摘要
对华北泥河湾盆地大长梁剖面更新世河湖相沉积序列进行的多参数岩石磁学研究表明磁铁矿和赤铁矿是大长梁剖面最主要的磁性矿物。磁铁矿的粒度主要为准单畴和多畴,主要来自盆地周围侵蚀的碎屑矿物;赤铁矿主要为风成成因,来源于中国西北部的黄土高原或沙漠地区的化学风化作用产物;某些沉积层中还含有主要由碎屑成因的粗颗粒磁铁矿经低温氧化形成的磁赤铁矿,该过程可能与泥河湾古湖的氧化环境演化密切相关。大长梁剖面的磁滞回线大多具有细腰特征,细腰的明显程度主要由样品中磁铁矿和赤铁矿的相对贡献控制。因此,细腰磁滞回线可以用来衡量泥河湾河湖相沉积物中赤铁矿的相对含量,并进而在一定程度上指示亚洲内陆干旱/半干旱地区化学风化作用的强弱以及相对温湿或冷干的气候环境变化。
The fluvio-lacustrine sequence of the Pleistocene age in the Nihewan Basin, North China, contains highresolution archives of early human evolution and paleoenvironment changes. In this study, a multi-parameter investigation such as the high temperature magnetic susceptibility （χ-T curves）and magnetization（Ms-T curves）, IRM acquisition and DC field demagnetization of IRM1.5T（IRM acquires at an applied field of 1.5T）, first order reversal curve （FORC） diagrams, magnetic hysteresis loops, and low temperature demagnetization of saturation isothermal remanent magnetization （SIRM）was carried on the samples collected from the Dachangliang section located at the east margin of the basin. 196 samples from the depth interval of 11 - 73m with a spacing of 20 - 40cm were collected for the present investigation. Results show that magnetite is the major magnetic materials and magnetic carrier of the characteristic remanent magnetization. Magnetite is dominated by pseudo-single-domain （PSD） or muhidomain （MD）grains, mainly the detrital input from the catchment regolith. Hematite, another important magnetic carrier of the characteristic remanent magnetization, is not well expressed in the χ-T curves due to its weak susceptibility. However, the combined characters of Ms-T curves, IRM acquisition and DC field demagnetization of IRMa.sT, the hysteresis loops and the FORC diagrams well demonstrate the presence of hematite. Hematite is dominated by the eolian hematite grains formed by chemical weathering in the Loess Plateau region or the deserts in Northwest China. Some χ-T curves show a susceptibility hump at about 300℃ , which suggests that maghemite may also exist in some samples. Our analysis shows that maghemite may mainly come from low temperature oxidation of coarse detrital magnetite grains. This low temperature oxidation process may relate to the oxidation/reduction condition in the Nihewan paleolake. Most of the magnetic hysteresis loops from the Dachangliang section show a wasp-waisted behavior, and the degree of wasp-waistedness is mainly controlled by the relative content of magnetite and hematite in the samples. Therefore, to some extent, the wasp-waisted hysteresis loops can he used to assess the relative content of hematite in the samples, which can he further used to retrieve the degree of chemical weathering and relatively warm/humid or cold/dry paleoclimate in the arid/semiarid areas of Asian interior. Our present study also demonstrates that the rock magnetic analyses can suggest some climatic significance, which may contribute to a better understanding of early human evolution and paleoenvironment changes at the high-latitude Nihewan Basin during the Early Pleistocene.
出处
《第四纪研究》
CAS
CSCD
北大核心
2007年第6期1072-1080,共9页
Quaternary Sciences
基金
国家自然科学基金项目(批准号:40221402和40325011)
中国科学院知识创新工程项目(批准号:KZCX3-SW-150)资助
关键词
泥河湾盆地
岩石磁学
环境磁学
细腰磁滞回线
Nihewan Basin, rock magnetism, environmental magnetism, wasp-waisted hysteresis loop
