新疆尼勒克黄土岩石磁学特征及变化机制研究

新疆黄土与黄土高原黄土相比，无论在物源还是后期成土环境方面都存在较大差异，因此二者的磁学特征有所不同。本文选取伊犁尼勒克地区的一个黄土-古土壤剖面进行了系统的岩石磁学及粒度研究。实验结果表明尼勒克剖面中磁性矿物具有以下特征：1）以亚铁磁性矿物磁铁矿和磁赤铁矿为主，并含有一定量的反铁磁性矿物赤铁矿和针铁矿；2）各黄土层成壤作用弱，磁性矿物以原始输入的粗颗粒MD和PSD为主。PS1古土壤层成壤作用强，以细颗粒SD为主。弱发育古土壤层PS0既包含SD颗粒，又有粗颗粒。各地层均不含SP颗粒；3）PS1古土壤层原始含铁矿物输入量略低于黄土高原地区，其他各地层均高于黄土高原地区，但受成土作用及其他因素影响，剖面中亚铁磁性矿物含量远低于黄土高原黄土-古土壤层。PS1古土壤层成壤作用强，在新疆地区较为少见，但其磁化率却低于各黄土层，说明尼勒克黄土-古土壤磁化率变化机制与黄土高原地区不同。古土壤层原始输入含铁矿物的量较黄土层低，但原始含铁矿物只是影响剖面磁化率变化的原因之一。PS1古土壤层在沉积后期受到间歇性水流作用，成土成因的强磁性SP磁铁矿/磁赤铁矿颗粒遭受破坏并转化为赤铁矿，导致PS1古土壤层磁化率的急剧降低，并使该层赤铁矿的相对含量增加。

STUDY OF ROCK MAGNETIC PROPERTIES AND ITS VARIATION MECHANISM OF LOESS IN NILEKE,XINJIANG

Xinjiang is located in the Central Asia which is the second largest loess distribution area of China. It is far from the ocean and the climate is controlled by the westerlies,so its climate is arid. The study of the loess magnetic properties in this area will contribute to our understanding of the evolution of the arid environment in northwest of China. Ili area is located in the west of Xinjiang,it is an ideal area for studying the paleoclimate change of the westerlies. We found a loess-paleosol section in Nileke(43°48'22"N, 82°38'38"E, altitude 1179m),northeast of Ili. Thickness of the section is 5.1m,and the paleosol layer is well developed. We collected samples across the section with 5cm intervals and got 103 samples in total. We have measured grain size and rock magnetic parameters of all samples. Grain size parameters mainly include median size d (0.5), the coarse grain size component(>63μm), the fine grain size component (χlf and χhf), isothermal remanent magnetization(IRM), saturation isothermal remanent magnetization(SIRM), anhysteretic remanent magnetization(ARM),hysteresis loop,temperature dependence of magnetization (M-T curve), temperature-dependent susceptibility curve (κ-T curve),low-temperature magnetic susceptibility curve,and the ratios of magnetic parameters(χARM and χARM/χ et al.).The analysis of grain size and rock magnetism in Nileke section show three characteristics:1)Ferrimagnetic minerals are the most important, such as magnetite and maghemite. At the same time there are amount of antiferromagnetic minerals, such as hematite and goethite, 2)Pedogenesis of the loess layers are weak, the MD and PSD grains in loess layers are brought by wind. PS1 paleosol is well developed, SD grains are the dominant magnetic component. SP grains are not found in all layers. 3)The lithogenic ferromagnetic minerals brought by wind from the source area in PS1 paleosol layer and loess layers at this section are respectively less and higher than that in the sections on the Loess Plateau. The content of ferrimagnetic minerals of Nileke section are less than that in the loess-paleosol layers of Loess Plateau because of the lower rate of pedogenesis and other factor. Pedogenesis of PS1 paleosol is very strong in Nileke section,and this is rare in Xinjiang region. But its magnetic susceptibility is less than loess layers. The mechanism of the susceptibility enhancement is more complicated. During the paleosol formation period,the west wind force is weak which will result in less iron input by wind in the paleosol layer than loess layers, and vice versa for the glacial periods, The SP magnetite and maghemite produced by pedogenesis are destroyed by intermittent fluviation. There are more hematite produced by the intermittent fluviation and strong chemical weathering in the warm interglacial period than glacial period. And some of the hematite are transformed from magnetite and maghemite.