Tectonically-controlled Evolution of the Late Cenozoic Nihewan Basin, North China Craton: Constraints from Stratigraphy, Mineralogy, and Geochemistry

The Late Cenozoic basins in the Weihe-Shanxi Graben, North China Craton are delineated by northeast-striking faults. The faults have, since a long time, been related to the progressive uplift and northeastward expansion of the Tibetan Plateau. To show the relation between the basins and faults, two Pliocene-Pleistocene stratigraphic sections （Chengqiang and Hongyanangou） in the southern part of the Nihewan Basin at the northernmost parts of the graben are studied herein. Based on the sedimentary sequences and facies, the sections are divided into three evolutionary stages, such as alluvial fan-eolian red clay, fan delta, and fluvial, with boundaries at -2.8 and -1.8 Ma. Paleocurrent indicators, the composition of coarse clastics, heavy minerals, and the geochemistry of moderate-fine clastics are used to establish the temporal and spatial variations in the source areas. Based on features from the middle- northern basin, we infer that the Nihewan Basin comprises an old NE-SW elongate geotectogene and a young NW-SE elongate subgeotectogene. The main geotectogene in the mid-north is a half-graben bounded by northeast-striking and northwest-dipping normal faults （e.g., Liulengshan Fault）. This group of faults was mainly affected by the Pliocene （before -2.8-2.6 Ma） NW-SE extension and controlled the deposition of sediments. In contrast, the subgeotectogene in the south was affected by northwest-striking normal faults （e.g., Huliuhe Fault） that were controlled by the subsequent weak NE- SW extension in the Pleistocene. The remarkable change in the sedimentary facies and provenance since -1.8 Ma is possibly a signal of either weak or strong NE-SW extension. This result implies that the main tectonic transition ages of -2.8-2.6 Ma and -1.8 Ma in the Weihe-Shanxi Graben are affected by the Tibetan Plateau in Pliocene-Pleistocene.

现代土壤磁化率空间分布特征及成壤影响因素分析——以桑干河阳原段为例

为了揭示影响成壤作用发生的主要因素,对河北省阳原县桑干河流域表层土壤开展常规土壤指标和环境磁学参数的地统计学分析,结果表明,土壤的频率磁化率(χfd20)、Eh值、有机质、黏土-粉砂含量等参数的空间分布特征受控于研究区的高程,而土壤的含水量、铁游离度和活化度的空间分布特征受控于坡向。因此,在影响成壤作用的五大因素(母质、气候、地形、有机质、时间)中,地形(高程、坡向)是影响研究区成壤作用强弱的主要因素。此外,虽然前人发现频率磁化率和铁游离度都能反映铁氧化物含量和成壤作用强弱,但本次研究通过对比空间分布特征发现,频率磁化率与有机质、Eh值等土壤指标有较好的相似性,且与耕地类型分布一致,所以,相对于铁游离度,频率磁化率更能反映研究区土壤发育状况。

Magnetic Fabric Study of Late Holocene Sediments in Huangqihai Lake,Inner Mongolia and its Sedimentary Significance

The detailed magnetic susceptibility anisotropy study on the sediments of two sections, which are located in the lake-center and the lakeshore of the Huangqihai Lake, Inner Mongolia, revealed their hydrodynamics and sedimentary environment changes in the recent 2000 years. The results show that the sediments of two sections both display normal depositional magnetic fabrics, of which the minimum susceptibility axes （K3） are oriented perpendicular to the bedding plane and the maximum magnetic susceptibility axes （K~） lie parallel with the bedding plane. In addition, the iakeshore presents higher magnetic anisotropy degree （P）, magnetic foliation （F） and current factor （F~） than the lake-center, showing that the lakeshore has higher flow velocity. For the west Huangqihai Lake （HQW） section in the lake-center, the magnetic matrix grain degree （Q） and the inclination of the minimum susceptibility axes sharply increased and decreased, respectively, at a depth of about 10 cm, 24 cm, 45 cm and 74 cm, which indicate abnormal depositional events with strong kinetic energy. Through comparative analysis, the HQW section can be divided into three stages from top to bottom. The upper stage was a stable depositional environment with strong hydrodynamic energy and low water level, which shows relatively higher Fs value and the Q value varies similarly with P and F. The middle stage was a stable depositional environment with high water level and low hydrodynamic energy, intermitted with high-energy abnormal events, which shows relatively lower Fs and the Q value was negatively correlated with P and F. The lower stage was an unstable depositional environment with low hydrodynamic energy and high water level, which shows relatively lower P, F, and F~ value.

Preparation of Biomorphic SiC/C Ceramics from Pine Wood via Supercritical Ethanol Infiltration

Biomorphic （wood derived） carbide ceramics with an overall composition in the SiC/C was produced by supereritical ethanol infiltration of low viscosity tetraethylorthosilicate/supercritical ethanol into biologically derived carbon templates （CB-templates） and in situ hydrolysis into Si（OH）4-gel, the Si（OH）4-gel was calcined at 1400℃ to promote the polycondensation of Si（OH）4-gel into SiO2-phase and then carbonthermal reduction of the SiO2 with the biocarbon template into highly porous, biomorphic SiC/C ceramics. The phases and morphology conversion mechanism of resulting porous SiC/C ceramics have been investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and Fourier transform infrared spectroscopy （FT-IR）. Experimental results showed that the biomorphic cellular morphology of pinewood charcoal was remained in the porous SiC/C ceramic with high precision that consisted of β-SiC with minority of α-SiC and the remain free carbon existed in amorphous phase.

Environmental magnetic comparisons between distal and proximal sediments of Huangqihai Lake,Inner Mongolia,China

Spatial variations in lake sediments are a product of their depositional heterogeneities.To better correlate synchronous strata in Huangqihai Lake,two sections were excavated from the lake-center and the lake-shore.The study involved detailed lithology,mineralogy,and rock magnetic analysis.The results show that the frequency-dependent susceptibility,which is related to the content of super-paramagnetic minerals,is relatively easy to contrast between the two sections.However,mass susceptibility and saturation isothermal remanence,which are related to the content of total magnetic minerals,are difficult to contrast.A possible reason for this difference is that small-particle super-paramagnetic minerals are formed mainly during pedogenic processes,which correlates well with climate change and avoids the effects of local factors when corrections are made at the whole-lake scale.Both sections can be divided into three stages,which appear to be well correlated with previous results obtained through other environmental proxies.

碳酸盐碳同位素揭示的泥河湾盆地上新世中晚期C_(4)植物扩张历史

植被中C_(4)植物比例与气候条件和生态系统类型密切相关,了解C_(4)植物扩张历史对于理解过去生态系统格局演化以及古气候变化具有重要科学意义。本研究在已有古地磁年代基础上,通过对泥河湾盆地郝家台NHA钻孔224.2~175.4 m段内243个沉积物样品的碳酸盐δ13C分析,结合δ18O、地球化学元素、粒度等环境代用指标测试,揭示了泥河湾盆地上新世3.66~2.89 Ma期间的C4植物扩张历史。研究结果显示:1)泥河湾盆地3.66~2.89 Ma期间,δ13C均高于-7‰,且δ13C值整体呈现波动上升的趋势,指示了研究时段内泥河湾盆地C3、 C_(4)植物共存,C_(4)植物总体呈现出明显扩张的趋势,其中约3.58 Ma和约3.10 Ma为研究时段内两次最为明显的C_(4)植物扩张重要时间节点;2)3.58 Ma之前,δ13C多低于-5‰,植被以C_(3)植物为主,3.58~3.10 Ma期间δ13C含量在-5‰至-3‰之间波动,气候趋向干旱化方向发展,C_(4)植物所占比例有所增加;3)3.10 Ma之后的全球性冷干事件导致泥河湾盆地气候显著变冷变干,尤其是干旱化程度显著增强,δ13C多高于-3‰,指示泥河湾盆地C_(4)植物出现显著扩张,以C_(4)植物为主的草本植被占比可能超越C_(3)植物,成为区域主要的植被类型;4)C_(4)植物扩张过程中,也存在数次波动,如在约3.46 Ma、约3.38 Ma、约3.28 Ma和约3.13 Ma等时段内出现数次持续时间较短的δ13C明显低值,分别可以与全球深海氧同位素阶段(MIS)明显冷时期对应的MG6、 MG4、 M2和KM2相一致,但这些低值呈现出随时间推移,δ13C值逐渐升高的趋势,显示C4植物的影响逐渐加强。

泥河湾盆地早更新世（2．6～2．1Ma）植被和气候变化

文章通过泥河湾盆地郝家台NHA钻孔约33 m岩芯（15800～12500 cm，2.6～2.1 Ma）共165个样品的高分辨率孢粉分析，并结合古地磁定年和PCA分析结果，重建了该地区早更新世2.6～2.1 Ma时段的植被和气候环境演化过程。研究结果显示：1）2.60～2.47 Ma，孢粉组合以偏暖的松属（多高于40%）和喜冷湿的云杉属（多高于60%）交替占优为特点，PCA axis 1和axis 2得分值均较低，但变化剧烈，植被应为松林和云杉林交替占优，气候冷暖交替为特征。2.60～2.57 Ma和2.53～2.51 Ma为两个寒冷期，大体对应于MIS 102和MIS 100阶段；2.49～2.47 Ma为干旱期，大体对应于MIS 98阶段；2.57～2.53 Ma为研究段温暖湿润阶段。2）2.47～2.33 Ma时段，总体云杉花粉（多高于40%），蒿属、藜科等喜旱草本花粉（多高于20%）含量较高，PCA axis 1得分值较高，表明研究区当时植被类型主要为森林草原或云杉林，气候寒冷，其中2.47～2.42 Ma为研究段最寒冷干旱时期，大体对应于MIS 96阶段。3）2.33～2.14 Ma时段孢粉组合中松属花粉多高于50%，阔叶乔木花粉含量多高于5%，PCA axis 1和axis 2得分值均为研究段最低时期，显示植被为松与阔叶树的混交林，或两者呈斑块状分布，气候最为暖湿。但2.29～2.24 Ma阶段，PCA axis 1得分值明显升高，气候变冷，大体对应于MIS 88和MIS 86阶段。