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Researchers at Utah State University have, for the first time, field-tested the idea that the intensity of quartz luminescence should be higher in sediments that had been exposed to wildfires.
Their goal was to use luminescence measurements from fire-affected soil to test the relationship between burn severity and luminescence intensity.
By analyzing samples taken 16 miles north of the North Rim of Grand Canyon National Park, where the 70,000-acre Magnum Fire took place in June 2020, as well as from surrounding areas, researcher April Phinney noticed that soil samples collected in wildfire-affected areas luminesced more than soil samples collected outside the fire perimeter.
The difference between burn severity areas was also clear, with high burn intensity samples luminescing up to twice as much as medium burn intensity samples. This field testing, thus, demonstrated that wildfire burn intensity is recorded in the magnitude of quartz luminescence.
In the scientist’s view, if quartz luminescence intensity is a fingerprint of fire exposure in surface soils, then it can be used to assess past fire intensity.
Burn intensity maps only exist for very recent fires, but quartz luminescence intensity can help look back in time up to two million years. These data can be used as a proxy for fire regimes, a measurement of how frequent and intense naturally occurring wildfires are in a particular ecosystem over a long period of time.
Having a grasp on these patterns is key to understanding and predicting current and future fire regimes, with important implications for hazard mapping and mitigation strategies.
Phinney explained that when a mineral is exposed to ionizing radiation, some of its atoms will eject an electron. Most of the time these electrons fall quickly back to their parent atom.
But in quartz, there are often structural defects in the crystal, for example, a titanium or sodium atom replacing a silicon atom, or a missing oxygen, which creates “positive traps.” The ejected electrons may thus be pulled to one of these defects, which will hold it (“trap it”) for millions of years – or until the crystal is exposed to light or heat.
When this happens, the electron is set free from the structural defect and can drop to a lower energy state (such as an atom missing an electron), releasing a photon in the process and resetting the luminescence clock.
Dating applications use this luminescence signal to determine the last time the mineral was exposed to light or heat.
In Phinney’s study, however, the age of the fire was already known, and the researchers instead used a measurement of the luminescence sensitivity – the light produced per dose of radiation – to identify quartz grains that had been exposed to elevated heat, which enhances the luminescence properties.
