Granule ripples in the Kumtagh Desert,China: Morphology,grain size and influencing factors

Granule ripples are found mainly in four regions of the Kumtagh Desert in China; they are characterized by an asymmetrical shape, with gentle lower slopes on both sides and abrupt crests. The ripples tend to be oriented perpendicular to the prevailing winds, except when they form near obstacles such as yardangs. The wavelengths (λ) range between 0·31 m and 26 m and heights (h) range from 0·015 m to 1 m. The relationship between wavelength and height can be described by a simple linear function, and the mean ripple index (λ/h) is about 20·4 for the study sites. The sediments are poorly sorted, with negative to very negative skewness at lee and stoss slopes and between‐ripple troughs, which confirms the ‘poured in’ and ‘shadow’ appearance described by previous researchers. The bimodal or trimodal distributions of grains (with modes of ?1·16φ, ?0·5φ and 3·16φ) and the enrichment of coarse particles at the ripple surface (with coarse granule contents ranging between 5·2% and 62·1%) indicate that the underlying layer is the original sediment source and that the granule ripples resist erosional processes. Although the impact of saltating particles and, consequently, the creep and reptation of coarse grains are responsible for granule ripple initiation at a micro‐scale, however, the characteristics of local sediments, wind regimes and topographical obstacles, as well as the feedbacks among bedform and airflow, more strongly affect the development and alignment of granule ripples at a macro‐scale.     

Geomorphological patterns in a linear dune field and ages of the linear dunes in the northern Kumtagh Desert, northwest China

The unique linear dunes in the northern Kumtagh Desert of northwest China have inspired enthusiastic debate among aeolian geomorphologists over their origins and classification. Their uniqueness lies not only in their morphology, but also in their geomorphological patterns. In this paper, the geomorphological patterns and ages of the linear dunes in the northern Kumtagh Desert are analyzed using the pattern analysis method that was recently adopted by geomorphologists. The analysis suggests that these linear dunes have developed since 13 ka BP, during the late part of the most recent deglaciation stage, when the region’s dry climate was favorable for dune formation. The differences in dune construction time within this region result from the complex factors that control their formation and spatial variations in the local environment, but these factors were not strong enough to prevent us from estimating the dune age recorded by the geomorphological patterns with acceptable precision. It is also suggested that the pattern analysis method can be used to estimate dune ages in the absence of more reliable dating methods.     

Gravel bodies in the Kumtagh Desert and their geomorphological implications

Although developed in predominantly arid environments, desert geomorphology sometimes bears information about past environmental change. China’s Kumtagh Desert has only recently been studied by researchers. These investigations revealed the existence of several unique features compared with other Chinese deserts, including gravel bodies distributed in the northern part of the desert. This paper summarizes the results of previous research in the region, as well as information obtained from remote sensing and field investigations, then discusses the geological background, underlying Quaternary geomorphology, and the types, development stages, and geomorphological implications of the desert’s gravel bodies. Four main types of gravel bodies and four developmental stages were identified. It is suggested that the great elevation difference between the Altyn Tagh Mountains and the Kumtagh Desert caused by uplift of the Qinghai-Tibetan Plateau constitutes the overall background for the development of gravel bodies, but subsequent climatic events were the direct causes of their evolution. The gravel bodies resulted from alluvial deposits on loose fine sediments mainly of aeolian origin that were subsequently subjected to wind erosion. Erosion cycles then progressively scattered these gravels to form gravel sheets. The gravel bodies have three main geomorphological implications: they record anomalous ancient climate events after the dune field formed and have little influence on the development of nearby seif dunes in the northeast, demonstrate that the Altyn Tagh Mountains are the main sediment source for the Kumtagh Desert, and suggest that the evolution of the environment and geomorphology of the Kumtagh Desert have been influenced by neotectonic movements.     

Geomorphology of desert sand dunes: A review of recent progress

Through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes. Alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes. As developments with equipment allowed, field measurements became more sophisticated. However, by the mid-1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported. Most importantly, the attempt to measure the stresses imposed by the wind on the sand surface proved impossible, and the use of shear (or friction) velocity as a surrogate for shear stress also failed to deliver. At the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux. In a development paralleled in fluvial geomorphology, aeolian geomorphologists have attempted to measure and model turbulent structures over dunes. Progress has recently been made through the use of more complex numerical models based on computational fluid dynamics (CFD). Some of the modelling work has also suggested that notions of dune ‘equilibrium’ form may not be particularly helpful. This range of recent developments has not meant that field studies are now redundant. For linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground-penetrating radar images of the internal structure of the dunes.

The paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form. At least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies. More recently, attention has shifted–although not completely–to modelling approaches, and very considerable progress has been made in developing models of dune development. It is clear, however, that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology.     

The dynamics of star dunes: an example from the Gran Desierto, Mexico

Observations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the Gran Desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type. Such interactions lead to deposition of sand in the central parts of the dune, giving rise to its pyramidal shape, as well as to some extension of the linear arms. The major arms of the dune studied are oriented NE-SW, or transverse to summer SSE and winter NNW winds. An avalanche face up to 10 m high develops during the course of each season. Flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune, where it is deposited as wind ripples migrate into zones of locally reduced flow velocity. Reattachment of the separated flow occurs on the lower part of the N or S arms, parallel to the flow. Spring westerly winds move sand obliquely up the S and N arms of the dune and outwards on the E arm. Large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms. Formation of star dunes in the Gran Desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge. Convergent leeside secondary flows are developed, which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune. Examples of star dunes at different stages of their development can be documented.     

Genesis and soil environmental implications of intact in-situ rhizoliths in dunes of the Badain Jaran Desert,northwestern China

Acta Geochimica - Desert rhizoliths are generally found as weathered, broken and scattered samples on dune field surface, but rarely in-situ in their initial states buried under the soil of desert...     

库姆塔格沙漠岩屑的岩石学分析及其物源指示

沙漠中的岩屑是沙源母岩风化的暂时性中间产物, 是判别母岩性质和来源的最可靠最直接的标志。采用沙薄片岩性显微鉴定, 对库姆塔格沙漠北部的堑谷阶地、边缘台地、羽毛状沙丘以及沙漠南部洪积台地、干河床、边缘沙丘和丘间地等地貌单元的岩块和粗碎屑进行了多样品系统性岩石学分析。结果表明：库姆塔格沙漠岩屑岩石类型复杂, 涵盖3大岩类、14种3级岩类、121种5级岩类, 以中酸性喷出岩类和区域变质岩类为主, 常见的岩石为流纹岩、霏细岩、安山岩、粗面岩、石英岩和石英化岩。碎屑岩矿组成与粒度关系密切, 砾石>90%、粗砂50%~90%是岩屑, 粒度1.0 mm左右的沙粒岩石类型最多, 沙粒细则碎屑单矿物含量高, 这种状况使得沙漠不同区域、不同地貌部位沙粒岩性分布存在差异。岩屑具有不同的颜色, 暗色颗粒主要由中酸性喷出岩组成, 而浅色颗粒主要由石英质岩类和长英质矿物组成, 其中部分同类岩屑既有暗色也有浅色。沙漠中岩屑的岩石特征与阿尔金山山区出露的岩性基本一致, 沙漠中不仅含有阿尔金山十分发育的动力变质岩类, 而且还含有红柳沟—拉配泉段特有的蛇绿岩套混杂岩系的各种岩类, 显示出库姆塔格沙漠的碎屑与阿尔金山母岩的渊源关系, 但母岩风化的差异性以及山脉隆起不同历史阶段碎屑物输出的差异, 使得岩屑中各类岩石的含量与山区母岩含量不尽相同, 细晶质及隐晶质且抗风化的岩石成为沙漠岩屑的主要组分。库姆塔格沙漠中粗晶质岩类和碎屑单矿物南多北少、与抗风化岩屑北多南少的分布格局是冲洪积物由南向北运移的结果, 显示了沙漠沙与冲洪积物岩屑特征的相近性和继承性, 揭示了库姆塔格沙漠具有近源就地起沙的物源属性。     

Relationship of spatial heterogeneity for vegetation and aeolian sand soil properties on longitudinal dunes in Gurbantunggut Desert, China

On longitudinal dunes in the southern Gurbantunggut Desert, the vegetation was investigated, and the texture, water content, organic matter, total nitrogen, salt content and pH of aeolian sand soil were measured. By means of geostatistical methods, the spatial heterogeneity for vegetation and aeolian sand soil properties was analyzed. The vegetation pattern and aeolian sand soil properties were moderately to strongly spatially heterogeneous as a whole, with a spatially dependent range of 8.8–74.8 m. Because both of them had a nest structure of grading system, their spatial heterogeneity needed to be described by the fractal dimensions at different sampling scales. The autocorrelated spatial range A ₀ and fractal dimensions of the cover of vegetation and the diversity of herberious synusium were similar to these of soil water contents (SM). The coarse grain size (φ1) and sorting (σ) of soil, the diversity of herbaceous synusium and the cover of vegetation had a closer dependent range A₀, being 37.8–57.8 m. The trends of spatial variation for organic matter and total nitrogen contents were very similar to that of vegetation cover. The spatial heterogeneities of soil pH and salt content were mainly restricted by terrain and basically not related to vegetation.     

Analysis of impact factors on scrubland soil respiration in the southern Gurbantunggut Desert,central Asia

Monitoring soil CO₂ respiration with chamber measurements and identifying controlling factors such as the diversity of vegetation species, moisture and temperature can help guide desert scrubland management. Soil CO₂ respiration and potential controlling factors at four sites in desert scrubland were examined along the Sangong River Basin (SRB) in northwestern China in 2004. Soil CO₂ respiration descended along the SRB as did the diversity of vegetation species, air temperature and air humidity. The two sites of the field station (FS) and the north desert (ND) and the low reaches of the SRB among these locations were monitored to analyze the effects of pH value, soil organic carbon (SOC), total nitrogen (TN) and calcium carbonate (CaCO₃) on soil CO₂ respiration during the growing season in 2005. The ND site was located at the southern edge of the Gurbantunggut Desert; the FS site was in the border area of the SRB Alluvial Fan. One-way ANOVA was performed. The result showed that air humidity and CaCO₃ content had a strong influence on soil CO₂ respiration; SOC content was a limitation to soil CO₂ respiration in the arid-desert zone. Effective management activities can attenuate soil CO₂ respiration and keep carbon balance trends at a desirabe level in desert scrublands.     

古尔班通古特沙漠蚀余沙丘的发现及其意义

我国贺兰山以东的沙漠多以流动沙丘为主,但新疆北部准噶尔盆地在年降水量不足150ｍｍ的古尔班通古特沙漠则以固定、半固定沙丘为主。因而研究这一沙漠的“准稳定态”,对我国沙漠理论的研究和风沙灾害治理、土地沙漠化防治均有重要意义。     

中国南方碳酸盐岩系层滑断裂控矿和成矿研究

碳酸盐岩系层滑断裂在中国南方古生代地层中广泛分布,是一种重要的控矿和成矿构造。厘定碳酸盐岩系层滑断裂的基本类型、形成机理和控矿作用对相关矿床的找矿具有重要意义。基于层滑断裂所属含矿岩系的岩性组合、构造样式及其与主干断裂的相互关系,初步将层滑断裂划分为4种基本类型,即碳酸盐岩与泥页岩界面+层滑断裂复合型,平行不整合面+层滑断裂复合型,逆冲推覆构造台阶式断层断坡+断坪复合型和陡倾斜主干断层+旁侧缓倾斜层滑断裂复合型,分别总结了不同类型层滑断裂的控矿特征,并列举其典型的矿床实例,讨论了层滑断裂的控矿作用和成矿作用,将层滑断裂的形成过程归纳为层间滑动、层滑断层、叠加断层3个阶段。     

库姆塔格沙漠东南缘BL砂黄土剖面的沉积过程与物源分析

通过对库姆塔格沙漠东南缘BL砂黄土剖面观测、描述和采样,分析了沉积物粒度、年代、沉积环境和地球化学元素特征,结果表明：1）BL剖面沉积物颗粒的93.34%为砂和粉砂,证明剖面为一砂黄土沉积地层;2）BL砂黄土剖面地层沉积的开始年代为全新世早期（8.3ka）,是阿尔金山东部冰川消融后在特定的地形环境条件下形成的。地表沉积环境在全新世开始到8.3ka之间,受到了阿尔金山冰川和冰缘带融水的泛滥冲刷,难以形成沉积,8.3ka之后冰川积雪融水减少后在高凸的合适地形开始了连续沉积,由此形成了BL砂黄土沉积地层。BL地层沉积速率约为4.22cm/100a,早期沉积速率较快,后期则有所减缓;3）对比BL剖面沉积物特征元素比值分布模式,其与库姆塔格沙漠表层砂、内部出露地层（全新世以来）的不同沉积物特征元素比值分布模式具有极其相似的特征,证明两者的物质具有同源性,BL剖面的沉积物来源就是库姆塔格沙漠表层或者内部地层翻出来的物质。BL砂黄土沉积的过程和物源的研究为今后研究全新世以来库姆塔格沙漠区域古气候过程奠定科学的基础。

     

Sediment provenance of the Wilderness barrier dunes,southern Cape coast,South Africa

Trace‐element geochemistry has been used to discern the provenance of sediments derived from geologically distinct sources found in a range of depositional settings. This study investigates whether this approach can also identify provenance signatures in sediments derived from clastic sedimentary rocks of similar origins, within a complex coastal transport environment. The potential local sediment sources of a coastal barrier dune in South Africa were analysed to assess their relationships with the barrier and whether these have varied with time. Results indicate that the oldest part of the barrier, partly formed during MIS 6 when sea levels were lower than at present, has a closer affinity to fluvial sediment provided by local rivers than either younger samples from the barrier′s seaward flank or the Holocene dunes atop its crest, both formed during sea‐level highstands. Constraints on the timing of barrier formation provided by OSL dating proved helpful in discriminating between provenance and weathering signals. Terra Nova, 00, 000–000, 2010     

The formation of dunes,antidunes, and rapidly damping waves in alluvial channels

Under the effect of a constant current for a long time, a water channel of infinitely long and constant depth interacting with a uniform sandbed of infinite thickness is used to simulate the formation of dunes, antidunes and rapidly damping waves in alluvial channels. The theory of potential flow is applied to the channel flow, while Biot's theory of poroelasticity is adopted to deal with erodible bed material. The governing equations, together with free surface, bed surface, and far field boundary conditions, form a complete boundary‐value problem without applying empirical sediment discharge formulas as in conventional researches. The comparison of the present result with Kennedy's (Journal of Fluid Mechanics, 1963; 16 : 521–544) instability analysis not only indicates the appropriateness of the present work, but also reveals the advantage of the present study due to its ability to find all kinds of bed forms (including the rapidly damping waves that Kennedy could not find) and of solving for the unclear lagged distance δ introduced in Kennedy's work. Copyright © 2001 John Wiley & Sons, Ltd.     

Spatial heterogeneity of cyanobacteria-inoculated sand dunes significantly influences artificial biological soil crusts in the Hopq Desert (China)

Artificial biological soil crusts (ABSCs), formed by inoculating Microcoleus vaginatus Gom. and Scytonema javanicum Born. et Flah. onto the topsoil of desert dunes, proved to be effective tools for the stabilization of moving dunes and promotion of soil fertility. As dominant driving forces in arid habitats, the abiotic environmental conditions of undulating dunes produce a gradient of abiotic stresses on cyanobacteria. Cyanobacteria are considered pioneering phototrophs in early soil crust communities in deserts. In this study, the development of ABSCs under various environmental site conditions was investigated using 16S rRNA-based polymerase chain reaction, denaturing gradient gel electrophoresis (DGGE), and soil property measurements. After inoculation in 2002 and long-term development, patchy moss crusts were observed on the dunes. All of the available sequenced bands in the DGGE gels belonged to Oscillatoriales and Nostocales. The dominant Nostocales genus in the ABSCs was still Scytonema; however, more Oscillatoriales genera were identified, which belonged to Microcoleus and Phormidium. The cyanobacterial compositions of different slope types were significantly distinct (p < 0.05), particularly those from windward slopes. The crusts of the top-dune slopes were more heterogeneous. In addition, the soil physicochemical properties and richness indices of the windward slopes were significantly lower than those of the leeward and interdune slopes (p < 0.05). Compared with uninoculated control dunes, all of the inoculated dunes had far higher biodiversities.     

论库姆塔格沙漠羽毛状断裂的成因 ——“八一泉运动”及其与青藏高原隆升、 气候变化的关系

通过野外调查和同位素测年, 讨论了库姆塔格沙漠羽毛状断裂的形成与"八一泉运动"有关, 纠正了前人所说的八一泉雅丹下部褶皱断裂地层属上新世地层的传统概念, 实际上是早更新世湖相地层于中更新世晚期因发生八一泉构造运动形成的褶皱断裂的变形地层带. 所称的上新世地层实际是中更新世晚期的变形地层, 这也是227 ka BP左右的"八一泉运动"的证据, 讨论了它在青藏高原的隆升过程中的特殊阶段, 及其对青藏高原以北地区气候环境的影响, 重新建立了青藏高原隆升与冰期、 间冰期气候变化的序列, 并首次探讨了与中国东部冰缘期的对比问题.     

青海南部冻土区天然气水合物成藏控制因素

基于开心岭和乌丽地区天然气水合物钻探试验井相关资料,在天然气水合物含油气系统理论的指导下,对青海南部冻土区天然气水合物的形成要素进行了分析。研究表明:青海南部冻土区具备较好的形成水合物的温度压力条件,冻土平均厚度达84m,连续稳定发育的冻土层可作为水合物天然的盖层;冻土层下平均地温梯度2.03℃/100m,甲烷水合物稳定带计算值为240~450m;那益雄组煤系烃源岩广泛发育、厚度大、TOC含量高且处于高-过成熟阶段,可以作为良好的烃源岩;区内褶皱、断层、裂隙发育,相互连通后不仅可以为流体提供高效的运移通道,还能为水合物提供良好的储集空间。钻探实践表明:研究区具有良好的天然气水合物勘探前景,天然气的来源及气体的运移-聚集是其成藏的关键控制因素,并形成以裂缝型天然气水合物为主的水合物矿藏。此外,青藏高原冻土层的发育深刻影响了天然气水合物的形成和保存,其与水合物含油气系统各要素在空间上的配置,共同控制了水合物的形成和分布。     

Age,origin and climatic controls on vegetated linear dunes in the northwestern Negev Desert (Israel)

The stabilized northwestern (NW) Negev vegetated linear dunes (VLD) of Israel extend over 1300 km² and form the eastern end of the Northern Sinai – NW Negev Erg. This study aimed at identifying primary and subsequent dune incursions and episodes of dune elongation by investigating dune geomorphology, stratigraphy and optically stimulated luminescence (OSL) dating. Thirty-five dune and interdune exposed and drilled section were studied and sampled for sedimentological analyses and OSL dating, enabling spatial and temporal elucidation of the NW Negev dunefield evolution.In a global perspective the NW Negev dunefield is relatively young. Though sporadic sand deposition has occurred during the past 100 ka, dunes began to accumulate over large portions of the dunefield area only at ～23 ka. Three main chronostratigraphic units, corresponding to three (OSL) age clusters, were found throughout most of the dunefield, indicating three main dune mobilizations: late to post last glacial maximum (LGM) at 18–11.5 ka, late Holocene (2–0.8 ka), and modern (150–8 years). The post-LGM phase is the most extensive and it defined the current dunefield boundaries. It involved several episodes of dune incursions and damming of drainage systems. Dune advancement often occurred in rapid pulses and the orientation of VLD long axes indicates similar long-term wind directions. The late Holocene episode included partial incursion of new sand, reworking of Late Pleistocene dunes as well as limited redeposition. The modern sand movement only reactivated older dunes and did not lengthen VLDs.This aeolian record fits well with other regional aeolian sections. We suggest that sand supply and storage in Sinai was initiated by the Late Pleistocene exposure of the Nile Delta sands. Late Pleistocene winds, substantially stronger than those usually prevailing since the onset of the Holocene, are suggested to have transported the dune sands across Sinai and into the northwestern Negev.Our results demonstrate the sensitivity of vegetated linear dunes located along the (northern) fringe of the sub-tropical desert belt to climate change (i.e. wind) and sediment supply.     

晋南临汾-运城盆地土壤氟含量及其影响因素

通过对临汾-运城盆地表层土壤F含量的空间变化规律和影响因素的研究,发现土壤平均含F量为520mg/kg,高于全国土壤背景值。表层土壤中F的空间分布不均匀,呈现出由盆地四周向盆地中心递增的趋势。研究表明,土壤F含量的分布主要受控于粉粒和有机质等土壤理化性质、元素间的相互作用、F的吸附特性等因素,人为因素对其也有一定影响。有机质和粉粒等土壤理化性质对土壤F含量的影响较大,说明研究区土壤F含量与F的特殊化学性质和母岩类型有关。因子分析发现,研究区表层土壤F的分布可能受自然地质作用和人为作用2种因素的影响,N、Cd、Se、Cu、Zn、Mo可能代表了F的原始共生元素;土壤F的含量随Mo、Zn、Fe、Mg等元素含量的增加而增加,可能与Fe3 离子在土壤中的吸附作用有关;而F-与OH-的配位置换作用可能是造成土壤中F-吸附的主导因素。     

Sand dunes as a major proximal dust source for late Pleistocene loess in the Negev Desert, Israel

Grain size analyses of three hilltop, primary eolian loess sequences in the Negev desert, southern Israel, show a bimodal grain-size distribution at 50-60 μm and 3-8 μm. Using analyses of mineralogy and OSL ages we demonstrate that the coarse mode is composed mostly of quartz grains and its relative magnitude increases regionally with time, suggesting an enhancement of a time-transgressive proximal dust source compared to a distal, Saharan fine-grain dust. The only proximal dust source for large amount of coarse silt quartz grains is the sands that advanced into Sinai and the Negev concurrently with the loess accretion during the late Pleistocene as a result of the exposure of the Mediterranean shelf. We therefore propose that the coarse silt quartz grains were formed through eolian abrasion within the margins of an advancing sand sea. This relationship between desert sand seas as a source for proximal coarse dust and desert margin loess deposits can be applicable to other worldwide deserts such as Northern Africa, China and Australia.