Improved AMS 14C Dating of Shell Carbonates Using High-Precision X-Ray Diffraction and a Novel Density Separation Protocol (Cards)
Improved AMS 14C Dating of Shell Carbonates Using High-Precision X-Ray Diffraction and a Novel Density Separation Protocol (Cards)
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使用高精度 X 射线衍射和新型密度分离协议改进壳碳酸盐的 AMS 14C 测年（卡）
DOI：
10.1017/s0033822200045756
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发表时间：
2010
期刊：
影响因子：
8.3
通讯作者：
T. Higham
中科院分区：
文献类型：
--
作者：
K. Douka;R. Hedges;T. Higham
放射性碳测年成功应用的一个关键变量是有效去除含碳污染物。就海洋碳酸盐而言，污染通常以次级低镁方解石的形式出现，这是碳酸钙的稳定多晶态，是死后再结晶或取代原生相的副产品，最初以高镁方解石或文石的形式出现。根据沉积环境的性质，次生相可能与原始壳碳酸盐同时代，甚至可能是通过溶解-再结晶过程产生的，也可能是年龄较小或较大的外源污染物。目前的预处理方案在测定碳酸盐年代之前检测和去除次生矿物学相的能力有限，这是海洋贝壳和珊瑚14C测定在可靠性方面往往难以验证的原因之一。我们已经开发了一种新的预处理方案，旨在实现更高的可靠性和准确性，在这种材料的年代测定。该方法需要两个步骤。第一项研究是利用x射线衍射改进文石中次生方解石的检测和定量，精度分别为~ 0.1%和~ 0.8%。接下来，在需要的地方，使用无毒重液体（卡片）的新型密度分离步骤应用于成岩样品。这使得方解石和文石能够清晰地分离，只有后者用于年代测定。我们已经将新的步骤，筛选和分离，应用于标准和考古实例，我们的初步结果表明，它是成功的和可复制的。在本文中，我们描述了方法和初步结果。
One critical variable in the successful application of radiocarbon dating is the effective removal of carbonaceous contaminants. In the case of marine carbonates, contamination appears usually in the form of secondary low-magnesium calcite, the stable polymorph of calcium carbonate and byproduct of the post-mortem recrystallization or replacement of the autochthonous phase, originally in the form of high-magnesium calcite or aragonite. Depending on the nature of the depositional environment, the secondary phase may be contemporary in age with the original shell carbonate and may have even been derived from it by dissolution-recrystallization processes, or can be an exogenous contaminant of younger or older age. The limited ability of current pretreatment protocols to detect and remove the secondary mineralogical phases prior to dating carbonates has been one of the reasons marine shell and coral 14C determinations are often difficult to validate in terms of their reliability. We have developed a new pretreatment protocol designed to achieve greater reliability and accuracy in the dating of this material. The method entails 2 steps. The first one involves the improved detection and quantification of secondary calcite in aragonite using X-ray diffraction, at a precision of ∼0.1% and ∼0.8%, respectively. Next, where this is required, a novel density separation step using non-toxic heavy liquids (CarDS) is applied to the diagenetic sample. This enables the clear separation of calcite and aragonite, with only the latter kept for dating. We have applied the new steps, screening and separation, on standard and archaeological examples and our initial results suggest that it is successful and reproducible. In this paper, we describe the method and initial results.