山西宁武高山湖泊湖相沉积物的环境磁学和古气候研究

Application of environmental magnetism to lacustrine sediments is very important to study the environmental and climatal changes and related mechanisms. One of the key questions is to establish any strong relationship between variation of magnetic properties and climatic and environmental changes. However, limited studies have been carried out on the mountain lakes. Ningwu Tianchi (including 15 lakes) is not only an important geological relic from the glacial age, but also a unique mountain lake swarm in North China. It has a very high value for scientific research, tourism and ecological protection. In recent years, lake levels have been significantly reduced and ecological environment has been greatly damaged, due to the changing anthropogenic influence or longer term natural factors. To conduct an effective and comprehensive research on the lacustrine sediments of Ningwu Tianchi Lake Swarm utilizing a combination of magnetic and non-magnetic methods, we need to select proper sediment profiles according to the altitude height and evolution history of lakes. The purposes are to: (1) investigate a link between magnetic properties and organic contents, and orientations of magnetic particles of sediments; (2) observe the possible effects on magnetic properties of the deposition environment and then establish a strong relationship between them; (3) investigate the mechanism of environmental and climatal changes that affects the sedimentary deposition process; and further (4) provide a basis to analyze the climate changes and mountain lakes evolution using environmental magnetism techniques. A research on Ningwu Tianchi Lake Swarm can provide the most critical information that indicates the late Pleistocene-Holocene climatic and environmental changes and evolution of mountain lakes, thus enabling us to achieve much in management and protection of this area.

应用环境磁学研究湖成沉积物及其携带的环境气候变化信息以及相关机制具有重要地位，研究关键就是把磁学性质和环境气候变化紧密联系。然而目前对高山湖泊湖成沉积物的环境磁学研究还非常有限。宁武天池形成于第四纪冰川期，是我国三大高山湖泊群之一，对科学研究、旅游业、生态保护有很高的研究价值。近年，由于人为因素和长期自然因素的影响，湖水面明显降低，生态环境遭到极大破坏。本项目拟依托宁武高山湖泊湖成沉积物，根据海拔高低和湖泊演化史选取典型沉积物剖面，结合磁学和非磁学方法，综合研究沉积物的磁性质变化及其与有机物含量之间的联系，以及沉积物中磁性颗粒的定向排列状态，考虑可能的各种影响因素，观察沉积环境对沉积物磁性质的影响，进而探讨影响沉积过程的环境气候变化机制，为应用环境磁学解析过去气候变化和高山湖泊演化提供依据。这项研究有助于人类更好地理解、控制、保护不断变化的环境，和在湿地管理保护方面取得更多成绩。

更新世以来的环境和气候变化是气候学研究的重点领域。我们选择宁武天池高山湖泊中湖相沉积物为研究对象， 首先应用环境磁学和磁化率各向异性参数，提取该区湖相沉积物中磁性矿物的类型、浓度、粒度信息，选择和确定有效的磁性参数组合，能够有效指示该区的沉积环境、气候和沉积物源的变化信息；应用地球化学等非磁学方法，研究了沉积物中磁性矿物的来源以及沉积后的次生变化和改造，揭示了湖泊沉积物的磁学指标对于气候和环境变化响应的复杂性。主要的载磁矿物是多畴（MD）磁铁矿并伴有沉积后蚀变，次生的磁性矿物是硫的铁化物。在晚更新世-中全新世 （~10,270–4,840 cal year BP），磁性颗粒相对较细，磁化率较高，磁化率各向异性（AMS）显示扁平的椭球体，最小轴K3竖直，最大轴K1和中间轴 K2平行于层面；此时，湖水相对平静， 湖泊处于封闭半封闭湖泊状态。在中晚全新世（~4,840–1,880 cal year BP），磁性颗粒较粗，磁化率相对稍低，AMS研究显示古流向北北西-南南东，磁化率各向异性程度高，最小轴K3倾斜较大，湖水动荡能量高， 湖水外流。从晚更新世到全新世早期，较高的频率磁化率值暗示超顺磁磁性颗粒的存在，较高的磁性浓度有降低趋势及有机质含量高，暗示了湖泊发育，湖泊沉积环境受夏季风及其周围环境影响，磁性矿物主要是风成来源，气候相对较为湿润。这项研究能够有助于我们加深理解华北干旱半干旱环境下湖泊沉积物中磁性矿物变化与古环境气候变化之间相互关系，有助理解高山湖泊－高原草甸－高原湿地－人类活动这个系统组分之间的相互作用。