In our warming world, wildfires are becoming bigger and hotter. But it’s difficult to make direct comparisons between modern and historic blazes. Now, researchers have found that hints about the intensity of historic fires may be baked into grains of sand.
“We don’t really have a good way to think about in the past how hot wildfires would have been or what would occur in different layers of the soil column,” said Shannon Mahan, a geologist at the U.S. Geological Survey who was not part of the work. Although tree rings can record fires, they can’t reveal the temperatures that soils experienced, and this proxy doesn’t work for treeless areas such as grasslands. But sand is everywhere. “Having a recording of that in a sediment grain—that’s quite new and useful.”
“We’re taking a twist on a previously known technique, known properties of quartz mineral—sand grains—and using them to record fire exposure,” said Tammy Rittenour, a geoscientist at Utah State University. Previous studies have taken advantage of quartz’s luminescence, or its production of light under certain conditions, to date archaeological materials. Knowing that the intensity of this light production is related to quartz’s previous exposure to light and heat, Rittenour suspected that the mineral could also log clues about wildfire severity.
To test the hunch, geoscientist April Phinney collected samples from sites within the perimeter of the 2020 Mangum Fire in Arizona that blazed near the Grand Canyon. Her soil cores came from spots where the team measured varying levels of soil burn severity on the basis of the amount of vegetation cleared by the fire. The researchers collected and cleaned the quartz bits from the top 3 centimeters of sand and used a lab technique called optically stimulated luminescence (the same technique used in archaeological dating) to coax the quartz samples to glow. The more severely an area had burned, the brighter its quartz shone. This was “really exciting,” said Phinney, who was part of the work while studying at Utah State University. No one had ever really investigated the relationship between fire intensity and quartz luminescence before, she said.
Little Fire Records Everywhere
The new technique is based on how quartz interacts with the heat of a wildfire. Quartz has flaws—the occasional missing oxygen, for instance—in its repeating pattern of atoms. Because of their positive charge, such spots trap electrons. Over time, the quartz’s collection of defects naturally accrues electrons. But exposure to intense heat can knock the electrons loose. Quartz that has been exposed to fire has more of these empty traps and, as a result, shines more brightly in response to optically stimulated luminescence.
This change in quartz’s luminescence preserves only the effects of an area’s most recent wildfire. But “once that signal is there, it’s not very easy to erase it,” Mahan said. However, although quartz is everywhere, it’s not all the same, she said. Variations in quartz samples may translate to differences in their light signals, so applying the technique broadly might require rigorous calibration.
With this new application of luminescence, researchers could get a handle on how hot past fires have burned, Phinney said. “We have more options out there to understand past fire history than perhaps we realized.” An understanding of how increasing aridity and rising temperatures have altered wildfires could help inform management practices, such as how frequently prescribed burns should be done, she said. Rittenour and Phinney presented their work in October at the Geological Society of America meeting in Pittsburgh.
And the method could provide insights about fires’ impacts deeper below the surface. Unless researchers have access to an area where they can plant temperature sensors prior to a controlled burn, it’s hard to study how different levels of soil are heated during a fire, said Luke McGuire, a geomorphologist at the University of Arizona who wasn’t part of the study. But with this technique, scientists could examine how heating changes with soil depth and how these variations influence phenomena common in the aftermath of fires, such as landslides and debris flows.
But, McGuire said, it’s not exactly clear how different wildfire conditions such as temperature, duration, and fuel load are related to soil burn severity. Controlled experiments in a prescribed burn could help flesh out some of those connections. Such experiments are part of the scientists’ next steps. They also plan to hone their technique by testing it in other locations and ecosystems.
Because of the mark that fire leaves on quartz, they may be able to go back hundreds or even thousands of years to get glimpses of how hot and long past fires burned.
“All of that’s recorded in a grain of sand,” Rittenour said.
—Carolyn Wilke (@CarolynMWilke), Science Writer