腾格里沙漠覆盖区晚第四纪大湖期的多手段测年及高湖面成因研究

MIS 3a was commonly considered a relatively humid period with high lake levels in large areas of western China, and Megalake Tengger (MTG) is a representative huge lake with high lake level at that time in the Tengger Desert. However the time of these high lake levels is being argued by recent researches. In this proposal, a comprehensive research focusing on timing and probing formation of the highest lake level based on complete sedimentary sequence as well as shorelines is proposed to be undertaken in the middle part of MTG, Lake Baijian. By using multiple dating methods including OSL, AMS radiocarbon and paleomagnetic dating, age of the sedimentary sequence as well as littoral sediments around Lake Baijian will be measured, and the definite age of the highest lake level will be determined based on cross-correction of the ages obtained. Through analysis of pollen assemblage and type of dominant soluble salt in the sedimentary documents, the ecological environment and salinity of lake water at the highest lake stage will be reconstructed. Basing on hydrological tracking, analysing the 87Sr/86Sr ratios of all potential water sources and water of the paleolake at different stage, the main water source of the highest lake level stage will be identified and formation of the highest lake will be discussed. This research will not only contribute to provide a definite age of the highest lake level in the area of Tengger Desert, helpful to answer whether climate of western China during MIS 3a stage is actually abnormally humid or not, but also provide insights to understand environmental evolvements in the present desert covered areas.

过去认为我国西部地区深海氧同位素三阶段晚期的气候较为湿润，普遍出现高湖面"大湖期"，"腾格里大湖"就是其中一个典型实例。近年来对"大湖期"的时间提出了异议。本项目拟在位于"腾格里大湖"中心的白碱湖地区钻取完整地层记录，并对其高湖岸进行系统调查，运用OSL、AMS C-14和古地磁方法测定高湖面地质记录的年代，在交叉检验可靠性的基础上厘定白碱湖地区高湖面的确切时代。通过孢粉组合恢复晚第四纪最高湖面时段的植被状态，通过介形虫优势种属和主要易溶盐类型恢复湖水盐度，重建高湖面时段的气候状况。通过可能补给水源及地层中古湖水载体的87Sr/86Sr比值测量，分析晚第四纪湖泊不同发展阶段的主要补给水源，解析腾格里沙漠区晚第四纪高湖面的成因。本研究不仅有助于明确回答腾格里沙漠覆盖区晚第四纪高湖面的确切时间和成因，而且能为回答MIS3a时段我国西部地区的气候是否真正有别于全球其它地区这一科学问题提供依据。

过去认为我国西部地区在深海氧同位素三阶段晚期的气候比较湿润，普遍发育有高湖面的“大湖期”。然而，最近的研究却对“大湖期”的时间提出了异议。本研究在位于腾格里沙漠腹地的白碱湖西北部地区开展了系统的湖岸考查和研究，获取了较全面的湖岸地貌和湖滨沉积记录；并在白碱湖的现代湖岸上钻取了深度为104 m的沉积岩芯。湖岸地貌、湖滨沉积、岩芯沉积记录及微体古生物学证据支持，现今的腾格里沙漠地区曾长时间发育湖泊环境。在沉积环境分析的基础上，综合运用石英OSL、ESR以及古地磁等多种测年手段对岩芯进行了沉积年代的确定，结果显示，该岩芯记录了最近2.6 Ma以来的湖盆演化。同时，对海拔介于1310-1350 m之间的多条湖岸线综合运用石英OSL、钾长石pIR IRSL及石英ESR信号进行了沉积年代确定，结果显示，“腾格里大湖期” 的湖岸线出现在早更新世晚期，明显早于早先报道的晚更新世。结合前人在较低海拔（1310-1295 m）的湖岸线上获得的测年结果，我们认为，腾格里沙漠区在晚更新世期间可能不存在较大范围的稳定湖泊，这一状态有别于相邻的吉兰泰地区。其次，研究区内沙丘堆积起始于距今20 ka，为亚洲夏季风减弱的结果；而沙漠景观的快速形成则是最近2 ka以来，暖干化气候背景下人类工农业活动的加剧所导致的。此外，钻孔中碎屑锆石的U-Pb年龄谱的初步结果显示，腾格里地区碎屑物源的变化对青藏高原的隆升历史有一定的指示意义。