Spatial distribution and geomorphology of lunette dunes on the High Plains of Western Kansas: implications for geoarchaeological and paleoenvironmental research

The High Plains are a vast, low relief landscape that contains thousands of upland-embedded depressional playa wetlands. Lunettes are isolated dunes that form from prolonged periods of aeolian deposition along the downwind margin of some larger playas. Playa-lunette systems are important landscape elements that have the potential to preserve high-resolution records of past environments, and several archeological sites have been associated with playa-lunette systems throughout the High Plains. While a comprehensive playa wetland geospatial database has been developed for the High Plains, lunettes have only been mapped for the Southern High Plains of Texas and New Mexico. The purpose of this research was to map and characterize the geomorphology of all lunettes associated with playas in the High Plains and bordering regions of western Kansas. Results indicate 129 lunettes are associated with 104 playas in Kansas. All lunettes are associated with playas larger than 5 ha, and about 10% of these larger playas have a lunette. Lunette height ranges from approximately 1.5–12 m, with a median height of 3 m. Most lunettes (60%) are in Finney, Gray, Lane, and Scott counties, which have broad, flat uplands with few major drainages upwind of playas and lunettes.     

Origin and evolution of playas and blowouts in the semiarid zone of Tierra de Pinares (Duero Basin, Spain)

A large number of blowouts and playas occur in the marginal sectors of the aeolian deposits located in the southern sector of the Duero Depression (Tierra de Pinares) in Spain. The blowouts are relict landforms that were developed on sand sheets by deflation during dry periods with lower vegetation cover and a deeper water table. The studied blowouts form complexes of NW–SE and NNW–SSE elongated hollows with accompanying dunes up to 4 km long in the leeward margins. Some hollows host lakes or swampy areas related to a shallow water table. The dunes formed by NE–ENE winds show steep windward slopes and gentle leeward slopes. The studied playas, with prevalent NNW–SSE orientations, result from the aeolian excavation of terrace deposits and the underlying marly bedrock. It is probable that the formation of these depressions in an initial stage was related to deflation processes affected preferentially NNW–SSE sandy channels perpendicular to the dominant wind direction. The precipitation of salts in the playas generates aggregates of clay particles (peloids) that are easily removed by the wind. Once the bottom reached the substratum, the deepening of the depressions progressed by the deflation of particles produced by weathering of the argillaceous bedrock.     

若尔盖盆地沙漠化及其景观格局变化研究

应用1975年的MSS数据、1990年和2005年的TM数据、以及2000年的ETM数据,通过遥感与地理信息系统技术,获得了若尔盖盆地在1975-2005年间的沙漠化发展特征,并采用景观指数运算软件FRAGSTATS3.3对该地区沙漠化土地的景观格局变化进行了分析。研究发现,在1975-1990年间,若尔盖盆地的沙漠化土地面积大幅增加;在1990-2000年间,沙漠化土地面积保持稳定;在2000-2005年间,沙漠化土地面积出现了小幅度的减小趋势。对若尔盖盆地沙漠化土地的景观格局变化进行研究发现,固定沙丘（地）和半固定沙丘（地）是若尔盖盆地主要的沙漠化土地景观类型。在1975-1990年间,沙漠化土地斑块数量和斑块平均面积均大幅增加,是该地区沙漠化快速发展时期;在1990-2000年间,沙漠化土地斑块数量下降,斑块平均面积增加,沙漠化景观破碎度有所降低;在2000-2005年间,沙漠化土地斑块数量增加,斑块平均面积减小,沙漠化景观破碎度增强。     

呼伦湖西南部咸水湖干涸湖滨带沉积物粒度特征

沙粒是区域地表沙化的重要物质基础，以呼伦湖西南部典型间歇性干涸盐湖的湖滨带为研究对象，通过野外采样和室内测定，分析沉积物粒度组成及平均粒径、标准偏差、偏度和峰态值等粒度参数，探讨粒度与可蚀性特征。结果表明：区内干涸盐湖湖滨带沉积物属沙壤土，干涸湖滨带是潜在的风蚀粉尘释放源；粉砂和细砂是湖滨带沉积物呈现空间变化的主要粒级，从湖心到湖滨带，沉积物粒径频率分布曲线呈三峰态向双峰态渐变，地表粗化现象明显；风力搬运侵蚀是表层沉积物粒度发生变化的主要原因。受主风向影响，呼伦湖西南部咸水湖干涸湖滨带存在沿主风向扩大趋势，如进一步连通成片可能发展为呼伦贝尔沙地新的风蚀区，应该引起重视。     

Riverine dunes on the Coastal Plain of Georgia, USA

Relict eolian dunes on the Georgia Coastal Plain line the north and east sides of many SE-trending streams and contain important paleoenvironmental information. A detailed analysis of four dune fields characterizes dune geomorphology, stratigraphy, sedimentology and soils. Georgia's inland dunes range from parabolic to infilled parabolic to irregular in form, typically merge to form long chains that parallel source streams and can occur as distinct bands. Typically, the dunes are composed of >95% quartz sand grains, with more than half the sand falling in the medium sand fraction. Dunes overlie flood plain and pointbar sands, backswamp and peat deposits, fluvial terraces and other dunes. In places, organic deposits occur on top of dunes. Cross-bedding is observable in dunes more than about 4 m thick and indicates eastward dune migration. Paleosols found within some dunes indicate Holocene reworking. Conditions favoring dune formation likely included at least seasonal aridity. Postdepositional modification of the dunes includes a smoothing of the dune surface, a clay increase in the C horizon, spodic horizon development, bioturbation, removal of weatherable minerals and fluvial incision.     

沙丘风蚀坑的形态及动力过程的研究进展

随着风沙活动对全球变化的响应研究不断深入,沙质海岸、湖岸和半干旱区草原环境中不同程度植被覆盖沙丘及其风蚀坑的动态得到了广泛的重视。本文对沙丘风蚀坑的形态及动力学特征的研究成果作了初步总结,着重阐述了沙丘风蚀坑内气流场、沙粒输移和蚀积变化。最后,提出了沙丘风蚀坑研究巾存在的问题和主要研究趋势。     

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.     

Complex systems in aeolian geomorphology

Aeolian geomorphology provides a rich ground for investigating Earth surface processes and landforms as complex systems. Sand transport by wind is a classic dissipative process with non-linear dynamics, while dune field evolution is a prototypical self-organisation phenomenon. Both of these broad areas of aeolian geomorphology are discussed and analysed in the context of complexity and a systems approach. A feedback loop analysis of the aeolian boundary-layer-flow/sediment-transport/bedform interactions, based on contemporary physical models, reveals that the system is fundamentally unstable (or at most meta-stable) and likely to exhibit chaotic behaviour. Recent field-experimental research on aeolian streamers and spatio-temporal transport patterns, however, indicates that sand transport by wind may be wholly controlled by a self-similar turbulence cascade in the boundary layer flow, and that key aspects of transport event time-series can be fully reproduced from a combination of (self-organised) 1/f forcing, motion threshold, and saltation inertia. The evolution of various types of bare-sand dunes and dune field patterns have been simulated successfully with self-organising cellular automata that incorporate only simplified physically-based interactions (rules). Because of their undefined physical scale, however, it not clear whether they in fact simulate ripples (bedforms) or dunes (landforms), raising fundamental cross-cutting questions regarding the difference between aeolian dunes, impact ripples, and subaqueous (current) ripples and dunes. An extended cellular automaton (CA) model, currently under development, incorporates the effects of vegetation in the aeolian environment and is capable of simulating the development of nebkhas, blow-outs, and parabolic coastal dunes. Preliminary results indicate the potential for establishing phase diagrams and attractor trajectories for vegetated aeolian dunescapes. Progress is limited, however, by a serious lack of appropriate concepts for quantifying meaningful state variables at the landscape scale. State variables currently used in the bare-sand models are far from capturing the rich 3D topography and patterns and are not sufficiently discriminative to distinguish different attractors. The vegetation component in the extended model, and indeed ecogeomorphic systems in general, pose even graver challenges to establishing appropriate state variables. A re-examination of older concepts, such as landscape entropy, perhaps complemented by recent developments in information theory, may be a potentially fruitful avenue for research, although the outlines of such an implementation are still rather vague.     

Aeolian dust in a saline playa environment, Nevada, U.S.A.

Saline playas in north-western Nevada, U.S.A., remnants of pluvial periods of the Pleistocene, represent a tremendous source of unconsolidated sediments available for aeolian transport. This study investigated the transport of aqueous-soluble solutes in dust from July 1994 through June 1996 along a transect from a barren salt-encrusted playa surface (elevation=1224 m), to a former pluvial lake beach (elevation=1228 m), to a dune-mantled upland (elevation=1248 m). The content of aqueous-soluble solutes in aeolian dust showed a significant (p≤0·05) interaction with dust trap location (playa, beach, dune) and time of collection. Dust collectors on the playa surface generally contained significantly more aqueous-soluble solutes and had greater total flux of solutes than either the beach or the dune locations. The solute content of aeolian dust was usually higher, in some cases several orders of magnitude, than that in the surface 5 cm of soil. Recent changes in playa hydrology may explain this result. Pulses of nitrate-rich dust, synchronous with spring emergence, and other nutrient additions via aeolian dust may have stimulated invasion of dune-mantled uplands by the weedSalsola paulsenii (barb-wire Russian thistle).     

科尔沁沙地沙漠化时空演变及其景观格局——以内蒙古自治区奈曼旗为例

利用1975 年MSS影像, 1992 年、2006 年、2010 年TM影像和1999 年ETM +影像等5 个时期遥感影像为数据源, 提取奈曼旗35 年来沙漠化土地动态变化信息, 并在此基础上, 探讨了区域沙漠化土地景观格局时空演变特征, 旨在为科尔沁地区沙漠化治理和区域生态环境恢复与重建提供参考。结果表明:(1) 35 年来, 奈曼旗沙漠化土地面积总体呈逆转的趋势, 沙漠化总面积减少了123.43 km², 沙漠化土地主要经历了先增加后减少的过程;(2) 景观格局呈波动性变化, 1999 年以前, 景观形状指数和多样性指数增加, 蔓延度和聚集度减小, 破碎化程度加强, 沙漠化程度上升, 沙漠化主导类型为中度和重度沙漠化, 1999 年以后破碎化程度变弱, 沙漠化程度明显下降, 沙漠化主导类型转变为中度和轻度沙漠化;(3) 1975-2010 年间轻度和中度沙漠化土地重心迁移最为明显, 轻度沙漠化土地重心向东北方向偏移了6.35 km, 中度沙漠化土地重心向西南方向偏移了7.24 km。     

CONCURRENT STABILIZATION OF SOME INTERIOR DUNE FIELDS IN MICHIGAN

Inland dunes occur over a large part of east-central lower Michigan, where they mantle glaciolacustrine and outwash surfaces that were exposed around 12,000 yrs. B.P. The dunes are parabolic, with northwest-oriented limbs, and occur in swampy landscapes, suggesting that paleoclimatic conditions at the time of their formation were much drier and possibly windier. In order to determine whether the dunes stabilized concurrently or randomly in time and space, surface soils were studied on 30 dunes in the area and quantitatively analyzed for relative differences. Soils data from the dunes indicate concurrent stabilization, following a period of regional mobilization of aeolian sand. Surface soils have formed by podzolization, in uniform parent materials, and are morphologically similar throughout the area. All the soils are weakly developed, with subtle variations on a A-E-Bs-BC-C horizonation sequence. Munsell colors of Bs horizons are remarkably uniform, with 27 of 30 sites exhibiting values of 4 and chromas of 6. Chemical data suggest that Fe and Al translocation has been uniform throughout the region. When compared with soils of known age in northwest lower Michigan, the data indicate that dunes in the region had stabilized at least by 4000 yrs. B.P., leaving an approximately 8000-yr. interval in which they could have formed. In contrast to the prevailing southwesterly winds of today, dune-forming winds were dominantly from the northwest. [Key words: aeolian sedimentation, dunes, soil, Great Lakes, Michigan.]     

鼠害在若尔盖草原沙漠化进程中的作用与机理

若尔盖草原是中国高寒湿润地区发生大面积沙漠化的典型案例,其沙漠化的成因及发展趋势一直受到极大关注。通过野外考察、遥感卫星影像解译等,着重探讨了鼠害在若尔盖草原沙漠化的形成和发展过程中的作用。结果表明：旱獭洞的分布与丘陵沙丘在坡度和坡向上有高度的相似性。地表致密草皮层发生机械性破坏,致使下伏松散沙暴露是若尔盖草原沙漠化的关键一步。旱獭洞的坍塌在草皮层的破坏过程中起到了至关重要的作用。草皮层破坏后,下伏松散沙被侵蚀,引起草皮层坍塌,导致古沙丘活化,形成流动沙丘。在此过程中,自然因素如风、降水、冻融、重力,人为因素如过牧、牲畜活动,都起到了不同的作用。小型鼠类如高原鼢鼠、高原鼠兔的严重鼠害区可以形成轻度沙漠化。     

包兰铁路沙坡头段防护体系内的风沙沉降规律

对沙坡头铁路防护体系内的风沙沉降进行了断面观测,据此分析了沉降速率、粒度组成和物源的时空变化规律。结果显示：防护体系前沿流沙区、防护距离50～300 m的植被区和300 m以外的植被区风沙沉降速率分别为108.6、17.1 kg·m^-2·a^-1和1.5 kg·m^-2·a^-1。随防护距离增大,沉降颗粒逐渐变细,分选变差,跃移组分含量逐渐减少而悬移组分含量逐渐增大,沉降来源逐渐由前沿流沙区近地面风沙运动颗粒转变为以大气降尘为主的悬移颗粒。年内风沙沉降的高峰期为3-5月,6-8月沉降减弱,9月至次年2月风沙沉降最弱,3个阶段内防护体系植被区月均沉降量分别为0.59、0.10 kg·m^-2和0.04 kg·m^-2。3-5月风沙沉降以上风向邻近区域的风蚀起沙为主,6-8月较远源的大气降尘相对增加而上风向邻近区域的风蚀起沙相对减少,9月至次年2月风沙沉降物源以大气降尘为主。     

中国北方草原沙漠化发展过程及其成因分析——以内蒙古苏尼特左旗为例

利用遥感监测和实地调查研究相结合的办法,研究分析了中国北方草原沙漠化动态变化趋势和发展原因。结果表明:苏尼特左旗沙漠化形势非常严峻,包括北部广阔草原地区的沙漠化和南部沙丘活化两大类型。苏尼特左旗在过去30余年里,沙漠化土地总面积在增加,而沙漠化指数（ADI）在2000—2005年间略有减小,反映出2000—2005年间研究区沙漠化程度略有恢复。研究区两大类沙漠化土地发展变化趋势并不相同,草原沙漠化在过去30余年里一直在发展,由斑块状向片状发展;而沙丘活化则在1973—1987年间强烈发展,在1987年以后则呈现出恢复逆转的态势,沙丘活化对气候变化表现出较高的响应,揭示出苏尼特左旗南部沙漠化的发展受人类活动影响较小,主要与气候变化有关。北部大面积的草原沙漠化与气候变化趋势不完全一致,主要随着人类过渡放牧活动或者大规模保护恢复活动,而发生沙漠化严重程度的加剧或减缓。苏尼特左旗的沙漠化发展状况表明,中国北方草原沙漠化发展以人类活动占主导地位,且以斑块状向连片发展为主要形式,其中围绕居民点和水源的沙漠化发展最为突出。实践表明,科学管理人类活动完全可以减缓甚至恢复中国北方草原区的沙漠化土地。     

黄河源区玛多盆地沙漠化土地粒度特征

为探讨黄河源区玛多盆地风沙沉积物分异特征,采集玛多盆地不同沙漠化程度、不同地貌部位、不同动力条件的沉积物样品进行粒度分析。结果表明：（1）玛多盆地沉积物随沙漠化程度增加,平均粒径增大,粗颗粒组分增多,分选性变好,偏度向负偏靠拢,峰态逐渐转向平坦,频率分布曲线也由双峰态转变为单峰态。（2）流动沙丘以中沙和细沙组分为主,分选性好,偏度近对称,峰度为中等尖锐。各个沙丘不同地貌部位沉积物粒度参数分异受风向改变的影响,变化规律不明显。流动沙丘沉积物相较于流动沙地沉积物的粒径更粗,分选性更好,可能是沙丘形成过程中风对沉积物颗粒的再次分选所致。（3）河流阶地剖面沉积物的主要粒级为中沙和细沙,分选中等偏差,偏度为正偏,峰尖锐,河流阶地沉积物的粒度组成与周围风成作用形成的流动沙丘（地）的粒度组成高度相似,极有可能是当地风沙活动的主要物源。     

风成沉积物粒度特征及其反映的青海湖周边近32 ka以来土地沙漠化演变过程

 湖东地区是青海湖周边土地沙漠化面积最大、程度最为严重的区域。选取青海湖湖东种羊场附近高度约为10 m的风蚀残丘（大水溏剖面）为研究对象,结合高密度采样和光释光测年,在对风成沉积物的粒度组成、粒度参数及粒度敏感指标进行分析的基础上,重建了32 ka以来青海湖周边的土地沙漠化演变过程。结果表明,32 ka BP以来青海湖周边土地沙漠化经历了末次冰期间冰阶（32～23.4 ka BP）的固结成壤期、末次冰期冰盛期（23.4～15 ka BP）的快速扩张期、末次冰期冰消期（15～10.4 ka BP）的缓慢减小期和全新世（10.4 ka BP至今）的快速缩小期4个阶段。     

青海湖湖东沙地沉积记录的全新世以来风沙活动变化

青海湖湖东沙地典型剖面的AMS¹⁴C测年数据表明该剖面记录了当地约10 ka BP的风沙活动及环境变化。对沉积物的粒度组成特征、粒度参数、粒度环境敏感指标等分析,并结合沉积物磁化率和青海湖盆地相关研究中风成砂、黄土和弱发育古土壤、古土壤的OSL、AMS¹⁴C年代数据等,进行全新世以来的风沙活动及气候环境变化讨论。结果表明：（1）粗颗粒敏感组分（138~156μm）可以用来指示风沙活动的强弱,细颗粒敏感组分（2~5 μm）可以用来指示成壤作用。（2）10~9ka青海湖盆地气候状况较全新世之前略有好转,暖湿程度增加,部分区域开始发育古土壤,但总体上古土壤、弱发育古土壤、黄土、风成砂共存;9~4.2 ka青海湖盆地水热组合条件较好,风沙活动弱,大部分沙丘固定成壤,但9~8 ka和4.2 ka左右风沙活动较为频繁的特征可能指示了8.2 ka和4ka冷事件;4.2 ka之后特别是2 ka之后风沙活动显著增强,气候朝干冷化发展。     

浑善达克沙地的土地沙漠化过程研究

借助TM/ETM＋影像和GIS平台,采用实地考察与野外实验和室内分析整理相结合的办法,对浑善达克沙地土地沙漠化过程进行了初步研究。结果表明,过去50 a来,浑善达克沙地经历了强烈的沙漠化发展。位于沙地中部正蓝旗的沙漠化土地从1987年的5 593.8 km²发展到2000年的6 015.3 km²,发展速率为32.4 km²·a^-1。2000年紧急启动的风沙源治理工程,一定程度改善了当地沙漠化发展态势,2005年正蓝旗共有沙漠化土地5 950.4 km²,有约330.7 km²的严重沙漠化土地得到了控制和逆转。其他大部分沙漠化土地虽有不同程度改善,但5 a的时间里尚未发生质的变化。沙漠化过程是原生植被逐步被沙生植被和沙生先锋植被取代的过程;同时也是土壤粗化、贫瘠化和空间重新分布过程。随着土地沙漠化程度的加剧,起沙风速降低,输沙强度增大,沙地近地表表现出相对高风能环境。沙漠化过程以固定沙丘活化为主,主要有发育“马蹄形”风蚀洼地、“鱼钩”型沙丘和“串珠状”风蚀槽三种沙丘活化形式,最终导致景观支离破碎、不少地区出现了流沙连片的现象。同时,农田开垦引起土壤风蚀的危险,也不容忽视。     

高寒沙地人工植被恢复区地表沉积物粒度特征

共和盆地位于青藏高原东北部,是典型的高寒干旱、半干旱荒漠区.青海省治沙试验站经过近50年的发展,形成以封沙育草区、固沙造林区和农田林网区为一体的绿洲防护体系,成为中国高寒沙地土地沙漠化综合整治的典型示范.本文选择典型固沙植物柠条(Caragana korshinskii)和乌柳(Salix microstachya),针对不同治理年限的沙丘和丘间地,探讨治理50年后人工植被恢复区地表60 cm深度土壤粒度的时空变化.结果表明:直播柠条能促进沙丘地表60 cm土层沉积物的细化,尤以地表5 cm更甚.土壤颗粒的变化与植被生长状况及物质来源有关.     

抛物线形沙丘的形态与演变的研究进展

抛物线形沙丘是一种重要的风积地貌类型,其形态及演变与气候变化密切相关,同时受到人类活动的影响,对其研究有助于理解全球变暖背景下的地貌响应和土地沙漠化正逆过程。本研究从沙丘形态类型、分布、成因、影响因素、动力过程及演变等几个方面回顾了近50 a来国内外对抛物线形沙丘的研究进展,并提出了未来的研究方向。