Charred micro-particles characterization in archaeological contexts: Identifying mixing between sediments with implications for stratigraphy

Charred micro-remains in archaeological sediments vary from large destruction layers, to localized lenses, and they are invaluable stratigraphic anchors when found in situ. Visual observations of black sediments are a good marker for primary depositions of burning events, however, an independent criteria is necessary to verify these sediments were not mixed with other contexts. In this study, a microarchaeological approach was used to quantify charred micro components in sediments from a pyrotechnological pit at Qubur el-Walaydah, allowing the assessment of their diffusion between sediments. Organic micro-particles were purified from black lenses in situ and their surrounding sediments, based on their density, showing the organic fraction is composed of particles that are lighter than a density of 1.7 g/mL and smaller than 1 mm³. Thermal and structural characterizations show that the micro-particles from black lenses are charred between 30 and 40%, compared with 5% in the surrounding sediments. A new method to quantify the charred micro-particles was developed based on microscopy, showing black lenses and hearths have between 40,000–110,000 charred micro-particles per gram of sediments, while their surrounding archaeological sediments have less than 20,000. These results show micro-particles could be purified and characterized, providing a quantifiable approach for assessing mixing between sediments based on particles concentrations. One application is micro-stratigraphic interpretations of primary depositions, allowing portable microscopes on-site to assess the mixing between sediments. Another application is to include the charred micro-particles in the radiocarbon dating assemblage as evidence for in situ burning. Dating charred micro-remains should be used only to solve chronological questions that do not require high resolution accuracy, when cultural material and macro-charred remains are not found.