Response of Clay Chemistry to Extreme Heating During Fire Events: Applications to Archaeology.

Abstract:

Fire in the natural environment has been shown not only to alter the above and below ground carbon stocks, but can also play a role in altering the mineralogy of the geologic material in which it comes in contact. Our work seeks to utilize the nature of these changes in mineralogy on a small scale to seek a relationship between fire intensity and clay mineralogy in the landscape by using locations of past fires and modern analogues. During approximately 1260-1400 AD, the Hopi Indian Tribe had a settlement named Chevelon located near Winslow, Arizona on the high plains of the Sonoran Desert. Due to abundant ash deposits and the highly oxidized nature of some of the walls of this structure, it is thought this location was burned and subsequently abandoned near 1400 AD. To help understand the burning process, archeologists created the Homolovi structure, which is a modern day analogue to the Chevelon structure, which was burned in 2006 by loading with the fuel that would have likely been used by the tribe during that time. Samples were collected from the wall and floor from both structures to help find possible correlations between the structures that could explain the type of fire that occurred at the Chevelon site. Our research is using clay chemistry as a tool to investigate fire intensity in an archeological context. Powder x-ray diffraction and scanning electron microscopy were used to help identify clay mineralogy. Our results suggest that majority of the clay mixture contains quartz, calcite, illite, and an iron oxide. Standards of these minerals were then placed in a muffle furnace for 6 hours at temperatures beginning at 100°C and increasing in increments of 100°C until reaching 600°C to simulate a high fire intensity that would occur from an intentionally set fire. The samples were then studied using a powder x-ray diffractometer and SEM to understand clay chemistry of these minerals at increasing temperatures, and then compared to the samples from the Chevelon and Homolovi structures to get an understanding of both fire intensities. Our work suggests that there are progressive changes in O/Si ratios with temperature in silicates that may be useful in tracing temperature of the sediments during fire events. This may provide additional insight to archeological studies at other sites where fire structures are present.