Application of XRF core scanning in varved lake sediments: A case study on Lake Xinluhai in the southeastern margin of Qinghai-Tibetan Plateau

Abstract

湖泊年纹层以其精确到年乃至季节尺度的高分辨率优势, 成为研究古气候环境变化的重要载体. X射线荧光(XRF)岩芯扫描由于其有分辨率高、分析快速等特点, 在湖泊年纹层研究中发挥了重要的作用. 本文以青藏高原东南缘新路海年纹层为研究对象, 采用X射线荧光(XRF)岩芯扫描, 对新路海湖泊纹层计年、纹层形成机理及古气候重建进行了研究. 结果表明: 利用X射线图像明暗层、Rad峰值、Zr和Fe元素峰值标记法获得的年代序列基本一致, 并且与独立的放射性测年(²¹⁰Pb/¹³⁷Cs)结果吻合, 证实了上述各种纹层计年方法的可行性和可靠性. 新路海年纹层层偶是由粗颗粒碎屑层和细颗粒碎屑层交互组成的, 较厚且Zr和Si元素高的粗颗粒层形成于春、夏季, 而较薄且Fe元素含量高的细颗粒层形成于秋、冬季. 纹层的厚度能够指示西南季风降水量的大小, 近100年以来新路海纹层厚度反映的西南季风演化, 与利用昆明地区历史文献重建的湿度记录(干旱/洪水指数)、树轮δ¹⁸O重建的尼泊尔喜马拉雅地区的季风降水基本一致. 近100年来新路海的纹层厚度具有7~8、4~5和2年的周期, 表明新路海纹层厚度记录的近100年的西南季风演化可能与厄尔尼诺—南方涛动、太平洋年代际振荡、印度洋偶极子和准两年周期震荡有关.;Varved lake sediment has become one of the important archives for paleoclimate and paleoenvironment studies due to its high-resolution at annual to seasonal timescale. X-ray fluorescence (XRF) core scanning plays an important role in the studies on varved lake sediments due to its high resolution and rapid analysis. In this paper, XRF core scanning was used to study the chronology and formation mechanism of varved lake sediments from Lake Xinluhai in the southeastern margin of the Qinghai-Tibetan Plateau, and the paleoclimate reconstruction based on varve thickness. The results show that varve counting produced using X-radiography, Rad peak, Zr and Fe peak method are essentially consistent with that using the independent radioactive dating (²¹⁰Pb/¹³⁷Cs), which confirms the feasibility and reliability of the above varve counting methods. The annual laminae in Lake Xinluhai were composed of coarse and fine clastic layers. The thick coarse layer with high Zr and Si contents was formed in spring and summer, while the thin fine layer with high Fe content was formed in autumn and winter. The varve thickness can be used to indicate the precipitation intensity of the Southwest Summer Monsoon. The evolution of Southwest Summer Monsoon reflected by the varve thickness in the past 100 years was basically consistent with that of monsoon precipitation reconstructed by humidity (drought/flood index) of Kunming, snow accumulation data from Dasuopu Glacier and the δ¹⁸O of tree ring in the Himalayan region of Nepal. In the past 100 years, the varve thickness of Lake Xinluhai had periodicities of 7-8, 4-5 and 2 years, which may be related with El Niño-Southern Oscillation, Pacific Decadal Oscillation, Indian Ocean dipole and Quasi-biennial Oscillation.