天山北麓河流下游冲积平原沉积记录的中晚全新世环境演变

Environmental Evolution of Middle-Late Holocene Verified by Sedimentary Evidences in the Alluvial Plain in the Northern Foothills of the Tianshan Mountains,China

天山北麓河流下游冲积平原与古尔班通古特沙漠交汇带,沉积环境独特,对全球气候变化响应敏感。囿于年代校正不确定性、代用指标的多解性以及地形地貌格局等因素的影响,该区域全新世时期沉积环境演变过程和气候演化模式研究,尚存在不同认识。重建该区域的中晚全新世环境演变历史及其对气候变化的响应过程,有助于加深对天山北麓冲积平原沉积环境演化时空特征的认识。本文选取天山北麓中部河流下游冲积平原四个典型冲积-湖积-风积地层剖面为研究材料,以光释光(OSL)测年确定地层绝对年代,综合沉积构造特征、沉积物粒度参数和石英砂表面微结构特征分析,探讨河流下游中晚全新世冲积-湖积平原沉积序列的演变过程和驱动因素。结果表明:(1)研究剖面主要由黏土、粉砂和极细砂含量较高的河湖相沉积构成,靠近北部沙漠边缘以极细砂、细砂为主的风成砂层明显增多,形成以冲积相、湖沼相和风沙相交替叠覆的沉积序列,其沉积相组合具有明显的时空变化特征;(2)沉积物石英颗粒表面常见冰川、流水、风力等多种外营力作用的痕迹,表明河流下游平原沉积物主要来源于山区冰川搬运、磨蚀的碎屑物质,并经流水和风力分选后成为古尔班通古特沙漠的重要物源;(3)7.59~6.0 ka河流活动频繁,广泛发育河湖相沉积,为相对暖湿气候期;6.0~4.25 ka期间,多地出现风沙活动,气候短暂转干,湿度有所降低;4.25 ka以来,发育冲洪积、湖积和风成沙交互沉积,气候“温湿-凉干”周期性波动。本文各剖面沉积序列综合揭示中全新世以来的沉积环境演变过程既有全球气候变化影响的共性特征,又有干旱区山麓地带大地貌控制的区域特点,这对预测未来沙漠边缘风沙活动变化趋势、合理利用淡水资源,从而构建区域生态安全屏障具有重要意义。

The sedimentary environment of the confluence zone between the alluvial plain and the Gurbantunggut Desert at the northern foot of the Tianshan Mountains is unique and sensitive to global climate change.Due to the complexity of factors such as the uncertainty of age correction,the multi-solution of proxy indicators,and the topographical and geomorphological pattern,there were different understandings of the evolution process of the sedimentary environment and the climate evolution model during the Holocene period in this region.It is quite necessary for science to reconstruct the sedimentaryevolution history of the middle-late Holocene in this area and its response to climate change,which would be helpful to deepen the understanding of the geo-environment of the northern Tianshan Mountains.In this study,four typical alluvial-lacustrine-eolian stratigraphic profiles in the alluvial plain formed by rivers in the northern foothills of the Tianshan Mountains were examined by optically stimulated luminescence(OSL)dating for determination of their absolute age of strata;then the evolution process and driving factors of these sedimentary sequences in the profiles were concluded after comprehensively analyzing their sedimentary structure characteristics,sediment grain size parameters,and quartz sand surface microstructure characteristics.It has the following findings:(1)The examined sedimentary profiles were mainly composed of fluvial and lacustrine deposits with high content of clay,silt,and very fine sand.Near the edge of the northern desert,the aeolian sand layer was dominated by very fine sand and fine sand increased significantly,forming a sedimentary sequence of alluvial facies,lake-marsh facies,and wind-sand facies alternately overlapping,and its sedimentary facies combination had obvious spatial and temporal variation characteristics.(2)Traces of glaciers,running water,wind,and other external forces could be identified on the surface of sediment quartz particles,indicating that the sediments downstream were mainly derived from the debris transported and eroded by glaciers in mountainous areas,and then became a general source of the Gurbantunggut Desert after sorted by running water and wind.(3)From 7.59 ka to 6.0 ka,it had been a relatively warm and wet climate period with frequent fluvial activities,and extensive fluvial-lacustrine sediments to be formed;in the period of 6.0 ka~4.25 ka,wind-sand activities occurred in many places,accompanied by a dry climate for a short time with decreased humidity;since 4.25 ka,alluvial,lacustrine and eolian sand have been deposited alternately,and the climate fluctuates periodically from dry-warm to wet-cold.The sedimentary sequences at all four profiles in this study comprehensively reveal that the evolution of the sedimentary environment since the Middle Holocene has not only the common characteristics of the influence of global climate change but also the regional characteristics of large geomorphology control in the piedmont of arid areas.This is of great scientific significance for predicting the future trend of wind and sand activities at the desert edge,rationally managing freshwater resources,and thus building a regional ecological security barrier.This study is also a solid reference for evaluating the ecological security of the northern Tianshan Mountains and the northern desert basin.