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1.
At the beginning of December 2003, one of the biggest floods for at least 150 yr was recorded on the Rhone River. In the lower part of the river, the peak flood reached 11,000 m3 s−1. The geomorphological and radioecological consequences of such an event were investigated downstream all the nuclear installations by using measured and calculated fluxes and the total export of suspended sediment and associated 137Cs. Results pointed out the major role played by large floods in the annual suspended sediment load, as 3.70 × 106 tons of silts, 0.85 × 106 tons of sands, and 0.84 × 106 tons of clays were transferred towards the coastal environment. Nevertheless, these solid loads were found to be lower than those expected as regards the liquid discharge reached during this event and suggested that previous floods that occurred on the river and on its main tributaries during the last decade have probably led to the removal of available sediment from the channels and their banks. Besides, the 137Cs activity measured within the suspended load was estimated at 14.9 ± 0.4 Bq kg−1, which is a level characteristic of the suspended sediments from the Rhone catchment area and demonstrated that nuclear installations located along the Rhone valley did not significantly contribute to any increase in 137Cs activity in the water during the flood. The total 137Cs particulate export amounted to 77 ± 17 GBq and was mainly associated with the silt fraction that contributes to around 70% of the total 137Cs export.  相似文献   

2.
A quantitative assessment is made of suspended sediment load, including particulate organic matter and organic carbon, in 2014 for the small streams of the Northern Yenisei region, in the taiga–tundra transition zone (near the city of Igarka, Krasnoyarsk krai). It was found that the suspended sediment concentration (SSC) of the streams under investigation fluctuated between 2 and 18 mg/L during the summer–autumn low-water period of 2014. The proportion of dissolved organic matter (DOM) in the total sediment yield varied from 16.4% to 74.1%, depending on landscape-geomorphological conditions for suspended sediment formation: it is higher for streams with tundra catchments and lower on forest watersheds underlain by sandy and clayey loams. The DOM content varies from catchment to catchment from 1.63 to 2.42 mg/L, and the mean concentration of dissolved organic carbon (DOC) is estimated at 0.73 to 1.09 mg C/L. It is shown that the local channel transformations serve as the main source of POM and DOC input to the water of two out of three streams under study. Surface runoff or fast subsurface flow in the organic soil horizon is the external source of DOM input to the water of the third stream during flooding. Regional empirical dependencies were obtained, which correlate the water discharge, total SSC and the proportion of DOM are obtained. The long-term proportion of DOM in the annual suspended sediment flow of the Graviika river makes up 25% and DOC, 11%, or, in absolute values, 406 and 183 t/year, and in units of layer 1.26 and 0.57 t/km2, respectively.  相似文献   

3.
A method was developed to quantify a suite of organic compounds from snow melt water samples present at trace level concentrations, using a dichloromethane liquid–liquid extraction and GC–MS. Samples from a 3-m snow pit sampled in 2005 from Summit, Greenland were analyzed using the method developed, and a profile of organics over the past 4 years was compiled. Supporting data including the concentrations of total organic carbon (TOC), low molecular weight acids, and trace elements were determined using well established methods. The results show that low molecular weight acids contribute a significant percentage, up to 20%, of the measured TOC. Hopanes were measured quantitatively for the first time in Greenland snow. Hopanes, as well as PAHs, are at very low concentrations and contribute 0.0002–0.004% to TOC. Alkanes and alkanoic acids were also quantified, and contribute less than 1% and up to 7%, respectively to TOC. No apparent seasonal pattern was found for specific classes of organic compounds in the snow pit. The lack of seasonal pattern may be due to post-depositional processing.  相似文献   

4.
Erosion and sediment characteristics were measured using simulated rainfall on two cultivated soils of contrasting lithology (Quaternary calcareous colluvium and Tertiary marls) in a semiarid Mediterranean area of SE Spain. Two rainfall intensities, high (56.0±2.4 mm h−1) and medium (31.4±1.4 mm h−1), were used in order to know the mechanisms involved in each selected condition. For each simulated event, runoff and sediment were sampled at 1-min intervals on a 1-m wide by 2-m long erosion plot. The erosion rate was calculated as the total amount of soil lost divided by the time period of the test. The duration of the test was that needed to reach steady state runoff, an average time of 24.5 min for Quaternary calcareous colluvium and 17.7 min for Tertiary marls. The size distribution of the transported sediment in the field (effective size distribution) was compared with equivalent measurements of the same samples after chemical and mechanical dispersion (ultimate size distribution) to investigate the detachment and transport mechanisms involved in sediment mobilisation. The results showed that the soil type determined the hydrological response, regardless of the rainfall intensity. The erosional response was, however, determined by the rainfall and soil surface characteristics.In Quaternary calcareous colluvial soils, the predominant erosion process depended on the rainfall intensity, with a prevailing detachment-limited condition in high-intensity events and prevailing transport-limited conditions in those events of medium intensity. These differences in the main erosion processes were reflected in the size of the transported material and in the change in sediment size within the storm. Thus, a time-dependent size distribution of the eroded material (decreasing coarse fractions and increasing fine fractions with runoff time) was observed for high-intensity events. In medium-intensity events, on the other hand, the time-independent size distribution of the eroded material indicated transport-limited erosion.Due to the rapid surface crusting on the Tertiary marl soil, no differences in the main erosion processes or in the sediment size distribution occurred for the different rainfall intensities tested. The erosion of marl soils was determined mainly by the limited quantity of available sediment.The effective size of material was a more sensitive parameter than the ultimate size of the sediment to study the way in which the sediment was transported.  相似文献   

5.
The effects of wetlands on hydrology, water quality, and wildlife habitat are internationally recognized. Protecting the remaining wetlands is one of the most important environmental issues in many countries. However wetlands in Japan have been gradually shrinking due to agricultural development and urbanization, which generally lowers the groundwater level and introduces suspended sediment and sediment-associated nutrients into wetlands. We examined the influences of channelization on discharge of suspended sediment and wetland vegetation in Hokkaido, northern Japan. The impact of river channelization was confirmed not only by the sediment budgets but also by river aggradation or degradation after the channelization and by the resultant vegetational changes. The budgets of suspended sediment demonstrated that wash load was the predominant component accounting for 95% of the total suspended load delivered into the wetland. This suspended sediment was primarily transported into the wetland by flooding associated with heavy rainfall. Twenty-three percent of the wash load and 63% of the suspended bed material load were deposited in the channelized reach, which produced aggradation of about 2 m at the end of the reach. A shorting of the length of the channel, due to channelization of a meandering river, steepened the slope and enhanced the stream power to transport sediment. This steepening shifted the depositional zones of fine sediment 5 km downstream and aggraded the riverbed. Development of the watershed may increase not only the water discharge but also the amount of suspended sediments. The aggradation reduced the carrying capacity of the channel and caused sediment ladened water to flood over the wetlands. The fine sediment accumulated on the wetlands gradually altered the edaphic conditions and wetland vegetation. A low percentage (10 to 15%) of organic contents of wetlands' soil is more evidence indicating that the present condition is far different from normal. Original vegetation such as sedges and Alnus japonica were disappearing from the adjacent areas of the river channel and were being replaced by willow trees (Salix spp.).  相似文献   

6.
西江水体有机碳含量变化及悬浮物碳同位素的意义   总被引：1，自引：0，他引：1  
以西江马口断面水体为研究对象,讨论其有机碳含量、悬浮物同位素组成(δ13C、Δ14C)与环境之间的关系。西江马口断面有机碳含量呈现季节性变化,其中颗粒有机碳(POC)含量范围介于0.13~4.98mg/L之间,溶解有机碳(DOC)含量范围介于0.98~4.17mg/L之间,DOC/POC比值为1.34,远低于世界多数河流之比值。近年来马口断面悬浮物POCδ13C的变化范围介于-21.3‰~-26.1‰之间,随时间出现整体性漂移现象,表明流域生态环境整体改善。POCΔ14C变化范围介于-132‰~-425‰之间,与取样期间的流域降水过程及土壤的侵蚀相关。  相似文献   

7.
2009年4～5月,对北部湾北部滨海湿地水体和表层沉积物进行了取样调查,研究水体中营养盐和表层沉积物中碳、氮和磷元素含量、分布及其影响因素,并在此基础上评价了滨海湿地水体富营养化水平和表层沉积物的污染状况.研究区水体中的溶解无机氮(dissolved inorganic nitrogen,DIN)、活性磷酸盐PO43-—P和化学需氧量(CODMn)平均含量分别为(211.84±37.44) μg/L、(11.01±12.11) μg/L和(0.92±0.32) mg/L;表层沉积物中总氮(TN)、总磷(TP)和总有机碳(TOC)的平均含量分别为(373±355) μg/g、(232.28±157.34) μg/g和(0.45±0.46)％.北部湾北部滨海湿地水体中的DIN含量和PO43-—P含量在茅尾海和廉州湾东部西场海区都很高,且水体富营养化比较严重,大规模的海水养殖可能是造成该现象的主要原因.研究区水体中CODMn含量空间分布呈近岸高、远岸低的特征.研究区铁山港顶部、南流江口、大风江口、钦江和茅岭江口较细的表层沉积物中的TN、TP和TOC含量较高.研究区表层沉积物的TOC/TN值分布表明,钦江口、钦州港区、防城港、企沙港区和珍珠湾东北部的沉积物有机质含量受到了强烈的陆源影响.根据加拿大安大略省环境质量评价标准,除了钦州湾顶部的龙门港区、大风江口中部和铁山港顶部西北角一小部分区域的表层沉积物中的TOC、TN和TP含量分别略高于1％、550μg/g和600μg/g,已被明显污染,而处于最低级别或严重危害级别,会对部分底栖生物产生影响外,其他绝大部分区域都属安全级别,为清洁或部分已被轻微污染.  相似文献   

8.
This study tests the hypothesis that Fourier-transform infrared spectroscopy (FTIRS) of lake sediments can be used to infer past changes in tree-line position and total organic carbon (TOC) content of lake water. A training set of 100 lakes from northern Sweden spanning a broad altitudinal and TOC gradient from 0.7 to 14.9 mg/l was used to assess whether vegetation zones and TOC can be modelled from FTIR spectra of surface sediments (0–1 cm) using principal component analysis (PCA) and partial least squares (PLS) regression. Preliminary results show that FTIRS of lake sediments can be used to reconstruct past changes in tree line and the TOC content of lake water, which is hardly surprising since FTIRS registers the properties of organic and minerogenic material derived from the water mass and the drainage area. The FTIRS model for TOC gives a root mean squared error (RMSECV) of calibration of 1.4 mg/l (10% of the gradient) assessed by internal cross-validation (CV) yielding an Rcv2 of 0.64. This should be compared with a near-infrared spectroscopy (NIRS) and diatom transfer function for TOC from the same set of lakes, which have a Rcv2 of 0.61 and 0.31, and RMSECV of 1.6 and 2.3 mg/l, respectively. The FTIRS-TOC model was applied to a Holocene sediment core from a tree-line lake and the results show similar trends as inferences from NIRS and pollen from the same core. Overall, the results indicate that changes in FTIR spectra from lake sediments reflect differences in catchment vegetation and TOC, and that FTIRS-models based on surface-sediment samples can be applied to sediment cores for retrospective analysis.  相似文献   

9.
River basin reservoir construction affects water and sediment transport processes in downstream reaches. The downstream impact of the Three Gorges Projects (TGP) has started to become apparent: (1) reduction in flood duration and discharge, and significant reduction in sediment load. Although there was some restoration in downstream sediment load, the total amount did not exceed the pre-impoundment annual average; (2) in 2003–2014, the d > 0.125 mm (coarse sand) load was restored to some degree, and to a maximum at Jianli Station, which was mainly at the pre-impoundment average. After restoration, erosion and deposition characteristics of the sediment was identical to that before impoundment. The degree of restoration during 2008–2014 was less than during 2003–2007; (3) after TGP impoundment, there was some restoration in d < 0.125 mm (fine sand) sediment load, however, it was lower than the pre-impoundment average; (4) due to riverbed compensation, the d > 0.125 mm sediment load recovered to a certain degree after impoundment, however, the total did not exceed 4400×104 t/y. This was mainly limited by flood duration and the average flow rate, and was less affected by upstream main stream, tributaries, or lakes. Restoration of d < 0.125 mm suspended sediment was largely controlled by upstream main stream, tributaries, and lakes, as well as by riverbed compensation. Due to bed armoring, riverbed fine suspended sediment compensation capability was weakened; (5) during 2003–2007 and 2008–2014, Yichang to Zhicheng and upper Jingjiang experienced coarse and fine erosion, lower Jingjiang experienced coarse deposition and fine erosion, Hankou to Datong had coarse deposition and fine erosion, and Chenglingji and Hankou was characterized by coarse deposition and fine sand erosion in 2003–2007, and coarse and fine erosion in 2008–2014. This difference was controlled by flood duration and number at Luoshan Station.  相似文献   

10.
Floodplains contain valuable stratigraphic records of past floods, but these records do not always represent flood magnitudes in a straightforward manner. The depositional record generally reflects the magnitude, frequency, and duration of floods, but is also subject to storm-scale hysteresis effects, flood sequencing effects, and decade-scale trends in sediment load. Many of these effects are evident in the recent stratigraphic record of overbank floods along the Upper Mississippi River (UMR), where the floodplain has been aggrading for several thousand years. On low-lying floodplain surfaces in Iowa and Wisconsin, 137Cs profiles suggest average vertical accretion rates of about 10 mm/year since 1954. These rates are slightly less than rates that prevailed earlier in the 20th Century, when agricultural land disturbance was at a maximum, but they are still an order of magnitude greater than long-term average rates for the Holocene. As a result of soil conservation practices, accretion rates have decreased in recent decades despite an increase in the frequency of large floods.The stratigraphic record of the Upper Mississippi River floodplain is dominated by spring snowmelt events, because they are twice as frequent as rainfall floods, last almost twice as long, and are sometimes associated with very high sediment concentrations. The availability of sediment during floods is also influenced by a strong hysteresis effect. Peak sediment concentrations generally precede the peak discharges by 1–4 weeks, and concentrations are usually low (<50 mg/l) during the peak stages of most floods. The lag between peak concentration and peak discharge is especially large during spring floods, when much of the runoff is contributed by snowmelt in the far northern reaches of the valley.The great flood of 1993 on the Mississippi River focused attention on the geomorphic effectiveness and stratigraphic signature of large floods. At McGregor, where the peak discharge had a recurrence interval of 14 years, the flood was most notable for its long duration (168 days above 1600 m3s−1), high sediment concentrations (three episodes >180 mg/l), and large suspended load (1.71 Mt). The flood of 2001, despite its greater magnitude (recurrence interval 70 years), was associated with relatively low sediment concentrations (<60 mg/l). The 1993 and 2001 floods each left 30–80 mm of silty fine sand on most low-lying floodplain surfaces, but the 2001 flood produced sandy levees near the channel while the 1993 flood did not. The stratigraphic signature of these recent floods is more closely related to the duration and total suspended load of the event than to the magnitude of the peak discharge.  相似文献   

11.
Organic geochemical record of environmental changes in Lake Dianchi,China   总被引：1，自引：0，他引：1  
In order to investigate the natural ecosystem of Lake Dianchi and to assess its anthropogenic impacts, a stratigraphic study of bulk and molecular compositions of organic matter was conducted using a 63-cm long sediment core. The results show that two apparent environmental changes occurred during the evolution of Lake Dianchi: (1) the first change occurred in the 43–63 cm sediment depth, and was revealed by the amount and the composition of organic matter in the stage. Natural changes were possibly major factors responsible for triggering the environmental change, but the influence of human activities could not be excluded. Subsequently, the lake entered into a relatively stable and oligotrophic stage, which maintained until 20-cm sediment depth. (2) Eutrophication started in the upper 20 cm depth. Human activities became a major factor influencing environmental changes in this stage. Vertical profiles of various organic geochemical variables in the upper 20-cm sediments show evidence that primary productivity of the lake increased progressively and that the lake started eutrophic. Especially in the uppermost 10 cm, notable excursions to less negative δ13Corg and δ15Ntotal and high TOC concentrations have recorded an abrupt change in the lacustrine environment, suggesting that the lake entered a hypereutrophic stage. In addition, enhancement of αβ-hopanes reflects the contribution of fossil fuels to the lake sediments.  相似文献   

12.
Over the past 150 years, major land use changes have occurred in the Stemple Creek Watershed in northern California that have caused erosion to move soils from the upland to the flood plain, stream channels, and the bay. The purpose of this study is to document the recent (1954 to present) sediment deposition patterns in the flood plain area adjacent to Stemple Creek using the 137Cesium technique. Sediment deposition ranged from 0.26 to 1.84 cm year−1 for the period from 1964 to 2002 with an average of 0.85±0.41 cm year−1. Sediment deposition rates were higher for the 1954 to 1964 period with a range of 0.31–3.50 cm year−1 and an average of 1.29±1.04 cm year−1. These data indicate that sediment deposition in the flood plain has decreased since the middle 1950s, probably related to reduction in row crop agriculture and an increase in pasturelands. This study shows that the flood plains in the Stemple Creek Watershed are a significant sink for the soils being eroded from the upland area. Given the significance of the flood plain for trapping eroded materials before they reach the stream channels or the bay, efforts need to be made to manage these flood plain areas to insure that they do not change and become a source rather than a sink for eroded materials as improved management practices on the upland areas reduce sediment input to the flood plain.  相似文献   

13.
黄河内蒙古段淤积泥沙洪水冲刷效应   总被引：1，自引：1，他引：0  
汪宏芳  贾晓鹏  王海兵 《中国沙漠》2014,34(4):1143-1149
为了探讨黄河内蒙古段淤积泥沙的洪水冲刷效应,于2012年对其三湖河口水文站河道监测断面汛期（7-10月）流量、悬移质泥沙含量以及洪水期间（2012年8月20日-2012年10月1日）悬移质泥沙含量、粒度百分含量的垂直变化特征与流量的关系进行了统计和分析。结果表明：（1）该次洪水具有峰高量大、洪峰过程在河段内持续时间长、洪水起涨和消退缓慢、峰形矮胖的特点;（2）洪水过程中,小于0.05 mm的细颗粒泥沙在2 000 m3·s-1左右的流量下就能输移通过,在2 000～2 400 m3·s-1时输沙强度最大,对河道淤积泥沙可以达到输沙最优的效果;而粒径大于0.05 mm的泥沙输移的效果不好。  相似文献   

14.
Fire can alter sediment sources and transport rates in river basins, changing landforms and aquatic habitats and degrading downstream water quality. Variability in the response between environments, between fires, and with time since fire makes predicting the catchment-scale effect of individual fires difficult. This study applies the fallout radionuclides 137Cs and 210Pbxs to trace the sources and transport of fine sediment through a river network following a wildfire of moderate to extreme severity in the 629-km2 eucalypt-forested Nattai River water-supply catchment near Sydney, Australia. The tracer analysis showed that post-fire erosion caused a switch in fine (< 10 µm) sediment sources from 80% subsoil derived from gully and river bank erosion to 86% topsoil derived from hillslope surface erosion. The fine sediment phosphorus content increased 4–10 fold over pre-fire levels. Annual post-fire sediment yields estimated from suspended solids rating curves were 109–250 times higher than they would have been without fire. A large additional amount of sediment remained stored within the river network for at least four years, particularly in lower-gradient reaches. Analysis of a sediment core showed that surface erosion following a previous fire had supplied at least 29% of total catchment sediment yield over the past 36 years. It is concluded that wildfire can alter catchment sediment budgets in two ways. Firstly, a spatially-diffuse pulse of elevated erosion is associated with moderate or intense rainfall events in post-fire years. Secondly, pulses of elevated catchment sediment yield are driven by the timing and river sediment transport capacity of runoff events. Severe post-fire erosion and high interannual hydrologic variability can result in large sediment stores persisting within the river network for many years. Fallout radionuclide tracers are shown to be useful in quantifying fine sediment sources and transport dynamics following wildfire, and the contribution of wildfire to catchment sediment yield.  相似文献   

15.
Groundwater dominance has important effects on the hydrological and geomorphological characteristics of river systems. Low suspended sediment concentrations and high water clarity are expected because significant inputs of sediment-free spring water dilute the suspended sediment generated by storms. However, in many Mediterranean rivers, groundwater dominance is characterised by seasonal alternations of influent and effluent discharge involving significant variability on the sediment transport regimes. Such areas are often subject to soil and water conservation practices over the centuries that have reduced the sediment contribution from agricultural fields and favour subsurface flow to rivers. Moreover, urbanisation during the twentieth century has changed the catchment hydrology and altered basic river processes due to its ‘flashy’ regime. In this context, we monitored suspended sediment fluxes during a two-year period in the Na Borges River, a lowland agricultural catchment (319 km2) on the island of Mallorca (Balearic Islands). The suspended sediment concentration (SSC) was lower when the base flow index (i.e., relative proportion of baseflow compared to stormflow, BFI) was higher. Therefore, strong seasonal contrasts explain the high SSC coefficient of variation, which is clearly related to dilution effects associated with different groundwater and surface water seasonal interactions. A lack of correlation in the Q-SSC rating curves shows that factors other than discharge control sediment transport. As a result, at the event scale, multiple regressions illustrate that groundwater and surface water interactions are involved in the sedimentary response of flood events. In the winter, the stability of baseflow driven by groundwater contributions and agricultural and urban spills causes hydraulic variables (i.e., maximum discharge) to exert the most important control on events, whereas in the summer, it is necessary to accumulate important volumes of rainfall, creating a minimum of wet conditions in the catchment to activate hydrological pathways and deliver sediment to the drainage network. The BFI is also related to sediment delivery processes, as the loads are higher with lower BFI, corroborating the fact that most sediment movement is caused by stormflow and its related factors. Overall, suspended sediment yields were very low (i.e., < 1 t km− 2 yr− 1) at all measuring sites. Such values are the consequence of the limited sediment delivery attributable to soil conservation practices, low surface runoff coefficients and specific geomorphic features of groundwater-dominated rivers, such as low drainage density, low gradient, steep valley walls and flat valley floors.  相似文献   

16.
Addition and incorporation of organic fertilizers is known to improve the properties of agricultural soils. Therefore, during 22 weeks of field incubation we studied the biodegradation (measured by static absorption of CO2) of two organic manures viz. municipal refuse and chicken manure incorporated into sandy soils, under sprinker irrigation, at the rates of 0, 16·5, 33·0, 49·5 and 66·0, and 0, 4·1, 8·25, 16·5, and 33·0 tonnes/ha, respectively.The results obtained indicated that the cumulative amount of CO2 respired was linearly correlated with the rate of organic material applied (p < 0.001). Statistical analysis of the data obtained showed that both the intercept (rate of decomposition) and the slope (rate of reaction with time) increased with increasing rate of application of organic manure. The magnitude of increment was highest at the application rates of 33·0 and 8·25 tonnes/ha for municipal refuse and chicken manure, respectively. The rate of increment, generally, declined thereafter thus indicating that these two rates are the optimum levels of incorporation of the appropriate organic manures under the conditions of this study.The maximum organic-carbon mineralized, accounting for 10·5 and approximately 50%, was obtained at the optimum rates for municipal refuse and chicken manure, respectively. Both simple and multiple regression equations describing the relationship between soil temperature and soil-moisture content each separately and in combination, as independent variables, and the cumulative rate of CO2 production were computed. A substantially strong relationship with soil temperature but inconsistent and contradicting results with soil moisture were obtained. Inclusion of soil temperature and moisture in a single model as an independent variable in relation to cumulative CO2 as a dependent variable improved the soil moisture-CO2 production relationship.  相似文献   

17.
长江口水沙入海通量的观测与分析   总被引：31，自引：0，他引：31  
在长江口用声学多普勒流速剖面仪（ADCP）进行走航式断面观测,结果表明,采集水样获取的悬沙浓度与ADCP记录的声学信号（后向散射强度）之间存在显著相关关系,因此可据ADCP声学数据获得沿观测断面的高时空分辨率的悬沙浓度剖面。分析结果表明．在徐六泾附近断面以往复流占优势．大潮期间单位时间最大水、悬沙通量值出现于涨潮期．落潮历时较长．落潮期水、悬沙通量分别大于涨潮期水、悬沙通量,净通量向海。长江口南槽拦门沙外侧主要为旋转流．涨潮期间内的水、悬沙通量均分别大于落潮期间的水、悬沙通量。2003年11月12日的观测结果表明．潮周期内通过徐六泾断面向海输运的水与悬沙通量分别为10^9m^3和10^8kg量级：径流量与悬沙输运率分别为10^4m^3／s和10^3kg／s量级。1998年以来徐六泾ADCP流量观测数据与同时间的大通径流量之间存在着显著的相关关系．故可由大通径流量估算徐六泾径流量。作为长江人海径流量．以作为历史资料整编的一种手段。结果表明,长江人海年径流总量略大于大通年径流总量：而在枯水期间人海径流量小于大通径流量。  相似文献   

18.
The objective of this study was to assess the contribution of the major soil/sediment surfaces in the Southern Aral Sea Basin to the dust generation potential of this region. Eight crusts and soils/sediments from seven sites, representative of these surfaces, were sampled in the field and their major characteristics (particle size distribution, organic carbon content, carbonate content, salt content and composition) that are related to dust generation, were determined. The PM10 and PM2·5 dust generation potential of the materials was determined in the laboratory using the Lubbock Dust Generation, Analysis and Sampling System (LDGASS). The highest amount of PM10 dust (579·3 mg.m−3) was generated from the Takyr crust material. The lowest by one Solonchak salt crust material (39·6 mg.m−3). Salt crusts from the desiccated Aral Sea bottom generated intermediate amounts of dust.The experimental results indicate that the Takyrs and Takyr-like soils, that occupy over 1 million ha in the Southern Aral Sea Basin, constitute the surfaces with the highest potential for being the source for the severe dust storms of the area. Second to the Takyr soils, the Solonchaks and Solonchak-like soils, also with an extent of over 1 million ha, contribute highly saline dust. To these must be added a large, as yet uncharted, proportion of the approximately 4 million ha of exposed sea bed, that exhibit Solonchak-like characteristics.  相似文献   

19.
Spatially sampled suspended sediment data from extra-channel surfaces of different ages during rainfall events were used to infer the timescale of landscape adjustment to Little Ice Age (LIA) deglacierization at Small River Glacier, Canada. Surface ages were determined by aerial photograph interpretation. Turbidimeters were installed in three ephemeral channels on “young,” “mature” and “old” surfaces; and their response to rainfall events of different magnitudes recorded. Results showed “young” ice proximal surfaces exposed since 1977 were increasingly vulnerable to mobilization during rainfall events. A suspended sediment response from “old” surfaces exposed since 1910 was recorded for only three events with 5- and 30-year return periods. The intensity and duration of rainfall events therefore had to increase in magnitude to mobilize sediment on surfaces of increasing maturity. Hysteresis in the channel suspended sediment response further confirmed the dependence of surface response on rainfall event magnitude. The rapid temporal decline in surface response indicated that surface armoring or sediment exhaustion is stabilizing surfaces within decades of exposure from the LIA maximum (ca. 1910) at Small River Glacier. However, further perturbation, for example by glacial advance or extreme climatic events, would likely modify the pattern of suspended sediment yields by changing terrain surface mobilization thresholds and reconfiguring channel sediment stores. We therefore suggest that it is difficult to determine when “paraglacial sedimentation” ceases to influence suspended sediment yields, and therefore significantly limiting the usefulness of the term to describe the period of suspended sediment yield adjustment following deglacierization.  相似文献   

20.
We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ13C and δ15N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ13Corganic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ15Norganic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ15N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l−1) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core.  相似文献   

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