Application and problems of <sup>210</sup>Pb and <sup>137</sup>Cs dating techniques in lake sediments

Abstract

湖泊沉积记录研究对了解流域侵蚀、揭示区域气候环境变化及人类活动影响、建立外生地球化学循环模式等均具有重要意义.放射性同位素²¹⁰Pb和¹³⁷Cs组合定年法是百年尺度湖泊沉积记录研究的首选方法，在过去几十年中得到了广泛应用，大力推动了湖泊沉积记录的高分辨率气候环境变化研究的开展.然而，如何理解和选择适用的计算模式来尽可能地提高定年准确度目前仍存在争议，这在一定程度上限制了该方法的应用.鉴于上述问题，本文基于本实验室近年来从青藏高原获取的若干支代表性湖芯的定年结果，从样品前处理、仪器及测试、数据分析及计算等全过程对²¹⁰Pb、¹³⁷Cs定年原理和方法进行了较为系统的阐述；在此基础上，提出了²¹⁰Pb定年样品封装原则，探讨了高纯锗γ谱仪在放射性活度测量中的影响因素，并建立了准确有效的比活度校正方法；明确指出了恒定初始浓度模式（constant initial concentration，CIC）可用于计算沉积物每层的年龄，而不是部分研究中认为的平均沉积速率，这对于深刻理解该定年模式并提供准确的定年结果具有重要的参考意义.同时，通过对比两种放射性核素的分布特征发现，相对于²¹⁰Pb而言，¹³⁷Cs更容易发生垂向迁移进而影响其时标准确性，这需引起研究者的充分重视，因此，当二者定年结果出现明显差异时，建议以²¹⁰Pb定年结果为准.;The study of lacustrine sedimentary records is of great significance for understanding basin erosion, revealing regional climatic and environmental changes and the influence of human activities, and establishing exogenous geochemical cycle patterns. The combined dating method of ²¹⁰Pb and ¹³⁷Cs is the preferred method for the study of centennial-scale lake sedimentary records. It has been widely used in the past decades, which greatly promotes the study of high-resolution climate and environmental changes in lake sedimentary records. However, it is still controversial how to understand and select the applicable calculation model to improve the accuracy of the dating as much as possible, which limits the application of this method to a certain extent. Because of the above problems, based on the dating results of several representative lake cores obtained from the Tibetan Plateau by our laboratory in recent years, the principles and methods of dating with ²¹⁰Pb and ¹³⁷Cs were systematically expounded from the whole process of sample pretreatment, instrument and sample measurement, data analysis and calculation. On this basis, the packing principle of ²¹⁰Pb dating samples was proposed, the influencing factors of High-purity germanium Gamma-Ray Spectrometer in radioactive activity measurement were discussed, and an accurate and effective specific activity correction method was established. It was pointed out that constant initial concentration (CIC) model could be used to calculate the age of each layer of sediments, rather than the average deposition rate in some studies, which is of great significance for understanding the model and providing accurate dating results. Meanwhile, by comparing the distribution characteristics of the two radioisotopes, it was found that ¹³⁷Cs was more likely to have vertical migration compared with ²¹⁰Pb, and thus affected the dating accuracy, which should be paid more attention to by researchers. Therefore, in case of the obvious difference between the two results, it was suggested that ²¹⁰Pb results should prevail.