Micro-contextual characterization of pyrogenic aragonite diagenesis in archaeological ash: implications for radiocarbon dating of calcium carbonate in combustion features

Pyrotechnological activities leave many traces in the archaeological record, most notably ash, which is the powdery residue of the combustion of organics such as wood. These traces have provided important insights into the biological and cultural evolution of humans. Given the common occurrence of ash layers at archaeological sites, the charred remains embedded within these features have been regularly targeted for radiocarbon dating. However, often charcoal does not preserve in sediments, and only the mineral fraction of ash is left. The latter is composed of calcium carbonate derived from the thermal decomposition of calcium oxalates produced by the plants used as fuel, and in principle can be dated using radiocarbon. Past attempts have shown that pyrogenic calcium carbonate in the form of calcite does not always preserve the radiocarbon content of the original plant, and that it is prone to recrystallization. Recently, pyrogenic aragonite (a metastable polymorph of calcium carbonate) in archaeological ash has produced accurate radiocarbon age determinations because its crystals did not recrystallize over time. In this paper, we report on the radiocarbon dating of an ash layer rich in aragonite identified at Tell es-Safi/Gath (Israel). Using a combination of infrared spectroscopy and micro-spectroscopy, X-ray diffraction, scanning electron microscopy, phytolith analysis, micromorphology, and radiocarbon dating, we reconstructed the formation processes of the combustion feature and showed that most of the pyrogenic aragonite crystals in the ash layer exchanged carbon with the environment, and thus cannot be considered a closed system suitable for dating. Therefore, we proposed an improved extraction method to isolate the smallest crystals of pyrogenic aragonite and calcite, which are more likely to keep their original isotopic composition based on independent age controls from the same depositional context.