Summary The acquisition of isothermal remanent magnetization (IRM) curves through the application of stepwise‐increasing uniaxial fields to a rock‐magnetic sample provides an important non‐destructive tool for the investigation of coercivity spectra (Dunlop & Özdemir 1997). We show that, through the use of an automated procedure based on the expectation–maximization algorithm (Dempster et al. 1977), both saturated and non‐saturated IRM acquisition curves can be effectively modelled into their individual coercivity contributions.

Mineral magnetic properties of soils and sediments are increasingly used as proxy parameters for environmental and palaeoclimate analysis. To investigate which magnetic minerals contribute to the environmental signal in the samples, chemical techniques such as the citrate-bicarbonate-dithionite (CBD) extraction method have been introduced in environmental magnetism studies. This technique is assumed to distinguish lithogenic (magnetite) from pedogenic (maghemite) mineral content in soils and sediments. Unfortunately, interpretation of the CBD extractions is not straightforward because the procedure is sometimes more suitable for distinction between grain size than for distinction between minerals. The procedure of the CBD extraction technique was investigated to determine the influence of extraction temperature and iron oxide concentration on the dissolution behaviour of the samples. Synthetic samples were extracted at three different temperatures (60º, 70º and 80ºC) at similar iron oxide concentration (5 wt%), and for three different concentrations (0.1 wt%, 1 wt% and 5 wt%) at the same temperature (60ºC). Our results show that a lower extraction temperature reduces the dissolution rate for all samples, while decrease in iron oxide concentration increases the dissolution rate. Thus, the parameters in the CBD procedure have a major influence on the dissolution behaviour of the samples. In practice this means that when natural samples of differing iron oxide concentration are extracted with this technique, the results of the extractions cannot be compared. Therefore, the outcome of this type of extraction experiment can only be accurately interpreted when the effect of the procedure on the dissolution behaviour is taken into account.

Summary Chemical extraction techniques are being introduced into environmental magnetism studies to aid in the interpretation of magnetic climate proxies. Previous studies have shown that the acid‐ammonium‐oxalate/ferrous‐iron (AAO–Fe2+) extraction technique can be used to selectively dissolve very fine‐grained magnetite and maghemite from synthetic samples. Here, we present the results of a study of this extraction technique to serve as a tool for selective dissolution of pedogenic magnetic minerals from a loess–palaeosol transect. Before and after extraction, the samples were subjected to classic mineral–magnetic methods (measurement of low‐field susceptibility and hysteresis parameters), as well as first‐order reversal‐curve analysis. In addition, acquisition curves of the isothermal remnant magnetization (IRM) were fitted with logarithmic distributions. These analyses showed the magnetic dominance of low‐coercivity magnetic minerals. By subtracting the IRM remaining after extraction from that before extraction, the magnetic fraction that was dissolved could be characterized as well, supplying extra information concerning the performance of the extraction technique. The AAO–Fe2+ method successfully dissolved the superparamagnetic and part of the single‐domain material from the palaeosol samples in one extraction step, repeating the extraction resulted in hardly any further changes to the magnetic content of the samples. The magnetic characteristics of the loess samples remained stable throughout the extraction experiment. The combination of the AAO–Fe2+ extraction with mineral–magnetic analysis has successfully identified the pedogenic contribution in our samples. Therefore, variations in the lithogenic fraction potentially present in pedogenically enhanced intervals can be assessed, improving the merit of mineral–magnetic climate proxy parameters.

Summary In soil magnetism, the magnetic parameters alone are not always sufficient to distinguish the lithogenic from the pedogenic magnetic fractions. Sequential extraction techniques have therefore been incorporated into magnetic studies to constrain the environmental interpretation. Here we report on the dissolution behaviour of magnetite and maghemite in the acid–ammonium oxalate method to see whether the method is suitable for specific dissolution of magnetic minerals from soils and sediments. To prevent changes in the extraction mechanism during the experiments (see Appendix A), we used an adapted version of the acid–ammonium oxalate (AAO) method, in which Fe2+ is added to the extraction solution prior to the experiment [the AAO‐Fe(II) method]. The procedure was divided into several 30 min extraction steps to check the dissolution progress. Synthetic samples containing a quartz matrix with 0.1 wt per cent of iron oxides were extracted with the AAO‐Fe(II) method. The iron oxides consisted of either magnetite or maghemite with grain sizes of < 0.5 µm (fine grained or SD/PSD) and < 5 µm (coarse grained or MD/PSD), or a 1 : 1 mixture of both minerals. Because only magnetite and maghemite were studied, the changes in magnetic characteristics could be monitored after each extraction step by analysis of the bulk susceptibility and hysteresis parameters measured at room temperature. The AAO‐Fe(II) method preferentially dissolved the smaller iron oxides from the samples. For samples containing iron oxides with coarse grain size there is a preference for dissolving maghemite rather than magnetite. Extractions of the samples containing mixtures of two different grain sizes or with different mineralogy show that the method preferentially dissolves the smaller grains before attacking the coarse grains in the sample.

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