Mark J. Dekkers

Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation

A new method of analysing isothermal remanent magnetisation (IRM) acquisition curves based on cumulative log Gaussian analysis [Robertson and France, Phys. Earth Planet. Inter. 82 (1994) 223^234] is proposed. It is based on the curve fitting of the IRM acquisition curve versus the logarithm of the applied field with: (i) the acquisition curve on a linear scale, (ii) the acquisition curve expressed as a gradient, and (iii) the acquisition curve on a probability scale. Even when a sample is not saturated, its magnetic properties can be defined, although with less certainty. The number of magnetic components required for an optimal fit to a measured IRM acquisition curve is evaluated statistically. The method discriminates on the basis of different mineral coercivity. Therefore, additional rock-magnetic tests are still required to separate minerals with similar coercivities. fl 2001 Elsevier Science B.V. All rights reserved.

Selected room temperature magnetic parameters as a function of mineralogy, concentration and grain size

Abstract A data set of room temperature magnetic parameters for several iron oxides and sulphides was compiled from the available literature. The aim was to propose the most effective methods for assessing mineralogy, concentration and domain state within environmental magnetic studies. Establishing the magnetic mineralogy is essential for a correct interpretation of concentration and grain-size indicative parameters. Hematite and in particular goethite are recognised by their high (remanent) coercivities. The ratio of saturation remanent magnetisation to susceptibility is suited for assessing mineralogy; for low coercivity minerals, high values of the ratio are indicative of pyrrhotite. Greigite and maghemite both have intermediate ratios, while very low values suggest the presence of (titano)magnetite. From the concentration-dependent parameters mass specific susceptibility, saturation remanent magnetisation and susceptibility of anhysteretic remanent magnetisation, susceptibility displayed the least grain-size dependence. Therefore, it is perhaps the best indicator of concentration, under the proviso of uniform mineralogy and when paramagnetic and diamagnetic contributions are taken into consideration. All minerals showed a decrease in coercivity and remanence ratios with increasing grain size for sizes larger than one micrometer. In contrast, the coercivity of goethite showed an increase with grain size. Assessment of domain state was complicated by very small and large grains displaying similar magnetic properties.

A new astronomical timescale for the loess deposits of Northern China

Here, we present a refined timescale for the entire sequence of Quaternary Chinese loess, which relies upon the correlation of detailed monsoon records to the astronomical solution of Laskar (1990) and the oceanic ODP677 δ18O record of Shackleton et al. (1990). The chronological scheme considers in detail the relative structures of the palaeoclimatic and palaeomagnetic records to produce an accurate timescale that is consistent with the current understanding of loess depositional and post-depositional processes. Analysis of this chronological framework demonstrates downward displacement of the palaeomagnetic horizons with respect to the climatic record.

Magnetic properties of natural pyrrhotite Part I: Behaviour of initial susceptibility and saturation-magnetization-related rock-magnetic parameters in a grain-size dependent framework

The grain-size dependence of the initial susceptibility (Xin), saturation magnetization (Js), saturation remanence (Jrs), coercive force (Hc), remanent coercive force (Hcr) and remanent acquisition coercive force (Hcr′), is reported for four natural pyrrhotites in a grain-size range from 250 μm down to < 5 μm. Xin decreases with decreasing grain size (range 7 × 10−5−1 × 10−5 m3 kg−1); Hc, Hcr and Hcr′ all increase with decreasing grain size (ranges respectively 7–80, 8–85 and 14–80 kA m−1). The magnitudes of Js (8.9–16.7 A m2 kg−1) and Jrs (2.0–6.0 A m2 kg−1) correspond with the trend indicated by the microprobe analysis (47.8 down to 46.9 at.% equivalent Fe). Jrs shows a maximum in the pseudo-single domain region (range 2.0–6.0 A m2 kg−1). Js shows a marked decrease for the finest grain-size fractions. Typical for pyrrhotite are a large Hc/Hcr ratio and a Jrs/Xin ratio which is larger than Hcr. When the presence of pyrrhotite has been established in a sample, its grain size is preferable determined by the Hcr trend because pyrrhotite compositional differences hardly show up in this trend.

An investigation of multi-domain hysteresis mechanisms using FORC diagrams

First-order reversal curve (FORC) diagrams provide a sensitive means of probing subtle variations in hysteresis behaviour, and can help advance our understanding of the mechanisms that give rise to hysteresis. In this paper, we use FORC diagrams to study hysteresis mechanisms in multi-domain (MD) particles. The classical domain wall (DW) pinning model due to Neel [Adv. Phys. 4 (1955) 191] is a phenomenological one-dimensional model in which a pinning function represents the interactions of a DW with the surrounding medium. Bertotti et al. [J. Appl. Phys. 85 (1999a) 4355] modelled this pinning function as a random Wiener–Levy (WL) process, where particle boundaries are neglected. The results of Bertotti et al. [J. Appl. Phys. 85 (1999a) 4355] predict a FORC diagram that consists of perfectly vertical contours, where the FORC distribution decreases with increasing microcoercivity. This prediction is consistent with our experimental results for transformer steel and for annealed MD magnetite grains, but it is not consistent with results for our MD grains that have not been annealed. Here, we extend the DW pinning model to include particle boundaries and an Ornstein–Uhlenbeck (OU) random process, which is more realistic that a WL process. However, this does not help to account for the hysteresis behaviour of the unannealed MD grains. We conclude that MD hysteresis is more complicated than the physical picture provided by the classical one-dimensional pinning model. It is not known what physical mechanism is responsible for the breakdown of the classical DW pinning model, but possibilities include DW interactions, DW nucleation and annihilation, and DW curvature.

Magnetic properties of natural pyrrhotite. II: High- and low-temperature behaviour of Jrs and TRM as function of grain size

Dekkers, Mi., 1989. Magnetic properties of natural pyrrhotite. II. High- and low-temperature behaviour of J~ and TRM as function of grain size. Phys. Earth Planet. Inter., 57: 266—283. Twelve grain-size fractions, ranging from 250 ~tm down to <5 ~m, of four natural pyrrhotite samples were homogeneously dispersed in a non-magnetic matrix to study their magnetic behaviour during thermal demagnetization and low-temperature treatment. Normalized decay curves of the isothermal saturation remanence (Jrs) obtained by continuous thermal demagnetization are identical for all samples studied. They are characterized by a continuous decay up to the maximum blocking temperature of 325°C. Small grain-size fractions have a blocking temperature spectrum slightly shifted to lower temperatures. Decay curves for ~rs obtained by stepwise thermal demagnetization yield a different behaviour for coarseand fine-grained fractions. The former show a monotonous decay up to the maximum blocking temperature, whereas the latter show an enhanced decay at 210—245°C, followed by a small remanence increase at 260—300°C. The enhanced decay is tentatively related to the formation of non-magnetic anomalous pyrrhotite; the increase with its upper stability temperature. The impact of pyrrhotite alteration and metastable preservation of high-temperature superstructures upon rapid cooling is discussed. The TRM intensity decreases with grain size, possibly owing to the absence of closure domains in pyrrhotite. Normalized TRM decay curves are virtually identical for all fractions. Stepwise thermal demagnetization indicates narrow blocking temperature spectra for coarse grain-size fractions, slightly broadening with decreasing grain size. Low-temperature cycling of ~rs shows a gradually increasing reversible remanence behaviour with decreasing grain size. This concurs with a gradual approach of the SD state in the smallest grain-size fractions (< 5 ~tm).TRM shows a reversible low-temperature behaviour throughout the investigated grain-size range.

Environmental magnetism: an introduction

In environmental magnetism, the properties of magnetic minerals are used as proxy parameters for many purposes. Examples are paleoclimate analysis, paleoceanographic studies, provenance studies of sediments, studies of anthropogenically-induced pollution, and archeological investigations. Mineral-magnetic techniques are sensitive, require little sample preparation, are rapid, often grain-size indicative, and usually non-destructive. These techniques involve ‘bulk’ properties which makes them complementary to geochemical micro-analytical techniques. Measurements include the field- and temperature-dependence of various types of induced and remanent magnetizations. Mineral-magnetic methods are continuously being improved. The underlying causal relations between observed mineral-magnetic properties and the processes that led to those properties, are becoming increasingly better understood, and the extended use of such properties as proxy parameters for many processes is foreseen. The following environmental magnetic applications are reviewed: the analysis of paleoclimatic variations in loess and other sediment types, the untangling of sedimentary features in piston cores, and the interpretation of the anthropogenic impact on the environment, in archeological studies and in studies of present-day pollution. The pathway between the provenance area and depositional site is shown to have a crucial impact on the magnetic properties.

The application of fuzzy C-means cluster analysis and non-linear mapping to a soil data set for the detection of polluted sites

Abstract It is important to map the distribution of pollutants and to trace their sources to assess potential environmental hazard. The present work concerns the application of multivariate statistical methods to a soil data base from the province of Styria (Austria) to delineate polluted areas and to distinguish between different types of pollution. The soil data base comprised pedological, geochemical and geological data and was extended by magnetic susceptibility measurements to further test the suitability of magnetic susceptibility as a tracer for pollution. Topsoil data from 521 locations were analysed by fuzzy c-means cluster analysis and non-linear mapping. Robust cluster solutions grouped the database according to the geological background and the land use at the sampling sites. The extraction of information on heavy metal pollution appeared to be possible by analysing the geological units separately and reducing the variables to those indicative for the pollution. The link between magnetic susceptibility and the heavy metal content, which was too complex to be described by bivariate statistics, was revealed by the multivariate methods.

Low-Temperature Oxidation of Magnetite in Loess-Paleosol Sequences: a Correction of Rock Magnetic Parameters

Low-temperature oxidation under atmospheric conditions affects the magnetic properties of magnetite in natural rocks: the coercivities of magnetite grains increase and other parameters change accordingly. It was recently shown that heating to 150°C largely removes the effects of low-temperature oxidation (van Velzen and Zijderveld, 1995). Heating may therefore serve as a detection tool for the presence of the effect of low-temperature oxidation.In the present study, a collection of loess and paleosol samples from various loess regions of the world is examined for the influence of low-temperature oxidation. In all samples of the collection a decrease of coercivities was found after heating to 150°C. Generally loess samples were affected to a larger extent than paleosol samples. The original range of remanent coercivities(Bcr)of 21-58 mT changed to 20-42 mT after heating. The IRM capacity of the samples decreased from 0 up to 25%. ARM showed changes between a decrease of 10% and an increase of 15%. The grain-size indicative parameter IRM/ARM is considerably influenced by the heating and therefore by low-temperature oxidation. The changes in susceptibility are limited and will not influence the interpretation of large-scale features of the susceptibility record as a paleoclimate proxy. Small variations, however, may be obscured by the varying influence of oxidation in the outcrop, which can significantly modify the rock-magnetic record.Rock-magnetic parameters used to determine magnetic mineral content and grain sizes should be corrected for the effect of low-temperature oxidation. To this end heating to 150°C is recommended. The occurrence of the changes is in itself already an indication for the presence of magnetite. Low-temperature oxidation will not only be due to recent weathering in the outcrop, but also to earlier oxidation processes in the source area, during transport and deposition of the loess and during pedogenesis. Truly fresh sediment samples are only influenced by this earlier oxidation. In that case heating will reveal the degree of ancient low-temperature oxidation, which may be related to climate at the time of deposition and pedogenesis.

Geochemical and paleomagnetic evidence for the occurrence of “missing” sapropels in eastern Mediterranean sediments

The cyclic occurrence of sapropels appears to be associated with maxima in the 65°N summer insolation target curve. Studies of the most recent sapropel have revealed extensive oxidation of organic matter or even the complete removal of this unit. Geochemical and magnetic signals allow for the detection of such a “missing” sapropel. In older sediments, these signals are altered by ongoing diagenetic processes. An important parameter to reveal missing sapropels is Ba. Elevated concentrations of Ba are always found in visible sapropels, and even after oxidation of part of the sapropel, the initial Ba profile remains visible. We use this tool to trace down missing sapropels in KC01B, a 37-m Kullenberg core from the Calabrian Ridge, eastern Mediterranean. At least 8 of 11 sampled intervals around insolation maxima contain evidence of a completely oxidized sapropel: Ba, trace metal distributions, and magnetic parameters are similar to those observed around visible sapropels.