末次冰盛期以来藏北当惹雍错湖泊沉积物反映的环境变化研究

Tibetan Plateau, which has been called "The Third Pole" of the Earth, is believed an ideal region for past environmental changes study. Numerous close lakes are important archives for environmental changes study on the Plateau, and a lot of case studies have been conducted in different areas of the Plateau by using lake sediments. Progresses have been achieved on such significant scientific issues as ancient monsoon evolution, paleo-lake-level fluctuation and Large Lake Period, etc. However, there is still lack of continuous environmental sequence with relatively long time scale in Qiangtang plateau of North Tibet. Tangra Yumco, a close big lake located in this area, is determined as the deepest lake ever known on Tibetan Plateau and shows notable potential on past environmental changes study. This project will use lake sediment core in Tangra Yumco as the main research object, establishing accurate age control by eliminating old carbon effect and clarifying the environmental significance of proxies by modern processes study. A continuous environmental change sequence since the Last Glacial Maximum will be established by using multi-proxies from the lake sediment core, from which monsoon evolution and lake level change information will be mainly taken out. Thus the climatic and environmental changes features and environmental events in typical periods and main transitions in this area will be depicted. Moreover, comparative studies between this record and the records from typical monsoon-dominated system as well as westerly-dominated system will also be conducted, to reveal the influence intensity of the ancient monsoon to this area, providing a new regional evidence for ancient monsoon research over wide range area on Tibetan Plateau.

青藏高原被称为地球"第三极"，是过去环境变化研究的理想区域。众多的封闭湖泊为青藏高原环境演变研究提供了重要的档案材料；在高原广大地区利用湖泊沉积已开展了大量研究，并在季风演化、古湖面变化及大湖期等重要科学问题上取得了较大的进展。然而，位于藏北的羌塘高原地区尚缺乏完整的较长时间尺度的连续环境变化序列。当惹雍错是位于该地区的一个封闭大湖，是目前已知青藏高原最深的湖泊，具有良好的进行过去环境变化研究的潜力。本项目拟以当惹雍错岩芯沉积物为主要研究对象，在剔除老碳效应的精确年代控制和现代过程研究的基础上，利用沉积物多指标建立末次冰盛期以来连续环境变化序列，并主要提取季风演化和湖面变化信息，刻画本地区典型时段和主要转换期的气候环境变化特征和环境事件；通过与典型季风和西风系统环境记录的对比，揭示古季风对本地区的影响程度，从而为青藏高原大范围古季风研究提供新的区域性的证据。

作为青藏高原目前已知最深的湖泊，地处羌塘高原腹地的封闭湖泊当惹雍错目前研究程度还较为薄弱。大量的高湖面遗迹表明该地区曾有剧烈的湖面波动，进而说明影响该地区的主要环流系统发生过巨大的变化，因此在当惹雍错开展古环境变化及湖面波动研究具有独特的优势，这对于揭示季风西风演化历史及影响机制具有重要的科学意义。.在本项目的支持下，我们在当惹雍错开展了湖泊现代过程监测与研究、湖泊沉积物14C年代学中的碳库效应研究、重建了不同时间尺度下基于多环境指标的古环境变化序列、利用光释光测年方法确定古湖岸阶地年龄从而对湖面波动历史进行了研究。项目已获取了大量的数据和样品，发表了一定数量的研究论文。结果显示当惹雍错流域现代水体水化学组成特征具有明显的季节差异，反映了印度季风降水对水化学组成的强烈影响，这能为利用沉积物中元素含量进行古环境研究提供科学依据。当惹雍错湖水水化学特征主要受蒸发和结晶过程控制，而河水和降水则主要受岩石风化作用影响，其中以碳酸盐的风化最为重要。经过多种方法对比，认为当惹雍错湖泊沉积物具有2200年左右的碳库效应，为建立可靠的年代框架提供了重要的依据。剔除碳库效应后建立的覆盖17.5 cal ka BP的多指标古环境序列显示，该地区在17.1 cal ka BP之前可能被冰覆盖，到16.0 cal ka BP时湿度增大，流域内植被快速转好，指示了印度季风出现第一次明显增强，之后直至12.0-11.4 cal ka BP期间季风开始减弱，对应于新仙女木冷事件。早全新世气候温暖湿润，在大约8.5 cal ka BP时湿度和温度都达到最大值，之后湖面一直处于总体上降低的趋势，气候变干，季风减弱。自6.0 cal ka BP以来当惹雍错湖泊生产力总体上处于持续降低的趋势，与水位下降具有一致性。当惹雍错晚冰期以来环境变化与青藏高原南部其他湖泊具有相似性，反映了印度季风在较大区域上是气候变化的重要控制因素，而当惹雍错地区的干旱事件则反映其了与北大西洋Bond事件之间的关联。本项目的研究为深入认识季风演化历史及机制提供了新的证据。