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More than 650 million tons/year of fluvial sediment are delivered from continental regions into the South China Sea (SCS). Previous studies have shown that the composition of the magnetic fraction of riverine sediments drained into the SCS is significantly variable from north to south. On the basis of this evidence, we now examine a full set of magnetic properties for a number of core‐tops taken at water depth comprised mostly between 800 and 3500 m. Room‐temperature magnetic parameters and thermal spectra are used to obtain information about the concentration and mineralogical magnetic composition. Spatial changes are observed in the relative proportion of magnetite and hematite with an increase of the latter toward the south, similarly to the observation on land. However, the N‐S contrast is much weaker in marine core tops than in river sediments, because of the role played by the shelf in partly trapping river‐borne sediments, in particular in the southern SCS. In part, sediments also reach the continental slope and the deep basins, being transported and mixed by surface and deep‐water currents, which yield the magnetite‐hematite mixing in the south. For the first time, we characterize a wide spectrum of magnetic properties of modern marine sediment in the South China Sea. Our results give important insights into the modern pathways of sediment particles, depicting the source‐to‐sink processes that affect the terrigenous sediment load.

Magnetotactic bacteria (MTB) synthesize ferrimagnetic crystals that contribute to the remanent magnetization in sediments, yet knowledge of how MTB populations vary in natural environments over time remains limited. We report abundances of three MTB morphotypes from nine sites collected and measured every month over a two-year period from a natural pond near Munich, Germany. Morphotype populations underwent coherent temporal trends among the nine sitesespecially at proximal sites with similar water depths. MTB populations varied independently of bottom water oxygen concentrations or temperature over the two-year period, except for spirilla, which flourished during the summer at some sites. Magnetic properties of the sediments did not reflect living MTB abundances, but instead varied with water depth. Deeper sites, which were also lower in organic carbon, nitrogen, and oxygen concentrations than shallower sites, had higher saturation magnetizations and were richer in single-domain particles.

Reconstructions of the past geomagnetic field provide fundamental constraints for understanding the dynamics of the Earth's interior, as well as serving as basis for magnetostratigraphic and archeomagnetic dating tools. Such reconstructions, when extending over epochs that precede the advent of instrumental measurements, rely exclusively on magnetic records from archeological artifacts, and, farther in the past, from rocks and sediments. The most critical component of such indirect records is field intensity because of possible biases introduced by material properties and by laboratory protocols, which do not reproduce exactly the original field recording conditions. Large biases are usually avoided by the use of appropriate checking procedures; however, smaller ones can remain undetected in individual studies and might significantly affect field reconstructions. We introduce a new general approach for analyzing geomagnetic databases in order to investigate the reliability of indirect records. This approach is based on the comparison of historical records with archeomagnetic and volcanic data, considering temporal and spatial mismatches with adequate weighting functions and error estimation. A good overall agreement is found between indirect records and historical measurements, while for several subsets systematic bias is detected (e.g, inclination shallowing of lava records). We also demonstrate that simple approaches to analyzing highly inhomogeneous and internally correlated paleomagnetic datasets can lead to incorrect conclusions about the efficiency of quality checks and corrections. Consistent criteria for selecting and weighting data are presented in this review and can be used to improve current geomagnetic field modeling techniques.

Hypervelocity impacts within the solar system affect both the magnetic remanence and bulk magnetic properties of planetary materials. Spherical shock experiments are a novel way to simulate shock events that enable materials to reach high shock pressures with a variable pressure profile across a single sample (ranging between ∼10 and >160 GPa). Here we present spherical shock experiments on basaltic lava flow and diabase dike samples from the Osler Volcanic Group whose ferromagnetic mineralogy is dominated by pseudo-single-domain (titano)magnetite. Our experiments reveal shock-induced changes in rock magnetic properties including a significant increase in remanent coercivity. Electron and magnetic force microscopy support the interpretation that this coercivity increase is the result of grain fracturing and associated domain wall pinning in multidomain grains. We introduce a method to discriminate between mechanical and thermal effects of shock on magnetic properties. Our approach involves conducting vacuum-heating experiments on untreated specimens and comparing the hysteresis properties of heated and shocked specimens. First order reversal curve (FORC) experiments on untreated, heated and shocked specimens demonstrate that shock and heating effects are fundamentally different for these samples: shock has a magnetic hardening effect that does not alter the intrinsic shape of FORC distributions, while heating alters the magnetic mineralogy as evident from significant changes in the shape of FORC contours. These experiments contextualize paleomagnetic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters. This article is protected by copyright. All rights reserved.

It is widely accepted that cooling rate can strongly influence the intensity of the thermal remanent magnetization (TRM) acquired by rocks during cooling to ambient temperatures. If ignored, this effect might lead to underestimates or overestimates of the ancient magnetic field intensity. To date, however, the cooling rate dependence of TRM acquired by particles with different domain states has never been systematically analyzed from the theoretical or experimental point of view. In this study, we present measurements of the TRM of synthetic magnetites with well-defined grain sizes that were quenched with constant cooling rates of 0.05, 0.1, 1, 3, 10, and 15 K/min. While single domain (SD) and small pseudo-single domain (PSD) samples are found to show larger TRMs after slow cooling, the TRMs of larger PSD and multidomain (MD) magnetites are not affected by an increase or decrease of the cooling rate. Overall, our results suggest that only smallest magnetite grains acquire a cooling rate-dependent TRM. Therefore, cooling rate corrections of paleointensity determinations are only necessary for samples dominated by SD remanence carriers, while rocks dominated by PSD and MD carriers, such as basalts, which are most commonly used for paleointensity studies, do not require such corrections.

[1] The widespread occurrence of magnetotactic bacteria (MTB) in several types of marine and freshwater sediment, and the role of fossil magnetosomes (magnetofossils) as main remanent magnetization carriers therein, has important paleomagnetic and paleoenvironmental implications. Despite numerous studies on MTB biology and on magnetofossil preservation in geological records, no detailed information is yet available on how magnetotaxis (i.e., the ability to navigate along magnetic field lines) is performed in sedimentary environments, and on how magnetofossils possibly record the Earth magnetic field. We provide for the first time experimental evidence for these processes. MTB living in sediment are poorly aligned with the geomagnetic field, contrary to what is observed in water. This can explain the seemingly excessive magnetic moment of most MTB. The observed alignment is sufficient for supporting magnetotaxis across the typical thickness of chemical gradients. Experiments with magnetofossil-rich sediment suggest that a natural remanent magnetization (NRM) is acquired by magnetofossils in the so-called benthic mixed layer, where natural MTB populations usually occur. The acquired NRM is proportional to the applied field at least up to ∼160 µT, and its intensity is compatible with values observed in nature for same sediment types. Therefore, if fossil magnetosome chains are not subjected to further alteration by early diagenetic processes, they can provide useful relative paleointensities. We propose a preliminary model to explain early stages of magnetofossil NRM acquisition as the result of a dynamic equilibrium between magnetic torques and randomizing forces due to sediment mixing.

Most rock magnetometers measure the bulk remanent magnetization of 1-inch cylindrical samples (centimeter scale) or even larger volumes of drill cores. High- Tc superconducting quantum interference device (SQUID) magnetometers are able to measure magnetic fields on rock surfaces at higher resolution on a millimeter scale: the fields of discrete rock-forming magnetic mineral grains or fine magnetic rock textures and structures can be observed when scanning across selected surface areas. The stability problems of a commercially available high- Tc SQUID magnetometer have largely been solved by improving the magnetic shielding and reducing the noise due to turbulent boiling of liquid nitrogen. Magnetizations as weak as 5×10−4 A/m can now be discriminated with a resolution of 1 mm. A software package has been developed to eliminate measurement errors arising from instrumental drift. The program also calculates the downwards continuation of the field data and offers full inversion for the vertical magnetization component. A synthetic sample demonstrates the performance of the SQUID sensor and the inversion software. The potential of high-resolution magnetic imaging is shown by measuring three rock samples with very different magnetic properties.

[1] Meteorites and diamonds encounter high pressures during their formation or subsequent evolution. These materials commonly contain magnetic inclusions of pyrrhotite. Because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report room temperature magnetic measurements on multidomain and single-domain pyrrhotite under nonhydrostatic pressure. Magnetic remanence in single-domain pyrrhotite is largely insensitive to pressure until 2 GPa, whereas the remanence of multidomain pyrrhotite increases 50% over that of initial conditions by 2 GPa, and then decreases until only 33% of the original remanence remains by 4.5 GPa. In contrast, magnetic coercivity increases with increasing pressure to 4.5 GPa. Below ∼1.5 GPa, multidomain pyrrhotite obeys Neel theory with a positive correlation between coercivity and remanence; above ∼1.5 GPa, it behaves single domain–like yet distinctly different from uncompressed single-domain pyrrhotite. The ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure, which can potentially be used as a geobarometer. Owing to the greater thermal expansion of pyrrhotite with respect to diamond, pyrrhotite inclusions in diamonds experience a confining pressure at Earth's surface. Applying our experimentally derived magnetic geobarometer to pyrrhotite-bearing diamonds from Botswana and the Central African Republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1.3 to 2.1 GPa. These overpressures constrain the mantle source pressures from 5.4 to 9.5 GPa, depending on which bulk modulus and thermal expansion coefficients of the two phases are used.

We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the Hc axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

[1] Variations in magnetic properties of buried soils can be used to reconstruct past climatic conditions during paleosol formation. Most methods, however, are based on comparisons between the magnetically enriched upper soil horizons and the magnetically unaltered parent material. In thin loess-paleosol sequences such a comparison can be problematic because all horizons, soil and underlying loess, may be affected to varying degrees by pedogenesis. We propose two direct estimates of pedogenic magnetite based on the analysis of anhysteretic remanent magnetization ratios (cARM/isothermal remanent magnetization) and coercivity distributions. These estimates are independent of any information regarding the parent material and are possible if pedogenic minerals have similar magnetic properties throughout the study region. This condition seems to be met throughout the Midwestern United States and a few loessic soils elsewhere. The remanence-carrying part of pedogenic magnetite is composed of single-domain particles with consistent, well-constrained magnetic properties. These particles are extremely well dispersed in the soil matrix as indicated by the absence of noticeable magnetostatic interaction effects. Our analyses of over 70 modern loessic soil profiles demonstrate that the abundance of pedogenic magnetite correlates well with modern climate and that the method is suited for reconstruction of past climates from paleosols.

[1] First-order reversal curve (FORC) diagrams are rapidly becoming a standard tool for characterizing magnetic particles because they simultaneously incorporate information regarding magnetostatic interaction and domain states. The simplest interpretation of FORC diagrams of single-domain (SD) particles is based on the Neel interpretation of Preisach theory, which predicts that the FORC function is the product of a coercivity and an interaction field distribution. Although the underlying assumptions of this interpretation are not correct, a strictly quantitative model of weakly interacting SD grains proves that the distributions of coercivities and interaction fields can be retrieved from a FORC diagram. To test this model, we present the possibility of a quantitative interpretation of FORC diagrams, and we present measurements of samples containing magnetosomes from cultures of magnetotactic bacteria and from a lake sediment. Two samples are investigated under the electron microscope to characterize the geometrical arrangement of the particles. We find that the clustering of otherwise similar particles has a strong influence on FORC diagrams. We also obtained a crude estimate of packing densities form the FORC diagrams, which were consistent with transmission electron microscopy observations and measurements of the anhysteretic remanent magnetization.

[1] Environmental problems linked to the concentration of atmospheric particulate matter with dimensions less than 10 μm (PM10) in urban settings have stimulated a variety of scientific researches. This study reports a systematic analysis of the magnetic properties of PM10 samples collected by six automatic stations installed for air quality monitoring through the Latium Region (Italy). We measured the low-field magnetic susceptibility of daily air filters collected during the period July 2004 to July 2005. For each station, we derived an empirical linear correlation linking magnetic susceptibility to the concentration of PM10 produced by local sources (i.e., in absence of significant inputs of exogenous dust). An experimental approach is suggested for estimating the percentage of nonmagnetic PM10 transported from natural far-sided sources (i.e., dust from North Africa and marine aerosols). Moreover, we carried out a variety of additional magnetic measurements to investigate the magnetic mineralogy of selected air filters spanning representative periods. The results indicate that the magnetic fraction of PM10 is composed by a mixture of low-coercivity, magnetite-like, ferrimagnetic particles with a wide spectrum of grain sizes, related to a variety of natural and anthropogenic sources. The natural component of PM10 has a characteristic magnetic signature that is indistinguishable from that of eolian dust. The anthropogenic PM10 fraction is mostly originated from circulating vehicles and is a mixture of prevailing fine superparamagnetic particles and subordinate large multidomain grains; the former are more directly related to exhaust, whereas the latter may be associated to abrasion of metallic parts.

[1] The anhysteretic remanent magnetization (ARM) is widely used in rock magnetism and paleomagnetism because of its sensitivity to the domain state of magnetic particles and the close analogy to natural remanent magnetizations. On the other hand, the ARM shares with other weak-field magnetizations the property of being extremely sensitive to magnetostatic interactions. Therefore it is desirable to model the effects of interactions on natural assemblages of magnetic particles. Direct micromagnetic calculations of the ARM acquisition process are not practicable; therefore an analytical approach is used calculate the ARM susceptibility of a system of interacting single-domain (SD) particles. The model is based on a statistical description of the interaction field. The equations obtained have been used to evaluate the dependence of the ARM on the packing fraction of the magnetic particles. The effect of interactions on the anisotropy of ARM (AARM) was evaluated as well. The AARM of densely packed particles is complex and depends critically on the microcoercivity. A physical interpretation of the AARM of highly interacting particles is therefore difficult.

[1] In 1965, D. J. Dunlop showed that the joint distribution of particle volumes and microcoercivities f(V, Hk0) can be determined for magnetically monomineralic, thermally stable single-domain (SSD) ensembles by taking advantage of the joint temperature and field dependence of relaxation time. We have developed a procedure that follows Dunlop's strategy to obtain f(V, Hk0) for ensembles containing both superparamagnetic and SSD grains, based on backfield remanence curves measured over a range of temperatures. Each point on the derivative curves represents the integrated contribution from grains that lie along a corresponding blocking contour on the Neel plot. A suitable set of such line integral samples can be used to reconstruct the f(V, Hk0) distribution using the methods of tomographic imaging. Samples of the basal Tiva Canyon Tuff have narrow size distributions of elongate Ti-poor titanomagnetite. Tomographic inversion of the low-temperature backfield spectra yield sharply peaked f(V, Hk0) distributions, from which we calculate modal grain dimensions in good agreement with those observed by transmission electron microscopy. Analysis of synthetic samples containing bimodal populations clearly distinguishes the two modes. Because our simplified forward calculations incompletely account for the effects of orientation distribution, the width of the coercivity distribution at each temperature is underestimated, and consequently, the inverse calculations yield grain distributions that are overly broad. Frequency- and temperature-dependent susceptibilities calculated for the inverted f(V, Hk0) distributions accord fairly well with measured susceptibilities for the weakly interacting Tiva Canyon samples, less well for a moderately interacting paleosol specimen, and poorly for a strongly interacting ferrofluid.

[1] A new method to calculate and analyze coercivity distributions of measured acquisition/demagnetization curves of remanent magnetization is presented. The acquisition/demagnetization curves are linearized by rescaling both the field and the magnetization axes. An appropriate filtering of the linearized curves efficiently removes measurement errors prior to evaluating the coercivity distributions. The filtered coercivity distributions are modeled using a set of generalized probability density functions in order to estimate the contributions of different magnetic components. An error estimation is calculated for these functions with analytical and numerical methods in order to evaluate whether the model is significantly different from the measured data. Three sediment samples from Baldeggersee (Switzerland) and three samples of urban atmospheric particulate matter (PM) have been analyzed using this method. It is found that the coercivity distributions of some of the magnetic components show significant and consistent deviations from a logarithmic Gaussian function. Large deviations are found also in the coercivity distributions of theoretical AF demagnetization curves of single-domain and multidomain particles. Constraints in the shape of model functions affect the identification and quantification of magnetic components from remanent magnetization curves and should be avoided as far as possible. The generalized probability density function presented in this paper is suitable for appropriate modeling of Gaussian and a large number of non-Gaussian coercivity distributions.

[1] Various magnetic parameters are in common use for estimating the grain size of magnetic particles. Among these, the ratio of the intensity of anhysteretic remanent magnetization (ARM) to that of isothermal remanent magnetization, as well as their alternating field (AF) demagnetization curves are used as an indicator of the domain state of the particles. Several models have been proposed to describe physically the acquisition of ARM in a biased AF field. Jaep [1969] first developed a semiquantitative theory based entirely on the thermal fluctuation analysis developed by Neel [1949, 1954, 1955]. Significant discrepancies were found between his model and experimental results on magnetite. A new, general theory of ARM based on the work of Jaep is presented here, with particular regard to the influence of various parameters like grain size, coercivity, and mineralogy on ARM intensity. An analytical expression for ARM intensity in the special case of very fine particles was derived from this theory, and a good agreement with experimental results and data from the literature was found. A new estimation of the atomic reorganization time was obtained from ARM measurements on a sample of the Yucca Mountain Tuff, which has well-known mineralogy and grain-size distribution. The results are in agreement with the value proposed by McNab et al. [1968] for magnetite. Some authors considered magnetic interactions as the key to understand the ARM in fine particles, and this is certainly true for strongly interacting samples. In this case, ARM would be useless for the characterization of magnetic grains. However, many sediments have a very low concentration of well-distributed magnetic grains. For these samples, the explanation of an ARM in terms of intrinsic properties of the grains, as qualitatively proposed by other authors, is more suitable.