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1.
The sandstones of the Ridge and Athleta members of Chari Formation(Callovian-Oxfordian)exposed at Jara have been analyzed for their petrographical and geochemical studies. Texturally, these sandstones are medium to coarse grained, poorly to well sorted, sub-angular to sub-rounded, and show low to medium sphericity.These sandstones were derived from a mixed provenance including granites, granite-gneisses, low and high-grade metamorphic, and some basic rocks of Aravalli range and Nagarparkar massif. The petrofacies analysis reveals that these sandstones belong to the continental block and recycled orogen tectonic regime. The studied sandstones are modified by paleoclimate, distance of transport, and diagenesis. Mineralogically and geochemically, sandstones are classified as quartzarenite, subarkose, arkose, sublithic arenite, and wacke, respectively. The A-CN-K ternary plot and CIA, CIW, PIA, and ICV values suggest that the similar source rocks suffered moderate to high chemical weathering under a hot-humid climate in an acidic environment with higher PCO_2. Generally good to strong correlations between Al_2O3 and other oxides in these sediments indicate clay mineral control. The K_2O/Na_2O versus SiO_2 diagram indicates that the studied samples occupy passive margin fields but the SiO_2/Al_2O_3 versus K_2O/Na_2O plot suggests that the Athleta Sandstone and Ridge Sandstone fall within the passive margin field, while Ridge Shale falls within the active continental margin field. 相似文献
2.
While saltation bombardment of sand grains on a fine substrate can produce considerable dust, the well-sorted nature of sand dunes tends to preclude them from consideration as major dust sources. Recent research, however, has revealed that sand dunes can, in some cases, be large sources of dust. We used the PI-SWERL(Portable In-Situ Wind Erosion Laboratory) to measure in the field the potential of sand dunes and other desert landforms to emit particulate matter 10 μm(PM-10) dust in the Tengger, Ulan Buh, and Mu Us deserts of northern China. Combined with high resolution particle size measurements of the dune sand, an assessment of sand dunes as a dust source can be made. Large active transverse dunes tend to contain little to no stored PM-10, yet they produce a low dust flux. Coppice dunes stabilized by vegetation contain appreciable PM-10 and have very high dust emission potential. There is a positive correlation between the amount of PM-10 stored in a dune and its potential dust flux. Saltation liberates loose fines stored in dunes, making them very efficient dust emitters compared to landforms such as dry lake beds and washes where dust particles are unavailable for aeolian transport due to protective crusts or sediment cohesion. In cases where large dunes do not store PM-10 yet emit dust when active, two hypotheses can be considered:(1) iron-oxide grain coatings are removed during saltation, creating dust, and(2) sand grains collide during saltation, abrading grains to create dust. Observations reveal that iron oxide coatings are present on some dune sands. PI-SWERL data suggests that low dust fluxes from dunes containing no stored dust may represent an estimate for the amount of PM-10 dust produced by removal of iron oxide coatings. These results are similar to results from dunes in the United States. In addition, PI-SWERL results suggest that dust-bearing coppice dunes, which cover vast areas of China's sandy deserts, may become major sources of dust in the future if overgrazing, depletion of groundwater, or drought destabilizes the vegetation that now partially covers these dunes. 相似文献
3.
The present study deals with the geochemistry of Late Quaternary ironstones in the subsurface in Rajshahi and Bogra districts, Bangladesh with the lithological study of the boreholes sediments. Major lithofacies of the studied boreholes are clay, silty clay, sandy clay, fine to coarse grained sand, gravels and sands with(fragmentary) ironstones. The ironstones contain major oxides, Fe_2 O_3*(*total Fe)(avg. 66.6 wt%), SiO_2(avg. 15.3 wt%), Al_2 O_3(avg. 4.0 wt%), MnO(avg. 7.7 wt%), and CaO(avg. 3.4 wt%). These geochemical data imply that the higher percentage of Fe_2 O_3* along with Al_2 O_3 and MnO indicate the ironstone as goethite and siderite, which is also validated by XRD data. A comparatively higher percentage of SiO_2 indicates the presence of relative amounts of clastic quartz and manganese-rich silicate or clay in these rocks. These ironstones also have significant amounts of MnO(avg. 7.7 wt%) suggesting their depositional environments under oxygenated condition. Chemical data of these ironstones suggest that the source rock suffered deep chemical weathering and iron was mostly carried in association with the clay fraction and organic matter. Iron concretion was mostly formed by bacterial build up in swamps and marshes, and was subsequently embedded in clayey mud.Within the coastal environments, the water table fluctuates and goethite and siderite with mud and quartz became dry and compacted to form ironstone. 相似文献
4.
Because of the important contributions of electrochemical redox reactions to biochemical cycles and their potential application for the in-situ remediation of contaminated sediment,the mechanisms of long-distance electron transport coupling spatially separated redox half reactions in sediment have drawn much attention.To explore a preliminary mechanism of long-distance electron transport in sediment,in the current study,two simplified composite systems are constructed consisting of spherical ferroferric oxide(Fe_3 O_4) nanoparticles and rod-like carbon nanotubes(CNTs) as conductive fillers and silica(SiO_2) particles as the matrix.Two different constructed composite systems(e.g.,SiO_2/Fe3 O4 and SiO_2/Fe3 O4/CNTs) were used to model a three-dimensional sediment framework instead of sediment with quite co mplex components.The effects of the loading of conductive fillers(e.g.,Fe_3 O_4,CNTs) and the particle size of SiO_2 matrix on the conductive behavior of the composite system were investigated.The results showed that both of the electrical properties of SiO_2/Fe3 O4 and SiO_2/Fe3 O4/CNTs composite systems typically exhibited a non-linear conductive behavior that the electrical conductivity increased with the increasing of filler loading and showed an abrupt increase at critical filler loading.The conductivity of the SiO_2/Fe_3 O_4 and SiO_2/Fe3 O4/CNTs composite systems with micro-sized SiO_2 as the matrix was higher than that of the composite systems with nano-sized SiO_2 as the matrix.Compared with the SiO_2/Fe_3 O_4 composite system,the electrical conductivity of the SiO_2/Fe3 O4/CNTs composite system was enhanced by several orders of magnitude and only a small loading of CNTs could make the conductivity of the SiO_2/Fe_3 O_4/CNTs composite system reach a higher level.The electrical conductivity predicted by the electrical conductivity model of a two-phase composite system showed a similar trend as the experimental results and the two-dimensional(2 D) percolation-based model filled with rods gave a good estimation of percolation probability. 相似文献
5.
A high-resolution mineral magnetic investigation has been carried out on the Jingbian loess/paleosol sequence at the northern extremity of the Chinese Loess Plateau. Results show that the magnetic assemblage is dominated by large pseudo-single domain and multidomain-like magnetite with associated maghemite and hematite. Variations in the ratios of SIRM 100mT/SIRM, SIRM 100mT/SIRM 30mT and SIRM 100mT/SIRM 60mT (SIRM is the saturation isothermal remanent magnetization; SIRM nmT represents the residual SIRM after an n mT alternating field demagnetization) have been used to document regional paleoclimate change in the Asian interior by correlating the mineral magnetic record with the composite δ18O record in deep-sea sediments. The long-term up-section decreasing trend in those ratios in both loess and paleosol units has been attributed to a long-term decrease in the relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema, respectively, which reveals a long-term decreasing trend in chemical weathering intensity in both glacial-stage source region (the Gobi and deserts in northwestern China) and interglacial-stage depositional area (the Loess Plateau region). We further relate this long-timescale variation to long-term increasing aridification and cooling, during both glacial extrema in the dust source region and interglacial extrema in the depositional area, over the Quaternary period. Changes in those ratios are most likely due to Quaternary aridification and cooling driven by ongoing global cooling, expansion of the Arctic ice-sheet, and progressive uplift of the Himalayan–Tibetan complex during this period. 相似文献
6.
Aeolian samples from deserts in northern China were studied using the optically stimulated luminescence (OSL) of quartz and potassium (K) feldspar separates. K-feldspar has shown advantages over quartz for dating samples younger than 100 years, due to its more homogeneous equivalent dose ( De) values, higher internal dose rate and higher luminescence sensitivity to dose. Without additional measurements, the luminescence characteristics of quartz obtained in De determination procedures can be used to reveal the source material and/or quartz thermal history for the deserts in northern China. The results indicated that the duration of paleosol formation in desert areas reflects regional paleoenvironments. 相似文献
7.
The desert and sandy land are the products of arid climate. The spatial distribution of modern deserts and sandy land in China and their relation to climate show following characteristics: arid and hyper-arid desert zones, at isohyet of less than 200 mm, are dominated by mobile dunes; semi-arid steppe and arid desert steppe with the precipitation between 200-400mm, are dominated by semi-fixed and fixed sand dunes; the precipitation of sub-humid forest grassland and humid forest zones with scattered fixed sand land is higher than 400 mm. With this as reference, in combination with considerable amount of paleoclimatic data in desert regions and adjacent regions, the distributions of desert and sandy land in China during the last interglacial period, the last glacial maximum (LGM), and the Holocene megathermal, were preliminarily reconstructed. The results compared with that of today show that the distribution of desert and sandy land in China was greatly dwindled during last interglacial period, and the mobile dune area was about two-thirds of that of today's, but greatly expanded during LGM. However, the dwindling area of desert and sandy land in the Holocene megathermal was smaller than that in the last interglacial period. The forcing mechanism was mainly related to the changes of East Asian winter and summer monsoon, south-northward swing of the westerlies and the variations of the Qinghai-Tibet Plateau monsoon intensity caused by global climate changes during the cold and warm intervals since the last interglacial period. 相似文献
8.
The desert and sandy land are the products of arid climate. The spatial distribution of modern deserts and sandy land in China and their relation to climate show following characteristics: arid and hyper-arid desert zones, at isohyet of less than 200 mm, are dominated by mobile dunes; semi-arid steppe and arid desert steppe with the precipitation between 200–400 mm, are dominated by semi-fixed and fixed sand dunes; the precipitation of sub-humid forest grassland and humid forest zones with scattered fixed sand land is higher than 400 mm. With this as reference, in combination with considerable amount of paleoclimatic data in desert regions and adjacent regions, the distributions of desert and sandy land in China during the last interglacial period, the last glacial maximum (LGM), and the Holocene megathermal, were preliminarily reconstructed. The results compared with that of today show that the distribution of desert and sandy land in China was greatly dwindled during last interglacial period, and the mobile dune area was about two-thirds of that of today’s, but greatly expanded during LGM. However, the dwindling area of desert and sandy land in the Holocene megathermal was smaller than that in the last interglacial period. The forcing mechanism was mainly related to the changes of East Asian winter and summer monsoon, south-northward swing of the westerlies and the variations of the Qinghai-Tibet Plateau monsoon intensity caused by global climate changes during the cold and warm intervals since the last interglacial period. 相似文献
9.
From Yulin, which is located in the transitional zone between the Loess Plateau and the Mu Us desert, to Weinan in the southernmost
part of the Loess Plateau, 9 loess sections were studied. Grain size analyses show that the advance-retreat changes of the
deserts in northern China may be the most important factor controlling changes in the sand particle percentage of the loess-soil
sequences during the last glacial-interglacial period. It is thus suggegted to use the sand grain content of loess deposits
as a proxy indicator of desert variations. Applying this indicator to the last glacial loess deposits in the northwestern
part of the Loess Plateau reveals that there were many millennial-scale cycles of the desert environments.
Project supported by the National Natural Science Foundation of China (Grant No. 49525230) 相似文献
10.
A land-surface physical process model was coupled with a mesoscale atmospheric model. This cou- pled model was then used to simulate the interactions between land and the atmosphere, including surface temperature, net radiation, sensible heat flux and latent heat flux over a desert/Gobi with an oasis in northwestern semiarid regions in China. Comparisons between observations and simulations were made over the oasis and the desert/Gobi, respectively. Both cold island effect and wet island ef- fect, the so-called oasis effect, were observed and simulated. Lower temperature, higher specific hu- midity and weaker turbulent transfer were present over the oasis than the desert/Gobi. A subsidence occurred over the oasis, leading to a thermally-generated mesoscale circulation. 相似文献
11.
Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Triassic alkaline magmatic belt along the northern margin of the North China Craton(NCC). Zircon U-Pb age, petrological, mineralogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteristics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside(cores)-aegirine-augite(rims), biotite, orthoclase and nepheline are the major minerals. The Baotoudong syenites have high contents of rare earth elements(REE), and are characterized by enrichment in light rare earth elements(LREE) and large ion lithophile elements(LILE; e.g., Rb, Ba, Sr), depletion in heavy rare earth elements(HREE) and high field strength elements(HFSE). They show enriched Sr-Nd isotopic compositions with initial ~87Sr/~86Sr ranging from 0.7061 to 0.7067 and ε_Nd(t) values from –9.0 to –11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO_2-undersaturated potassic-ultrapotassic, and is characterized by high contents of Ca O, Fe_2O_3, K_2O, Na_2O and fluid compositions(H_2O), and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area(80 km). The occurrence of the Baotoudong syenites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic. 相似文献
13.
According to the bioclimatic zones, dune mobility and the fabric characteristics of stratigraphic sedimentary facies, the
deserts in China are divided into Eastern, Western, Central and Northwestern deserts. Based on the records of stratigraphical
facies, climatic proxies, historical data, etc. in each desert region, the evolution of deserts with climatic changes in time
and space since 150 ka B. P. in China are dealt with; then the evolution of deserts in relation to the glacial climatic fluctuations
caused by solar radiation changes, underlying surface variation and their feedback mechanism is discussed through comparison
with global records; finally, in consideration of global warming due to increasing of greenhouse gases such as C0 2, the possible tendency of the evolution of deserts and the climatic changes is discussed. 相似文献
14.
湖泊沉积物的粒度分析是一种研究古环境变化常用的手段,但是其环境意义具有多解性,尤其是沙(荒)漠干旱区的湖泊沉积物粒度变化,代表的是沙尘天气的多寡强弱还是其它环境因素的变化值得细致研究。在对我国最大的内陆淡水湖—新疆博斯腾湖的沉积物短岩芯进行了^210Pbex,^137Cs定年的基础上,对近50多年沉积物的粒度进行分析,结果表明平均粒径与粗颗粒含量与区域最大河流—开都河大山口站的最大一天洪量、洪峰流量、天山巴音布鲁克站年降水量、湖泊水位变化以及湖泊附近焉耆站3-6月风力强度变化记录等环境因素对比发现,湖泊中部沉积物中粗颗粒含量变化主要反映的是入湖河流径流量、区域洪水强度及频率、山地降水,而并不反映大风天气或者湖泊水位的变化。对其近千年沉积岩芯的多指标分析结果表明,即使在数百年时间尺度上,博斯腾湖沉积物粒度的环境意义依然与十年时间尺度上一致。 相似文献
15.
The chemical variation of the Earth’s mantle rocks has been interpreted to reflect multiple episodes of partial melting. With the increasing of melt generation and extraction, the readily molten minerals and incompatible elements decrease in the residual mantle peridotite. The present-day gladiate of the Earth, however, cannot cause mantle batch melting[1], nor 40% partial melting that allows pyroxenes to be completely dissolved into melt and forms dunite[2,3]. Recent studies show that mantl… 相似文献
16.
By utilizing the data of the intensive observation period (May-June, 2000) of Dunhuang land-surface process field experiment that belongs to "Land-atmosphere Interactive Field Experiment over Arid Region of Northwest China", the bulk momentum transfer coefficient Cd and bulk sensible heat transfer coefficient Ch between surface and atmosphere over desert and Gobi in the arid region are determined according to three different methods. The result shows that, though these bulk transfer coefficients are different, they are in the same order. Especially, the means of Cd and Ch are close. Moreover, through analyzing the wind direction, the interference of the building near the observational station with the data is eliminated. From this, the relation between the bulk transfer coefficients and the bulk Richardson number and the range of the typical values of the bulk transfer coefficients over desert and Gobi in the typical arid region are obtained. 相似文献
17.
Palynological records were selected from the profiles of three research sites in the mountainous interior of southern China(Dajiuhu at Shennongjia in the western part of Hubei Province, Dahu in the Nanling Mountains, and Gantang in the northern part of Fujian Province). It can be inferred that the forest vegetation growing in the south of the Qinling Mountains-Huaihe River was luxuriant during the late glaciation. The species succession with ecological significance in palaeoflora(Abies sp., Fagus sp. and Alnus sp.) revealed that there was a certain amount of precipitation and effective humidity in the mountain lands between Qinling Mountains-Huaihe River and Nanling Mountains. This ecological environment favored the zonal distribution of mountainous forest vegetation. The late glacial maximum featured a cold, wet climate that completely differed from the cold-dry climate on the Loess Plateau, and on the steppe and desert steppe of the Qinghai-Tibet Plateau. Besides being influenced by the predominant factors driving the Northern Hemisphere climate, the cold-wet climate feature of the mountainous interior of southern China was closely associated with some geographical factors such as the latitudinal position, proximity to the ocean, and the topography and landforms. 相似文献
18.
The Gobi Altai region of southwestern Mongolia is a natural laboratory for studying processes of active, transpressional, intracontinental mountain building at different stages of development. The region is structurally dominated by several major E—W left-lateral strike-slip fault systems. The North Gobi Altai fault system is a seismically active, right-stepping, left-lateral, strike-slip fault system that can be traced along the surface for over 350 km. The eastern two-thirds of the fault system ruptured during a major earthquake (M = 8.3) in 1957, whereas degraded fault scarps cutting alluvial deposits along the western third of the system indicate that this segment did not rupture during the 1957 event but has been active during the Quaternary. The highest mountains in the Gobi Altai are restraining bend uplifts along the length of the fault system. Detailed transects across two of the restraining bends indicate that they have asymmetric flower structure cross-sectional geometries, with thrust faults rooting into oblique-slip and strike-slip master faults. Continued NE-directed convergence across the fault system, coupled with left-lateral strike-slip displacements, will lead to growth and coalescence of the restraining bends into a continuous sublinear range, possibly obscuring the original strike-slip fault system; this may be a common mountain building process. The largely unknown Gobi-Tien Shan fault system is a major left-lateral strike-slip fault system (1200 km + long) that links the southern ranges of the Gobi Altai with the Barkol Tagh and Bogda Shan of the easternmost Tien Shan in China. Active scarps cutting alluvial deposits are visible on satellite imagery along much of its central section, indicating Quaternary activity. The total displacement is unknown, but small parallel splays have apparent offsets of 20 + km, suggesting that the main fault zone has experienced significantly more displacement. Field investigations conducted at two locations in southwestern Mongolia indicate that late Cenozoic transpressional uplift is still active along the fault system. The spatial relationship between topography and active faults in the Barkol Tagh and Bogda Shan strongly suggests that these ranges are large, coalescing, restraining bends that have accommodated the fault's left-lateral motion by thrusting, oblique-slip displacement and uplift. Thus, from a Mongolian perspective, the easternmost Tien Shan formed where it is because it lies at the western termination zone of the Gobi-Tien Shan fault system. The Gobi-Tien Shan fault system is one of the longest fault systems in central Asia and, together with the North Gobi Altai and other, smaller, subparallel fault systems, is accommodating the eastward translation of south Mongolia relative to the Hangay Dome and Siberia. These displacements are interpreted to be due to eastward viscous flow of uppermost mantle material in the topographically low, E–W trending corridor between the northern edge of the Tibetan Plateau and the Hangay Dome, presumably in response to the Indo-Eurasian collision 2500 km to the south. 相似文献
19.
Episodic dune formations during the Quaternary are found in many deserts of China. The causes of desert expansions on different time scales are not the same. Desert extension at about 1.1 and 0.9 Ma ago were the response to the active tectonic movements, whereas the desert evolutions on the ten-thousand years time scale were the response to the orbital scale climatic changes. Spatial scale studies on desert evolution indicate that desert margins shifted greatly during the last glacial maximum (LGM) and the Holocene optimum, its changing from 125°E of the LGM to 105°E of the climatic optimum. Historical desertification in the semiarid China is not a response to climate drought but largely associated with the human impacts (mainly over-cultivation) since about 2300 years ago, which leads to the reworking of the underlying LGM sands. 相似文献
20.
阐述了我国西北部戈壁砂砾石层的划分及其时代确定等问题,以近几年西北地区地震小区划和重大工程建设项目中的实际观测资料为依据,对西北戈壁滩砂砾石场地的脉动特征进行了分析和总结,在此基础上探讨了场地脉动频谱特性与场地有关参数的关系,同时还就地脉动测试在工程中的推广应用等问题提出了几点看法。 相似文献
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