Search

Reconstructions of the global production rate of the cosmogenic isotope 10Be from sedimentary records of authigenic 10Be/9Be ratios have been successfully used to obtain independent estimates of geomagnetic dipole moment variations caused by field excursions or reversals. In this study, we assess the reliability of 10Be/9Be as a proxy for the cosmogenic 10Be production rate by evaluating two potential biasing sources represented by sediment composition and climatic modulation. For this purpose, we compare five high-resolution 10Be/9Be records of the Matuyama-Brunhes (M-B) field reversal from sediment cores of the Indian, West Pacific, and North Atlantic oceans. Significant increase of 10Be/9Be ratios at ∼ 774 ka is explained in terms of the dominant control of geomagnetic modulation during the M-B reversal. Results do not support the existence of a direct proportionality between measured sedimentary 10Be/9Be ratio and cosmogenic 10Be production rate, as shown by 10Be/9Be records that offset relative to each other during and outside the M-B reversal. Residual differences between offset-corrected rescaled records do not appear to be related to an incomplete correction of variable sediment scavenging efficiencies by 9Be normalization. Instead, these differences can be explained by a common climatic modulation model, assuming a linear relation between 10Be/9Be and the global 10Be production rate with site- and time-dependent additive and multiplicative coefficients. These coefficients are linear functions of a single global climate proxy identified with the benthic δ 18 O record. Additive coefficients are almost constant in time and can represent up to ∼ 60% of the average 10Be/9Be value during periods of stable field polarity. Multiplicative coefficients are also site-specific, with mean values representing the bulk scavenging efficiency of the site, and variations about this mean expressing a multiplicative climatic modulation of the 10Be production rate. The amplitude of this modulation amounts to 10–15% of the maximum variations recorded during the M-B reversal and is sufficiently large to mask minor variations of the dipole moment during stable polarity periods. Reconstructions of the geomagnetic dipole intensity can benefit from the information about climatic modulation effects gained with our modelling approach. Best suited sites for magnetic field reconstructions should be characterized by minimal Be-recycling contributions from ancient 10Be reservoirs and minimal climatic modulation, as far as it can be determined from relative comparisons with other records. These conditions are most likely encountered in open basins at sites (1) with 2.8 km water depth, (2) > 200 km offshore, and (3) located underneath a large current system extending over regions with minimum terrigenous inputs. Scaling all records with respect to a chosen reference enables to produce 10Be/9Be stacks with reduced noise and short-term local environmental effects. Differences between stacks obtained in this manner highlight global climatic effects that need to be considered when generating calibrated reconstructions of the geomagnetic dipole moment.

Changes of the Earth's magnetic field have been observed with varying spatial and temporal resolution since the middle ages by means of compasses, and later by refined mechanical inclination and intensity instruments, until the modern use of quantum mechanical devices and satellite observations. Ancient records, on the other hand, rely on the natural magnetization of archaeological artifacts and other materials, such as rocks and sediments, being therefore indirect, less accurate and sparser. The combination of such heterogeneous records into a single, self-consistent, global model of the geomagnetic field is very challenging: the highly uneven data coverage, both in space and time, requires a careful handling of data uncertainties and error correlations. Previous models dealt with this problem using separated treatments of instrumental data and indirect records, respectively, as well as a different handling of the dipole field component, with respect to non-dipole terms. Here we present a global geomagnetic field model based, for the first time, on the simultaneous inversion of historical, archaeomagnetic, and volcanic records, using a Bayesian approach with minimal-committing time regularization that minimizes the energy of secular variation. A detailed assessment of data uncertainty and error correlation is used to minimize artifacts generated by the appearance of an overwhelming number of incomplete, mostly declination-only records, associated with shipboard measurements in the early colonial period. Our model yields lower dipole energies, which better match modern values, as well as higher non-dipole energies and a stronger secular variation of the dipole moment. Some model artifacts associated with extremely heterogeneous distributions of records in time and space are not completely eliminable and might be caused by incorrect a-priori uncertainty assessments. Alternatively, they might represent an intrinsic limit that can be overcome only by adding new records at critical locations and times.

Near-shore marine sediments deposited during the Paleocene-Eocene Thermal Maximum at Wilson Lake, NJ, contain abundant conventional and giant magnetofossils. We find that giant, needle-shaped magnetofossils from Wilson Lake produce distinct magnetic signatures in low-noise, high-resolution first-order reversal curve (FORC) measurements. These magnetic measurements on bulk sediment samples identify the presence of giant, needle-shaped magnetofossils. Our results are supported by micromagnetic simulations of giant needle morphologies measured from transmission electron micrographs of magnetic extracts from Wilson Lake sediments. These simulations underscore the single-domain characteristics and the large magnetic coercivity associated with the extreme crystal elongation of giant needles. Giant magnetofossils have so far only been identified in sediments deposited during global hyperthermal events and therefore may serve as magnetic biomarkers of environmental disturbances. Our results show that FORC measurements are a nondestructive method for identifying giant magnetofossil assemblages in bulk sediments, which will help test their ecology and significance with respect to environmental change.

Levinson-Lessing Lake in northern Central Siberia is a sedimentary archive characterized by continuous, widely constant sedimentation at high rates (0.7 m ka−1 for >32 ka). This study provides the first evidence of the suitability of the lake′s sediments for paleomagnetic analyses using the 46-m-long core Co1401. Although the lowermost 8 m are disturbed, the upper 38 m of Co1401 provide the preconditions for an exceptional, high-resolution paleomagnetic record located within the tangent cylinder of the inner core. High-resolution analyses of magnetic susceptibility, anhysteretic remanent magnetization, isothermal remanent magnetization, and hysteresis parameters show largely uniform mineral magnetic properties. First-order reversal curves indicate magnetite particles in pseudo-single domain state are the main remanence carrier, supplemented by single-domain particles, originating likely from magnetotactic bacteria. Above 6.7 m, the bulk magnetic mineralogy is slightly harder than below and initial greigite formation occurs. However, the main remanence carriers are still of detrital origin.

Since their introduction in Earth sciences in 1990, first-order reversal curve (FORC) measurements have become an important tool for characterizing geologic materials. The visualization of irreversible magnetic processes, such as transitions between magnetic states in individual crystals, in a two-dimensional map provides a unique tool for discriminating complex mixtures of magnetic particle assemblages according to their composition and size, thereby reducing the intrinsic non-uniqueness of simpler magnetic characterization tools. The use of FORC measurements in Earth sciences has rapidly evolved, both technically (e.g., measurement resolution, data processing) and theoretically. For instance, numerical unmixing techniques have been recently implemented on FORC measurements (FORC-PCA), and micromagnetic calculations are now used to model the FORC signature of specific magnetic particle assemblages. The scope of this work is to provide, for the first time, a didactic and comprehensive review of FORC diagrams and their application in Earth sciences which includes (1) the theoretical foundations; (2) the connection with Preisach theory and with other magnetic characterization tools; (3) detailed instructions for optimal planning, execution, and processing of FORC measurements; and (4) an in-depth analysis of the fundamental FORC signatures commonly occurring in rocks and sediments.

Using 12-year-long series of data (2001-2012) from geomagnetic observatories and repeat stations in Austria and its neighboring countries, a regional spatial-temporal (ST) model is developed based on the polynomial expansion consisting of latitude, longitude, and time of the geomagnetic field components and total magnetic field F. Additionally, we have used three different global models (CHAOS-5, POMME-9, and EMM2015), which are built on spherical harmonics up to a maximum degree Lmax and give the core field and crustal field separately. The normal field provided by the ST model and its “model bias”, which comprise the residuals of the differences between measured and predicted values, are calculated and the respective maps are shown. The residuals are considered an estimate of the local crustal field. In the case of global models, we have applied for each of these three methods to calculate the “model bias”: residuals of the differences between observed values and predicted values of the model, residuals of the differences between observed values and core field values of the model, and the average bias for the period 2001-2012. The normal field of the region of Austria provided by each global model is also calculated. Generally, the regional and global models yield relatively similar crustal fields for the Austrian region, especially when the first method is used. The normal fields calculated by them are in good agreement with each other. Each of the global models directly provides the crustal field, and they are compared with the aeromagnetic data provided by aeromagnetic surveys over the Austrian region. The ST model is in better agreement with aeromagnetic data. We have also analyzed the secular variation over the region, which is calculated from the rate of change of normal field given by the ST and global models.

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.

Hypervelocity impacts occur on bodies throughout our solar system, and play an important role in altering the mineralogy, texture, and magnetic properties in target rocks at nanometer to planetary scales. Here we present the results of hypervelocity impact experiments conducted using a two-stage light-gas gun with 5 mm spherical copper projectiles accelerated toward basalt targets with ~6 km s−1 impact velocities. Four different types of magnetite- and titanomagnetite-bearing basalts were used as targets for seven independent experiments. These laboratory impacts resulted in the formation of agglutinate-like particles similar in texture to lunar agglutinates, which are an important fraction of lunar soil. Materials recovered from the impacts were examined using a suite of complementary techniques, including optical and scanning electron microscopy, micro-Raman spectroscopy, and high- and low-temperature magnetometry, to investigate the texture, chemistry, and magnetic properties of newly formed agglutinate-like particles and were compared to unshocked basaltic parent materials. The use of Cu-projectiles, rather than Fe- and Ni-projectiles, avoids magnetic contamination in the final shock products and enables a clearer view of the magnetic properties of impact-generated agglutinates. Agglutinate-like particles show shock features, such as melting and planar deformation features, and demonstrate shock-induced magnetic hardening (two- to seven-fold increases in the coercivity of remanence Bcr compared to the initial target materials) and decreases in low-field magnetic susceptibility and saturation magnetization. © The Meteoritical Society, 2017. Russian Science Foundation, RSF: 17-17-01279 Rochester Academy of Science, RAS Acknowledgments—We thank the anonymous reviewer for the review and constructive suggestions and Prof. Oliver Tschauner for useful suggestions and the editorial handling, which helped to improve the manuscript. The work is supported by Act 211 Government of the Russian Federation, agreement no. 02.A03.21.0006 and is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. We gratefully acknowledge support by the U.S. National Science Foundation IRM Visiting Fellowship to N.S.B. D.D.B. was supported by Program no. 7P of Presidium RAS and a Russian Science Foundation Grant 17-17-01279. D.D.B. acknowledges the staff at the Center of Microscopy and Nanotechnology of the University of Oulu (Finland) for assistance with some aspects of electron microprobe and SEM works.

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.

Sediments continuously record variations of the Earth's magnetic field and thus provide an important archive for studying the geodynamo. The recording process occurs as magnetic grains partially align with the geomagnetic field during and after sediment deposition, generating a depositional remanent magnetization (DRM) or post-DRM (PDRM). (P)DRM acquisition mechanisms have been investigated for over 50 years, yet many aspects remain unclear. A key issue concerns the controversial role of bioturbation, that is, the mechanical disturbance of sediment by benthic organisms, during PDRM acquisition. A recent theory on bioturbation-driven PDRM appears to solve many inconsistencies between laboratory experiments and palaeomagnetic records, yet it lacks experimental proof. Here we fill this gap by documenting the important role of bioturbation-induced rotational diffusion for (P)DRM acquisition, including the control exerted on the recorded inclination and intensity, as determined by the equilibrium between aligning and perturbing torques acting on magnetic particles., Sediments record variations of the Earth's magnetic field via the alignment of magnetic grains during and after deposition, yet the role of post-depositional processes remains unclear. Here, the authors present experiments showing how microbially-induced bioturbation controls the alignment process.

Geomagnetically induced currents (GICs) in power systems, which can lead to transformer damage over the short and the long term, are a result of space weather events and geomagnetic variations. For a long time, only high-latitude areas were considered to be at risk from these currents, but recent studies show that considerable GICs also appear in midlatitude and equatorial countries. In this paper, we present initial results from a GIC model using a thin-sheet approach with detailed surface and subsurface conductivity models to compute the induced geoelectric field. The results are compared to measurements of direct currents in a transformer neutral and show very good agreement for short-period variations such as geomagnetic storms. Long-period signals such as quiet-day diurnal variations are not represented accurately, and we examine the cause of this misfit. The modelling of GICs from regionally varying geoelectric fields is discussed and shown to be an important factor contributing to overall model accuracy. We demonstrate that the Austrian power grid is susceptible to large GICs in the range of tens of amperes, particularly from strong geomagnetic variations in the east–west direction.

Geophysical Journal International, 168 (1), ISSN:0956-540X, ISSN:1365-246X

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.

High-resolution first-order reversal curve (FORC) diagrams are increasingly employed to characterize magnetic minerals in sediments, and especially as a magnetofossil detection tool. Conventional processing protocols, however, are not well suited for this purpose because the opposed needs of high resolution and smoothing of noisy data over different regions of the FORC diagram cannot be reconciled. This problem is particularly severe in samples containing non-interacting single-domain particles, whose FORC signature is described by an infinitely narrow, divergent feature called a central ridge. In this paper, a new FORC processing protocol called VARIFORC (VARIable FORC smoothing) is presented for the analysis of non-regular FORC functions, such as those featuring a central ridge. This protocol is based on weighted polynomial regression of rectangular arrays of measurement points whose size is determined by the local properties of the FORC function (i.e. small arrays where high resolution is needed and vice-versa). The resulting FORC diagram is characterized by improved signal-to-noise ratios that pass significance tests over much larger domains. This enables simultaneous quantitative analysis of FORC signatures corresponding to different magnetization processes (e.g. the central ridge produced by single-domain particles and a weak, extended background due to detrital magnetic minerals). VARIFORC has been successfully tested on a magnetofossil-bearing pelagic carbonate sample that has a non-regular FORC function, and on a volcanic ash sample with a typical pseudo-single-domain signature. An unexpected minor central ridge contribution, which is invisible to traditional processing, has been detected in the volcanic ash sample. This finding demonstrates the effectiveness of the new FORC processing protocol and potential new applications of high-resolution FORC measurements.

Massive stars, which terminate their evolution as core collapse supernovae, are theoretically predicted to eject more than 1E-5 solar masses of the radioisotope 60Fe. If such an event occurs sufficiently close to our solar system, traces of the supernova debris could be deposited on Earth. Herein, we report a time resolved 60Fe signal residing, at least partially, in a biogenic reservoir. Using accelerator mass spectrometry, this signal was found through the direct detection of live 60Fe atoms contained within secondary iron oxides, among which are magnetofossils, the fossilized chains of magnetite crystals produced by magnetotactic bacteria. The magnetofossils were chemically extracted from two Pacific Ocean sediment drill cores. Our results show that the 60Fe signal onset occurs around 2.6 Ma to 2.8 Ma, near the lower Pleistocene boundary, terminates around 1.7 Ma, and peaks at about 2.2 Ma., Figures from the Supplementary Information are not included due to file size restrictions. Download the, now open access, original article for those details (see doi)

Experimental results on the change in the Verwey transition temperature in magnetite under compression exhibit puzzling variability, with slopes ranging from − 6 to + 16 K/GPa. Our thermodynamic analysis of the Verwey transition in magnetite explains much of this variability in terms of the transformation strain as magnetite changes in crystal structure from cubic to monoclinic. Because this strain involves a much larger change in shape than in volume, the change in temperature of the phase boundary ( T v ) can be much more sensitive to nonhydrostatic stress than to hydrostatic pressure. Uniaxial compression and tension both are predicted to increase T v , an effect that is opposite in sign and, for favorable orientations, up to six times greater per gigapascal than for pressure. Moreover, because the monoclinic twin orientation with highest T v is thermodynamically favored, our treatment also shows how any desired twin may be selected uniquely by applying stress of appropriate orientation, thereby removing obstacles to understanding many kinds of low-temperature phenomena in magnetite caused by the presence of multiple twins. A similar analysis for the thermodynamic effects of strong magnetic fields is outlined, predicting that T v will generally be lowered. Again, the twin for which T v is highest (i.e., is lowered the least) will be the most stable. Twin selectivity per gigapascal of stress is up to two to three times stronger than that per Tesla of magnetic field.

There is agreement that ultrafine maghemite grains (

SUMMARY Magnetic properties of suspended material in the water columns of freshwater and marine environments provide snapshots of magnetic biomineralization that have yet to be affected by the eventual time-integration and early diagenetic effects that occur after sediment deposition. Here, we report on the magnetism, geochemistry and geobiology of uncultured magnetite- and greigite-producing magnetotactic bacteria (MB) and magnetically responsive protists (MRP) in Salt Pond (Falmouth, MA, USA), a small coastal, marine basin (∼5 m deep) that becomes chemically stratified during the summer months. At this time, strong inverse O2 and H2S concentration gradients form in the water column and a well-defined oxic–anoxic interface (OAI) is established at a water depth of about 3.5 m. At least four morphological types of MB, both magnetite and greigite producers, and several species of magnetically responsive protists are found associated with the OAI and the lower sulphidic hypolimnion. Magnetic properties of filtered water were determined through the water column across the OAI and were consistent with the occurrence of magnetite- and greigite-producing MB at different depths. Sharp peaks in anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) and single-domain (SD) values of ARM/SIRM occur within the OAI corresponding to high concentrations of MB and MRP with magnetically derived cell densities of 10 4 –10 6 ml −1 . Low-temperature ( 1 per cent) is present within magnetite magnetosomes, produced either

Natural sediments are a complex mixture of magnetic minerals with different origins and different geochemical history, each of which is called a magnetic component. Magnetic components practically never occur in isolated form, and their characterization using bulk magnetic measurements relies on the individuation of the systematic variation of some parameters within a large group of samples. These variations can be interpreted either as a mixing trend or as the result of natural processes, which affect the physical and chemical properties of the magnetic particles. An alternative approach is offered by the analysis of magnetization curves using model functions, which are supposed to represent the magnetic properties of individual components. The success of this approach relies on (1) the choice of model functions that can reproduce the natural properties of a component with sufficient accuracy by varying a minimum number of parameters and (2) on very precise and accurate measurements, which are necessary to overcome the extreme sensitivity of the method to noise. In this paper, the analysis of remanent magnetization curves proposed by Egli (2003) is applied to a large set of representative sediments from the most variable environments and to a set of artificial magnetite samples. Despite the variety of materials and natural processes involved in the formation of these sediments, seven groups of magnetic components with well-defined and consistent properties could be identified. It has been found that both lacustrine and marine sediments contain two magnetically distinct groups of magnetosomes, which react differently to changes of the redox potential. The effects of some natural processes, such as weathering, reductive dissolution and transport could be observed on the individual components.

The characterization of individual magnetic components in sediments and sedimentary rocks is difficult, as these natural materials are often a complex mixture of magnetic mineral sources. The analysis of magnetization curves with model functions is the only practicable method of unmixing the magnetic components and characterizing their magnetic properties, if a priori information is not available. Unfortunately, such analyses rely on time consuming measurements and on the choice of appropriate model functions. The simplification of the unmixing problem is closely related to the number of parameters required to fully characterize a magnetic component, and the significance of these parameters in rock magnetic terms. A systematic analysis of synthetic and natural samples shows that a combination of four parameters, here called magnetic fingerprints, is sufficient for this purpose. The fingerprints of individual magnetic components isolated from a wide range of natural sediments and sedimentary rocks form well-defined groups with specific properties. These groups reflect common processes of formation, transport and dissolution of magnetic particles. A clear distinction can be made between two sorts of biogenic magnetite, atmospheric dust, urban pollution and ultrafine magnetite produced in soils and lacustrine/marine sediments. � 2004 Elsevier Ltd. All rights reserved.

The analysis of magnetization curves with model functions has been proposed independently in several works as a method to unmix and characterize magnetic minerals assemblages in sediments and sedimentary rocks. Unfortunately, a successful result of such analysis relies on time-consuming measurements and on the choice of appropriate model functions. However, once the magnetic properties of individual magnetic components have been determined on a small set of selected samples, a simpler and faster analysis of similar samples is possible. The fast analysis of a large number of samples allows investigation of the effect of natural processes on the properties of single magnetic components. The simplification of the unmixing problem proposed in this paper is based on an iterative linearization procedure, which considers the variability of magnetic components. Any simplification of the unmixing problem is limited by a minimum number of parameters, which are required to fully characterize a magnetic component. It has been shown that a combination of four parameters, so-called magnetic fingerprints, is sufficient for a complete characterization of the remanent magnetization of a component. The usefulness of magnetic fingerprints in tracking natural processes is demonstrated exemplarily for lake sediments. The response of Baldeggersee (Switzerland) to environmental changes has been investigated, with special regard to the role of bacterial magnetite in the iron cycle and its possible use as a sensitive paleoredox indicator. The relation between the magnetic properties of lake sediments on one hand, and climatic, tectonic- and human-driven environmental changes on the other, is strongly non-linear. Therefore, a classic correlation between so-called magnetic proxies and environmental signals should be considered with care.

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.

We present a novel technique for quantitative unmixing of primary and secondary ferrimagnetic minerals in sediments. Hysteresis and high-resolution first-order reversal curve (FORC) measurements are performed on sediment samples before and after digestion in a citrate–bicarbonate–dithionite (CBD) solution optimized for maximum selective extraction of secondary fine-grained iron oxides. The difference between magnetic measurements of untreated and CBD-treated sample materials is used to calculate the original magnetic signature of CBD-extractable minerals. A combination of selective chemical extraction and magnetic measurements suited for the detection of single-domain particles provides a cross-check between chemical and magnetic unmixing of primary and secondary iron oxides and resolves the non-uniqueness problem of numerical unmixing methods. A quantitative magnetic characterization of secondary ferrimagnetic minerals in a magnetofossil-rich pelagic carbonate is presented for the first time. It can be used for calibration of recently developed fast magnetic unmixing techniques. CBD-based Fe extraction from sediments with minimal clastic and/or aeolian inputs, such as pelagic carbonates, is particularly suited for the search for cosmogenic 60Fe signatures from supernova explosions, because 60Fe dilution by dissolved primary Fe-bearing minerals is minimized. Fil: Ludwig, P.. Technische Universitat Munchen; Alemania Fil: Egli, R.. Central Institute for Meteorology and Geodynamics; Austria Fil: Bishop, S.. Technische Universitat Munchen; Alemania Fil: Chernenko, V.. Technische Universitat Munchen; Alemania Fil: Frederichs, T.. Universitat Bremen; Alemania Fil: Rugel, G.. Helmholtz-Zentrum; Alemania Fil: Merchel, S.. Helmholtz-Zentrum; Alemania Fil: Orgeira, Maria Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; Argentina

[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] 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.

The kinematic field of central western Europe is characterized by relatively small movements (around 1-2 mm/year) and diffuse seismicity with earthquakes occurring mostly in the shallow crust (within 15 km), prevalently concentrated along the Alps and the European Cenozoic Rift System (ECRIS). In order to study and constrain the current crustal kinematic field we reconstructed the velocity and the strain field using permanent GPS stations, belonging to different networks (AGNES, EUREF, REGAL, RGP). The 2D strain rate tensor has been calculated using the method of least-squares collocation. Our results show that the area of maximum compression is located along the Alpine chain, where maximum values of 7 +/- 2 nstrain/year are found, while maximum extension is measured between the Armorican Massif and the Massif Central, where values of 4 +/- 2 nstrain/year are reached. The earthquakes with M > 3.0, have been used to estimate the seismic strain rates, while the style of the seismic deformation was reconstructed from the fault plane solutions (FPS) available from the literature. Relatively high values of seismic strain rates (around 10 nstrain/year) are measured along the Alpine Chain and the ECRIS. Results obtained by geodetic and seismic data are quite in agreement and reflect the different tectonic evolution of the geological features characterizing the area of study. The orientation of the compressional geodetic and seismic strain axes are NW-SE in most of the area of study, on account of the action of plate boundary forces. A rotation of the same axes to N-S direction along the eastern Alps, possibly related to the Adria convergence, is found. (c) 2006 Elsevier Ltd. All rights reserved.

An interdisciplinary investigation of the Pleistocene clay succession of Fornaci di Ranica (Italian Pre-Alps) aimed to reconstruct the palaeoenvironmental history and its chronostratigraphic position and to consider the implications for the Quaternary evolution of the southern alpine borderland near the Early/Middle Pleistocene transition. A detailed survey of Quaternary deposits in boreholes and exposed sections provided a complete record of the basin. The most complete core was analyzed for palynology and magnetic properties. Plant macrofossils were obtained by flotation. Pollen and spectrometric analysis allowed a precise stratigraphic placement of a moose antler and braincase kept in museum collections. The basin originated from fluviatile obstruction of a tributary valley cut into bedrock. The basal deposits formed as a terrestrial hydromorphic soil under a dense Tsuga-forest (zone RNC 1). The overlying palustrine gyttja shows a marked forest withdrawal with a climatic signature (zone RNC 3), followed by a re-establishment of mixed conifer forests (zones RNC 4-6). The occurrences of pollen of cold-adapted taxa (Saxifraga oppositifolia type, Saxifraga stellaris type, Larix), are the oldest so far known south of the Alps and suggest that mean summer temperature was near or even below the thermic tree limit. High herb diversity, partially represented by tall forbs, characterizes this zone. A further increase of the water table can be assumed in zones RNC 4 and RNC 5, since pollen of aquatic plants increases. The find of Larix decidua L. cones demonstrates the existence of the European larch in the Early Pleistocene of the Alps, previously not identified at the species level. After the Serio river had reached the maximum Quaternary aggradation, the lake was rapidly filled up with turbiditic sediments. The identification of Cervalces latifrons (Johnson, 1874) by Azzaroli (Palaeantogr. Italica 71(1979) 48) was confirmed. Pollen spectra obtained from clay fragments preserved in the braincase allowed us to constrain the stratigraphic position of these fossil remains in the lower palustrine portion of the sequence. Pollen and moose palaeoecology corroborate the reconstruction of a boreal landscape. The succession of Fornaci di Ranica is attributed to the late Early Pleistocene on the basis of the following elements: (i) the occurrence of an acme pollen zone of Tsuga, with Carya and Pteroearya; (ii) the low representation of Cedrus; (iii) the biochronology of Cervalces latifrons; (iv) normal fossil magnetic polarity, interpreted as part of the Jaramillo subchron on the basis of correlations with other studied sections from the same region, and, finally, (v) indications from the regional geological evolution. The basin deposits spanned the final stage of a temperate period and the subsequent transition to a cold phase during the Jaramillo subchron or the Cobb cryptochron. The basin originated by fluvioglacial aggradation, triggered by glaciation in the northern part of the Serio river catchement. The lake succession may represent an episode of marked increase of polar ice volume related to MIS (36) 30, or 28. The correlation with the Leffe sequence also suggests that cold climate conditions inferred for pollen zone RNC 3 did not occur earlier in the Early Pleistocene of this region. (C) 2004 Elsevier Ltd and INQUA. All rights reserved.

International Journal of Earth Sciences, 94 (4), ISSN:1437-3254, ISSN:1437-3262

[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.

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.

[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.

Pliocene volcanic rocks from south-east Austria were paleomagnetically investigated. Samples were taken from 28 sites located on eight different volcanoes. Rock magnetic investigations revealed that magnetic carriers are Ti-rich or Ti-poor titanomagnetites with mainly pseudo-single-domain characteristics. Characteristic remanent magnetization directions were obtained from alternating field as well as from thermal demagnetization. Four localities give reversed directions agreeing with the expected direction from secular variation. Another four localities of the Klöch–Königsberg volcanic complex (3) and the Neuhaus volcano (1) have reversed directions with shallow inclinations and declinations of about 240° while the locality Steinberg yields a positive inclination of about 30° and 200° declination. These aberrant directions cannot be explained by local or regional tectonic movements. All virtual geomagnetic pole positions are located on the southern hemisphere. Four virtual geomagnetic poles lie close to the geographic pole, while all others are concentrated in a narrow longitude sector offshore South America (310°–355°) with low virtual geomagnetic pole latitudes ranging from − 15° to − 70°. The hypothesis that a transitional geomagnetic field configuration was recorded during the short volcanic activity of these five localities is supported by 9 paleointensity results and 39Ar/40Ar dating. Virtual geomagnetic dipole moments range from 1.1 to 2.9·1022 Am2 for sites with low VGP latitudes below about 60° and from 3.0 to 9.3·1022 Am2 for sites with higher virtual geomagnetic pole latitudes. The new 39Ar/40Ar ages of 2.51 ± 0.27 Ma for Klöch and 2.39 ± 0.03 Ma for Steinberg allow for the correlation of the Styrian transitional directions with cryptochron C2r.2r-1 of the geomagnetic polarity time scale. Graphic abstract