Optically stimulated luminescence dating of aeolian sand in the Otindag dune field and Holocene climate change

The dune system in Otindag sand field of northern China is sensitive to climate change, where effective moisture and related vegetation cover play a controlling role for dune activity and stability. Therefore, aeolian deposits may be an archive of past environmental changes, possibly at the millennial scale, but previous studies on this topic have rarely been reported. In this study, thirty-five optically stimulated luminescence (OSL) ages of ten representative sand-paleosol profiles in Otindag sand field are ob-tained, and these ages provide a relatively complete and well-dated chronology for wet and dry varia-tions in Holocene. The results indicate that widespread dune mobilization occurred from 9.9 to 8.2 ka, suggesting a dry early Holocene climate. The dunes were mainly stabilized between 8.0 and 2.7 ka, implying a relatively wet climate, although there were short-term penetrations of dune activity during this wet period. After ~2.3 ka, the region became dry again, as inferred from widespread dune activity. The "8.2 ka" cold event and the Little Ice Age climatic deterioration are detected on the basis of the dune records and OSL ages. During the Medieval Warm Period and the Sui-Tang Warm Period (570-770 AD), climate in Otindag sand field was relatively humid and the vegetation was denser, and the sand dunes were stabilized again. These aeolian records may indicate climate changes at millennial time scale during Holocene, and these climatic changes may be the teleconnection to the climate changes elsewhere in the world.     

横向沙丘气流平均速度变化规律的风洞模拟

在沙丘动力系统中,存在沙丘形态、气流、沙粒运移三者之间复杂的相互作用。通过风洞实验的方法,针对不同形态的6组横向沙丘模型,采用粒子图像测速系统,测量了模型沙丘周围气流水平速度和垂直速度的变化规律。实验结果表明,横向沙丘迎风坡水平气流存在1.28～1.89之间的加速率,垂直气流存在上扬趋势,这二者均有随沙丘迎风坡坡度增大而增大的趋势。在横向沙丘背风坡,由于气流的分离,水平气流速度减小并出现反向,其大小约为自由风速的17％;垂直气流速度存在下沉趋势,其最大沉速出现在气流重附点附近;背风坡气流速度的变化受沙丘迎风坡坡度影响较小,受自由风速的影响较大。沙丘对气流速度的改变在近地层较为显著,随着高度的增加地形影响逐渐减小。     

库姆塔格沙漠大尺度风场特征分析

利用美国NCEP/NCAR1979—2006年27 a地面10 m高度的日平均风场再分析-2资料以及46 a的地面常规气象站观测资料和1 a的沙漠中自动气象站观测资料,对库姆塔格沙漠地区风场进行了统计与诊断分析。结果表明：沙漠地区,一年四季北风和东北风为近地面主要存在的风向,春季风速最强,夏季次之,秋冬季最弱;在夏秋冬季节,由于北风的逐渐减弱,加上受到南侧阿尔金山和青藏高原风场的影响,南下的气流在沙漠南侧分为三支,分别流入新疆、青海和甘肃。春夏季节在沙漠北侧基本都是以东北风或者西北风吹向库姆塔格沙漠,秋冬季节,沙漠地区出现西南风。根据沙漠周围6个气象站风场风速显示,库姆塔格沙漠地区风速存在明显的周期变化,在过去的10多年中库姆塔格沙漠周围风场风速处于一个低谷期,但最近几年风速有不断加强的趋势。     

Development and setting of the Alcudia Bay beach–dune system (Mallorca, Spain)

The beach–dune system of Alcudia Bay is located in the north of the island of Mallorca. The system includes both simple and compound parabolic dunes formed by N to NNE winds and is made up of two dune areas with different dynamics: 1) the northern area, where, despite the fact that the source of beach sediment is from the south, dunes are formed by northerly winds and develop a narrow and linear barrier which separates a marsh from the bay; and 2) the southern area, where complex parabolic dunes formed by north-northeasterly winds, prograde south-southwest landward extending several kilometers inland. The broad pattern of the dunefield size is limited to the southeast by a mountain range and to the west by the effects of a topographic corridor, oriented north–south, which channels the prevailing northerly wind, causing a southward dune progression and limiting the westward extension of the dunefield.     

库姆塔格沙漠的“羽毛状沙丘”形态的观测

参照卫星照片和地形图,实地观测了库姆塔格沙漠“羽毛状沙丘”分布区域的地貌。新月型“沙垄”是该区域的主导性地貌类型,垄间地形波状起伏。如果将“沙垄”喻为“羽轴”,垄间的波状起伏地形为“羽枝”,则可构成卫星照片所显示的“羽毛状沙丘”。新月型“沙垄”的走向为NE-SW,由新月形沙丘前一沙丘的迎风坡与后一沙丘的东翼相连而成,高3~19m,宽约50m,长15~22km。“垄”间起伏地形的波长为100~300m,振幅为40~100cm,与“沙垄”成60~105°夹角。在波状起伏地形的波峰与波谷过渡区堆积颜色较浅的细沙,厚2~7cm,构成了“大沙波”。实地观测的“羽毛状沙丘”的形态与卫星照片显示的形态有一定差异。     

普若岗日冰原毗邻地区风沙地貌及其环境演变

冰川作用形成的大面积的冰碛物沉积及寒冻风化是高寒区风沙沉积的主要物质来源.风沙地貌的发育与高寒区环境密切相关,冻结作用使沙丘发育以加积作用为主,形成了高大的新月形沙丘.沙丘的侵蚀与移动主要发生在暖季,新月形沙丘运动速度相当缓慢,年平均水平移动量约1.8～0.5cm·a^-1,加积速率约0.3～0.08cm·a^-1.普若岗日冰原毗邻地区的环境变化主要受温度条件制约,沙丘腐殖质层沉积年代与我国东部沙区古沙丘、敦德冰芯具有一定的相关性,可以认为西南季风的强度对本区具有重要的作用.高原下垫面的变化造成太阳辐射差异,是引起西南季风强弱,造成高原腹地温度与降水随之变化的重要原因.     

从冰前风沙地貌初看普若岗日冰原的形成演变

野外实地考察和室内样品分析表明,位于普若岗日冰原西侧冰川前缘带的大片风沙地貌,直接发育在冰碛之上,并以冰碛为主要物质来源,与冰川运动和冰原环境具有密切的联系,是相关冰川和冰原形成演化过程的良好反映与记录.结合沙丘沉积序列中的沉积构造测量、粒度分析及腐殖质夹层的¹⁴C测年等结果,初步得出:普若岗日冰原至少形成于18kaBP;冰原降水可能主要来自西风降水;18kaBP以来,冰原在总体上处于收缩过程,在约108kaBP来,冰原西缘的零平衡线的年均水平退缩速率约为088～102m·a^-1,铅直升高速率约为24～32mm·a^-1。     

Relationships between deflation and near surface wind velocity in a coastal dune blowout

Detailed field measurements were made of the degree of surface level change in a blowout, situated in the coastal dune area of Meijendel, The Netherlands. A formula was established to express the erosivity of the wind for the corresponding 34 measurement intervals, over a period of about 3 years. Having established, for 12 wind sectors, the relationship between wind velocity at the nearest standard weather station and at five locations in the blowout, correlations were derived between the deflation rate in the blowout and the wind erosivity using standard hourly wind data. The winter season, although the most windy, is by far the least effective season: the threshold shear wind velocity is disproportionally increased because the moisture content of the surface sand is high. The soil moisture conditions are described as a function of the daily precipitation and evaporation rate. The two most relevant upper and lower threshold shear wind velocities are determined empirically. During the summer season, deflation rate is even higher than indicated by the net surface level change because deflation is partly compensated by deposition of sand transported by rainwash from the blowout margin. With regard to spatial variation in the deflation rate within the blowout, it appears that locations with relatively high velocities coincide with higher deflation rates. However, the spatial variation is commonly less than is expected from the horizontal wind velocity distribution. In view of this the role of wind direction, blowout morphology and algal coverage of the soil in the deflation rate is discussed.     

Identifying storm impacts on an embayed, high-energy coastline: examples from western Ireland

Large sections of the western Irish coast are characterised by a highly compartmentalised series of headland-embayment cells in which sand and gravel beaches are backed by large vegetated dune systems. Exposure to modally high-energy swell renders most of these beaches dissipative in character. A mesotidal range (c. 3.5–4.5 m) exists along much of the coast. Analysis of instrumental wind records from three locations permitted the identification of a variety of storm types and the construction of storm catalogues. Few individual storms were recorded at all three stations indicating a lack of regional consistency in storm record. Of the total storms recorded, only a small percentage are potentially damaging (onshore directed) and even fewer span a high tide and thus potentially induce a measurable morphological response at the coast.

Through a combination of historical records, meteorological records, field observations and wave modelling we attempt to assess the impact of storms. Quantifiable records of coastal morphology (maps, air photos and beach profiles) are few in number and do not generally record responses that may be definitely attributed to specific storms. Numerical wave simulations and observations at a variety of sites on the west Irish coast, however, provide insights into instantaneous and medium term (decadal) storm responses in such systems.

We argue that beaches and dunes that are attuned to modally high-energy regimes require extreme storms to cause significant morphological impact. The varying orientation of beaches, a spatially nonuniform storm catalogue and the need for a storm to occur at high water to produce measurable change, impart site-specific storm susceptibility to these embayments. Furthermore, we argue that long-period wave energy attenuation across dissipative shorefaces and beaches reduces coastal response to distant storms whereas short-period, locally generated wind waves are more likely to cause major dune and beach erosion as they arrive at the shoreline unrefracted.

This apparently variable response of beach and dune systems to storm forcing at a decadal scale over a coastline length of 200 km urges caution in generalising regarding regional-scale coastal responses to climatic change.     

秦皇岛市海岸风成沙丘的研究

秦皇岛市北戴河向南直至滦河河口发育着我国最长的沙丘海岸，沙丘高约20～30m，最高42m。在沙丘发育最充分的地区自岸向陆，沙丘按形态可分成岸前沙丘，沿岸主沙丘链，斜向沙丘链和平坦沙席等4种类型。沙丘由分选极好的中细砂组成，具有微弱的负偏态和双跃移特征。沙丘内部发育有高角度板状交错层理、低角度楔状交错层理等层理构造。滦河新三角洲和沿岸海滩的丰富沙源、向岸和离岸两盛行风的频繁交替以及适宜的空旷地形空间是塑造沙丘的主要条件。加强管理，保护沙丘的自然面貌是当务之急。