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1.
The variation of the Asian winter monsoonal strength has seriously affected the climate and environmental conditions in the Asian monsoonal region, and even in marginal islands and the ocean in the East Asian region. However, relevant understanding remains unclear due to the lack of suitable geological materials and effective proxies in the key study areas. Here, we present a grain-size record derived from the palaeo-aeolian sand dune in the southeastern Mu Us Desert, together with other proxies and OSL dating, which reflect a relatively detailed history of the winter monsoon and abrupt environmental events during the past 4.2 ka. Our grain-size standard deviation model indicated that >224 μm content can be considered as an indicator of the intensity of Asian winter monsoon, and it shows declined around 4.2-2.1 ka, enhanced but unstable in 2.1-0.9 ka, and obviously stronger since then. In addition, several typical climate events were also documented, forced by the periodic variation of winter monsoonal intensity. These include the cold intervals of 4.2, 2.8, 1.4 ka, and the Little Ice Age (LIA), and relatively warm sub-phases around 3.0, 2.1, 1.8 ka, and the Medieval Warm Period (MWP), which were roughly accordant with the records of the aeolian materials, peat, stalagmites, ice cores, and sea sediments in various latitudes of the Northern Hemisphere. Combined with the previous progresses of the Asian summer monsoon, we preliminarily confirmed a millennial-scale anti-correlation of Asian winter and summer monsoons in the Late Holocene epoch. This study suggests that the evolution of the palaeo-aeolian sand dune has the potential for comprehending the history of Asian monsoon across the desert regions of the modern Asian monsoonal margin in northern China.  相似文献   

2.
The sub-cloud evaporation effect refers to the evaporation process for raindrops that fall from the cloud base to the ground, which is usually accompanied by depleted light isotopes and enriched heavy isotopes in the precipitation. Based on 461 event-based precipitation samples collected from 12 weather stations in the Qilian Mountains and the Hexi Corridor from May to August of 2013, our results indicated that sub-cloud evaporation has a great influence on the δ18O of precipitation, especially in small-amount precipitation events. In May, June, July, and August the δ18O composition was enriched by 35%, 26%, 39%, and 41%, respectively, from the cloud base to the ground. This influence clearly strengthened with temperature rise, from the Qilian Mountains to the Hexi Corridor. When falling raindrops are evaporated by 1.0% in the Qilian Mountains and the Hexi Corridor, the composition of δ18O would be enriched by 1.2% and 2.6%, respectively. Temperature dominated the sub-cloud evaporation in the Qilian Mountains, whereas relative humidity controlled it in the Hexi Corridor. These results provide new proofs of the evolutional process of stable isotopes in precipitation in arid regions.  相似文献   

3.
We chose five communities, representing a mild to severe gradient of grassland desertification in a semi-arid area of Ordos Plateau, northwestern China, to explore the spatial relationships among plant species, above-ground biomass (AGB), and plant nutrients (N and P). Community 1 (C1) was dominated by Stipa bungeana; Community 2 (C2) by a mix of S. bungeana and the shrub Artemisia ordosica; Community 3 (C3) by A. ordosica; Community 4 (C4) by a mix of Cynanchum komarovii and C. komorovii; and Community 5 (C5) by C. komorovii. Quantitative methods, including geostatistics, were used to compare community composition, structure, and indicators of ecosystem function (i.e. AGB, plant N and P) in five 16-m2 plots. The highest AGB, plant nitrogen (N) and plant phosphorus (P) were found in lightly degraded community C2. With increasing desertification effects from C3 to C5, the AGB, N, and P decreased significantly while plant density remained unchanged. The spatial variations of AGB were higher in shrub-dominated communities (C1 and C5) than in grass-dominated communities (C2–C4). Strong spatial relationships were detected within and among the communities, with stronger relationships between AGB and density than between AGB and species richness. Spatial patterns of plant N and P were different from those of AGB, reflecting different N and P contents of individual plants and different species that can redistribute soil resources in these communities. The AGB was positively correlated with soil nutrients (TOC, TN, TP, and IN), except for soil AP. We concluded that several specific aspects of ecosystem properties were directly associated with the conversion of the grass and shrub “functional types” in these degraded grasslands.  相似文献   

4.
在干旱区,雨滴在云下降落易受到二次蒸发的影响,明确降水从云层底部降落到地面过程中稳定氢氧同位素的变化在同位素水文学研究中很有必要。基于新疆地面气象站逐小时的观测资料,采用改进后的Stewart模型,研究了新疆雨滴云下蒸发剩余比（从云下到近地面雨滴蒸发后剩余体积占原体积的百分率f）、雨滴中δD变化量（△δD）和过量氘变化量（△d）的时空特征,并分析了△d与气象要素的关系。结果表明：（1）新疆降水中△d和蒸发剩余比存在明显的空间差异。（2）当在气温低、相对湿度大、降水强度大、雨滴直径大的情况下,蒸发剩余比较大,△d接近于0,蒸发剩余比与△d间的线性关系明显,斜率较低。不同气象条件下,蒸发剩余比与△d的关系并不固定,利用这一线性关系反推蒸发剩余比应慎重。（3）敏感性分析可以得出,如果各气象站的气温升高2℃,△d降幅为0.26%~3.10%;如果相对湿度升高5%,△d升幅为1.23%~8.34%;如果雨强增大10%,△d升幅为0.06%~0.89%;如果雨滴直径增加0.2 mm,△d升幅为0.98%~8.16%,但雨滴直径增加量大于1.2 mm时,△d变化量趋于稳定。  相似文献   

5.
中国城市环线道路间距模型与案例   总被引：2，自引：0，他引：2  
贾文毓 《地理学报》2007,62(6):571-578
通过对中国16 座城市环路的分析, 总结出了中国城市环路间距原理: 中国城市环路由外而内的间距之比符合黄金分割数(ω= 0.618033988?) 或其倒数(1/ω = 1.618033988?)。 根据间距比值可将中国环路分为A、B、C 三种类型: A 型标准比值为1/ω; B 型标准比值为 ω; C 型在纵(横) 向上标准比值为1/ω, 在横(纵) 向上标准比值为ω。通过实证分析, 中国城市环路间距之比基本符合这一原理。该原理适用于不同规模、不同性质和不同形态的城市环路。运用该原理可对中国城市环路进行规划建议和合理性评价。  相似文献   

6.
尹海龙  田长彦  任婧  陈春秀  黄建 《中国沙漠》2014,34(5):1277-1284
以盐生植物盐角草(Salicornia europaea)为材料,以NaCl模拟不同盐度环境,盆栽试验了氮(0.3g·kg-1,N1;0.6g·kg-1,N2;1.2g·kg-1,N3;2.4g·kg-1,N4)、盐(2.5g·kg-1,S1;5.0g·kg-1,S2;7.5g·kg-1,S3)处理对其生长发育及氮素吸收利用的影响。结果表明:(1)不同盐度下施氮均可以显著促进盐角草的生长,地上部干质量均在N2处理下达到最大,而株高均在N1时达到最高,且施氮对盐角草生长的影响与盐度有关;(2)不同盐度环境下施氮所能达到的最高干物质产量及最高施氮限量不同,表现为S3S1S2,随着施氮量的增加,氮素生产力与氮素农学利用效率均表现出下降的趋势;(3)施氮显著增加了盐角草各器官含氮量及氮吸收量,同一施氮水平下盐角草各器官含氮量及氮吸收量均表现为同化枝茎根;(4)同一施氮水平下,随着盐度的增加,盐角草同化枝渗透势显著下降,同一盐度环境下,随着施氮量的增加,同化枝渗透势呈现出下降趋势,渗透调节能力增大;(5)3个盐度环境下,施氮均增加盐角草同化枝光合色素含量,从而提高光合效率,增强其对盐渍环境的适应能力。  相似文献   

7.
Pore water sampling from lake and estuary sediments using Rhizon samplers   总被引：1，自引：0，他引：1  
Rhizon samplers have been used for several years for pore water extraction from soils (e.g. Knight et al. in Environ Pollut 99:293–298, 1998; Tye et al. in Geochim Cosmochim Acta 67:375–385, 2003; Sigfusson et al. in Sci Total Environ 369:203–219, 2006). With a few exceptions (e.g. Song et al. in Chemosphere 50:711–715, 2003; Seeberg-Elverfeldt et al. in Limnol Oceanogr Methods 3:361–371, 2005), they are rarely used for the extraction of pore water from sediments. The current study highlights the advantages and disadvantages of Rhizon samplers and outlines a simple, but effective method for pore water collection from lacustrine and estuarine sediment cores. This method can be used in the field or laboratory and does not require the use of a glove box to prevent change in pore water redox status.  相似文献   

8.
Measurement of samples from 154 sites in the continental sector of the Cameroon Volcanic Line yielded six palaeomagnetic poles, at 243.6°E, 84.6°N, α 95 = 6.8°; 224.3°E, 81.2°N, α 95 = 8.4°; 176.1°E, 82.0°N, α 95 = 8.5°; 164.3°E, 86.4°N, α 95 = 3.4°; 169.4°E, 82.6°N, α 95 = 4.6° and 174.7°E, 72.8°N, α 95 = 9.5°, belonging to rocks which have been dated by the K–Ar method at 0.4–0.9  Ma, 2.6  Ma, 6.5–11  Ma, 12–17  Ma, 20–24  Ma and 28–31  Ma, respectively. The results are in general agreement with other palaeomagnetic poles from Oligocene to Recent formations in Africa.
  The first three poles for rocks formed between 0.4 and 11  Ma are not significantly different from the present geographical pole. Together with other African poles for the same period, this suggests that the African continent has moved very little relative to the pole since 11  Ma. The other three poles for rocks dated between 12 and 31  Ma are significantly different from the present geographical pole, showing a 5° polar deviation from the present pole in the Miocene and 13° in the Middle Oligocene.  相似文献   

9.
New palaeomagnetic data from the Lower and Middle Cambrian sedimentary rocks of northern Siberia are presented. During stepwise thermal demagnetization the stable characteristic remanence (ChRM) directions have been isolated for three Cambrian formations. Both polarities have been observed, and mean ChRM directions (for normal polarity) are: Kessyusa Formation (Lower Cambrian) D = 145°, I = -40°, N = 12, α95= 12.8°; pole position: φ= 38°S, A = 165°E; Erkeket Formation (Lower Cambrian, stratigraphically highly) D = 152°, I = - 47°, N = 23, α95= 6.8°; pole position: φ= 45°S, A = 159°E; Yunkyulyabit-Yuryakh Formation (Middle Cambrian) D = 166°, I = - 33°, N = 38, α95= 4.6°; pole position: φ= 36°S, L = 140°E. These poles are in good agreement with the apparent polar wander path based on the bulk of existing Cambrian palaeomagnetic data from the Siberian platform. In Cambrian times, the Siberian platform probably occupied southerly latitudes stretching from about 35° to 0°, and was oriented 'reversely' with respect to its present position. Siberia moved northwards during the Cambrian by about 10° of latitude. This movement was accompanied by anticlockwise rotation of about 30°. The magnetostratigraphic results show the predominance of reversed polarity in the Early Cambrian and an approximately equal occurrence of both polarities in the part of the Middle Cambrian studied. These results are in good agreement with the palaeomagnetic polarity timescale for the Cambrian of the Siberian platform constructed previously by Khramov et al. (1987).  相似文献   

10.
Downstream hydrologic effects since the closure of Glenbawn Dam, a large dual-purpose storage for water conservation and flood mitigation, include: (i) a reduction in mean annual runoff of about 21 × 106 m3; (ii) a change in the probability distribution of mean daily flows involving a truncation of flows >;8 × 106 m3 d?1, a much reduced frequency of flows >7 × 105 m3 d?1 and an increased frequency of flows <7 × 105 m3 d?1; and (iii) a reduction in flood magnitude of at least 80 per cent for all probabilities of exceedance. From suspended sediment samples collected before and after dam closure, sediment trap efficiency has been estimated at 99 per cent.An accommodation adjustment of the channel has occurred upstream of the first unregulated tributary because the bed is armoured, the banks are well vegetated, some bedrock and concrete controls are present and all regulated releases are incompetent to transport the bed material. Immediately downstream of the first unregulated tributary, the channel has contracted by up to 45 per cent and degraded by up to 69 per cent during lateral migration. Further downstream no channel changes were recorded although the bed material has progressively coarsened over time.  相似文献   

11.
The frequency and timing of Holocene paleofloods in the hilly terrain of New Hampshire and Maine are identified using 14C and high-resolution (cm-by-cm) particle size analysis of sediment cores taken from six post-glacial lakes (~0.1–1.4 km2). A total of nine sediment cores (4.5–6 m long) were taken near the base of stream delta foreslopes. End-member modeling of the particle-size frequency distributions from each core produces 3–5 representative end member distributions, or end members (EMs). Concurrent increases in mean and median particle size, and in the relative abundance of the coarsest EM(s), indicate increased transport capacity of inflowing tributaries, resulting from rainstorms. In all 9 cores, particle size data show clear signs of episodic, high-energy sediment transport events where proxy measurements such as loss-on-ignition and magnetic susceptibility do not, demonstrating the sensitivity of particle size analysis in paleostorm investigations made using lake sediment cores. Floods caused by storms in this region peaked around 1.4, 2.1, 3.0, 3.9, 6.8, 8.2, and 11.5 ka cal BP, and presently appear to be increasing in frequency. Periods of storminess in New Hampshire and Maine correlate well with other records of precipitation and climate in the northeastern United States during the Holocene, further supporting modern records which show tropical air masses as a primary driver of extreme precipitation events in New England (Ludlum 1996; Konrad 2001; Sisson and Gyakum 2004).  相似文献   

12.
《Geomorphology》1997,18(2):61-75
In badland areas of the Ebro Basin, in a semiarid climate, two erosion plots (257 m2; 5° slope and 128 m2; 23° slope) on exposed Tertiary clays were monitored over two years (Nov. 1991–Nov. 1993). This material is characterized by high sodium absorption ratios which lead to high soil dispersivity. The dominant erosion processes in both plots are rilling and sheet erosion. Rainfall intensity was recorded at a weather station, connected to a data-logger, sediment production for single events was collected in tanks, and ground lowering was measured every six months by erosion pins and microtopographic profile gauge techniques. Significant runoff was produced only by rainfall events above 5 mm. Another threshold at 20 mm rain was noted. For rainfalls higher than 20 mm, the 23° slope plot shows a greater runoff response than the 5° one. Rainfall events exceeding this threshold showed a higher sediment production for the steeper slope. In the relationship between precipitation and sediment concentration, an envelope curve can be drawn indicating that any rainfall event of a given amount and intensity has a maximum sediment concentration which we speculate to be a function of the runoff sediment transport capacity. Runoff response and sediment yield in the studied plots are controlled by the rainfall and soil characteristics and their seasonal variations. In both plots, the erosion pins show that erosion rates in rill areas are 25–50% higher than in the interrill areas. Sediment yield recorded by collector devices was higher than the rates measured by erosion pins. The erosion rates based on rill cross-sections by profilometers were higher than the ones recorded by collectors.  相似文献   

13.
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14.
This paper examines how extreme weather events affect the mobility of low-income urban residents in Ghana. Bringing together scholarship on extreme weather and mobilities, it explores the differential impact of flooding on their everyday lives as they navigate the cities of Accra and Tamale. A range of qualitative methods were drawn on, including semi-structured interviews, focus group discussions, and follow-along-participant observations in selected communities of both cities. Three key themes emerged: disrupted road and transport infrastructure, everyday mobility challenges, and coping/adaptive strategies. In flooding conditions, residents experienced difficulties leaving/returning home, engaging in income-generating activities, and accessing transport services and other key urban infrastructure. Conceptually, the paper reveals how disruption to urban residents’ daily movements and activities (re)produces new forms of mobilities and immobilities, which have three relational elements: postponed, improvised and assisted. Throughout the analysis, we show how these mobilities/immobilities vary by age and gender: all urban residents, (though women in particular), experience postponed mobility; young people especially engage in improvised mobility; and children and the elderly are in greatest need of assisted mobility. The paper thus contributes to scholarship on extreme weather events and mobility by providing a more spatially nuanced understanding of the multi-faceted domains in which flooding, socio-economic conditions and adaptive strategies intersect to influence urban mobility in resource poor settings.  相似文献   

15.
The Heihe River Basin is the second largest inland river basin in Northwest China and it is also a hotspot in arid hydrology, water resources and other aspects of researches in cold regions. In addition, the Heihe River Basin has complete landscape, moderate watershed size, and typical social ecological environmental problems. So far, there has been no detailed assessment of glaciers change information of the whole river basin. 1：50,000 topographic map data, Landsat TM/ETM＋ remote sensing images and digital elevation model data were used in this research. Through integrated computer automatic interpretation and visual interpretation methods, the object-oriented image feature extraction method was applied to extract glacier outline information. Glaciers change data were derived from analysis, and the glacier variation and its response to climate change in the period 1956/1963–2007/ 2011 were also analyzed. The results show that：（1） In the period 1956/1963–2007/2011, the Heihe River Basin＇s glaciers had an evident retreat trend, the total area of glaciers decreased from 361.69 km2 to 231.17 km~2; shrinking at a rate of 36.08%, with average single glacier area decrease 0.14 km~2; the total number of the glaciers decreased from 967 to 800.（2） Glaciers in this basin are mainly distributed at elevations of 4300–4400 m, 4400–4500 m and 4500–4600 m; and there are significant regional differences in glaciers distribution and glaciers change.（3） Compared with other western mountain glaciers, glaciers retreat in the Heihe River Basin has a higher rate.（4） Analysis of the six meteorological stations＇ annual average temperature and precipitation data from 1960 to 2010 suggests that the mean annual temperature increased significantly and the annual precipitation also showed an increasing trend. It is concluded that glacier shrinkage is closely related with temperature rising, besides, glacier melting caused by rising temperatures greater than glacier mass supply by increased precipitation to  相似文献   

16.
Summary. From nine Upper Cretaceous—Lower Tertiary (85 ± 5–66 ± 5 Ma) volcanic hills in Central Argentina (33°S, 65°W), 26 hand samples were collected yielding a palaeomagnetic pole at 45°E 70°s ( A 95 = 12.1°; k = 13.6; N = 12) after AC cleaning. Three sites show normal and nine reversed polarity. This pole is close to the pole for the late Cretaceous (69 Ma) Andacolo Series.  相似文献   

17.
The degradation of permafrost stability in China over the past 30 years is evaluated using a new, high-resolution near-surface air temperature reanalysis dataset. Results show that the permafrost extent clearly decreased by 22% from 1980 to 2010, that is, a loss of 12.684;104 km2. The degradation occurred not only in the transition regions between permafrost and seasonally frozen ground, but also and more importantly, in the interior of the permafrost regions. The degradation in the interior of permafrost regions accounted for 87% of the total degraded areas. The degradation occurred mainly during the 1980s to 1990s in the northeast permafrost area and the Qilian Mountains, and during the 1990s to 2000s in most areas of the Qinghai-Tibet Plateau (QTP). This degradation will have systemic impacts on engineered infrastructures in permafrost regions, the water balance, and the global carbon budget. A more robust physical model should be used to evaluate the permafrost thermal stability at finer resolution in the future.  相似文献   

18.
Artemia populations were sampled in four hypersaline ecosystems in continental evaporitic basins from the Argentinean provinces of La Pampa and Buenos Aires. Biometrics of cysts and nauplii were performed. The morphometrics of adult females under standard culture conditions, studied by multivariate discriminant analysis, provided evidence that these populations belonged to the speciesA. persimilis; this was further supported by cross-breeding tests established between one of these populations (Salinas Grandes de Hidalgo, province of La Pampa) and a population ofA. franciscana from San Francisco Bay (California, USA), which showed reproductive isolation and barriers to gene flow between both populations.  相似文献   

19.
基于距离与规模的中国城市体系规模结构   总被引：2，自引：0，他引：2  
城市规模结构是城市体系研究的重要内容。经济基础理论相关的研究忽视了城市体系中某一城市规模往往受到其他城市规模、距离的影响。本文以第五次人口普查的"市人口"作为城市人口规模标度指标,以铁路运行距离为两城市间距离的标度指标,证明了距离要素对解释中国城市体系结节结构的有效性。讨论了全国城市体系、北方城市、南方城市体系中城市i的规模Pi与最近距离城市n的规模Pn、城市体系首位城市s的规模Ps、区域中心城市c的规模Pc,及与相应城市间距离Din、Dis、Dic的定量关系,以及Din及其位序的定量关系。研究发现:(1)全国城市体系中,Pi可以用Pn、Din、Dic来说明,其中,Pn、Din为正向影响,Dic为负向影响,Pn影响最大,Din次之,Dic再次之。(2)北方城市体系中,Pn、Din、Pc均对Pi有正向影响,Din影响力最大,Pn次之,Pc再次之。(3)南方城市体系中Pi的解释因素及各因素的重要性与全国城市体系相同。南方城市体系与北方城市体系中Pi的解释因素的差异性源自南方城市体系中Dic、Pc/Pi较小。(4)Din与其位序R间存在与城市位序-规模律相同的内在定量关系。  相似文献   

20.
首先从人口、经济、用地3个维度综合考察武汉市增长与收缩的全貌,并采用县区及街道2个尺度的数据定量描述了武汉市增长与收缩的特征与空间格局,发现武汉市下辖青山区、硚口区、汉阳区和蔡甸区存在局部较严重的收缩现象,空间上形成集聚,形态上呈“穿孔式”。进一步以青山区为案例,着重从资本视角探讨发生局部收缩的内在机制,发现其存在老龄化、少子化趋势,但局部收缩的主因是资本从产业部门的“逃逸”。  相似文献   

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