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Wildfire heat may be recorded in grains of sand.

Wildfire heat may be recorded in grains of sand

15 November 2023

Quartz sand from the forest floor gets more reactive to light after being heated. The behavior could help geoscientists unearth evidence of past fires.

A woman with long brown hair squats and hammers a tube into the ground, surrounded by scorched trees.
April Phinney hammers a tube into the ground to collect soil from the area of the 2020 Mangum Fire north of the Grand Canyon in Arizona. Credit: Tammy Rittenour

Six months after the 2020 Mangum Fire had razed 70 000 acres just north of the Grand Canyon, April Phinney was on the ground measuring burn intensity from charred vegetation and collecting soil samples from locations within and outside the burn area. The Utah State University geoscience graduate student wanted to field-test an idea: Would quartz grains in the soil record the intensity of the heat from the fire?

Analysis of those soils offers promising but preliminary evidence that the quartz is indeed changed by the fire in a quantifiable way, Phinney’s adviser Tammy Rittenour reported last month at the Geological Society of America conference in Pittsburgh, Pennsylvania. And because quartz sticks around for much longer than other proxies of past fires, it could become a useful new tool for reconstructing wildfire histories.

For decades, scientists have used quartz to estimate the burial age of soils. The longer a quartz crystal sits underground, the more its electrons get knocked out of atoms by background radiation and trapped in defects in its crystal structure. When the crystal is brought to the surface and light is shone on it, the electrons trapped in the quartz defects are liberated, and as they drop back to a lower energy state, they emit photons. From the strength of the light emitted by the quartz, researchers can estimate how long the quartz has been buried.

A graph shows the soil burn severity index on the x-axis and luminescence sensitivity on the y-axis, with nonburned samples having lower sensitivity and highly burned samples having high sensitivity.
The amount of light emitted by quartz from soil correlates with the severity of burning in the area where it was collected. Credit: Adapted from a figure by Tammy Rittenour and April Phinney

But not all quartz behaves the same way—some shines brighter. After the quartz has had its burial age “reset” by exposure to light, its background sensitivity is measured through the return signal it releases after being hit with a controlled dose of radiation. Previous research has shown that quartz that has been heated tends to be more sensitive and gives off more light per dose. That was the signal Phinney hoped to get to measure the fire from the dirt.

The 11 samples from inside and outside the burn area display a correlation, plotted in the figure, between the crystal luminescence sensitivity and the burn intensity of the area the quartz came from. The relationship indicates that the mineral could be used to infer whether a fire had occurred in a particular area and how intense it was. By using quartz luminescence to estimate the age and intensity of past fires, researchers may be able to better unpack how wildfires have changed over time. Unearthing that history could be key to understanding how climate change and modern-day fire suppression have affected wildfire burn intensity.

Andrew Murray, a geoscientist at Aarhus University in Denmark, expresses some skepticism about the results, given the small number of samples and lack of prefire sampling. “We need people to take advantage of controlled burns and take measurements of the sediment before and after,” he says. “That would convince me.” Phinney and Rittenour agree that more studies are needed.

Those studies could establish quartz as an important new marker of wildfires. “The fire proxies that we've got at the moment are not very reliable,” Murray says. In contrast to charcoal, which, he says, “gets blown around a lot,” quartz grains in the soil are less likely to be moved away from the location of a fire. And quartz could provide a proxy that goes back hundreds of thousands of years, as compared with at most a few thousand years for scorched tree rings, a trustworthy marker of fires.

“Quartz is found everywhere,” Phinney says. “So this would provide an enduring, ubiquitous source to understand fire behavior in ecosystems that we don’t know much about.”

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