Nanominerals Of The Loess-red Clay Sequences In Chinese Loess Plateau

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.