Abstract
Anthropogenic carbonates are pyrotechnological products composed of calcium carbonate, and include wood ash, lime plaster/mortar, and hydraulic mortar. These synthetic materials are among the first produced by humans, and greatly influenced their biological and cultural evolution. Therefore, they are an important component of the archeological record that can provide invaluable information about past lifeways. One major aspect that has been long investigated is the possibility of obtaining accurate radiocarbon dates from the pyrogenic calcium carbonate that makes up most of these materials. This is based on the fact that anthropogenic carbonates incorporate atmospheric carbon dioxide upon the carbonation of hydrated lime, and thus bear the radiocarbon signature of the atmosphere at a given point in time. Since plaster, mortar, and ash are highly heterogeneous materials comprising several carbon contaminants, and considering that calcium carbonate is prone to dissolution and recrystallization, accurate dating depends on the effectiveness of protocols aimed at removing contaminants and on the ability to correctly identify a mineral fraction that survived unaltered through time. This article reviews the formation and dissolution processes of pyrogenic calcium carbonate, and mineralogical approaches to the definition of a ‘dateable fraction’ based on its structural properties.
Highlights
The ability to harness fire, called pyrotechnology, had a major impact on the cultural trajectory of modern humans, in terms of food production, warmth, and protection, and in the development of new synthetic materials, such as lime plaster, ceramics, metals, and glass [1]
Acidic groundwater undersaturated with respect to calcite induces the dissolution of anthropogenic carbonates buried in sediments [67], with wood ash being more affected due to its higher porosity compared to lime plaster and mortar
Toffolo et al [86,87] showed that experimental pyrogenic binders exhibit blue CL due to the absence or low occurrence of MnCO3 sites, whereas geogenic calcites exhibit orange CL, and the same trend is verified in archeological plasters that are well preserved according to Fourier transform infrared spectroscopy (FTIR) (Figure 3)
Summary
The ability to harness fire, called pyrotechnology, had a major impact on the cultural trajectory of modern humans, in terms of food production, warmth, and protection, and in the development of new synthetic materials, such as lime plaster, ceramics, metals, and glass [1]. Besides the wealth of information that these materials provide about human evolution, subsistence and adaptation strategies, ancient technology, and historic architecture [2,3,5], lime plaster and wood ash may contribute to the establishment of absolute chronologies Both materials contain carbon of atmospheric origin, including the radiocarbon (14C) isotope, and offer the possibility to obtain accurate age determinations when organic materials are not available [6]. Given the inherent heterogeneity of anthropogenic carbonates, it follows that a tailored characterization approach should be deployed in order to identify the desired target phases and avoid contaminants This important aspect has been long understudied and limited to assumptions; only in the last decade have major efforts been put forward to assess the state of preservation of anthropogenic carbonates and understand how this information can be used to achieve accurate dates [19,32,33,34,35,36,37,38,39,40,41,42]. This review will focus on establishing criteria for sample selection based on the basic structural properties of pyrogenic CaCO3 crystals
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