Infrared spectroscopy refines chronological assessment, depositional environment and pyrolysis conditions of archeological charcoals

Abstract

Based on infrared spectral characteristics, six archeological sample sets of charcoals from German (5) and Brazilian (1) sites, covering the time span from the nineteenth century CE to 3950 BCE, were compared to a chronological (present to the fifteenth century BCE) series of Austrian charcoals. A typical chronological trend of several bands (stretch vibrations: O-CO of carboxylates at 1,585-1,565 and 1,385-1,375 cm −1 , CO carboxylic acids at 1,260-1,250 cm −1) that indicate oxidation and subsequently increasing hydrophilicity (O-H stretch vibration at about 3,400 cm −1) was also contained in the archive samples. Three sample sets fit in the typical band development according to their age. For three sample sets this conformity was not observed. Despite the age of two sample sets (3950-2820 BCE), most charcoals were assigned to the Modern Period. Apart from the high degree of carbonization, anaerobic depositional conditions over a longer period of time seem to contribute to the surprising conservation. Non-removable mineral components in charcoals, as observed in a third sample set, strongly influence infrared band intensities and positions of organic compounds. The role of inorganic components in terms of charcoal aging, and the information we can obtain from spectral characteristics in an archeological context, are discussed. Black carbon (BC), a product of incomplete combustion, comprises a wide range of charred organic materials with different chemical and physical properties. Charcoal represents a main component. Apart from its function as remains in the historical and archeological context, its behavior, stability and turnover under different environmental conditions are the focus of numerous investigations in diverse research fields. Due to its recalcitrance, BC is considered to be a relevant sink in the carbon cycle 1. Stable and relatively persistent BC is found in deep marine sediments 2 , at archeological sites 3,4 and as component of Chernozems 5,6. Charcoals are archeological evidences for former land use and deforestation 7 and support the reconstruction of the long-term development of forest ecosystems 8,9 , forest fires 10-12 and anthropogenic activities associated with fire 13. Chemical changes in soils due to the aging of pyrogenic carbon from slash-and-burn practices and the evolution of stable carbon stocks were observed in the Fujian Province 14. The aim to quantify the stable carbon pool in soils and sediments and to understand the behavior of black carbon, its residence time, degradation, turnover rates and the associated influencing factors in the environment has prompted many studies, both lab and field experiments. These have led to apparently contradictory observations due to the individual chemistry of pyrogenic organic matter and different storage conditions in soils in terms of temperature, moisture and oxygen access 15. Some fundamental facts can be deduced from numerous

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“…The production and storage conditions of cremation slag samples could also be compatible with this complexation and stabilisation by the soil with which they came into contact. Smidt et al [43,44] analysed archaeological wood charcoal samples using FTIR and compared them with modern samples. Their analysis revealed the presence of peaks due to carboxylates in the regions of 1585-1565 cm −1 (asymmetric O-C-O stretching) and 1385-1375 cm −1 (symmetric O-C-O stretching), the intensity of which increases with increasing age of the archaeological sample.…”

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