一種地下水14C測年沉淀法采樣新程序及對以往測年成果可靠性的評估

齊繼祥+張之淦+彭玉榮+郭華良+段寶謙

摘要：國際原子能機構推薦的地下水14C測年沉淀法采樣程序因未能有效控制大氣CO2污染，常使古老地下水14C測年結果偏年輕許多。在分析可能存在的大氣CO2污染途徑及影響因素的基礎上，開發了一種融入多種大氣CO2污染控制技術的地下水14C測年沉淀法采樣新程序。新采樣程序在保持操作簡便的前提下，使大氣CO2污染得到有效控制；通過專門設計的樣品瓶實現了采樣程序與制樣程序的有效銜接，有利于降低制樣程序中的大氣CO2污染。以苯合成-液體閃爍計數法測定14C比活度，新采樣程序使古老地下水14C測年結果（在測年上限50×103年以內）不再產生可識別的偏差。在河北平原地下水14C測年采樣中的初步應用顯示，早期取得的古老地下水14C測年結果存在較大的負偏差，例如對14C表觀年齡約為47×103年的地下水，可偏年輕20×103年以上。因此，有必要對以往取得的古老地下水14C測年結果及由此得出的研究結論是否可靠重新進行評估。地下水14C測年沉淀法采樣新程序的推出使傳統放射性測量法重新得到重視，因采用的樣品量千倍于加速器質譜法，使制樣程序中的大氣CO2污染更易控制，古老地下水14C測年結果更具可靠性。

關鍵詞：地下水；14C測年；偏差；大氣CO2污染；采樣方法；測量方法；測年上限；河北平原

中圖分類號：P641 文獻標志碼：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