An investigation of the magnetic carriers and demagnetization characteristics of the Gulang loess section, northwestern Chinese Loess Plateau

Abstract

We report the results of a rock magnetic and paleomagnetic investigation of unit L9 of the Gulang (GL) loess section, in the northwestern Chinese Loess Plateau (CLP). We demonstrate that the dominant magnetic remanence carriers are coarse-grained pseudo single-domain/multidomain magnetite particles, and hematite. The application of either thermal (THD) or alternating-field demagnetization (AFD) yielded several normal polarity intervals interbedded with intervals of reversed polarity, but which cannot be correlated with standard geomagnetic polarity time scales. A greater number of normal polarity intervals, however, were evident after THD than after AFD. We propose that the normal polarities are either the result of overprinting of a viscous remanent magnetization (VRM) carried by coarse magnetite particles on the original depositional remanent magnetization (DRM); or that they represent genuine geomagnetic excursions (such as the Kamikatsura and Santa Rosa excursions) which are difficult to identify. In addition, we propose that in the case of the GL loess section, the effect of thermal demagnetization tails of a VRM is a possible reason why THD treatment yielded a greater number of normal polarity intervals than does AFD. However, reversed polarities obtained by either THD or AFD may be credible for sedimentary intervals with a relatively high content of hematite particles which preserve the primary DRM. Although numerous studies have proposed that THD provides more reliable paleomagnetic directions for loess sediments from the CLP, our results suggest that careful attention needs to be paid to the type of demagnetization method chosen, especially in the case of coarse-grained loess deposits.

Tectonic Forcing of the Extreme Aridification of the East Asian Interior at Around 900 ka–Insights From the Spatially Inconsistent Magnetization of Chinese Loess

“…The higher values of HIRM and NRM 585°C /NRM in the coarsest part of L9 also indicate a higher proportion of hematite in this layer and its significant contribution to the NRM (Figure 3a and Figure S4 in Supporting Information ). Similar results were reported for the Gulang loess section, which is located on the northwestern margin of the CLP (Chen et al., 2014). Hematite has long been recognized as a reliable and stable magnetic carrier of the NRM in sediments (Butler, 1992; Hu et al., 2013; Turner, 1980).…”

“…Numerous paleomagnetic studies of loess sections in the eastern CLP (east of the Liupan Mountains) have documented a remagnetized normal polarity within the coarsest part of L9, instead of the expected reverse polarity (Heller & Liu, 1984; Jin & Liu, 2011; Wang et al., 2005; Xiong et al., 2001; Yue, 1995; Zheng et al., 2007). However, in several recent publications, we noted that the remagnetized normal polarity was absent in the coarsest part of L9 in loess sections in the western Loess Plateau (west of the Liupan Mountains) (Chen et al., 2014; Sun et al., 2019; Wang et al., 2014). Unfortunately, the spatial pattern of the observed polarity anomalies has not been comprehensively evaluated and the underlying cause is unclear.…”

“…However, it is surprising that our paleomagnetic results for the Caotan section reveal an overall reverse polarity in the coarsest part of L9 (Figure 2a). For the other three sections located in the northwestern marginal areas of the CLP, stable reverse polarities are also easily recognizable in the coarsest part of loess unit L9 (Chen et al., 2014; Sun et al., 2019; Wang et al., 2014) (Figure 1 and Figure S3 in Supporting Information ). By contrast, the paleomagnetic results for the Luochuan section reveal a stable normal polarity resulting from complete remagnetization within the coarsest part of L9 (Figure 2b) (Jin & Liu, 2011).…”

Environmental magnetic properties of core sediments in the Yellow River subaqueous delta and their chronologicalapplications

“…指示三角洲前缘河口沙坝沉积环境 [6,14] 。90%以上黄河泥沙来源于黄土高原 [30] ， 黄土成壤过程产生大量次生细颗粒 SD 和 SP 磁铁矿 [31] 。因此，该阶段 细颗粒 YD01 孔 Unit III 阶段，岩性主要以黄灰色黏土质粉砂为主，夹大量粉砂薄 层(2-5 cm)或纹层，判断为三角洲侧缘沉积环境 [14] HIRM 值高 [34] ，且也有与指示细颗粒磁性矿物含量参数相反的特征 [35] 。赤铁矿 等常见高矫顽力磁性矿物多在风力作用下以碎屑岩的形式从西北沙漠地区搬运 到黄土高原 [31,36]…”