No abstract

A magnetic study was carried out on lacustrine sediments from the Zoigê basin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisition is unexpectedly observed during static three‐axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples. X‐ray diffraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning electron microscopy (SEM) reveals that the greigite particles are in the grain size range of 200–300 nm, possibly in the single‐domain state. Greigite clumps of about 3 μm size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15 nm, indicating that the particles seen in the SEM are polycrystalline.  GRM intensities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar. Variation of the demagnetization sequence confirms that GRM is mainly produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for detailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found along the demagnetization axis. Behaviours of acquisition and AF demagnetization of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.

Summary Further studies of gyroremanent magnetisation (GRM) and rotational remanent magnetisation (RRM) have been conducted on lake sediments from the Zoigê Basin in the Eastern Tibetan Plateau, in which greigite is the main carrier of gyroremanent magnetisation. Greigite has the greatest effective gyrofield (Bg) of all magnetic minerals studied so far, being several hundred μT for a peak AF of 80 mT. This high Bg value has the potential to be used as an indicator for greigite. The GRM produced during static alternating field (AF) demagnetisation became close to its maximum at a peak AF of 150 mT. Attempts to extract the natural remanent magnetisation (NRM) by algebraic elimination of the GRM were unsuccessful above fields of about 30 mT because the GRM became much larger than the remaining NRM. The GRM of a crushed sample was much reduced because of the destruction of the sample's anisotropy, although as expected, both RRM and Bg> were similar before and after crushing, thus demonstrating that RRM and Bg are independent of anisotropy. Measurement of the anisotropy of two samples from different depths showed that the deeper sample, which acquired the higher GRM, also had the higher anisotropy presumably as a result of greater sediment compaction. Study of anisotropy of GRM may help to elucidate the preferred alignment of greigite within the sample, which is difficult to ascertain by other means.

No abstract

An expressway-side soil profile 22 cm long was sampled from the grassland of the expressway linking Beijing and the Capital International Airport. Magnetic measurements, geochemical and multivariate statistic analyses were performed on the soil samples. The results reveal that the soil profile can be divided into two parts with significant difference in magnetic proxies and heavy metal concentration. The uppermost soil horizon (0-8 cm) represents the pollution-rich layer with higher concentration of ferrimagnetic phases and metallic elements. The values of x are very high with an average of 141.60 6 10 28 m 3 ?kg 21 in the layer. We explain that the anthropogenic dust input from traffic is the predominant cause for strong signals of magnetic phases and heavy metals. Below the profile depth of 8 cm, there is minor pollution in the soil with lower concentration of magnetic minerals and heavy metals compared to the natural background values. x remains quite stable and relatively low with an average of 49.44 6 10 28 m 3 ?kg 21 . S-ratio also generally decreases with depth, and it changes from 0.93 in the 0-8 cm layer to 0.87 below the depth of 8 cm. It indicates that the soil samples are overwhelmingly predominated by ferrimagnetic minerals in the upper part soil, while the contribution of imperfect antiferromagnetic components is stronger in the lower part. Rock magnetic experiments show MD magnetite as the main magnetic carrier both in the upper and lower parts. The magnetic grain size in the upper part is, however, a bit coarser than that in the lower part. Cluster analysis shows a positive correlation between magnetic properties (x, ARM, SIRM) and heavy metal pollutants of Pb, Zn, Cu. Fuzzy C-means cluster analysis can clearly help divide the soil profile into two different layers and distinguish their characteristics. It can be concluded that these magnetic concentration-related parameters can be used as proxies for pollution investigation in a fast, sensitive, low-cost and highly efficient approach to screening heavy metal pollution.