黄土高原风成沉积物磁化率各向异性研究

摘要

The magnetic susceptibility of the Chinese loess-paleosol sequence in the Northwest China represents a proxy climate index related to the East Asian summer monsoon intensity. In contrast to the loess deposits, the magnetic susceptibility is almost independent of each other in the red clay, thus challenging the validity of the readily measurable magnetic susceptibility in describing the monsoon evolution history recorded by the red clay. Concentration variation of pedogenic magnetite and maghemite produced during pedogenesis is considered as the main reason for variation of magnetic susceptibility of the loess-red clay sediments. However no identical view on formation of magnetite and maghemite via inorganic precipitation or organic process was proved,the latter including the mediation of Fe-reducing bacteria which may be termed a ' Biologically Induced Mineralisation' ( BIM ) process and magnetotactic bacteria process called a ' Biologically Organised Mineralisation ' ( BOM ) process. The organic matter content and morphological characteristics of magnetic mineral in the eolian sediments have great potential for the study of magnetic minerals genesis and variation of magnetic susceptibility.rnIn this paper,the Zhaojiachuan section (35°45'N, 107°49'E) in Xifeng town is selected for this study. This section is located in the centre of Dongzhiyuan which is the largest Yuan in the Chinese Loess Plateau, Gansu Province. The present day mean annual temperature is about 8. 7℃ with a range of -22. 4 ~35. 1℃ ,and the mean annual precipitation is 555mm in this region. The profile contains an upper about 170m alternating loess-paleosol sequence and a lower red clay with a thickness of about 55m. The loess-paleosol sequence is traditionally subdivided into five major stratigraphic units. From the surface downward these are; Holocene Black Loam ( S0 ) , late Pleistocene Malan Loess(L1 ) , Middle Pleistocene Upper Lishi Loess ( S1-L9) and Lower Lishi Loess ( S9 ~ L15 ) , and early Pleistocene Wucheng Loess( S15 ~ L33) . The Late Miocene red clay lies on a base of lacustrine sediments. Magnetostratigraphic measurements have dated the lower boundary of the red clay deposit at 7. 6Ma which indicates aeolian dust accumulation began at least 7. 6Ma ago. In this study,about 2240 dry mass samples were taken at 10cm intervals for organic matter measurements that were quantified by Potassium bichromate-Ferrous ammonium sulfate-heating method. In addition,and magnetic minerals from the paleosol units S1 and S5 together with the loess layers L1 were characterized using transmission electron microscopy ( TEM). The result indicates that the loess-paleosol sediments above the paleosol unit S5 have higher values of organic matter greatly varying from 0. 25% to 0. 50% in the whole section, and variation of the organic matter concentration of the loess-red clay was positively related to the alternation of magnetic susceptibility of the loess-red clay horizon. The correlation indicates that nearly all the organic matter maxima corresponds with susceptibility peaks, and organic matter minima corresponds with susceptibility troughs, according with the loess and paleosol above the paleosol unit S5. The content from S5 to the bottom of the red clay correlates poorly with the signatures of the magnetic susceptibility compared with the records above the paleosol unit S5. All of these facts demonstrate that the magnetic minerals are mainly in pedogenic origin and the variation of organic matter content of the aeolian sequence may be regarded as an effective proxy of biogeochemical intensity during deposits weathering. The microbiology is a bridge between the magnetic susceptibility of aeolian sediments in North West China and paleoclimate. Thus it is suggested that the magnetic susceptibility of the Late Miocene red clay has same paleoclimatic implication as the Quaternary loess-paleosol sequence, which shows biogeochemical process intensity during aeolian sediment pedogenesis. In addition, the traditional viewpoint is that the Quaternary loess-paleosol sequence form under alternation of cold-dry and warm-wet climate while the red clay results from warm-dry climate which results in the variation of magnetic susceptibility. Thus climate setting should be taken into account for paleoclimatic reconstruction using environmental magnetism.%中国黄土高原粉尘堆积以往注重从物源区携带粉尘的古风向,对粉尘沉积→固结期的古风向缺乏研究.本文评述了近年来应用黄土高原黄土、红粘土磁化率各向异性研究古季风风向变迁所取得的一些重要进展并指出磁化率各向异性最大轴和最小轴的方位和倾伏均与沉积→固结期风向变化密切相关.黄土磁组构结果与利用各种统计方法计算出的古风向在多个剖面与现代夏季风方向相似,由此推论黄土是冬季风带来的沙尘在经历夏季风携带的降雨洗礼后才固结而成.而在黄土之下的红粘土地层中的初步研究发现磁组构显示明显的西风带沉积特征,指出这一现象将在今后研究中继续深入探讨.