湖泊沉积物年纹层的研究方法及其意义

文章总结了前人对湖泊沉积物年纹层类型的划分,并根据年纹层的形成过程和组分特征分为3个大类,即碎屑年纹层、生物成因年纹层(如硅藻年纹层等)和化学成因年纹层(如方解石年纹层、菱铁矿年纹层、黄铁矿年纹层、蒸发盐年纹层等)。介绍了目前应用于湖泊沉积物年纹层研究的主要方法和技术包括:1)新鲜沉积物表面照相、X射线照相技术;2)光学显微镜观察;3)数字化图像分析;4)扫描电子显微镜技术等。而岩相学大薄片是目前年纹层研究工作中应用非常广泛的材料,对目前制作大薄片比较普遍采用的快速冷冻-冷冻干燥和水-丙酮-环氧树脂交换这两种方法进行了介绍并比较了各自的优劣。湖泊沉积物年纹层研究的古环境意义主要体现在两个方面,一是提供了高精度的纹层年代学时间标尺,在诸如气候突变事件的时限、大气¹⁴C浓度变化、火山灰层定年、古地磁场长期变化主曲线重建等方面有重要意义;二是年纹层性质研究如年纹层厚度和年纹层微相变化本身所蕴藏的高分辨率古气候环境变化信息,在太阳活动周期、ENSO等气候事件的研究中也有重要意义。最后,文章简单介绍了我国东北龙岗火山区四海龙湾玛珥湖沉积物中年纹层的特征并展望了其研究潜力。

METHODOLOGY OF STUDYING ON VARVED LAKE SEDIMENTS AND ITS SIGNIFICANCE

Varved lake sediments are one of the high-resolution geological archives on the continent. Based on the independent, inherent and continuous calendar varve chronology, high resolution paleoenvironmental information can be obtained at annual or even seasonal scale. The types of varve in lake sediments can be classified into three main types according to their formation process and components, i.e. clastic varve, biogenic varve (e.g. diatom-rich varve), and chemically induced varve (such as calcareous varve, ferrogenic varves, and evaporative varve) . The research methodology on varved sediments are: (1) surface core photography and X-ray radiography, (2) optical microscopy, (3) digital image analysis, and (4) scan electron microscope. The petrological large thin section, which becomes more and more important in obtaining detailed information from varve, attracts more attention. We evaluate two ways of embedding sediments (shock-freeze and freeze-dry technique and water-acetone-epoxy resin exchange technique) for thin sections. Among these methods and techniques, digital image analysis and scan electron microscope will play more important roles in obtaining varve chronology and high resolution paleoenvironmental records. To achieve a high quality varve chronology, the annual feature of one varve must be built firstly and demonstrated by internal and external evidences, and the possible error must be estimated and minished. The paleoenvironmental implications of varve studies represent at two ways: the first one is high precise varve chronology timescale, while the other is high resolution paleoclimate and paleoenvironment information from the nature of varve . Based on long and continuous varve chronology, the duration of abrupt climatic event during the Last Deglaciation can be counted; in addition, reconstructing atmospheric radiocarbon concentration, dating of tephra layers, building of regional paleomagnetic secular variation, and etc can be achieved by the method of high resolution varve chronology. The varve thickness data can be applied to search for cyclic patterns in lake sediments and to compare with known periodicities of solar activity. In addition, the history of climate event such as ENSO can be traced. Compared with Finland, Sweden,Germany,Swiss,Poland and North America, where much more lakes with varved sediments are found and more research results are reported, the varved lake sediments found in other parts of the world are infrequent. Because of great potentials in high resolution paleoclimatic and paleoenvironmental reconstruction, the research of the varved lake sediments should be strengthened in future. Maar lakes, usually closed and deep, is suitable for the formation and preservation of varve. Much more efforts are paid to look for maar lakes with varved sediments in China. Here, we report the characteristics of varve found in the Sihailongwan maar lake in northeastern China.