Temporal patterns of arid-humid transitions in the south-eastern Arabian Peninsula based on optical dating

The Rub Al Khali aeolian system of the Arabian Peninsula is the world's largest erg. While the region is presently hyper-arid, evidence for multiple past humid periods has been documented in previous studies. Radiocarbon dating of lacustrine, travertine and palaeogroundwater deposits suggests that the climate was humid at 10–6 and 35–25 ka. These phases have been associated with summer rainfall increases occurring in tandem with an intensification and northward migration of the monsoon system during Northern hemisphere precessional maxima. Published optical dating studies have focused on preserved sand dune and aeolianite deposits to establish ages for the intervening arid phases. Here we present a summary of this work and additional optical dating results for samples of aeolian sediment from a deep drill core, which provides insights into the style of barchan dune accretion in the Liwa region of the United Arab Emirates. The large dune accreted rapidly during the mid-late Holocene. The results suggest that the transitions from humid to arid conditions and resulting accumulation of aeolian sediment in the form of large bedforms occurred abruptly at around 6 ka. Further aeolian sedimentation has not been constant since that time, with initial gradual dune growth followed by rapid vertical accretion, and possible termination of accumulation around c. 2 ka.     

Variable vegetation cover and episodic sand movement on longitudinal desert sand dunes

Longitudinal (linear) sand dunes of the Simpson and Strzelecki dunefields in eastern central Australia present a paradox. Low levels of activity today stand in contrast to luminescence dating which has repeatedly shown deep deposits of sand on dune crests dating to within the late Holocene. In order to investigate the nature of dune activity in the Simpson–Strzelecki dunefield, vegetation and sand mobility were investigated by detailed vegetation survey and measurement of rippled area and loose sand depth of dunes at three sites along a climatic gradient. The response of both vegetation and sand movement to inter-annual climate variability was examined by repeat surveys of two sites in drought and non-drought conditions. Projected plant cover and plant + crust cover were found to have inverse linear relationships with rippled area and the area of deep loose sand. No relationship was found between these measures of sand movement and the plant frontal area index. A negative exponential relationship between equivalent mobile sand depth on dune surfaces and both vascular plant cover and vascular + crust cover was also found. There is no simple threshold of vegetation cover below which sand transport begins. Dunes with low perennial plant cover may form small dunes with slip faces leading to a positive feedback inhibiting ephemeral plant growth in wet years and accelerating sand transport rates. The linear dunefields are largely within the zone in which plant cover is sufficient to enforce low sand transport rates, and in which there is a strong response of vegetation and sand transport to inter-annual variation in rainfall. Both ephemeral plants (mostly forbs) and crust were found to respond rapidly to large (> 20 mm/month) rainfall events. On millennial time-scales, the level of dune activity is controlled by vegetation cover and probably not by fluctuations of wind strength. Land use or extreme, decadal time-scale, drought may destabilise dunes by removing perennial plant cover, accelerating wind erosion.     

Late Pleistocene dune construction in the Central Sand Plain of Wisconsin, USA

Wisconsin's Central Sand Plain east of the Wisconsin River is composed of eolian sand forming high-relief dunes surrounded by sand sheets and scattered low-relief dunes. To establish a maximum age for dune formation, three samples for optical dating were taken from glacial Lake Wisconsin lacustrine sediment that underlies eolian sand. These age estimates range from 19.3 to 13.6ka. Age estimates taken from within or at the base of the dunes range from 14.0 to 10.6ka. Samples taken from < 2m of the ground surface were slightly younger, indicating dunes were stabilized between 11.8 and 5.5ka. The younger ages near the surface of some dunes were most likely the result of pedoturbation or localized problems with applying the optical dating method. The majority of the optical age estimates from dunes (18 of 21) indicated that most of the dunes were active between 14 and 10ka and that most dune activity ended by 10ka. These ages suggest that localized activity on dune crests may have occurred in the Holocene but would have been limited to < 1m of sand accumulation. The timing of dune activity and the lack of any significant Holocene reactivation suggest that dune activation in this setting cannot be attributed solely to changes in aridity. Instead, we attribute dune formation to changes in sediment availability from either sand inputs from the Wisconsin River or the melting of permafrost.     

Stratigraphic evidence for late-Holocene aeolian sand mobilization and soil formation in south-central Kansas, U.S.A.

Stratigraphic evidence and radiocarbon dating of sediments from the Great Bend Sand Prairie in Kansas indicates that significant deposits of aeolian sand have accumulated in the region during the late Holocene. Radiocarbon ages obtained from total humates in buried soils suggest that five periods of late-Holocene stability and soil formation are preserved in dune fields at approximately 2300, 1400, 1000, 700, 500 and 300 years B.P. Reactivation of aeolian sand in the past 1000 years has resulted in a variety of well defined, parabolic dunes. In general, events in the region correspond with established chronologies elsewhere on the Great Plains and in particular correlate well with dune fields in north-eastern Colorado. Overall, results indicate that the threshold of landscape stability on the Great Bend Sand Prairie can easily be crossed in the current climatic regime.     

毛乌素沙地风沙环境变化研究的理论和新认识

作为地球环境系统的重要组成部分,沙漠和沙地对气候变化响应敏感。沙漠沙地的环境变化关系到区域的可持续发展,并且通过一系列反馈作用,会影响区域与全球的气候环境。位于中国北方中部的毛乌素沙地是全球中纬度地区的典型沙地,其环境变化与全球其他沙漠沙地具有相似性和同步性。中国学者对毛乌素沙地风沙环境变化的研究开展较早,在国内外有重要的影响。本文系统梳理了该方面的相关研究,围绕“风沙环境变化的过程与机制”这一核心问题,总结学术界在毛乌素沙地晚新生代形成发展、晚第四纪扩张与多旋回变化、全新世中期固定以及晚全新世沙丘再活化等方面取得的重要进展,尤其是获取了毛乌素沙地不同阶段风沙环境变化的关键证据,并据此提出“构造—轨道尺度上沙地形成与变化的全球温度变化驱动”“千年—百年尺度沙丘系统的非线性响应”等新的理论认识。未来应继续加强沉积证据的积累、新测试手段的拓展应用、基于过程分析的精细重建、数值模拟与预测等研究,着力探究风沙环境变化的动力学机制、人与干旱区环境复杂系统的耦合关系以及各圈层的互馈作用,这对于自然地理学学科传承、交叉学科的融合发展以及地球系统科学的理论构建都具有深远意义。     

青海湖湖东沙地不同沙丘降雨入渗研究

降雨入渗对干旱、半干旱区土壤水分的影响很大,降雨量、降雨强度、土壤前期含水量等要素都会影响水分的入渗过程。本文通过监测青海湖湖东沙地3种类型沙丘的土壤水分和降雨情况,对区内降雨特征以及不同要素对沙丘水分入渗的影响进行了分析。结果表明：降雨量只有达到某一临界值才发生下渗,流动沙丘、固定沙丘、经治理（人工植被+麦草方格沙障）的流动沙丘发生下渗的临界降雨量分别为5.6 mm、1.6 mm、0.2 mm。水分累积入渗量随降雨量增大而增加,降雨量相同的情况下,入渗量大小表现为经治理的流动沙丘>固定沙丘>流动沙丘。当降雨量和土壤前期含水量相近时,入渗量随降雨强度的增大而增加,尤其在小降雨事件下,降雨强度是影响入渗的关键因素,大降雨事件下降雨量则成为影响入渗的决定因素。降雨量和降雨强度相近的情况下,入渗量与土壤前期含水量呈负相关关系。随着降雨量增大,入渗水分全部消耗所需要的时间逐渐增加,尤其当降雨量大于10 mm时,入渗水分消耗所需时间将随着降雨量的增大迅速增加。     

GEOMORPHOLOGICAL AND DENDROCHRONOLOGICAL ANALYSES OF A COMPLEX LANDSLIDE IN THE SOUTHERN APENNINES

Complex landslides, capable of reactivation, are typical slope movements in high relief areas. Due to their distribution, size and kinematics, these landforms represent a major hazard, posing a high risk to populations, settlements and infrastructures. This paper integrates geomorphological analyses, instrumental measurements and dendrochronological approaches in assessing a large, reactivated landslide system on the southern piedmont of Monte Sirino (southern Italy). The landslide system is associated with weak geological structures, earthquake activity, and rapid recent incision of the mid-Pleistocene Noce lake deposits. Potential reactivation triggers include a higher regional annual rainfall, one of the highest in southern Italy, and more frequent heavy snowfalls in recent decades. Reactivation of the Sirino landslide system has important implications for the motorway connecting Salerno and Reggio Calabria, which crosses it. The results of our study show that the slide is reactivated with an almost decadal frequency and that major reactivations are correlated to prolonged snowfall, which occurs with increasing frequency in the southern Apennines. The last observation suggests the need for similar studies on the behaviour of other landslide systems in the southern Apennines, performing integrated approaches such as geotechnical and dendrogeomorphological analysis.     

Visualizing and quantifying the movement of vegetative drought using remote-sensing data and GIS

Remote-sensing-based drought monitoring methods provide fast and useful information for a sustainable management strategy of drought impact over a region. Common pixel-based monitoring methods are limited in the analysis of the dynamics of this impact at regional scale. For instance, these hardly allow us to quantify the movement of drought in space and time and to compare drought with rainfall deficits without losing the variability of these events within a region. This study proposed an object-based approach that allowed us to visualize and quantify the spatio-temporal movement of drought impact on vegetation, called vegetative drought, in a region. The GIS software Dynomap was used to extract and track objects. Measures of distance and angle were used for determining the speed and direction of vegetative drought and rainfall deficit objects, calculated from the National Oceanic and Atmospheric Administration's (NOAA's) normalized difference vegetation index and rainfall estimates data. The methods were applied to the two rainy seasons during the drought year 1999 in East Africa. Results showed that vegetative drought objects moved into the southwestern direction at an average angle of??138.5° during the first season and??144.5° during the second season. The speed of objects varied between 38 km dekad^?1 and 185 km dekad^?1 during the first season and between 33 km dekad^?1 and 144 km dekad^?1 during the second season, reflecting the rate of spread between dekads. Vegetative drought objects close to rainfall deficit objects showed similar trajectories and sometimes regions overlapped. This indicated that the two events are related. We conclude that a spatiotemporal relationship existed between the two types of events and that this could be quantified.     

Characteristics of Australian droughts under enhanced greenhouse conditions: Results from 14 global climate models

This paper presents characteristics of droughts simulated by global climate models (GCMs) under enhanced greenhouse gases conditions. We used a drought index called the Reconnaissance Drought Index (RDI) which takes both rainfall and potential evapotranspiration into account to investigate variations of droughts among 12 regions in Australia. The RDI was applied to simulated climate variables from 14 GCMs performed for the IPCC 4th Assessment Report.The results show a general increase in drought areal extent and/or frequency for most regions. However, the increases are not significant over the North West, North Queensland, Queensland East Coast and Central Queensland. For most regions, the change beyond 2030 is larger than that prior to 2030, but the uncertainty in the projections also increases with time. By 2030, there is a likely (>66% probability) risk of twice or more drought affected area and/or twice as often drought frequency over South West Western Australia. By 2050, this will include the Murray-Darling Basin, South Australia and Victoria, and by 2070 this will extend to New South Wales and Tasmania. For North Queensland such a risk is unlikely (<33% probability) for the next 100 years. This information can be considered indicative in long-term planning focussing on sustainability.     

The formation and evolution of the barrier islands of Inhaca and Bazaruto, Mozambique

The barrier islands of Inhaca and Bazaruto are related to the extensive coastal dune system of the Mozambican coastal plain, south-east Africa. Optically stimulated luminescence (OSL) dating of key stratigraphic units indicates that accretion of sediment within these systems is episodic. Both islands appear to have been initiated as spits extending from structural offsets in the coastline. Superposition of significant quantities of sediment upon these spits during subsequent sea-level highstands formed the core of the islands, which were anchored and protected by beachrock and aeolianite formation. At least two distinct dune-building phases occurred during Marine Oxygen Isotope Stage (MIS) 5, tentatively attributed to marine transgressions during sub-stages 5e and 5c. Although some localized reactivation of dune surfaces occurred prior to the Holocene, large quantities of sediment were not deposited on either island during the low sea-levels associated with MIS 2. Significant dune-building and sediment reworking occurred immediately prior to and during the Holocene, though it is not clear whether these processes were continuous or episodic. Significant erosion of the eastern shoreline of Bazaruto suggests that it is far less stable than Inhaca and may suffer further large-scale erosion. A model is presented for the formation of barrier islands along the Mozambican coastal plain.     

晚更新世以来长江三角洲长尺度气候干旱事件 ——来自沉积物环境磁学的记录

选择位于长江三角洲南部区域的3个钻孔CSB4、CSB6和CSB8,开展环境磁学方面的研究,以揭示该地区晚更新世以来沉积物记录的显著气候事件。结果显示：3个钻孔都存在明显的S300低值区域,其变化规律为当S300为低值时,磁化率、ARM和SIRM的值较低,而IRMAF80mT/SIRM和ARM/SIRM的比值相对较高,表明磁性矿物浓度降低、粒度变细。同时S300低值区域沉积物以黄色粉砂质黏土为主,含铁锰质结核。IRM获得曲线定量分析结果揭示：沉积物中赤铁矿等高矫顽力组分的含量相对增加,指示长江三角洲区域3次显著的干旱气候事件。在磁性地层及其AMS 14C、光释光测年基础上,推断3次干旱事件发生的时间分别为163―121 ka B.P.（事件I）、57―42 ka B.P.（事件II）和14―10 ka B.P.（事件III）,对应海平面相对较低时期,季风减弱,降水减少,气候干旱,形成以氧化作用为主的环境。     

Twenty years of land use and livelihood changes in SE-Niger: Obsolete and short-sighted adaptation to climatic and demographic pressures?

This paper explores the evolution of land use and natural resource management strategies over the past twenty to fifty years in a remote Sahelian region. The empirical example is Karagou village in SE Niger. Building on an in-depth survey from 1992 and a targeted, follow-up analysis of contemporary land use strategies in 2011, the change process is scrutinized.The analysis uses the conceptual lenses of land systems science, human–environmental timelines, and the notion of multiple exposures. Enabling and constraining conditions for local livelihoods in terms of the resource base (landscape, water, and population) are described. Results characterize how land use strategies have developed and how local people opportunistically use different landscape elements such as dune landscapes, valley bottoms (bas-fonds) and oases (cuvettes). Major concerns are rainfall variability, saturation of cropland, and perceived dwindling groundwater resources. It is concluded that the land use and livelihood strategies have remained remarkably stable in the face of the changing socio-ecological fringe conditions, but that this situation may hamper a sustainable transformation.     

The evolution of Holocene coastal dunefields, Jutland, Denmark: A record of climate change over the past 5000 years

Coastal dunefields have developed on the west coast of Jutland in Denmark over the past 5000 years. The dunefields are situated in a temperate climate zone with frequent high energy wind events. Dunefield development was characterized by repeated periods of transgressive dune formation punctuated by periods of dune stabilization and soil formation. The chronology of dunefield evolution is based on Accelerator Mass Spectrometry (AMS) radiocarbon dating of peaty palaeosols (24 samples) and Optically Stimulated Luminescence (OSL) dating of aeolian sand deposits (19 samples). These dates indicate that the completeness of the stratigraphic record varies considerably, but that the timing of aeolian activity events was identical in the dunefields examined here. Initiation of aeolian activity occurred around 2200 BC, 800 BC, AD 100, AD 1050–1200, and between AD 1550 and 1650. Proxy-climate data from bogs in southwestern Sweden suggest that these periods of dunefield activity were initiated during wet/cool summers. Most likely these climatic situations were associated with a more frequent passage of cyclones across Denmark in the summer seasons (increased storminess) causing aeolian sand movement and dune formation. The continued removal of vegetation in Jutland between 4000 BC and AD 1850 allowed the aeolian sand, when first activated, to move more and more easily across the coastal plains; the final phase of aeolian sand movement between AD 1550–1650 and 1900 had catastrophic implications for many coastal inhabitants in the region.     

Dunefield activity response to climate variability in the southwest Kalahari

Aeolian activity on the crests of linear dunes in the arid southwest Kalahari Desert is primarily controlled by vegetation cover. Cover is temporally variable in response to rainfall, with mobility index (MI) values and field based studies demonstrating the dynamic nature of aeolian processes in this dunefield. Landsat TM data for a drought (1984) and wetter (1993) year were analysed for two subscenes in the dune field, one for a pastoral landscape and one for a conservation area, to investigate the spatial dynamics of dune activity, using the empirically determined 14% cover threshold as the limit to crestal aeolian activity. TM band 3 data were found to best represent vegetation cover, including senesced material, at the end of the dry season, the period when the potential for aeolian activity is normally high due to the wind strength at this time. The extent of 0–14% cover areas ranged from 10% (1984) to 3% (1993) for the pastoral area and 16% (1984) to 6% (1993) for the conservation landscape. The wetter (1993) image residuals have different spatial characteristics in the two subscenes, with bare active dunes in the pastoral lands associated with locations of livestock pressure and those in the conservation area related to natural fire events. The study demonstrates the spatial dynamics of temporally variable linear dune activity in the southwest Kalahari, and also shows that areas devegetated by drought can recover sufficient vegetation in wet years to limit the spatial extent of aeolian transport on dune crests.     

若尔盖盆地沙漠化驱动机制

作为世界上最大的高寒泥炭沼泽湿地,若尔盖盆地是黄河水源的重要补给区,素有黄河"蓄水池"之称.作为生态敏感区,在自然环境和不合理的人为活动的影响下,若尔盖盆地出现了湿地萎缩、草场退化、沙地增加等一系列严重的生态环境问题.本文通过分析若尔盖盆地沙漠化现状及其驱动因素,进而阐述和分析沙漠化驱动机制.结果表明:若尔盖盆地广泛分布的古风沙沉积物是现代沙漠化的沙物质来源,地表致密草皮层的机械破坏,使下伏松散的古风沙沉积物暴露,是形成沙漠化的关键一步;随后在风蚀、雨水冲刷及生物因素的共同作用下,松散的古风沙沉积物被侵蚀,引起草皮层坍塌,导致古沙丘活化,风沙活动加剧,形成风沙沉积;若尔盖沙漠化是人为因素与自然因素共同作用的结果,其中人为因素是其主导驱动因素,鼠类活动和热融沉陷、滑塌是沙漠化的诱发因素.     

小冰期福建海岸沙丘的沉积环境

选取福建东南沿海典型海岸沙丘——东海沙丘,运用GPR（Ground Penetrating Radar）技术探测海岸沙丘内部构造,辅助天然剖面和人工探槽观测,进行系统的年代采样与OSL（Optically Stimulated Luminescence）测年,并选择物理意义明确的粒度参数作为主要环境代用指标,获得了亚热带季风区砂质海岸带典型海岸沙丘的沉积年代和沉积结构。结果表明：（1）东海沙丘是由若干灌丛沙丘组成的横向沙丘,0～6 m主要是～0.45 ka BP以来形成的海岸风成沙丘,可以划分为～0.45 ka BP和0.22～0.12 ka BP两个阶段。（2）东海沙丘沉积过程是多因素共同作用的结果,冷干的气候环境是沙丘发育的适宜条件,入境台风频率增加可能加快了风沙的沉积过程。对比历史文献资料发现,气候异常期往往与社会动荡期、重要历史事件相对应。     

Significance of variations in the wind energy environment over the past 50 years with respect to dune activity and desertification in arid and semiarid northern China

Climate data from 339 meteorological stations collected from the 1950s to the early 2000s was employed to discuss aeolian activity in arid and semiarid northern China. The results show that at decadal time scales, erosivity varied greatly in this region. Most of arid and semiarid northern China was characterised by environments with moderate to low wind energy. After the 1980s, the erosivity was only 20 to 50% of that beforehand, and this difference had a significant impact on the environmental changes observed during the two periods. The dune mobility index was consistent with the observed dune activity. After the 1980s dune activity decreased and in some deserts with vegetated dune systems during the 1970s most of the dune plinths were active until the 1980s, after which only the crests were active. Some mobile dunes that had developed at the margins of mobile deserts were replaced by semi-anchored or anchored dunes. Because most desert areas with vegetated dune systems in arid and semiarid northern China are used for farming or grazing, the results of our monitoring show that the desertification trends were consistent with the trends in erosivity and dune activity in this region. Desertification was controlled much more by climatic changes than has previously been acknowledged, and especially by fluctuations in wind energy.     

DROUGHT DISASTERS AND ENVIRONMENTAL CHANGE DURING PAST 30 YEARS IN AFRICA

AfricaisadevelopingcontinentwherequiteafeWcoUnnesarepoliticallyunstable,economicallydifficultor/andhavehadsenousdisasters,especlallydroughdisasters.ThedroughdisastersinAfricahavehadfarmoreseriousresults,farlargeraffededareasthan0thercontinatsdunngpastafeWdecades.Thedeatht0llbecauseOfthemwasstrildng.Inffospaperthebasicsituationandtheresultsofdroughsanddroughdisasters,therootcausesOfthedisastersandmeasuresf0rcontrollingthemareconsideredandanalysedfromthevieWPolntofenvironmentalchmp.klltheseas…     

古尔班通古特沙漠风沙土土壤蒸发特征

土壤表面蒸发是降水的无效损失,其大小直接决定着降水转化为有效土壤水的效率,从而间接决定了植物可利用水分的多寡。在降水稀少的沙漠区,定量认识土壤蒸发的大小和其所占降水的比例显得尤为重要。在2004年全生长期(5~9月)应用自制的微型蒸渗仪(Micro-Lysime-ter)测定了风沙土(有结皮覆盖的丘间低地和裸露沙丘顶部)和对照的荒漠盐碱土的土壤蒸发。结果表明:在相似的气象条件下,风沙土区的丘间低地和荒漠盐碱土的日平均蒸发量分别约是裸露沙丘(风沙土区)的2倍和3倍。整个生长期裸露沙丘的累积蒸发量为18.7mm,丘间低地为38.8mm,荒漠盐碱土为52.1mm。根据整个观测期的累积蒸发量和累积降雨量计算得出的蒸发降雨比分别为:裸露沙丘0.20,丘间低地0.42,荒漠盐碱土0.62,表明裸露沙丘由降水转化为土壤水而储存的比率最高。同时,从数据分析中可以看出:在风沙土区0~5cm土层的含水量对土壤蒸发起决定作用,而盐碱土区的土壤蒸发则受到更深层次土壤水分的影响。裸露沙丘低的土壤蒸发、高的降水转化效率,为植被恢复奠定了良好的基础。因此,裸露沙丘的低蒸发量可以视为植被状况对生态水文过程的一种良性反馈。     

关于沙漠环境演变几种研究指标的探讨

沙漠环境演变研究过程中,地层沉积序列和沙漠环境信息的重建是其研究重点。大量关于沙漠环境演变研究所取得的成果显示：在沙漠沉积地层时间序列测定中,光释光（OSL）测年比较适用,但其在高分辨率方面欠缺、精度稍差,实际研究中应该视实际情况把OSL测年与^14 C测年结合使用。在环境代用指标选择方4面,沉积粒度及地球化学元素比较适合沙漠环境演变研究;磁化率、碳酸钙、有机碳作为环境代用指标有一定的局限性,但研究中仍多使用;孢粉分析在较湿润区域的环境演变研究中作为环境代用指标效果较好,但在沙漠这个极度干旱、植被稀少的环境下适用性不强。