腾格里沙漠边缘表土磁学性质及其意义

通过对腾格里沙漠边缘表土进行岩石磁学的测试和分析,发现样品中的磁性矿物主要是铁的氧化物。主要为磁铁矿,其次为磁赤铁矿和赤铁矿。磁颗粒以多畴和准单畴为主。样品之间低频磁化率值相差很大,靠近雅布赖山附近的样品明显高于其他样品。频率磁化率普遍很弱,最大3.7%,反映了该沙漠边缘区的干旱条件下所进行的以物理风化为主的地质过程。结果表明,本地区磁化率几乎不受成土作用的影响。磁化率增强机制与黄土高原不同,可能主要反映了局部地区的母质、地形和微气候的影响。

THE MAGNETIC PROPERTIES OF TOPSOIL FROM THE EDGE OF TENGGER DESERT,AND ITS ENVIRONMENTAL SIGNIFICANCE

Based on systematical analysis on rock magnetism,this paper studies the topsoil from the edge of Tengger Desert in Northwestern China.The desert is the fourth largest desert in China,located in the northeast of the Chinese loess plateau.The most parts locate in Ala Shan,Neimenggu Province,the rest parts locate in the Gansu and Ningxia Province.The area has a small precipitation of 102.9mm but a high evaporation of 2258.8mm.The temperature changes greatly.The maximal temperature is 39℃,and the lowest temperature is -29.6℃ during a year.The west wind blows nearly all over the year whose average speed reaches 4.1m/s.On the edge of the desert,18 samples were collected along the desert highway.The sample sites have a least distances of 500m away from the highway,and depths of 2~5cm under the surface,for reducing human disturbances.A 30km distances was adopted between the nearest two samples.A series of data were measured in the Key Laboratory of Western China ’s Environmental Systems, Lanzhou University,such as Xlf, Xfd%,XARM,Hcr,J -T curves,hysteresis loops.The average value of Xlf is 6.38×10-7 m3/kg.The maximum is 31.49×10-7m3/kg from sample 9,and the minimum is 1.83×10-7 m3/kg from sample 11.An interesting phenomenon shows that,the samples near the Yabulai Mountain have higher values.Xfd% have low values between 0.1% to 3.7%,which show the samples lack SP(superparamagnetic)grains,and the pedogenesis was very weak.Hysteresis loops and the Hcr show all samples contain ferromagnetic minerals.J -T curves show the samples have a clear Curie point of 580℃,which indicates that the dominant mineral of samples is magnetite.And the samples have a faint Curie point around 680℃,which indicates that the samples also contain hematite. Heating curve was beyond cooling curve in thermomagnetic curves. This demonstrates that there are some maghemite translating hematite in those samples. The measured magnetic parameters indicate that main magnetic minerals of those topsoil is magnetite,the other may have maghemite and hematite.The grain size of magnetic minerals is coarse,mostly within range of multi-domain.The magnetic susceptibility has little relation with the pedogenesis but closely relates with landform and parent materials.The topsoils from such arid area or aeolian dust,show similar magnetic mineralogy between them,and do not undergo clearly pedogenesis.Their enhancement mechanism of magnetic susceptibility differs from that in the Loess Plateau.The magnetic susceptibility in such dried region indicates information of parent material,landform and microclimate,instead of pedogenesis.