PAST WILDFIRE EXPOSURE REVEALED BY QUARTZ LUMINESCENCE AND ROCK-COLOR ALTERATION

Wildfire occurrence, size, and destructive threat have increased in the last several decades, suggesting a fire-regime shift. Records of fire severity prior to recent forest management practices and climate change are needed to create wise land stewardship plans that promote healthy ecosystems, reduce human health risks, and avoid infrastructure damage. However, data characterizing past fire behavior is limited to recent decades covered by satellite imagery and a few metrics that assess only specific aspects of fire behavior.

Fire exposure is a useful metric of past fire behavior as it represents the amount of fire intensity (heat, duration, and fuel burned by fire) that a given sample receives, and corresponds with soil burn severity (loss of organic matter). We investigate the use of luminescence signal resetting (lower equivalent dose/apparent age), luminescence sensitivity (signal brightness per applied radiation dose), and thermal activation characteristics (TAC) which are applied to soil and surface rock samples and color alteration of surface rocks. The ability of these methods to detect fire exposure is tested by comparing samples from different soil burn severity categories and depths below ground. Samples include soil and surface rocks from the 2020 Mangum Fire, located on the Kaibab Plateau, northern Arizona.

Results indicate that luminescence sensitivity of soil and rock samples increases, equivalent doses of rock samples are reset to low values, TAC temperature estimates of past heating slightly vary, and rocks from the Kaibab Plateau redden in color when exposed to fire. Luminescence sensitivity and rock-color alteration appear to be the most effective metrics to detect past fire exposure and are best linked with mapped soil burn severity. Luminescence and color properties of surface rocks and sediment may provide a durable record of past fire behavior.