Burned phytoliths absorbing black carbon as a potential proxy for paleofire

Developing and refining fire proxies is paramount for reliable reconstructions and the inferences that they gain about fire in the Earth System. Burned phytolith index is an important tool for fire reconstruction. However, the source of the darkened color which appears on burned phytoliths is controversial and requires additional study to understand the relationship between phytolith characteristics and fire activity. By simulating burning of six grass species under open conditions, we extracted phytoliths from the ashes using a microwave digestion method. Then, we measured the carbon content of the ashed phytolith and the unburned phytolith (from modern plant). Next, we measured the carbon content of burned phytolith when treated with bleach. Our results show that the carbon contents of ashed phytoliths are higher than phytoliths extracted from plants, and ashed phytoliths after bleaching. The increased carbon content probably resulted from adsorption of black carbon by phytoliths exposed to open flames. We conclude that phytolith- related carbon might be a potential indicator of paleofire using soils and sediments.