Fire Events and Soil Thermometry: The Applications of Clay Chemistry for Tracing Temperature Changes in Soils and Sediments Below Surface Fires

Fires in the natural environment affect the physical, chemical, and biological properties of soils. However, fires may also alter the mineralogy of the geologic material with which it comes in contact. Previous experiments on high-temperature alteration of clays indicate that dehydration, oxidation, and hydroxylation in clay minerals can occur progressively, in that order, at increasing temperatures up to 500 °C. It is also well known that wildfire events can allow soils several centimeters deep to reach these temperature ranges. In this experiment, alterations in clay chemistry were used as a tool to investigate fire intensity, along with the changing morphology of clay minerals. For data collection, small camp fires were set in York County, South Carolina, and temperatures were recorded using a datalogger system to 5 cm deep during the fire event. Control samples were taken adjacent to the fires to compare the changing morphology of the minerals when heated. Powder x-ray diffraction and scanning electron microscopy were used to identify the clay mineralogy. The clays from soil samples were identified as hydrous kaolinite, anhydrous kaolinite, and varying types of goethite. To observe the dehydration, oxidation, and hydroxylation of clay minerals, scanning electron microscopy with emission dispersive spectroscopy was used to identify the O/cation ratios present, which would indicate changes in the oxidation state of the clay minerals. By mapping the changes in O/cation ratios with temperature in silicates, we are able to trace the temperature of the sediments during fire events. This research suggests it may be possible to utilize these geochemical trends to aid in soil and sediment temperature investigations, in both archaeological and modern soil and surface process investigations