一种地下水14C测年沉淀法采样新程序及对以往测年成果可靠性的评估
齐继祥+张之淦+彭玉荣+郭华良+段宝谦
摘要：国际原子能机构推荐的地下水14C测年沉淀法采样程序因未能有效控制大气CO2污染，常使古老地下水14C测年结果偏年轻许多。在分析可能存在的大气CO2污染途径及影响因素的基础上，开发了一种融入多种大气CO2污染控制技术的地下水14C测年沉淀法采样新程序。新采样程序在保持操作简便的前提下，使大气CO2污染得到有效控制；通过专门设计的样品瓶实现了采样程序与制样程序的有效衔接，有利于降低制样程序中的大气CO2污染。以苯合成-液体闪烁计数法测定14C比活度，新采样程序使古老地下水14C测年结果（在测年上限50×103年以内）不再产生可识别的偏差。在河北平原地下水14C测年采样中的初步应用显示，早期取得的古老地下水14C测年结果存在较大的负偏差，例如对14C表观年龄约为47×103年的地下水，可偏年轻20×103年以上。因此，有必要对以往取得的古老地下水14C测年结果及由此得出的研究结论是否可靠重新进行评估。地下水14C测年沉淀法采样新程序的推出使传统放射性测量法重新得到重视，因采用的样品量千倍于加速器质谱法，使制样程序中的大气CO2污染更易控制，古老地下水14C测年结果更具可靠性。
关键词：地下水；14C测年；偏差；大气CO2污染；采样方法；测量方法；测年上限；河北平原
中图分类号：P641 文献标志码：A
Abstract： The precipitation method sampling procedure of 14C dating of groundwater recommended by IAEA is suffering from atmospheric CO2 pollution. As a result， the dating results tend to be much younger. After analyzing the possible sources of atmospheric CO2 pollution and related influencing factors， a state-of-the-art precipitation method sampling procedure of 14C dating of groundwater was developed to reduce the atmospheric CO2 pollution. This sampling procedure not only integrates various CO2 pollution controlling techniques， but also enjoys easy operation. A specially-designed sample bottle links the field sampling operation and the sample preparing operation effectively. After adopting the new procedure， the negative deviations of the dating results obtained by using benzene-synthesis-liquid-scintillation spectrometer （for the dating results below the upper limit of 50 000 years） are negligible. By comparing the previously-obtained 14C dating results of ancient groundwater with the results obtained by adopting the new sampling procedure in Hebei Plain， the former exists significant negative deviations. For the groundwater with 14C apparent age of approximately 47×103 years， the dating results tend to be more than 20×103 years younger. Therefore， it is necessary to conduct new evaluations for the formerly-obtained 14C dating results of ancient groundwater and the research conclusions based on the dating results. The new precipitation method sampling procedure of 14C dating of groundwater makes researchers attach importance to the traditional radioactivity measurement method again. Because the sampling quantity is a thousand times more than accelerator mass-spectrometry， it is easier to control the atmospheric CO2 pollution during sample preparation， and the 14C dating results of ancient groundwater are more reliable.
Key words： ground water； 14C dating； deviation； atmospheric CO2 pollution； sampling method； measurement method； upper limit of dating； Hebei Plain
摘要：国际原子能机构推荐的地下水14C测年沉淀法采样程序因未能有效控制大气CO2污染，常使古老地下水14C测年结果偏年轻许多。在分析可能存在的大气CO2污染途径及影响因素的基础上，开发了一种融入多种大气CO2污染控制技术的地下水14C测年沉淀法采样新程序。新采样程序在保持操作简便的前提下，使大气CO2污染得到有效控制；通过专门设计的样品瓶实现了采样程序与制样程序的有效衔接，有利于降低制样程序中的大气CO2污染。以苯合成-液体闪烁计数法测定14C比活度，新采样程序使古老地下水14C测年结果（在测年上限50×103年以内）不再产生可识别的偏差。在河北平原地下水14C测年采样中的初步应用显示，早期取得的古老地下水14C测年结果存在较大的负偏差，例如对14C表观年龄约为47×103年的地下水，可偏年轻20×103年以上。因此，有必要对以往取得的古老地下水14C测年结果及由此得出的研究结论是否可靠重新进行评估。地下水14C测年沉淀法采样新程序的推出使传统放射性测量法重新得到重视，因采用的样品量千倍于加速器质谱法，使制样程序中的大气CO2污染更易控制，古老地下水14C测年结果更具可靠性。
关键词：地下水；14C测年；偏差；大气CO2污染；采样方法；测量方法；测年上限；河北平原
中图分类号：P641 文献标志码：A
Abstract： The precipitation method sampling procedure of 14C dating of groundwater recommended by IAEA is suffering from atmospheric CO2 pollution. As a result， the dating results tend to be much younger. After analyzing the possible sources of atmospheric CO2 pollution and related influencing factors， a state-of-the-art precipitation method sampling procedure of 14C dating of groundwater was developed to reduce the atmospheric CO2 pollution. This sampling procedure not only integrates various CO2 pollution controlling techniques， but also enjoys easy operation. A specially-designed sample bottle links the field sampling operation and the sample preparing operation effectively. After adopting the new procedure， the negative deviations of the dating results obtained by using benzene-synthesis-liquid-scintillation spectrometer （for the dating results below the upper limit of 50 000 years） are negligible. By comparing the previously-obtained 14C dating results of ancient groundwater with the results obtained by adopting the new sampling procedure in Hebei Plain， the former exists significant negative deviations. For the groundwater with 14C apparent age of approximately 47×103 years， the dating results tend to be more than 20×103 years younger. Therefore， it is necessary to conduct new evaluations for the formerly-obtained 14C dating results of ancient groundwater and the research conclusions based on the dating results. The new precipitation method sampling procedure of 14C dating of groundwater makes researchers attach importance to the traditional radioactivity measurement method again. Because the sampling quantity is a thousand times more than accelerator mass-spectrometry， it is easier to control the atmospheric CO2 pollution during sample preparation， and the 14C dating results of ancient groundwater are more reliable.
Key words： ground water； 14C dating； deviation； atmospheric CO2 pollution； sampling method； measurement method； upper limit of dating； Hebei Plain
