全新世以来腾格里沙漠风沙活动地质记录与模拟数据综合研究

EVOLUTION OF AEOLIAN ACTIVITIES IN THE TENNGER DESERT DURING THE HOLOCENE:COMPREHENSIVE RESEARCH BASED ON GEOLOGICAL RECORDS AND SIMULATED DATA

自晚更新世以来,地处干旱与半干旱过渡区的腾格里沙漠,在亚洲季风的作用下,经历了几度流沙出现、扩大和固定、半固定乃至弱成壤的沙漠正、逆交替变化过程。东亚季风在时空尺度上的强弱变化通常被解释为驱动腾格里沙漠环境变化的主要原因。用于重建古环境和风沙活动的变化多依赖于腾格里沙漠边缘区域的黄土、湖泊、孢粉和冲洪积物,然而涉及到沙漠腹地的古环境记录较少,并且不同地点的沉积剖面所记录的地质信息具有区域局限性。本研究在腾格里沙漠西南边缘至腹地采取4个典型风沙沉积序列,共采集97个沉积样品和11个风沙沉积物光释光年代数据,并收集整理了约14 ka以来已发表的69个风沙沉积物光释光年龄数据以及137个古土壤、湖相沉积物和钙质根管的14C年龄数据;另外,为了减少沉积序列在解读和重建沙漠环境演变的或然性,运用TraCE-21 ka古气候模拟数据分别计算了14 ka以来腾格里沙漠地区地表有效湿度和沙丘活化指数的变化趋势,以及春冬和夏秋季节沙丘活化指数对全年平均值的贡献量,综合探讨了全新世以来腾格里沙漠近地表风沙活动历史以及驱动因素。结果表明:1)早全新世时期(11.7~8.0 ka),较粗的平均粒径和偏多的风沙年代数据表明该时期风沙活动较强;模拟数据结果显示该时期风沙活动呈现减弱趋势,这主要是由于减弱的东亚冬季风引起的近地表风速下降所导致的,从而发生大量的风沙堆积;2)中全新世时期(8~3 ka),较高的低频磁化率和较多的14C年代数据指示该时期古土壤发育,地表湿润条件转好,风沙活动较弱。模拟数据结果表明该时期地表有效湿度较高,风沙活动处于较低水平,这主要是由于增强的东亚夏季风带来了大量的降水,促使地表有效湿度较高,沙丘逐渐被植被固定;3)晚全新世以来(3~0 ka),较粗的平均粒径和风沙年代数据增多表明风沙活动再次活跃。模拟数据结果显示该时期风沙活动呈现增加趋势,表明东亚夏季风减弱和腾格里沙漠的气候干旱导致沙漠地表有效湿度逐渐下降,植被退化,固定沙丘再度活化。

The Tengger Desert(37°24′~40°1′N,102°45′~105°37′E)is located not only in the transitional zone between arid and semi-arid regions but also in the margin of the modern East Asian monsoon domain.The variation of the East Asian monsoon on the spatiotemporal scale is commonly regarded as one of the essential exterior forces driving the alternating expansion and shrinkage of the Tengger Desert.To reconstruct the evolution of the paleoenvironment and aeolian activity,loess,lacustrine sediments,pollen,and alluvial sediments in the margin of the Tengger Desert were widely used.However,there are few geological records in the hinterland of the desert.Additionally,geological information recorded by stratigraphic sequence at different locations in the Tengger Desert has regional limitations.In this study,4 profiles(TGL-A(37°03′8″N,103°21′07″E;2.6 m),TGL-B(37°51′33″N,103°23′49″E;2.5 m),TGL-C(37°39′16″N,103°10′59″E;2.6 m),and TGL-D(37°36′46″N,103°36′45″E;2.0 m))in the southwestern of the Tengger Desert were studied,and 11 optically stimulated luminescence(OSL)samples were collected and measured.Then,we collect the OSL age of 69 aeolian sediments and the 14 C age of 137 paleosol,lacustrine sediments and calcareous root canals since ca.14 ka.Moreover,in order to reduce the uncertainty of using stratigraphic sequence to reconstruct the paleoenvironmental evolution of the Tengger desert,the TraCE-21 ka(Transient Climate Evolution Simulation over Last 21000 Years)paleoclimate simulation data was also used to calculate the effective moisture and dune mobility index in the warm season(JJASON)and cold season(DJFMAM).The evolution and driving factors of aeolian activity in the Tengger Desert during the Holocene are discussed comprehensively.The results indicate as following:(1)During the Early Holocene(11.7~8.0 ka),the coarser mean grain size and a larger number of aeolian sediment OSL ages indicated that the aeolian activity was strong.The simulation results suggest that the aeolian activity appeared a weakening trend in this period,which is mainly due to the decrease of near-surface wind speed caused by the weakening East Asian winter monsoon,then resulting in a large amount of aeolian accumulation.(2)During the Middle Holocene(8~3 ka),the higher low-frequency magnetic susceptibility and more 14 C age indicated the development of paleosol,the improvement of effective moisture,and the weakening of aeolian activity.The simulation results show that the effective moisture was relatively high,and the aeolian activity was weak in this period.The enhanced East Asian summer monsoon(EASM)brought more rainfall,which increased the effective moisture and stabilized the dunes.(3)During the Late Holocene(3~0 ka),the coarser mean grain size and the increasing number of aeolian sediment OSL ages implied that aeolian activity was reactive.The simulation results show that the aeolian activity displayed an increasing trend.The weakening of EASM and the drying of the Tengger desert induced the effective moisture gradually decreased,vegetation degraded,and fixed dunes reactivated.