地下水补给型湖泊表层沉积物矿物组成及其形成机制——以巴丹吉林沙漠湖泊群为例

盐湖中的矿物沉积记录着丰富的环境气候变化信息,是古环境研究的重要对象.在无地表径流补给的盐湖中,其矿物组成及沉积特征与有地表径流补给的湖泊相比是否有一定的特殊性,是值得探讨的问题.采集巴丹吉林沙漠33个不同矿化度地下水补给型湖泊的表层沉积物和10个地表风积砂样品,通过X衍射的方法,分析样品的矿物组成.结果显示:湖泊表层沉积物主要为石英、长石、辉石、云母等碎屑矿物,部分湖泊含有少量的碳酸盐和氯化物盐类矿物.湖泊沉积物的矿物组成与湖水矿化度的关系较为密切,淡水湖仅分布碎屑矿物,微咸水湖含有碎屑矿物和碳酸盐类矿物,盐湖含有碎屑矿物、碳酸盐类矿物和氯化物.风积砂样品中主要为碎屑矿物,占总矿物含量的90%,对湖泊沉积物的矿物组成影响较大,但对湖泊沉积物中的盐类矿物没有贡献,表明湖泊表层沉积物中盐类矿物主要是自生作用形成的.虽然本地区湖泊边缘的沉积物中盐类矿物种类相对较少并且含量较低,但其盐类矿物组成与分布能够响应湖水矿化度的变化,其环境指示意义与有径流补给的盐湖相同,可以指示其湖水的盐度.因此,可以从巴丹吉林沙漠地下水补给型湖泊沉积的盐类矿物中提取相应的古环境信息,用于恢复古气候和古环境的研究.     

Analysis of groundwater–lake interaction by distributed temperature sensing in Badain Jaran Desert,Northwest China

The Badain Jaran Desert is the second largest desert of China with a total area of 49 200 km². At least 72 perennial lakes are scattered throughout the desert, sustaining a unique desert–lake ecosystem. Groundwater of various origins was believed to play an essential role in maintaining those desert lakes, but hydrological measurements are lacking due to difficult field conditions. This study applied the distributed temperature sensing technique to continuously measure temperature variations in one of the desert lakes – the Badain Lake – to identify groundwater discharge to the lake based on the temperature differences between groundwater and lake water. Because temperature may be influenced by various unforeseen and temporary factors, it is critical to discern those factors that may affect the temperature such as solar radiation and vertical temperature stratification and to ensure that the temperature variations of the lake water as detected by the distributed temperature sensing are mainly caused by groundwater discharge. A time window was identified during which the groundwater discharge is the dominant factor that determines the temperature pattern of the lake water. The results show that the temperature near the eastern and southeastern lakeshore of the eastern Badain Lake is colder than the average, indicating that this area is the main groundwater discharge zone. Near the northwestern lakeshore adjacent to a sand dune, a weak cold abnormal area was identified, indicating that the sand dune is another recharge source to the lake through channelling the local precipitation toward the lake. The contribution from the sand dune, however, appeared to be less than that from the regional groundwater flow. This study provides the first identification of the temperature abnormal areas, which imply groundwater discharges into desert lakes and contributes to a better understanding of the unique desert–lake ecosystem. Copyright © 2015 John Wiley & Sons, Ltd.     

1973-2010年巴丹吉林沙漠腹地湖泊面积空间变化的遥感分析

利用1973、1990、2000、2010年4个时期遥感影像数据,提取并统计了1973-1990、1990-2000、2000-2010、1973-2010年4个时期湖泊面积变化信息,分析了巴丹吉林沙漠湖泊面积变化的空间特征.结果表明,湖泊数量与面积整体表现为减小,同时也存在湖泊面积增大与湖泊新增;1973-2010年间,沙漠腹地共干涸19个湖泊、新增7个湖泊,43个湖泊萎缩、6个湖泊扩张,26个湖泊面积基本保持稳定;从湖泊面积的空间变化看,面积减小的湖泊在整个湖泊区域均有分布,面积增大的湖泊较多分布于研究区东侧,且多在中东部区域外围,面积减小剧烈的湖泊在空间上有聚集分布的态势;湖泊面积变化量在整个空间上并没有表现出显著的正相关关系.湖泊面积变化所表现的空间特征可能由于不同湖泊区域补给来源不同或不同湖泊补给方式存在差异造成的.     

The discovery of surface runoff in the megadunes of Badain Jaran Desert,China, and its significance

The Badain Jaran Desert exhibits the greatest difference in altitude of all of the world’s deserts. On the slopes of megadunes in the desert, there are physical and chemical deposits produced by surface runoff. In addition, we have observed rarely-seen infiltration-excess surface runoff in the megadune depressions as well as spring streams at the base of megadunes. We used electron microscopy, energy spectrum analysis, infiltration experiments, moisture content determinations and grain-size analysis to study the mineral and chemical composition of the runoff precipitates, and grain-size of the deposits associated with the runoff, together with the hydrological balance in the megadune area, and the atmospheric precipitation mechanism responsible for groundwater recharge and for supplying water to lakes. The observations of shallow runoff and infiltration-excess surface runoff indicate the occurrence of strong and effective precipitation in summer, which would provide an important source for groundwater recharge. Several lines of evidence, such as the physical and chemical deposits resulting from shallow subsurface runoff, spring streams, infiltration-excess runoff, and gravity capillary water with a moisture content of 3–6%, demonstrate that precipitation reaches the base of the megadunes through infiltration and subsequently becomes groundwater. The chemical deposits, such as newly-formed calcite and gypsum, and gray-black physical deposits, as well as different stages in the development of fan-shaped landforms resulting from shallow subsurface runoff, indicate that groundwater recharge in the area is the result of long-term precipitation, rather than intermittent individual major rainfall events. Fine sand layers with a low infiltration capacity lead to subsurface runoff emerging at the ground surface. Five factors play an important role in maintaining a positive water balance and in replenishing groundwater via rainfall: effective rainfall as a water source, the high infiltration capacity of the sands enabling rainfall to rapidly become capillary water in the dunes, low evapotranspiration rates due to the sparse vegetation, the fact that the depth of the sand layer influenced by evaporation is shallow enough to maximize the deep infiltration of rainfall, and rapidly-moving gravity capillary water in the sandy dunes. These five factors together constitute a mechanism for groundwater recharge from rainfall, and explain the origin of the groundwater and lakes in the area. Our findings represent a significant advance in research on the hydrological cycle, including groundwater recharge conditions and recharge mechanisms, in this desert region.     

Effect of permafrost thaw on the dynamics of lakes recharged by ice‐jam floods: case study of Yukon Flats,Alaska

Large river floods are a key water source for many lakes in fluvial periglacial settings. Where permeable sediments occur, the distribution of permafrost may play an important role in the routing of floodwaters across a floodplain. This relationship is explored for lakes in the discontinuous permafrost of Yukon Flats, interior Alaska, using an analysis that integrates satellite‐derived gradients in water surface elevation, knowledge of hydrogeology, and hydrologic modelling. We observed gradients in water surface elevation between neighbouring lakes ranging from 0.001 to 0.004. These high gradients, despite a ubiquitous layer of continuous shallow gravel across the flats, are consistent with limited groundwater flow across lake basins resulting from the presence of permafrost. Permafrost impedes the propagation of floodwaters in the shallow subsurface and constrains transmission to ‘fill‐and‐spill’ over topographic depressions (surface sills), as we observed for the Twelvemile‐Buddy Lake pair following a May 2013 ice‐jam flood on the Yukon River. Model results indicate that permafrost table deepening of 1–11 m in gravel, depending on watershed geometry and subsurface properties, could shift important routing of floodwater to lakes from overland flow (fill‐and‐spill) to shallow groundwater flow (‘fill‐and‐seep’). Such a shift is possible in the next several hundred years of ground surface warming and may bring about more synchronous water level changes between neighbouring lakes following large flood events. This relationship offers a potentially useful tool, well suited to remote sensing, for identifying long‐term changes in shallow groundwater flow resulting from thawing of permafrost. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrological cycle and water balance estimates for the megadune–lake region of the Badain Jaran Desert,China

The hydrology and water balance of megadunes and lakes have been investigated in the Badain Jaran Desert of China. Field observations and analyses of sand layer water content, field capacity, secondary salt content, and grain size reveal 3 types of important natural phenomenon: (a) vegetation bands on the leeward slope of the megadunes reflect the hydrological regime within the sandy vadose zone; (b) seepage, wet sand deposits, and secondary salt deposits indicate the pattern of water movement within the sandy vadose zone; (c) zones of groundwater seeps and descending springs around the lakes reflect the influence of the local topography on the hydrological regime of the megadunes. The seepage exposed on the sloping surface of the megadunes and gravity water contained within the sand layer confirm the occurrence of preferential flow within the vadose zone of the megadunes. Alternating layers of coarse and fine sand create the conditions for the formation of preferential flows. The preferential flows promote movement of water within the sand layer water that leads to deep penetration of water within the megadunes and ultimately to the recharging of groundwater and lake water. Our results indicate that a positive water balance promotes recharge of the megadunes, which depends on the high permeability of the megadune material, the shallow depth of the surface sand layer affected by evaporation, the occurrence of rainfall events exceeding 15 mm, and the sparse vegetation cover. Water balance estimates indicate that the annual water storage of the megadunes is about 7.5 mm, accounting for only 8% of annual precipitation; however, the shallow groundwater per unit area under the megadunes receives only 3.6% of annual precipitation, but it is still able to maintain a dynamic balance of the lake water. From a water budget perspective, the annual water storage in the megadunes is sufficient to serve as a recharge source for lake water, thereby enabling the long‐term persistence of the lakes. Overall, our findings demonstrate that precipitation is a significant component of the hydrological cycle in arid deserts.     

Palaeomonsoon variability in the southern fringe of the Badain Jaran Desert,China, since 130 ka BP

Taking the Chagelebulu Stratigraphic Profile as a typical example, a comprehensive study has been conducted to elucidate the palaeoclimatic and geomorphic evolution patterns in the southern fringe of the Badain Jaran Desert, which were found to be complex and polycyclic in the past 130 ka. However, the fluctuating magnitude is not as remarkable as that in the eastern China sandy region. The shift in climate from interglacial to glacial and the uplift process of the Qinghai–Xizang Plateau are the two leading forces driving the evolution of the climate and desert landforms in this area. Seventeen cycles of cold, dry and warm, humid climatic stages were recognized in the Upper Pleistocene Series of the profile. The sharp uplift of the Qinghai–Xizang Plateau superimposed a cool and arid climatic trend in this area. As a result of the climatic changes, the desert in this area has undergone multiple stages of expansion and contraction since 130 ka bp . The middle Holocene Epoch and the early stage of the Late Pleistocene Period were the main periods when the sand dunes became stabilized, and the early and late phases of the Holocene Epoch and late phase of the Pleistocene Epoch were the main periods when the previously stabilized sand dunes became mobile. The late phase of the Pleistocene Epoch was the most mobile stage, when the aeolian sand activities formed the essential geomorphic pattern of the Badain Jaran Desert. Copyright © 2005 John Wiley & Sons, Ltd.     

Detailed measurements of thickness and grain size of a widespread onshore tsunami deposit in Phang‐nga Province,southwestern Thailand

Measurements of thickness and grain size along flow‐parallel transects across onshore deposits of the 2004 Indian Ocean tsunami revealed macroscopic horizontal variations and provided new insights into tsunami sedimentation. The tsunami caused severe erosion of beaches, river mouths, and the shallow seafloor along the coast of southwestern Thailand and supplied sufficient sediment to deposit a kilometer‐wide blanket of sand on the land surface. The tsunami deposits generally fine landward with some fluctuations caused by local entrainment and settlement of sediments. Sediments of medium and fine sand are restricted to a few hundreds of meters inland from their source, whereas finer grained sediments were suspended longer and deposited 1 km or more inland. Although the thickness of the tsunami deposits is strongly influenced by local topography, they generally thin landward. In areas of low‐relief topography, the rate of landward thinning is exponential and reflects the dominance of sediment supply to nearshore areas over that to areas farther inland.     

The influence of sediment size,relative grain size and channel slope on initiation of sediment motion in boulder bed rivers. A lichenometric study

Sediment transport of four boulder bed rivers is studied using lichenometry. The presence of lichens on boulders in the river channel is used to date the last mobilization of the blocks. Using size frequency diagrams and regional growth curves calibrated with dated reference points it is possible to determine the flood event responsible for the last mobilization of each boulder with lichens present. The specific stream power of flood events over the last 60 years is then calculated, and thresholds of sediment transport based on the sediment size are calculated. The results from the four studied rivers are compared to similar relationships in the literature. Sediment motion thresholds appear to be very variable within the same type of river (mountainous boulder bed rivers). The critical specific stream power necessary to mobilize a particle of a given diameter may vary by up to 10 times from one river to the next. Bed sediment size and river slope may explain this large range of stream powers. Calculation of the relative size of the transported particles (D_i/D₅₀) also shows that both hiding and protrusion effects, as well as channels slope, are important factors in sediment transport. Copyright © 2010 John Wiley & Sons, Ltd.     

The threshold effect of image resolution on image‐based automated grain size mapping in fluvial environments

It has recently been demonstrated that surficial grain sizes in fluvial environments could be derived with automated methods applied to airborne digital imagery having a ground resolution of 3 cm. This letter seeks to further examine the potential of digital imagery for automated grain size mapping. In order to broaden the application of automated grain size mapping from airborne imagery, the effect of image resolution needs further study. Automated grain size mapping was attempted on an airborne digital image with a ground resolution of 10 cm. The results show that meaningful grain size information can be derived from 10 cm imagery. However, the ground resolution of the image acts as a size threshold below which no grain size information is detectable. Therefore, these results strongly suggest that future applications of automated grain size mapping will always be dependent on the ground resolution made available by the technology in use at the time of image acquisition. Copyright © 2005 John Wiley & Sons, Ltd.     

The causes of bedload pulses in a gravel channel: the implications of bedload grain‐size distributions

Unsteady bedload transport was measured in two c. 5 m wide anabranches of a gravel‐bed braided stream draining the Haut Glacier d'Arolla, Switzerland, during the 1998 and 1999 melt seasons. Bedload was directly sampled using 152 mm square Helley–Smith type samplers deployed from a portable measuring bridge, and independent transport rate estimates for the coarser size fractions were obtained from the dispersion of magnetically tagged tracer pebbles. Bedload transport time series show pulsing behaviour under both marginal (1998) and partial (1999) transport regimes. There are generally weak correlations between transport rates and shear stresses determined from velocity data recorded at the measuring bridge. Characteristic parameters of the bedload grain‐size distributions (D₅₀, D₈₄) are weakly correlated with transport rates. Analysis of full bedload grain‐size distributions reveals greater structure, with a tendency for transport to become less size selective at higher transport rates. The bedload time series show autoregressive behaviour but are dif?cult to distinguish by this method. State–space plots, and associated measures of time‐series separation, reveal the structure of the time series more clearly. The measured pulses have distinctly different time‐series characteristics from those modelled using a one‐dimensional sediment routing model in which bed shear stress and grain size are varied randomly. These results suggest a mechanism of pulse generation based on irregular low‐amplitude bedforms, that may be generated in‐channel or may represent the advection of material supplied by bank erosion events. Copyright © 2003 John Wiley & Sons, Ltd.     

The use of grain‐size distribution patterns to elucidate aeolian processes on a transverse dune of Thar Desert,India

The grain‐size distribution of aeolian dune sands in the Thar Desert, India was analyzed and compared with three model distributions – log‐normal, log‐hyberbolic and log‐skew‐Laplace – to determine the best‐fit statistical model. In total, 51 samples were collected along a single transect over a transverse dune, of which 15 were from the stoss side, 12 from the crest and 24 from the lee side. Samples were collected during a calm period in the afternoon of a winter's day. It was observed that of these 51 samples, 33 fit best to a log‐hyperbolic distribution, 14 fit best to a normal distribution and only four fit best to a Laplace distribution. However, it was further observed that of 24 samples from the lee side, 13 fit best to a normal distribution, eight fit best to a hyperbolic distribution, and three fit best to a Laplace distribution. Of 12 samples from the crest of the dune, 11 fit best to the log‐hyperbolic distribution, only one to the Laplace distribution but none to a normal distribution. Of 15 samples from the stoss side of the dune, only one sample best‐fits a normal distribution, 14 fit best to a log‐hyperbolic distribution, and none best fit to a Laplace distribution. During sample collection a calm period prevailed and there was no dusty wind. It was therefore assumed that in the initial stage a mixture of coarse, medium and fine sands was laid down on the stoss side of the dune. As wind speeds increased and saltation started, the coarser fractions were segregated and lagged behind on the stoss slope. In the final stage when the remaining intermediate and finer fractions reached the dune crest, the finer fractions were winnowed away to suspension from the crest of the dune. As a result, a narrow range of intermediate sized sediments was deposited by rolling down the lee side to explain the development of log‐normality. In such a situation, both the coarser and finer fractions, to which the skewed distributions can be attributed, are separated from the initial mixture of coarse, intermediate and fine fractions. Hence the main criteria for the development of a normal distribution is the lack of skewed fractions and the concentration of the narrow, intermediate size fractions in the final grain size distribution. This is also corroborated with the index of symmetry, which is a measure of the difference between the angle of two slopes of the hyperbolic distribution as represented by the coarser and finer fractions. Copyright © 2010 John Wiley & Sons, Ltd.     

Regional grain size variations in aeolian sediments along the transition between Tibetan highlands and north‐western Chinese deserts – the influence of geomorphological settings on aeolian transport pathways

Sediment distribution is investigated applying grain size analysis to 279 surface samples from the transitional zone between high mountains (Qilian Shan) and their arid forelands (Hexi Corridor) in north‐western China. Six main sediment types were classified. Medium scale (10³ m) geomorphological setting is carefully considered as it may play an important role concerning sediment supply and availability. A tripartite distribution of sedimentological landscape units along the mountain to foreland transition is evident. Aeolian sediments (e.g. loess and dune sands) are widespread. They are used to identify aeolian transport pathways. The mU/fS‐ratio (5–11 µm/48–70 µm) among primary loess opposes the two grain size fractions being most sensitive to varying accumulation conditions. The first fraction is attributed to long‐distance transport in high suspension clouds whereas the latter represents local transport in saltation mode. The ratio shows strong correlation with elevation (R² = 0.77). Thus, it indicates a relatively higher far‐traveled dust supply in mountainous areas (>3000 m above sea level [a.s.l.]) compared to the foreland. The contribution of westerlies to high mountain loess deposits is considered likely. Hereby, the influence of the geomorphological setting on grain size composition of aeolian sediments becomes apparent: the contribution from distant dust sources is ubiquitous in the study area. However, the far‐distance contribution may be reduced by the availability of fine sand provided in low topography settings. Plain foreland areas support fine sand deflation from supplying river beds, allowing the formation of sandy loess in foreland areas and intramontane basins. In contrast, high mountain topography inhibits strong sand deflation into loess deposits. Eastern parts of the Hexi Corridor show higher aeolian sand occurrence. In contrast, the western parts are dominated by gravel gobi surfaces. This is attributed to higher sand supply in eastern parts provided by the Badain Jaran Desert and fluvial storages as sand sources. Copyright © 2014 John Wiley & Sons, Ltd.     

The particle size characteristics of fine‐grained channel deposits in the River Exe Basin,Devon, UK

Deposition and storage of fine‐grained (<62·5 μm) sediment in the hyporheic zone of gravel bed rivers frequently represents an important cause of aquatic habitat degradation. The particle size characteristics of such fine‐grained bed sediment (FGBS) exert an important control on its hydrodynamic properties and environmental impact. Traditionally, particle size analysis of FGBS in gravel bed rivers has focused on the absolute size distribution of the chemically dispersed mineral fraction. However, recent work has indicated that in common with fluvial suspended sediment, significant differences may exist between the absolute and the in situ, or effective, particle size composition of FGBS, as a result of the existence of aggregates, or composite particles. In the investigation reported in this paper, sealable bed traps that could be remotely opened to sample sediment deposited during specific storm runoff events and a laser back‐scatter probe were used to quantify the temporal and spatial variability of both the absolute and effective particle size composition of FGBS, and the associated suspended sediment from four gravel bed rivers in the Exe Basin, Devon, UK. The absolute particle size distributions of both the FGBS and suspended sediment evidenced c. >95%<62·5 μm sized primary particles and displayed a seasonal winter–summer fining, while the opposite trend was displayed by the effective particle size distribution of the FGBS and suspended sediment. The effective particle size distributions of both were typically highly aggregated, comprising up to 68%>62·5 μm sized particles. Spatial variation in the effective particle size and aggregation parameters was of secondary importance relative to temporal variation. The effective particle size distribution of the FGBS was consistently coarser and more aggregated than the associated suspended sediment and there was evidence of aggregate break‐up in samples of resuspended bed sediment. The implications of these findings for sediment transport modelling are considered. Copyright © 1999 John Wiley & Sons, Ltd.     

沉积物金属元素变化的粒度效应--以太湖沉积岩芯为例

水体沉积物中金属元素含量变化除了与人为污染有关之外,在很大程度上受沉积物粒度、矿物组成等沉积物性质的影响．在进行沉积物金属元素研究中,要充分考虑沉积物金属元素含量变化的粒度效应．本文通过对太湖MS岩芯中17种金属元素、沉积物粒度、矿物组成等指标的分析,研究了金属元素变化特征以及与沉积物粒度组成的关系．MS岩芯金属元素变化可分为两类,第一类主要为Al、Fe、K等,该类元素在沉积岩芯中下部含量较高,岩芯上部含量较低;第二类为Na元素,在沉积岩芯中下部含量较低,岩芯上部含量升高．MS岩芯中金属元素与粘土含量之间具有显著相关关系,经沉积物粒度(粘土含量)校正后,沉积岩芯中金属元素含量趋于稳定．因此,沉积物粒度组成(粘土含量)是影响金属元素含量的主要因素,在研究太湖沉积物金属元素变化规律及进行金属污染评价时,应对金属元素含量进行粒度校正．     

Grain‐size distribution patterns of a point bar system in the Usri River,India

Grain‐size distribution patterns in a point bar system of the Usri River, India, were critically analysed in the light of log‐normal, log‐hyperbolic and log‐skew‐Laplace distribution models. Sand samples were collected from the cross‐bedding foreset of different sizes of bedform; the objectives were to (i) study whether bedform heights have any role in grain‐size distribution patterns, (ii) offer a best‐fit statistical model, (iii) study the downstream variation of size‐sorting in a point bar system, and (iv) study the mechanism of grain sorting. The results indicate that the bedform heights have no role in grain‐size distribution patterns. Quantitatively when the errors in three distribution models were analysed, it was observed that the log‐normal distribution is the best‐fit statistical model and the next one is the log‐skew‐Laplace. However, in the upper reaches of the river, log‐normal distribution is the best‐fit model in the case of large bedforms, whereas in the lower reaches the log‐normal model is the best‐fit one in the case of small bed forms. It is also observed that within a point bar, for large and small bedforms, there is a tendency for mean grain size to decrease downstream. Between point bars for large bedforms there is no consistency in decreasing grain size downstream, whereas for small bed forms the decrease of grain size downstream is observed except near the confluence at Palkia. With distance of transport, the coarser and finer fractions of sediments are gradually chopped off. The coarser fractions are buried below the advancing bedforms on the lee sides and the finer ones are transported further downstream. Thus the finer admixture giving rise to the fining‐upward sequence overlies a carpet of coarser materials. This mechanism provides a clue to the process of grain sorting in the fluvial environment. An interpretation has been offered for the log‐normality of the grain‐size distribution pattern. During prolonged transportation in a fluvial environment, the larger grain‐size fractions are gradually chopped off and buried below the advancing bedforms on their lee sides. On the other hand, the finer fractions are transported further downstream in suspension. Thus the narrow, intermediate size fraction takes active part in the distribution patterns leading to the generation of unimodality and a symmetric distribution pattern downstream, which are the main criteria for log‐normality. Similarly, increase of bedform size is the effect of increase of stream power and Froude number leading to the selective segregation of bed materials. Thus the intermediate size fractions take a more active part than the coarser and the finer size fractions in developing log‐normality. Besides the hydrodynamic parameters of the Usri, coarsening of grain size downstream has been attributed to (i) the aggrading nature of the Usri downstream, and (ii) the contribution of coarser materials to the Usri by its tributaries and bank erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

The behavior of specific sediment yield in different grain size fractions in the tributaries of the middle Yellow River as influenced by eolian and fluvial processes

Based on data from 35 stations on the tributaries of the Yellow River, annual specific sediment yield (Y_s) in eight grain size fractions has been related to basin‐averaged annual sand–dust storm days (D_ss) and annual precipitation (P_m) to reveal the influence of eolian and fluvial processes on specific sediment yield in different grain size fractions. The results show that Y_s in fine grain size fractions has the highest values in the areas dominated by the coupled wind–water process. From these areas to those dominated by the eolian process or to those dominated by the fluvial process, Y_s tends to decrease. For relatively coarse grain size fractions, Y_s has monotonic variation, i.e. with the increase in D_ss or the decrease in P_m, Y_s increases. This indicates that the sediment producing behavior for fine sediments is different from that for relatively coarse sediments. The results all show that Y_s for relatively coarse sediments depends on the eolian process more than on the fluvial process, and the coarser the sediment fractions the stronger the dependence of the Y_s on the eolian process. The Y_s–D_ss and Y_s–P_m curves for fine grain size fractions show some peaks and the fitted straight lines for Y_s–D_ss and Y_s–P_m relationships for relatively coarse grain size fractions show some breaks. Almost all these break points may be regarded as thresholds. These thresholds are all located in the areas dominated by the coupled wind–water process, indicating that these areas are sensitive for erosion and sediment production, to which more attention should be given for the purpose of erosion and sediment control. A number of regression equations were established, based which the effect of rainfall, sand–dust storms and surface material grain size on specific sediment yield can be assessed. Copyright © 2007 John Wiley & Sons, Ltd.     

Flocculation: a key process in the sediment flux of a large,glacier‐fed lake

This study investigates the consequences of flocculation for sediment flux in glacier‐fed Lillooet Lake, British Columbia based on density, fractal dimension, in situ profiles of sediment concentration and size distribution, and settling velocity equations presented in the literature. Sediment flux attributed to macroflocs during the late spring and summer accounts for a significant portion of sediment flux in the lake, equivalent to at least one‐quarter of the average annual sediment flux. Fine sediment is reaching the lake floor faster in flocs than occurs if settling as individual grains. This flux varies both spatially and temporally over the observation period, suggesting a link between deposition via flocculation and the properties of bottom sediments. Macrofloc flux increased through June, reached a peak during July, and then declined into August. Macrofloc flux was greatest in the distal end of the first basin, approximately 10 km from the point of inflow. Relatively high excess densities (～0·1 g cm^–3 at 500 µm) for flocs in situ are consistent with a composition dominated by inorganic primary particles. Microlaminations within Lillooet Lake varves have been linked by earlier workers to discharge events, and the action of turbidity currents, emanating from the Lillooet River. While turbidity currents undoubtedly occur in Lillooet Lake, these results demonstrate flocculation as an adjunct process linking discharge, lake level, macrofloc flux, bulk density and microlaminations. In situ measurements of sediment settling velocity in glacier‐fed lakes are required to better constrain flux rates, and permit comparison between flocculation in lacustrine environments with existing studies of estuarine, marine and fluvial flocculation. Copyright © 2009 John Wiley & Sons, Ltd.     

结合粒度和137Cs对小流域水库沉积物的定年——以黔中喀斯特地区克酬水库为例

对取自黔中喀斯特地区克酬水库32cm长沉积物柱样进行^137Cs比活度以及粒度测定,结合气象站点降雨数据进行比对分析,对沉积物不同层位的年代进行了划分,并以此结合质量深度推算了1960-2004年间该水库的沉积速率。结合实例分析了^137Cs沉降、运移和沉积,改进了仅依靠^137Cs峰值定年方法,认为在流域面积较小、^137Cs比活度测试绝对数值较小的流域,降水和其他条件造成的土壤侵蚀程度差异可能是造成土壤中^137Cs进入湖泊沉积物多少的更主要原因,并表现为沉积物中^137Cs不同层位的差异。