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

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。

New Precipitation Method Sampling Procedure of 14C Dating of Groundwater and the Evaluation of the Formerly-obtained 14C Dating Results

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.