Radiocarbon (14C) dating of anthropogenic carbonates (CaCO3) such as ash, lime plaster and lime mortar, has proven a difficult task due to the occurrence of a number of contaminants embedded within the CaCO3 pyrogenic binder. These include 14C-free geologic components and/or secondary phases bearing an unknown amount of 14C, and thus the alteration of the original pyrogenic isotopic signature of the material results in major age offsets when carbon recovery is performed through acid hydrolysis. Here we present a characterization/quantification approach to anthropogenic carbonates that includes Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, thin section petrography, thermogravimetric analysis and scanning electron microscopy coupled with high-resolution cathodoluminescence, with which we identified the pyrogenic CaCO3 fraction in an aerial lime plaster and two hydraulic mortars. The preserved pyrogenic component was then isolated by density separation and its purity checked again using FTIR. Carbon was recovered through thermal decomposition in vacuum. The resulting 14C age matches the expected age of the lime plaster, whereas hydraulic mortars are slightly offset due to the carbonation of calcium hydroxide lumps. This approach highlights the importance of a dedicated characterization strategy prior to dating and may be applied to aerial lime plasters to obtain accurate ages.

中文翻译：

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石灰粘合剂的结构表征和热分解允许准确测定空中石灰石膏的放射性碳年龄

放射性碳 (14C) 人为碳酸盐 (CaCO) 的年代测定3)，例如灰烬、石灰灰泥和石灰砂浆，已被证明是一项艰巨的任务，因为 CaCO 中嵌入了许多污染物3热解粘合剂。这些包括14不含碳的地质成分和/或次生相，含有未知量的14C，因此当通过酸水解进行碳回收时，材料原始热解同位素特征的改变导致主要的年龄偏移。在这里，我们提出了一种人为碳酸盐的表征/量化方法，包括傅里叶变换红外光谱 (FTIR)、X 射线衍射、薄片岩相学、热重分析和扫描电子显微镜以及高分辨率阴极发光，我们通过这些方法确定了热解 CaCO3在空中石灰石膏和两个液压砂浆中的一部分。然后通过密度分离分离保存的热解组分，并使用 FTIR 再次检查其纯度。通过在真空中热分解回收碳。所结果的14C 年龄与石灰灰泥的预期年龄相匹配，而水硬砂浆由于氢氧化钙块的碳化而略有偏移。这种方法强调了在测年之前专门的表征策略的重要性，并且可以应用于空中石灰膏以获得准确的年龄。