【作者】 谢巧勤；

【导师】 徐晓春； 陈天虎；

【作者基本信息】 合肥工业大学 ， 矿物学岩石学矿床学， 2009， 博士

【摘要】 中国黄土高原的风尘序列包括第四纪黄土-古土壤序列和晚第三纪红粘土序列,该套沉积是全球气候变化最好的陆相记录之一。从上个世纪五十年代开始,中国黄土就吸引了大批中外第四纪地质学家和气候学家。他们对黄土的成因、形成时代和蕴含的古气候信息进行广泛深入的研究,从而将中国的黄土推向了全球古气候研究的前沿。本文以高分辨电镜为主要研究手段,结合粉晶x射线衍射、土壤化学分析等方法,从纳米尺度对黄土高原风尘序列中成土强磁性矿物种类、磁性矿物粒径、磁性矿物存在形式和演化以及成壤过程中新生的纳米非磁性矿物种类及其古气候意义进行了深入系统的研究,并探讨了黄土磁化率的古气候本质以及在红粘土中的适用性。论文在以下方面取得了新的进展和认识:磁铁矿、磁赤铁矿和赤铁矿是黄土-红粘土的磁信号载体,成土强磁性矿物主要为磁赤铁矿而不完全为磁铁矿,这些成土磁赤铁矿是由成土磁铁矿低温氧化而来;亚微米的强磁性颗粒为具有纳米多孔超微观结构的复合磁性颗粒,主要由纳米磁赤铁矿和磁铁矿构成。黄土和古土壤中磁性矿物的粒径和磁化率载体是显著不同的,黄土以粉尘粗颗粒磁性矿物为主,50%左右的磁化率由风尘磁性矿物携带,而古土壤磁性颗粒以成壤新生亚微米-纳米颗粒为主,70%左右的磁化率由成土纳米磁性矿物携带;实验同时表明,CBD实验不仅能完全溶解成土磁性颗粒,而且能够部分溶解风尘磁性颗粒。黄土高原黄土、古土壤和红粘土的磁性矿物种类没有差别,但其中的磁性矿物相对含量差别显著。黄土磁性矿物以风尘磁铁矿和赤铁矿为主,少量成土磁赤铁矿和针铁矿。古土壤磁性矿物以成土磁赤铁矿和磁铁矿为主,风尘磁铁矿、成土赤铁矿、针铁矿次之。红粘土磁性矿物以风尘磁铁矿和成土赤铁矿为主,成土磁赤铁矿次之。黄土、古土壤和红粘土中磁性矿物以包裹磁性颗粒、黏附磁性颗粒和独立磁性颗粒三种主要方式存在,独立磁性颗粒为磁性矿物最主要的赋存形式。根据磁性矿物种类和微观结构,独立磁性颗粒可以分为单一物相和复合颗粒两大组合形式,以及磁铁矿核-赤铁矿边、磁赤铁矿核-赤铁矿边的不同微结构。黄土、古土壤和红粘土磁性矿物存在形式和微结构的差别,是磁铁矿粒径以及粉尘沉积后成壤阶段古气候性质的差异所致。黄土-红粘土序列磁化率与生物、生物活动密切相关。黄土磁化率古气候意义在红粘土中同样适用,其古气候意义不仅仅是成壤强度指标,更确切地说是成壤强度控制因素,即生物地球化学强度的指标。风尘序列磁化率与古气候密切联系的媒介是生物,二者通过下列途径联系:古气候影响生物产率和生物活动强度,生物产率、生物活动强度和古气候影响黄土磁性矿物的形成和转化速率,磁性矿物的形成和转化速率决定黄土-古土壤磁化率的变化。第四纪黄土中存在大量的新生纳米方解石,这种纳米方解石一维延长,呈棒状,长度不定,从几纳米到几微米,少见弯曲形态,直径均一,一般在40nm左右,有的两根棒共生在一起形成“T”字形态。纳米棒状方解石在黄土层中的含量高于古土壤中的含量。纳米棒状方解石是黄土成壤过程中赖干冷环境生物活动产物,是干冷环境指示矿物。利用新生的纳米棒状方解石碳氧同位素可以反演古气候。与第四纪黄土不同的是,晚第三纪粘土中大量出现的新生矿物为一维纳米凹凸棒石,这种纳米矿物主要以面型和在粒间间隙中以束状、交织缠绕状分布,舒展形态显示其良好的柔韧性和弯曲度,典型的自生成因,是干旱-半干旱环境的指示矿物。系统的高分辨电镜调查研究发现,凹凸棒石在灵台红粘土中不是均匀分布的,但重要的是凹凸棒石含量的突变点与红粘土演化史上的古气候突变点或转折点相一致。中国黄土高原红粘土中凹凸棒石的大量出现标志了西北内陆干旱化的开始,凹凸棒石含量的降低显示了干旱化的进一步加剧。更多还原

【Abstract】 The eolian sequence in Chinese Loess Plateau, including the Quaternary loess-paleosol sequence and Tertiary red-clay sequence, is an excellent continent paleoclimate proxy. Since 1950’s, the genesis, dating and paleoclimate significance of the loess depositions have been lucubrated by oversea and inland geologists, climatologists. Now the Chinese loess deposits have become an important part of world paleoclimate agents. In the paper, magnetic mineralogical types, grain size distribution, assemblages, concentration, microcharacteristic, evolution and paleoclimate implying were examed at nano scale by applying high resolution electron microscopy, X-ray powder diffraction, and CBD method, as well nanometer fibroid minerals (main calcite and palygorskite ) resulting from pedogensis. The major findings and conclusions are listed as the following:The principal carriers of magnetic properties in the loess-red clay deposits are magnetite, maghemtite and hematite. The ferromagnetic mineral formed during pedogenesis is mainly maghemite resulting from pedogenic magnetite lower- temperature oxidation, next to magnetite. The sub-micrometer ferromagnetic particles mainly consist of nanometer maghemtite and magnetite, and have pecial morphological characteristics and microstructures of nanoporous textures.For loess samples, coarse grains (≥2μm) carry more than 50% of the magnetic signal. A different pattern is found for paleosol samples, in which the≤0.3μm fraction contributes > 60% (before magnetic extraction) or 70% (after extraction) to the bulk MS. In addition, the CBD technique is absolutely able to extract pedogenic superfine-grained magnetic particles and some part of coarse dust magnetic particles.The Chinese loess sequences have some identical magnetic minerals. However, the content of ferromagnetic minerals is varied with pedogenesis intensity and paleoclimatic properties. In the loess deposits, eolian magnetite and hematite are main magnetic minerals, low content of maghemite and goethite. For the paleosol, the concentration of pedogenic maghemite and magnetite is higher than that of eolian magnetite and pedogenic hematite. And the paleosol maybe have few eolian hematite and goethite. To the red clay, eolian magnetite and pedogenic hematite are the most important magnetic minerals, next to pedogenic maghemite.There are three types of magnetic mineral formation including inclusive magnetic particles, coating magneitc particles and single magnetic particles, a key formation. According to magnetic mineral species and microstructures, the third formation is divided into only one species of magnetic mineral (either magnetite or maghemite) and magnetic minerals complexes. The first type of complex, comprising magnetite and maghemite, developed as follows. Sub-micrometer eolian magnetite was partially oxidized to form nanoporous texture consisting of nano-crystalline maghemite in the case that oxidation was incomplete; alternatively, nanometer-scale pedogenic magnetite was oxidized to maghemite. The former types of grains have been identified in the present study, but not the latter, perhaps due to complete oxidation resulting from their fine grain size. The second type of complex comprises magnetite and hematite, resulting from partial LTO of micrometer-scale eolian magnetite. The third type is a maghemite–hematite complex that mainly originated from the oxidation of pedogenic maghemite. The loess, paleosol and red clay have different magnetic minerals formation and microstructures resulting from difference of magnetite grain size and paleoclimatic property during pedogenesis.The loess-red clay magnetic susceptibility has a consanguineous relationship with biology and its action. The loess susceptibility has been regarded as a good proxy of pedogenesis intensity. However more exact expression should be an index of biogeochemistry process intensity. Biology correlates magnetic susceptibility to paleoclimate, and accomplished by following processes: firstly, the paleoclimate affects biology production and its action intensity, then in turn controls magnetic minerals formation and minerals phase transition, at last production and transformation rate of magnetic mineral restricts magnetic susceptibility value of the loess-red clay sequences. So biology is a key bridge to the eolian susceptibility and paleoclimate.The Quaternary loess sequences have plenty of nanometer calcite with nod morphology, in diameter of 40 nm and the crystal length of n. nm - n.μm. The calcite content in loess is usually higher than that in paleosol, and nanometer calcite in the paleosol suffers from more intensity eluviation than that in loess. These nano nod calcite may be origin from biology process during eolian dust pedogenesis, and these biology withstands aridity-cold climate condition. So the calcite content in the loess and paleosol is especially suitable for the studies of paleoclimatic changes. The higher-resolutionδ13C andδ18O values of nano nod calcite can elucidate the paleoclimatic fluctuation in the Chinese Loess Plateau during Pleistocene.Palygorskite is in rich Mg-Al silicate with chain-layer structure, in diameter range from 30nm to 50nm and the crystal length of n. nm - n.μm, a typical indicator mineral of arid and strong evaporation environment. The result of SEM images in suit show that palygorskite distributes in intergranular space of the red clay or shows laminar spreading, and arranges in bunch resulting from crystal parallel intergrowth, winding and twisting. Under high resolution SEM (nanoscale),palygorskite fiber crystal grows around the smectite edges, and sometimes the whole slice consists of palygorskite. Therefore, palygorskite is not only autogenic but also may be origin from illite-smectite transformation. The autogenic palygorskite extensively occurs in red clay deposition before about 3.6 Ma( a few during 3.2 Ma-3.6 Ma ) at Lingtai profile, which reveals that the late Tertiary climate during the red clay formation should be dominated by dry-warm climate condition characterized with strong evaporation, and indicates the important shift of the East Asian paleomonsoom system in this period. This research provides mineralogy evidence for paleoclimate evolution, which is in good agreement with previous researches. Meanwhile, the shift from dry-warm to dry-cold climate condition results in much colder and drier of the Asian interior since 3.6MaBp even if eolian deposits have accumulated on the Chinese Loess Plateau since ~7.0MaBp or much early.更多还原