Inferring Wildfire Intensity from Quartz Luminescence
Pittsburgh, Pa., USA: On 8 June 2020, the Mangum Fire ignited 16 miles
north of the North Rim of Grand Canyon National Park. By the time it was
mostly contained, about a month later, the fire had burned over 70,000
acres of land.
April Phinney, a M.Sc. candidate at Utah State University, immediately
started drafting a burn intensity map based on remote sensing data. Six
months later, she set boots on the burned ground and started collecting
soil samples, hoping they would contain quartz grains. This research would
become the basis of
a presentation
being given by Phinney’s advisor, Tammy Rittenour, on Sunday, 15 October at
the Geological Society of America’s GSA Connects 2023 meeting.
Quartz may be the most common mineral on Earth, but it’s anything but
boring. The focus of Phinney’s research was quartz’s ability to luminesce
(i.e., emit light). 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 (e.g., a titanium or sodium atom replacing
a silicon atom, or a missing oxygen), which create “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 this
study the age of the fire was already known, and the researchers instead
used a measurement of the luminescence sensitivity (light produced per dose
of radiation) to identify quartz grains that been exposed to elevated heat,
which enhances the luminescence properties.
Wildfires, of course, provide an enormous amount of heat to the soil they
raze; it was therefore conceptually understood that the intensity of quartz
luminescence should be higher in sediments that had been exposed to
wildfires. However, this had never been field-tested before, and Phinney
set out to do so for the first time. Using luminescence measurements from
fire-affected soil, she would test the relationship between burn severity
and luminescence intensity.
In the field, Phinney collected soil samples inside the fire perimeter in
different burn intensity zones. To establish quartz’s baseline luminescence
level, she also collected a few samples outside the fire perimeter, in
areas that had not burned for at least 70 years, according to U.S. Forest
Service records.
Her results show that soil samples collected in wildfire-affected areas
luminesce more than soil samples collected outside the fire perimeter. The
difference between burn severity areas is also clear, with high burn
intensity samples luminescing up to twice as much as medium burn intensity
samples. Phinney’s field testing demonstrates that wildfire burn intensity
is recorded in the magnitude of quartz luminescence.
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 us 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. Understanding these patterns is key to understanding
and predicting current and future fire regimes, with important implications
for hazard mapping and mitigation strategies throughout the southwestern
United States.
Quartz as a Recorder of Fire Intensity: A New Proxy Record
from Quartz Luminscence Intensity
Contact: April Phinney, april.ivy.phinney@gmail.com
14: T100. Advancements in the Science and Management of Wildfire
Impacts on the Critical Zone
Monday, 16 October 2023, 11:20–11:35 a.m.
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