关键词: 牛轭湖沉积, 沉积特征, 粒度分析, 磁化率分析, 长江中游

Abstract:

Recent lake sediments (last 50-100 years) are essential for quantitative paleoclimate research. However, there is less research on oxbow lake sediments at preasent. This paper describes grain size and magnetic parameters data from the ²¹⁰Pb-aged sediment cores TE(85-cm long) and ZZ-2(89-cm long) recovered from two oxbow lakes, located in the middle reaches of the Yangtze River, China. Combined with statistical methods and local chronicles, the basic sediment sequence of the oxbow lake sediments was established. Two cores disclosed the sedimentary characteristic and evolution process of oxbow lakes. The following conclusions are drawn: (1) By the analyses of grain-size parameters and magnetic parameters of cores TE and ZZ-2, two stages of oxbow lake sediments have been distinguished. They were lower (layer A) which reflected fluvial facies, and upper (layer B) which reflected lacustrine facies. According to the grain-size parameters, two sediment cores could be divided into three types: TypeⅠ(lake sediments) corresponding to layer B, Type Ⅱ (fluvial sediments) and Type Ⅲ (fluvial-lake sediments) corresponding to layer A. Vertically, they are coarse-fine-coarse gradations in the grain sizes of the oxbow lake sediments from bottom to top. (2) For modern oxbow lake, age models based on radioisotope analysis is still the effective means of the short-core sediments, but the average sedimentary rate will produce larger errors, which need to be checked by environmental change events through the identification of sediments in a specific event. (3) Modern oxbow lake sediments have good response to the river basin flood events, but the mechanism still needs to be studied further. Thus, it is of great significance to reveal the ancient flood event information in the ancient oxbow lake sediments.