呼伦贝尔沙地末次冰盛期晚期至全新世风成沙表面矿物特征及环境意义

The environmental implication from microscopic texture of eolian sand of Hulun Buir duneland centred on late last glacial maximum and Holocene

运用扫描电镜与能谱分析,研究呼伦贝尔沙地末次冰盛期晚期以来全样风成沙的微区形态和粘土矿物成分。呼伦贝尔沙地的沙粒磨圆度较好,在扫描电镜观察下保存了水成沙粒的特征,意味着沙地物源曾经流水作用到达该区域。颗粒表面同时留有风力搬运的碟型坑等撞击痕迹,为干枯河床沙丘活化、风力搬运流动扩张的过程提供证据。末次冰盛期晚期风成沙颗粒大且表面光滑洁净,未见次生粘土矿物,反映出冰期地表无植被发育,区域沙漠活化、流动性加强。而全新世适宜期的褐色沙质土壤砂粒表面同时发育鳞片状自生粘土矿物和同沉积粘土颗粒及团块。能谱分析表明:全新世适宜期,风化自生粘土矿物有蒙脱石和伊利石,在相对温湿气候条件下,发育干旱碱性土壤,反映出冰后期植被发育、土壤化作用增强。

Scanning electron microscope(SEM) and energy dispersive spectrometry(EDS)have been applied simultaneously in order to observe micromorphology and clay mineral for the whole grains from aeolian sand of Hulun Buir dunefield since the late last glacial maximum. Based on the OSL age of the alternate layers, climatic proxies including grain size,magnetic susceptibility, organic carbon content and chroma have also been measured to correlate with the environmental indications from SEM and EDS. The fairly roundness indicated the fluvial sedimentary origin. The collision pits were also been observed which are typical eolian evidence. The coexistence of fluvial and eolian surface texture of the depositing sands in this duneland implies that the dried fluvial sand in river- bed was reworked by wind erosion. Glacial sand has clear surface without any clay mineral attachment, which suggests slightly weathering resulting from intensified winter monsoon erosion and barren mobile desert. However, sandy soil of Holocene optimum has not only developed flaky authigene clay,but also adhered immigrated clay grain and block. The authigene clay minerals are montmorillonite and illite evidenced from EDS, which are typical indicative clays of alkaline soil. The abundance of clay minerals states that relatively humid and warm climate and developed vegetation cover. The depositing alternations of aeolian sand of last glacial maximum and sandy soil of Holocene optimum were also developed in other eastern China dunefields. The combination of SEM and EDS aiming to observe clay mineral in this study can definitely be referred to the environmental research in other dunefields. The analyses of grain surface are effective methods to reveal the sand source and transportation process apart from environmental information. Fixed dunes under the vegetation cover in Holocene optimum not only decreased the output of sand grains, but also degraded the grain- size resulting from physical and chemical weathering. Hence, being finer of accumulated sandy soil was mainly affected by sand decrease and degradation of regional output rather than weakened winter monsoon during the humid and warm Holocene optimum.