中亚地区现代表土磁学特征及其古环境意义*

在中国西北地区主要沙漠、绿洲和戈壁及蒙古国南部戈壁等亚洲中部粉尘源区采集了表土(包括沙漠和土壤)样品,系统分析了样品的磁学性质(包括磁化率、无磁滞磁化率、等温剩磁、磁滞回线和热磁曲线)。结果表明,中亚地区表土样品中主要磁性矿物是铁磁矿,含有赤铁矿和磁赤铁矿,并伴有少量顺磁性矿物;磁性矿物粒度主要是准单畴(PSD)和多畴(MD),超顺磁性颗粒含量较低,接近于中国西部黄土地层含量。中亚地区表土磁性矿物含量总体都较低,磁性矿物含量在空间上与降雨量成正比。研究结果指示西部地区(准噶尔盆地和南部塔里木盆地)表土样品较北部地区(鄂尔多斯高原、阿拉善高原以及蒙古高原)表土样品,其粗颗粒磁性矿物含量高,而细颗粒磁性矿物含量低。本研究结果为黄土地层古土壤磁性增强源自成土过程提供了最直接的证据。

MAGNETIC CHARACTERISTICS OF SURFACE SOIL IN ARID REGION OF CENTRAL ASIA AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE

In this study, magnetic techniques were used to characterize surface soils from different geomorphology (i.e. sand desert, oasis, Gobi, and dry lake) in Northwest China and Mongolia. Results demonstrate that the main magnetic minerals in surface soil are magnetite, maghaemite and haematite with some paramagnetic material. Cross plots of Mrs/Ms vs. Bcr/Bc and χ_fd % vs. χ_ARM/SIRM indicate that the main magnetic grain sizes in surface soil are pseudo single domain (PSD) and multidomain (MD). No significant difference has been found in some magnetic parameters including χ_lf, SIRM, SOFT and SIRM/χ_lf, from surface soils from sites from the West area (i.e. Tarim Basin and Junggar Basin) and the North area (i.e. Alxa Plateau, Erdos Plateau and Monlia Plateau). However, other magnetic parameters, i.e. χ_fd ％, χ_ARM,χ_ARM/SIRM,χ_ARM/χ_lf and HIRM generally have lower values in the West. Samples from the West are dominated by magnetic minerals with lager grain size while those from the North are dominated by fine magnetic minerals. The similarity in magnetic mineral constitutions between Chinese loess and surface soils from Central Asia implies that the loess might originate from a vast arid to semi-arid region in Central Asia. The low value of concentration-dependent magnetic parameters indicates that the low concentration of magnetic minerals in surface soils from Central Asia and the magnetic enhancement from pedogenic take place in both the loess and the paleosols, although the progress is stronger in the later.