| JIA Jianjun,YANG Yang,CAI Tinglu,GAO Jianhua,XIA Xiaoming,LI Yan,GAO Shu. 2018. On the sediment age estimated by 210Pb dating: probably misleading “prolonging” and multiple-factor-caused “loss”. Acta Oceanologica Sinica, 37(6):30－39 |
| On the sediment age estimated by 210Pb dating: probably misleading “prolonging” and multiple-factor-caused “loss” |
| 210Pb测年的“延年”与“折寿” |
| Received:January 05, 2018 Revised:February 09, 2018 |
| DOI：10.1007/s13131-018-1214-4 |
| Key words:210Pb dating sedimentation rate sediment flux |
| 中文关键词: 210Pb测年 沉积速率 沉积通量 |
| 基金项目:The National Natural Science Foundation of China under contract Nos 41376068 and 41776068. |
| Author Name | Affiliation | E-mail | | JIA Jianjun | State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China | | | YANG Yang | Key Laboratory of Coast and Island Development of MOE, Nanjing University, Nanjing 210023, China | yangy@nju.edu.cn | | CAI Tinglu | State Research Centre for Island Exploitation and Management, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China | | | GAO Jianhua | Key Laboratory of Coast and Island Development of MOE, Nanjing University, Nanjing 210023, China | | | XIA Xiaoming | State Research Centre for Island Exploitation and Management, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China | | | LI Yan | College of the Environment and Ecology, Xiamen University, Xiamen 361102, China | | | GAO Shu | State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China | |
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| Abstract: |
| The radionuclide 210Pb is suitable for century-scale dating and has been used to calculate the sedimentation rate in a variety of environments. However, two common ways to apply 210Pb dating techniques may give misleading results. One is "prolonging of age", i.e., using the calculated sedimentation rate to date back to 200 or 300 years. This practice must be treated with caution because the 210Pb dating techniques do not guarantee direct dating for ages much older than 100 years. Another is "loss of age", i.e., the calculated time span between the topmost layer and the 210Pb background layer in cores is less than 100 years when an apparent sedimentation rate is used in the calculation. Here, we propose that based on the principle of 210Pb dating, the upper limit of age suitable for direct 210Pb dating is between 110 and 155 years. The "prolonging" application is acceptable only if the sedimentary environment in the past several hundred years was stable and the sedimentation rate was generally constant, and verification with independent evidence (such as historical records or biomarker methodology) is needed. Furthermore, after analyzing many published and collected data, we found four possible reasons for the "loss of age". First, the compaction effect of sediment should be corrected in laboratory analysis or else the calculated age will be underestimated. Second, the accuracy and uncertainty of 210Pb activity measurement affect the judgment of the background. To be cautious, researchers are apt to choose a background activity with a younger age. Third, use of a slightly smaller value of supported 210Pb activity in a calculation will lead to considerable underestimation of the time span. Finally, later-stage erosion and migration are common for sedimentation, which lead to loss of sedimentary records and are often reflected as a "loss of age" in cores. We believe that proper use of 210Pb dating data may provide helpful information on our understanding of sediment records and recent environmental changes. |
| 中文摘要: |
| 放射性同位素210Pb适于百年尺度的定年和沉积速率计算，已经在冰芯、土壤、湖泊、水库、河口、潮滩、潟湖、海湾和浅海陆架等多种环境得到广泛应用。目前，210Pb测年方法的应用有两种现象值得注意：一种是“延年”，即根据测算的沉积速率回溯定年的时间，有时回溯的时间跨度可达二、三百年；另一种是“折寿”，即根据测算的表观沉积速率计算得到的210Pb本底层位之上的时间跨度不足百年。根据210Pb测年的原理和有关仪器的检出限来分析，本文作者认为，210Pb直接定年的上限以110 yr~155 yr为宜。因此，“延年”的运用有一定的前提条件，即研究区的沉积环境在回溯的数百年尺度内应该保持稳定，同时回溯的定年结果最好有其他独立的定年方法（如历史文献记录或生物标志物等）来印证、以增加其可靠性。对于210Pb测年的“折寿”现象，可能的原因有四种。第一，沉积及取样的压实效应没有在分析和计算过程中得到校正。第二，210Pb活度测量的精度和不确定度影响了对本底的判断；为稳妥起见，研究者选定的本底层位一般会比真实的本底年龄要年轻一些，导致定年结果的时间跨度缩小。第三，210Pb定年计算模式决定了本底活度值的微小差异将带来计算出来的时间跨度产生显著差异。第四，沉积过程事实上发生了后期的侵蚀和迁移，导致保存在柱样中的沉积记录不完整。为了有效地减少210Pb测年法的结果误差，可以采取减小210Pb活度测量子样的间距、并同步测量226Ra活度等技术手段；另外，在计算时应该使用沉积通量而不是表观沉积速率。我们相信，210Pb测年方法的正确运用和解译将对物源判断、沉降后的侵蚀和扰动等问题提供一些有益的信息。 |
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