel at the tip of the continental plate, the dehydration-hydration cycles in and above the downgoing plate, the assumed melting of the asthenospheric wedge to the rise of melts toward the volcanic arc and the magma chambers beneath the volcano edifices. Further targets include fault zones in the forearc, accommodating tensional stress, as well as hydrothermal and mineral deposits, to mention a few. The following chapters emphasize on a variety of structures along continental margins and show the potential of deep EM in this geodynamic setting.

Inherent ambiguity in the estimations of the subsurface structure made using single geophysical method is well recognized. Use of more than one method to map the same area greatly reduces the ambiguity. Recognizing this fact, there is an increasing trend for the application of multiple geophysical methods and integration of results to develop more confidence on the interpreted model. An earlier study based on integrated geophysical data along the Kuppam- Palani geo-transect in the Southern Granulite Terrain (SGT) revealed a four-layered crustal structure with a seismic low velocity, electrically conducting mid-crustal layer (LVCL). Reliable estimates of LVCL parameters are often difficult to achieve because of equivalence problem. We apply previously developed 1-D joint inversion algorithm to a segment of this transect to analyze the efficacy of the algorithm in reducing the ambiguity in the estimates of LVCL parameters. We take seismic refraction and reflection data corresponding to one shot point, and coincident magnetotelluric data at one location from the part of the profile where the crustal structure is interpreted as layered. Inversion with a large number of initial guesses and model appraisal indicates that LVCL parameters are better constrained with joint inversion than with individual inversions.

Earthquake (EQ) prediction, in particular short-term prediction, is one of the topmost challenges in modern science. However, the general view of the community is pessimistic. EQ prediction research has been rather heavily biased toward seismology for the last several decades. In addition to seismics, however, the importance of other methods is being recognized. We intend to evaluate the possible role of electromagnetic (EM) approach to this end by introducing examples of precursors needed for short-term prediction. Recent advances in the physics of critical phenomena to be applied to EQ generation mechanism and the possibility of EQ triggering effect of EM pulses will also be mentioned.

During the past 10 years, marine electromagnetics has developed from infancy into a sizable geophysical industry. While this is feasible in the time and the frequency domains, most of the commercial marine hydrocarbon applications operate in frequency domain, i.e. Controlled-Source Electromagnetic (fCSEM). Until 5 years ago, it was generally assumed that time domain (tCSEM^TM) methods would not be of any use in oil exploration. Since then, however, many such measurements have been recorded with several independent tCSEM^TM systems that already exist or under development. Time domain measurements can be used everywhere and more suitable for shallow water. They have large anomalous responses than those in fCSEM. From the physical viewpoint, time domain measurements are complementary to frequency domain measurements, as they focus on different spatial regions. With recent advances in electronics time-domain CSEM data can reliably be acquired in an offshore environment. Multiple surveys using autonomous receiver nodes have successfully acquired marine time domain CSEM data. Our work takes the technology a step further, by developing a high-density marine cabled system with novel 5-component sensor package.

It is widely believed that the main geomagnetic field is created by the dynamo action of motions in the Earth’s fluid core that are driven by thermal and compositional buoyancy. Early numerical simulations of the geodynamo that succeeded in generating strong, Earth-like dipole magnetic fields had to assume, for computational reasons, an unrealistically high viscosity for the core fluid. Some recent high-resolution models have used more realistic, smaller viscosities, but have unexpectedly produced only non-dipolar or dipolar but comparatively weak magnetic fields, which are less Earth-like. We recently advanced a possible explanation for this paradoxical behavior: we argued that these models had used the geophysically unrealistic outer boundary condition of uniform temperature on the core-mantle interface. In support of this opinion, we integrated two otherwise identical models, in one of which we applied the uniform temperature condition and in the other the more realistic condition of horizontally uniform heat flux. In the latter model, we obtained large-scale convective flows and a comparatively strong dipole-type magnetic field; for the former, we found solutions resembling those obtained by other models that had assumed uniform temperature on the core-mantle boundary. Further explanations for the very different character of the solutions are given here.

Using the most recent global database of paleomagnetic directions for the past 4 Myr we have tested whether the far-sided effect of Wilson (1970, 1971) remains a stable feature of the time-averaged field. We found out that this characteristic persists for all sub-time intervals as well as for different sites distributions. The U-shaped pattern of the mean inclination anomaly (deviation from the inclination of the axial dipole) as a function of latitude is described by a small quadrupole contribution that amounts 5% of the dipole. There is no need for other terms which in any case cannot be properly described given the overall dispersion of the data. We have analyzed the evolution of the quadrupole/dipole ratio for periods characterized by different mean axial dipole strength using composite curves of relative paleointensity. We report that periods of weaker dipole field are effectively characterized by a larger mean inclination anomaly and thus by a larger quadrupole/dipole ratio. We infer that the mean value of the inclination anomaly could potentially be an indirect indicator of the mean dipole strength.

Volcanic records of reversals are mostly exempt of complications linked to their magnetization process and thus potentially tell us the most significant story about the field variations prevailing during these periods. We have found no convincing indication supporting the presence of long-term non-zonal features governing the transitional field. A few VGP paths seem to be controlled by flux patches of the present non-axial dipole field lying immediately below the sites, but the detailed reversal records are characterized by scattered VGPs that are not related to anomalies of the present non axial dipole field. Assuming that clusters of VGPs over Australia would be associated with an hypothetical time persistence of the present anomaly in this area, then the geometry of the transitional field would have to be controlled by the equatorial dipole, since it is responsible for the present Australian patch. This is difficult to reconcile with our present knowledge of the variability of the equatorial dipole as well as with the structure of most detailed VGP paths. In fact, the existence of complex directional changes with rebounds and precursors in the detailed volcanic records reflect the persistence and the amplification of secular variation following the collapse of the axial dipole. We have now learned much more about the evolution of the axial dipole from studies of relative paleointensity in sediments but also from the records of absolute paleointensity that have been obtained for a few volcanic records. The data converge to indicate asymmetrical pre- and post-reversal phases but also a systematic overshoot marking the end of the recovery phase. These features can be explained by a dynamical model assuming a coupling of the Earth’s dipole with the quadrupolar mode during reversals.

Coring at Site 1256 (6.736°N, 91.934°W, 3635 m water depth) during Ocean Drilling Program (ODP) Leg 206 and Integrated Ocean Drilling Program (IODP) Expeditions 309 and 312 successfully sampled a complete section of in situ oceanic crust, including sediments of Seismic Layer 1, lavas and dikes of Layer 2, and the uppermost gabbros of Layer 3. The crust at this site was generated by superfast seafloor spreading (>200 mm/yr full spreading rate) along the East Pacific Rise some 15 Ma ago. One goal of drilling a complete oceanic crust section is to determine the source of marine magnetic anomalies. For crust generated by fast seafloor spreading, is the signal dominated by the upper extrusive layer, do the sheeted dikes play any role, how significant is the magnetic signal from gabbros relative to that at slow spreading centers and what is the timing of acquisition of the magnetization? To address these questions, we have made a comprehensive set of rock magnetic and paleomagnetic measurements that extend through the igneous interval. Continuous downhole variations in magnetic grain size, coercivity, mass-normalized susceptibility, Curie temperatures, and composition have been mapped. Compositionally, we have found that the iron oxides vary from being titanium-rich titanomagnetite (TM60), which are commonly partially oxidized to titanomaghemites, to titanium-poor magnetite as determined semi-quantitatively from Curie temperature analyses and microscopy studies. Skeletal titanomagnetite with varying degrees of alteration is the most common magnetic mineral throughout the section and is often bordered by large iron sulfide grains. The low-Ti magnetite or stoichiometric magnetite is present mainly in the dikes and gabbros and is associated with higher Curie temperatures (550°C to near 580°C) and higher coercivities than in the extrusive section. Magnetic grain sizes predominantly fall in the pseudo single domain (PSD) grain size region on Day diagrams, with only a small numbers of samples falling within the single domain (SD) or multi-domain (MD) regions. Overall the magnetic properties of this hole are strongly influenced by post-emplacement alteration, particularly the lower part of the section from the gabbros up into the transition zone. Some of the more prominent features of the rock magnetic data are the gradual increase in Curie temperatures with depth from about 200–350°C at the top of the extrusives to about 425°C just above the transition zone, the more variable Curie temperatures and less variable susceptibility and coercivity of remanence in the upper half of the extrusives relative to the lower half the near constant composition (x = 0.6) and oxidation (z = 0.6) of the iron oxide grains (>5 μm) in the extrusives (Chapter 12 this volume), the highly irreversible nature of thermomagnetic curves in the extrusives, in which the cooling curve has Curie temperatures higher (generally >500°C) than indicated by the heating curve, the abrupt change in rock magnetic properties across the transition zone, particularly the Curie temperature., a somewhat finer grain size and increased intensity in the sheeted dike zone relative to the extrusives and gabbros, and the nearly constant Curie temperatures (530 and 585°C) for the dikes and gabbros.

Oceanic crust is the carrier of the marine magnetic anomalies and is therefore a valuable archive of geomagnetic information. ODP/IODP Hole 1256D was the first to sample an entire sequence of oceanic crust down to the gabbro. We studied the vertical variation of magnetic remanence carriers by means of scanning electron microscopy, microanalysis and rock magnetic measurements. The extrusive layer contains dendritic, low-temperature oxidized titanomagnetites (TMs), i.e. titanomaghemite, with initial compositions close to values previously reported for mid-ocean ridge basalts (MORB). The degree of low-temperature oxidation (maghemitisation) remains fairly constant across the extrusives. We explain the observed increase in Curie temperature with depth by submicron inversion of titanomaghemite to intergrowths of titanomagnetite and nonmagnetic phases, where the Ti-content of titanomagnetite is decreasing with depth. In the underlying sheeted dikes, TMs are again the primary magnetic mineral. Due to slower cooling, they are in most cases oxy-exsolved into lamellar intergrowths of Ti-poor TMs and ilmenite. The magnetominerals are altered to a much higher degree than in the extrusives. In the gabbroic part of the section, TMs reach sizes up to several mm, although the magnetic grain size remains consistently in the pseudo-single-domain range because of grain subdivision by exsolution lamellae. The extrusives carry a thermoremanent magnetisation (TRM), retaining the primary paleomagnetic direction but with a reduced remanence intensity. The sheeted dikes hold a thermo-chemical remanent magnetization (TCRM) or secondary TRM acquired during hydrothermal alteration, whereas the underlying gabbro acquired a TCRM significantly after emplacement due to slow cooling at this depth.

We have investigated the magnetic mineralogy and absolute paleointensity of basalt samples from Site 1256 cored during ODP Leg 206 and IODP Expeditions 309 and 312. The site is located on the Cocos Plate 5 km east of the transition zone between marine magnetic anomalies 5Bn.2n and 5Br (~15 Ma). The deepest hole, Hole 1256D, extends 250 m through sediments and 1257 m into the igneous upper oceanic crust generated by superfast seafloor spreading (>200 mm/yr) along the East Pacific Rise. This is the first drill site to penetrate an in situ and intact section of crust. The section consists of about 811 m of basaltic sheet flows and massive lavas, 346 m of sheeted-dike complex, and 100 m of gabbros and granoblastic dikes. Rock magnetic investigations included thermomagnetic analyses, alternating field, thermal demagnetization, saturation IRM, magnetic grain-size and coercivity analyses. Curie points identified titanomagnetites and titanomaghemites as the magnetic carriers and grain-size studies indicate that the carriers are mixtures of single domain (SD) and pseudosingle domain (PSD) grains. Using the Thellier-Coe method, we have attempted paleointensity determinations for 82 specimens sampled from different “stratigraphic” levels of the core. Partial thermal remanent magnetization (pTRM) checks were performed systematically one temperature step down from the last pTRM acquisition in order to document magnetomineralogical changes. The determinations were obtained from the slope of the pTRM gained vs. natural remanent magnetization lost in the Arai diagrams. Only about 6% of the samples (i.e. 5 samples) yielded marginally acceptable results. The paleofield estimated ranges from 16 to 28 μT and has a mean virtual axial dipole moment (VADM) of 5 × 10²² A/m², which is concordant with the average intensity for the period between 0 and 160 Myr (4 ± 2 × 10²² A/m²) and is about 2/3 of the strength of the present field (~8 × 10²² A/m²).

The Kilauea 1955 and 1960 lava flows (Big Island of Hawaii, USA), both emplaced in a field of ∼36 μT, were studied using the multi-specimen parallel differential partial thermoremanent magnetization (pTRM) paleointensity (PI) method. In nineteen specimens from an upper cooling unit of the 1955 flow, the pTRMs were acquired at a temperature of 450°C and a PI of 34.3 ± 1.5/1.6 μT was obtained, while 13 specimens of the lower cooling unit heated to 230°C resulted in a PI of 38.5 ± 3.3 μT. The 1960 flow was studied at various temperatures of 400, 440, 480, 500 and 550°C. At 400 and 550°C overestimates of the PI were obtained: 47.3 μT and 41.5 μT respectively. The other temperatures yielded PI values ranging from 32.2 to 34.2 μT, just 6–11% lower than the expected field and similar to the 1955 flow results. The 550°C PI result is biased because of mineral alteration as indicated by an 18% susceptibility decrease which reduces the pTRM capacity leading to an overestimate of the PI. The overestimate of the PI at 400°C may be due to the comparatively small pTRM acquired and thus to larger high-temperature pTRM tails. Results obtained with the multi-specimen method for both flows compare favorably with the best other PIs obtained by the Thellier–Coe, Thellier–Thellier and microwave methods. The success rate is high with more than 90% of specimens contributing to a PI. Only the Thellier–Coe method used on single plagioclase crystals and the microwave method have similar success rates.

During the last two decades, important advances in archaeomagnetic research in Italy have been made, both in the acquisition of new data and in the improvement of methodologies and data elaboration techniques. Nowadays, 73 directional and 23 intensity results are available, mainly obtained from archaeological sites situated in southern Italy. Most of the data come from the study of ancient kilns and ceramics and their ages range from 1300 BC to 1600 AD. The quantity and quality of the available Italian directional data have permitted the construction of reference secular variation (SV) curves for declination and inclination, using the latest improvements on curve building techniques. These curves describe, with reasonable accuracy, the variations of the Earth’s magnetic field in Italy for the 600 BC to 600 AD period, for which many data are available. For older BC periods and for the Medieval times data are still very scarce. Archaeomagnetic dating of in situ archaeological materials is now possible but still caution is needed for the time periods where the reference SV curves are accompanied by large error envelopes. Certainly more new, high quality directional and intensity data of well dated archaeological material are necessary to better describe the variations of the Earth’s magnetic field during the past and to make reliable archaeomagnetic dating possible, based on the full description of the Earth’s magnetic field vector.

We present a detailed investigation of the geomagnetic polarity transition that terminated the Olduvai subchron as recorded by loess/paleosol sediments at Lingtai in the central Chinese Loess Plateau (CLP). The polarity transition occurs within loess layer L25, where mineral magnetic parameters show considerable variations and sedimentation rate changes occur. The magnetic record obtained after thermal cleaning exhibits more than twenty apparent polarity flips, most of which occur within a stratigraphic distance corresponding to no more than ∼15,000 years. We argue that these results do not represent the actual behavior of the geomagnetic field. Instead, we propose that the combined effect of detrital and pedogenic remanences—which almost always co-exist on the central CLP—are responsible. These cause significant, lithologically-controlled, delays in the acquisition of the total remanence. In effect, the sediments act as a filter that generates noisy magnetic output from possibly simple input. We conclude that loess/paleosol sediments from the central CLP are poor candidates for tracking short-term geomagnetic field behavior such as polarity transitions, geomagnetic excursions and paleosecular variation.

In Brazil, there are many Precambrian and Phanerozoic mafic dike swarms of variable length, chemistry and structural trend. Phanerozoic dike swarms are more abundant and widely distributed than those of Precambrian ages. The longest dikes and the densest swarms are Mesozoic, e.g. the Ponta Grossa swarm in Paraná State. It is well accepted that magnetic anisotropies yield the most efficient methods to determine petrofabric orientation in rocks, particularly where standard petrofabric techniques are inadequate or inefficient, and even in rocks that are visually isotropic. Magnetic fabrics, also called magnetic anisotropies, can be determined using either low-field anisotropy of magnetic susceptibility (AMS), which is the most popular method, or anisotropy of magnetic remanence (AMR). These are powerful tools for structural geology and have been applied in many geological situations. Magnetic fabrics were determined in many dike swarms of Precambriam and Mesozoic ages together with extensive rock magnetic studies. The main magnetic fabric for these swarms is related to magma flow, and the relative position between magma sources and emplacement fractures could be inferred. In some swarms AMS and AMR tensors are coaxial, whereas in others the AMS fabric is primary in origin but the AMR fabric is tectonic and acquired after dike emplacement.

Significant differences between granites and lava flows can require different basic assumptions when interpreting AMS results. Among the differences between both types of rocks, perhaps the earliest in being recognized was the wider range of mineral compositions found in granites. Such difference can result in complex mineral assemblages that, in turn, can complicate the interpretation of AMS results in granites relative to the AMS measured in lava flows. Closely linked to this mineralogic effect is the distinction between “primary” flow fabrics and “secondary” effects. Such distinction is a matter of concern in most granites whereas the AMS of lava flows is usually considered “primary” without further examination. As the increasing evidence obtained from lava flows shows, however, the AMS of lavas is not as simple as the general model of hydrodynamic alignment of particles would suggest and much can be learned from lava flows that can be applied directly to the interpretation of AMS in granites. In this work, the better understanding of the fabric of lava flows that has been obtained in recent years is used as the basis for a reassessment of the basic assumptions needed for the correct interpretation of AMS in both lava flows and granitic rocks.

Theory of the Anisotropy of Magnetic Susceptibility (AMS) assumes field-independent rock susceptibility in the low fields used by common AMS meters. This is valid for rocks whose AMS is carried by diamagnetic and paramagnetic minerals and also by pure magnetite, while rocks with pyrrhotite, hematite or titanomagnetite may show significant variation of susceptibility in common measuring fields. Consequently, the use of the contemporary AMS theory is in principle incorrect in these cases. Fortunately, it has been shown by practical measurements and mathematical modelling of the measuring process that the variations of the principal directions and of the AMS ellipsoid shape with field are very weak, which is important in most geological applications. The degree of AMS, however, may show conspicuous variation with field and, if one wants to make precise quantitative fabric interpretation, it is desirable to work with the AMS of the field-independent component. Three methods exist for simultaneous determination of the field-independent and field-dependent AMS components, all based on standard AMS measurement in variable fields within the Rayleigh Law range. The field-dependence of the AMS can be used in solving some geological problems. For example, in volcanic and dyke rocks with inverse magnetic fabric, one can decide whether this inversion has geological (special flow regime of lava) or physical (SD vs. MD grains) causes. In rocks consisting of two magnetic fractions, one with field-independent susceptibility (magnetite, paramagnetic minerals) and the other possessing the field-dependent susceptibility (titanomagnetite, hematite, pyrrhotite), one can separate the AMS of the latter fraction and in favourable cases also of the former fraction.

A new MFK1-FA Kappabridge is introduced that precisely measures the magnetic susceptibility of rocks and the anisotropy of susceptibility. The instrument has the following features: separation of the in-phase (real) and out-of-phase (imaginary) components, auto-ranging and auto-zeroing, automated measurement of the field variation of the bulk susceptibility, AMS measurements using the spinning specimen method, built-in circuitry for controlling the non-magnetic furnace (CS-4) and cryostat (CS-L), full instrument control by an external computer, sophisticated hardware and software diagnostics. Examples are shown to illustrate the capability of the instrument for rock magnetic and palaeomagnetic research.

Rema6W is a Microsoft Windows software package that provides full control of the AGICO JR-6 series dual speed spinner magnetometers (models JR-6 and JR-6A) together with instant data visualization. The main features include measurements in two speeds of rotation and three different acquisition times using automatic, semi-automatic, or manual specimen holders. In addition to a fully graphical user interface, the main functions of the program can be controlled using simple keyboard shortcuts. Acquired data are stored in an ASCII text data file allowing for easy viewing, editing, and further processing using Remasoft, PMGSC Paleomagnetism Data Analysis, or SuperIAPD programs. Acquired data are automatically sorted according to the specimen names and/or magnetic states, thus enabling an instant control on paleomagnetic demagnetization process. Plots for each specimen or magnetic state can be directly printed, copied into the clipboard, or exported as a Windows metafile graphics.

The crystal symmetry of hematite in the basal plane predicts three easy magnetization axes for the sublattice spin orientation above the Morin transition. Spin canting then leads to three preferred magnetization axes perpendicular to these easy axes. By combining detailed crystallographic orientation by EBSD measurements with dense magnetic hysteresis and remanence curves as a function of rotation angle, the relation between crystallography and magnetic properties has been experimentally verified for the basal plane of a natural hematite crystal. The measurements lead to a better understanding of the interplay between spin canting, remanence and magnetic susceptibility at different field strengths. The measurement results coincide qualitatively with theoretical predictions, and provide experimental evidence for quantitative evaluation by more complex micromagnetic modeling.

Magnetic anomalies provide information about location, size and composition of earth structures, ore bodies and tectonic features even in bodies containing only a few percent magnetic minerals. Here we investigate the magnetic properties and oxide mineralogy of anorthosites, rocks rich in plagioclase (>90%), and compare their magnetic signatures to aeromagnetic anomaly maps of the regions. Two of the anorthosite complexes have large negative anomalies associated with them; both have low susceptibility and high remanence related to hemo-ilmenite mineralogy and remanent directions antiparallel to the present field. One complex has appreciable natural remanent magnetization quasi-parallel to the present field, and strong susceptibility, creating an enhanced positive anomaly. The fourth anorthosite has little or no magnetic anomaly over much of its area, in accordance with the weak remanence, low susceptibility and variable magnetic mineralogy observed. The anorthosite samples producing significant anomalies, and maintaining strong and stable natural remanent magnetization over geologic time all contain oxides of the hematite-ilmenite series. This study adds support to ‘lamellar magnetization’ whereby exsolved phases in the ilmenite-hematite system produce strong and stable magnetization with only minor amounts of oxide material.

Dating cave sediments by the application of the palaeomagnetic method – magnetostratigraphy – is a difficult and sometimes risky task, as the method is comparative in its principles and does not provide numerical ages. For dating clastic cave sediments and speleothems it is limited by the complex conditions occurring underground so that it is often necessary to combine it with other methods that offer supplementary absolute-, calibrate-, relative- or correlate-ages. Interpretation of magnetostratigraphic results faces other serious problems that may endanger palaeomagnetic studies in given caves if they are not detected. The sedimentary fills of a number of profiles are separated into individual sequences and cycles, divided by breaks in deposition (unconformities). The dynamic character of cave fill deposition is reflected in the start or termination of individual magnetozones at unconformities in a number of profiles. The general character of cave depositional environments with their numbers of post-depositional changes, hiatuses, reworking and re-deposition does not allow precise calculation of the temporal duration of individual interpreted magnetozones. All these factors contribute to the fact that exact calibration of the geometric characteristics of the magnetostratigraphic logs with the GPTS cannot be attained at all or only with problems, if it is not adjusted using results of other dating methods.

We present a quantitative model for the climatic dependence of magnetic enhancement in loessic soils. The model is based on the widely accepted hypothesis that ultrafine magnetite precipitates during alternating wetting and drying cycles in the soil micropores. The rate at which this occurs depends on the frequency of drying/wetting cycles, and on the average moisture of the soil. Both parameters are estimated using a statistical model for the soil water balance that depends on frequency and intensity of rainfall events and on water loss by evapotranspiration. Monthly climatic tables are used to calculate the average soil moisture and the rate of pedogenic magnetite production, which is proportional to a new parameter called magnetite enhancement proxy (MEP). Our model is tested by comparing MEP calculated for known present-day climates with the magnetic enhancement of modern soils. The magnetic enhancement factor, defined as the ratio between a given magnetic enhancement parameter and MEP, is expected to be a site-independent constant. We show that magnetic enhancement differences between soils from the Chinese Loess Plateau and from Midwestern U.S. are explained by our model, which yields similar magnetic enhancement factors for the two regions. Our model is also successful in predicting the mean annual rainfall threshold above which magnetic enhancement declines in a given type of climate.

The role of hematite and goethite as palaeoenvironmental indicators in loess-palaeosol sediments is studied by diffuse reflectance spectroscopy (DRS) and rock magnetic methods. Forty five selected samples from four loess-palaeosol profiles in Bulgaria were used to deduce the behaviour of the hematite/goethite ratio in loess and palaeosol units, which were formed under contrasting palaeoclimate conditions – cold (glacial) and warm (interglacial). According to DRS, the pedogenesis is accompanied by preferential formation of hematite over goethite in all sites. At the same time, rock magnetic data prove that this process is concomitant with in-situ formation of a strongly magnetic ferrimagnetic fraction, responsible for the observed magnetic enhancement of palaeosol units. Systematically higher amount of hematite in the loess-palaeosol profile Orsoja, situated at the fifth Danube river terrace and the Durankulak profile at the Black sea coast is supposed to be due to additional coarse-grained hematite in the aeolian dust blown from local dust sources during glacial periods. Component analysis of the curves of stepwise acquisition of isothermal remanence reveals the presence of two components – pedogenic (P) with median coercivity at half-width of the Gaussian function of 31 ± 3.2 mT; detrital unweathered component D₁ in loesses from Lubenovo and Koriten profiles with B_1/2 of 58 ± 4 mT, and weathred detrital component D₂ (B_1/2 of 123 ± 46 mT) in all palaeosols and weathered loess units from Orsoja and Durankulak profiles. Mineral phases, carrying these coercivity components are supposed to be pedogenic maghemite (P), aeolian (titano)magnetites (D₁) and hematite (D₂).

Soil magnetometry has proved to be a helpful auxiliary method for outlining potentially contaminated areas. Magnetic susceptibility of topsoils may reflect concentration of atmospherically deposited anthropogenic iron oxides, which often coexist with harmful substances, such as heavy metals. Magnetic mapping of topsoils yields unambiguous results in areas with high concentration of pollutants and soils developed on iron-poor geologic basement. In this chapter we review the approach and results of magnetic mapping in a relatively clean area, characterized by rather complex topography. Surface measurements of magnetic susceptibility were carried out after previous examination of its vertical distribution in topsoils and complex laboratory measurements of other magnetic parameters as well as scanning electron microscopy observations. Our results show that over the whole area in concern topsoil magnetic susceptibility is enhanced due to atmospherically deposited anthropogenic iron oxides (prevailing from local sources), and soil magnetometry can thus be used for delineation of anthropogenic effect on soils also in this rather complex and difficult area.

Magnetic measurements of soil and tree bark adjacent to a busy highway revealed a significant variation in the concentration of magnetic particles with distance from the highway. Furthermore, forest-facing tree-bark contains significantly more magnetic particles than road-facing tree-bark. Magnetic particles were detected both on the bark of the maple trees and in the first centimeter of the soil cover (O/A horizon). Stability of the Saturation Isothermal Magnetization (SIRM) and the hysteresis parameters of the soil indicates the presence of Single-Domain/Pseudo-Single-Domain (SD/PSD) magnetic carriers. Measurements of the tree bark hysteresis parameters and SIRM detect a significantly lower coercivity component that we interpret to be an indication of more abundant PSD-type magnetic grains. Magnetic measurements around the perimeters of eight tree trunks reveal magnetic carriers whose distribution is antipodal to the source direction (highway). We interpret our observation by adopting an air circulation model, where suspended PSD/SD particles are carried in the air stream. The air stream from the heavy traffic lowers the amount of moisture on the tree trunk surfaces facing the highway and thus reduces an adhesive potential on this side. Therefore, more particles can stay on the moist side of the trunk protected from the direct airflow. A magnetic signature of tree rings was tested as a potential paleo-climatic indicator. We have examined wood from sequoia tree, located in Mountain Home State Forest, California, whose tree ring record spans over the period 600–1700 A.D. We have measured low and high-field magnetic susceptibility, the Natural Remanent Magnetization (NRM), Saturation Isothermal Remanent Magnetization (SIRM), and stability against thermal and Alternating Field (AF) demagnetization. Magnetic investigation of the 200 mm long sequoia material suggests that the magnetic efficiency of natural remanence (=natural remanent magnetization normalized by saturation remanence) may be a sensitive paleoclimate indicator because it is substantially higher (in average > 0.01=1%) during the Medieval Warm Epoch (700–1300 A.D.) than during the Little Ice Age (1300–1850 A.D.) where it is < 0.01=1%. Diamagnetic behavior has been noted to be prevalent in regions with higher tree ring density. The mineralogical nature of the remanence carrier was not directly detected but maghemite is suggested due to low coercivity and absence of Verwey transition. Tree ring density, along with the wood’s magnetic remanence efficiency, records the Little Ice Age (LIA), which is well documented in Europe and elsewhere. Magnetic analysis of the thermal stability reveals the blocking temperatures near 200°C. This phenomenon suggests that the remanent component in this tree may be thermal in origin and was controlled by local thermal conditions.