| 摘要: |
| 石英颗粒表面微形态分析是判断沉积物成因和来源的重要方法之一。对伊犁盆地尼勒克黄土石英颗粒表面微形态特征进行了研究，并将其与冰川、河流、沙漠样品进行比较，结果表明尼勒克黄土石英颗粒磨圆较差，冰川和水流作用的微形态特征明显，系近源风力搬运沉积。通过对比分析讨论了石英粉砂的可能产生机制，认为流水和寒冻风化作用为伊犁黄土的形成提供了重要动力来源。尼勒克黄土的物质来源和匈牙利黄土类似，而与黄土高原黄土有较大差别，这主要表现在经受风力搬运-沉积的次数上或者风力搬运的距离上。该研究为解译伊犁地区黄土的古气候环境意义提供了重要基础。 |
| 关键词: 伊犁黄土 石英颗粒表面结构 成因与来源 |
| DOI：10.7515/JEE201604005 |
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| 基金项目:国家自然科学基金项目（41572162，41172166）；中国科学院地球环境研究所自主部署重点项目（ZZBS1301）；中国科学院科技创新“交叉与合作团队”项目（中科院人字[2013]47号）；中国科学院国际合作局对外合作重点项目（132B61KYS20160002） |
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| Micromorphological characters of quartz grain from Nilke loess-paleosol sequences and their implications of origin and provenance |
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LI Yue, SONG Yougui, ZHAO Jingdong1,2
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment,
Chinese Academy of Sciences, Xi’an 710061, China;2. Cold and Arid Regions Environmental and Engineering Research Institute,
Chinese Academy of Sciences, Lanzhou 730000, China
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| Abstract: |
| Background, aim, and scope Xinjiang is also one of the most significant loess regions in China with the exception of Chinese Loess Plateau, and the loess is mainly present in the northern piedmont of the Tianshan and Kunlun Mountains and Ili Basin. The loess deposits in Ili Basin unconformably cover the river terraces, the low uplands and the slopes of the Tianshan Mountains, and the researches on them have the merit of enabling reconstructions of past climate change in westerly dominant area of inland Asia. Although many predecessors have investigated the spatial distribution and studied the ages, pedostratigraphy, rock magnetism, elemental and mineral compositions and their implications for paleoclimate in details with respect to the Ili loess, there are many urgent questions that need to be solved: How were the materials of Ili loess generated? Which geological process was experienced by the loess silt particles before they reached the sediment area? And where is the provenance of Ili loess? We have had no understanding of the answers by far, which hampers the interpretation of climate change. For this reason, this paper aims to solve the problems above mentioned with the micromorphological characters of quartz grains. In recent study, we compared the micromorphology of quartz grains from Nilka loess with those from glacial tills, riverbed deposits and desert sands to determine the mechanisms of loess-sized quartz silt production. These works will lay the foundation for the subsequent studies on the Ili loess. Materials and methods The 4 loess samples were collected from Nlika section located at the second-order terrace of Kashi River in the east of Ili Basin with the depths 1.50 m, 9.10 m, 11.06 m and 19.56 m, respectively. Glacial till was sampled from the terminus of Arqialeter Glacier located in catchment area of Xiate River; riverbed deposits were sampled from Kashi River and the sampling sites lay in the west of Nilka loess section; desert sands were sampled from Kyzylkum, Muyunkum and Saryeslk Atyrau desert within Kazakhstan respectively. With regard to the loess, 2 g dried sample was weighed, then hydrogen peroxide and hydrochloric acid were added to remove the organic matters and carbonates, and finally the quartz minerals were separated by hydrofluosilicic acid (H2SiF6) treatment. We chose the quartz grains within the grain sizes of 20 — 70 μm for observation. For glacial till, riverbed deposits and desert sands, first, organic matters and carbonates were removed from these samples with the same method; second, remnant materials were dried and sieved through 0.125 mm; third, quartz grains of < 0.125 mm were selected under binocular microscope. All the quartz grains were mounted on the SEM (scanning electron microscope) stubs with conductive tapes and sputtered with gold, and morphologic observation were conducted with PhenomTM proX (energy spectrum version) at the Environmental Mineralogy Laboratory, State Key Laboratory of Loess and Quaternary Geology. After that, frequency of each texture’s occurrence was calculated. Results The results reveal that the quartz grains from Nilka loess are poorly rounded; microtextures produced by glaciation and fluviation are obvious but those by aeolian abrasion are rare; the chemical dissolution developed well in the surface of quartz grains but very few chemical precipitation could be observed. Discussion In spite of the surface with sharp feature, we are quite confident that Nilka loess is the eolian sediment according to previous literatures and the grain size distribution characteristics. With the Euclidean distances, we found that the loess was nearest to riverbed deposit and farther away from glacial till, and had farthest distance to desert sand. This result suggested the significant impact of flow action on the generation of loess quartz silt and a certain contribution from glaciation. By analyzing the characteristics of tectonic environment in Tianshan region, combined with silt generating mechanisms reported by predecessors, we also considered the freezing weathering in high elevation area as one of the silt generating mechanisms. With comparative analysis to probable sequence of events indicating the formation of loess, it holds that the origin of Nilka loess are similar to those of loess in Hungary but different from those of loess in Chinese Loess Plateau. Conclusions With the studies of surface micromorphological characters of quartz grain and the grain size, we believe that Nilka loess also belongs to eolian sediment, and the shape of particle and morphology by mechanical effect indicate that Nilka loess materials have experienced glaciation and flow action before transportation by wind.The flow action played an important role in the generation of quartz silt from Nilka loess and glacial abrasion might make a certain contribution. However eolian erosion leaved no obvious traces in surface of quartz particles, indicative of the shorter transporting distance by wind between loess deposit area and provenance. Moreover it is inferred that the weather denudation was also the important silt generating mechanism. The origin of Nilka loess are similar to those of loess in Hungary, but different from those of loess in Chinese Loess Plateau, which displays mainly in the time of transportation-deposition by wind or distance of wind transport. Recommendations and perspectives This paper analyzes the geologic processes experienced by Nilka loess materials and gives a understanding of the loess silt generating mechanism with plotting the probable sequence of events indicating the formation of loess. This work will lay an important foundation for interpreting climatic significance of Ili loess. |
| Key words: Ili loess quartz grain textures origin and source |
