Earliest record of wildfires provide insights to Earth’s past vegetation
and oxygen levels
Boulder, Colo.: While wildfires over recent years have raged across much of
the western United States and pose significant hazards to wildlife and
local populations, wildfires have been a long-standing part of Earth’s
systems without the influence of humans for hundreds of millions of years.
“Wildfire has been an integral component in earth-system processes for a
long time and its role in those processes has almost certainly been
underemphasized,” said Ian Glasspool, lead author of a study published
yesterday in Geology that describes the earliest record of
wildfire found yet to date.
In the study, Glasspool and co-author Robert Gastaldo document
430-million-year-old charcoal produced by wildfires found in samples from
Wales and Poland. Their discovery pushes back the earliest record of
wildfire by an additional 10 million years.
Glasspool explained that wildfire has three essential ingredients: a source
of fuel, a source of ignition (which comes in the form of lightning
strikes), and sufficient atmospheric oxygen.
“It looks now as though our evidence of fire coincides closely with our
evidence of the earliest land plant macrofossils. So as soon as there’s
fuel, at least in the form of plant macrofossils, there is wildfire pretty
much instantly,” said Glasspool.
However, the types of plants that existed 430 million years ago during the
Silurian period would have looked starkly different from the plants we see
and are familiar with today. Instead of grasses, trees, and flowers,
flat-lying plants barely even an inch tall would have covered much of the
landscape, with the occasional waist-height or knee-height plant. In
contrast to much of the diminutive plant cover, the ancient fungus Prototaxites would have stood nearly 30 feet (9 meters) tall,
towering over the landscape. These Silurian plants would have been strongly
dependent on water for their reproduction and likely would not have been
found in seasonally dry areas.
“The Silurian landscape had to have enough vegetation across it to have
wildfires propagated and to leave a record of that wildfire,” said
Gastaldo. “At points in time that we’re sampling windows of, there was
enough biomass around to be able to provide us with a record of wildfire
that we can identify and use to pinpoint the vegetation and process in
time.”
In addition to a sufficient source of fuel, which Silurian plant life was
able to provide, the other crucial factor in producing early wildfires is
atmospheric oxygen levels. At the present day, oxygen makes up
approximately 21% of the gasses in the planet’s atmosphere. Atmospheric
oxygen levels have changed greatly over Earth’s history, with essentially
zero oxygen in Earth’s atmosphere for the first part of the planet’s
history.
As the research study describes, modern burn experiments indicate that
wildfires are unlikely to occur below levels of 16% atmospheric oxygen.
“If you drop below that level you might initiate a fire but it’s not going
to propagate,” said Glasspool. “So when you look at the probability of
finding charcoal in the record, you’re really only going to find charcoal
if that fire was able to propagate, and you can put a minimum threshold
value on atmospheric oxygen when you find charcoal.”
Based on the charcoal analyzed in their study, they concluded that
atmospheric oxygen during the Silurian attained levels equivalent to, or
possibly above, those of the present. Oxygen would have been raised to near
present levels by increased photosynthesis from terrestrial plant life
impacting the oxygen cycle. Thus, wildfires would likely have been a
significant global phenomenon during the Silurian, playing an important
role in sediment movement and carbon and phosphorus cycling.
FEATURED ARTICLE
Silurian wildfire proxies and atmospheric oxygen
Authors: Ian Glasspool and Robert Gastaldo
Author contact: ian.glasspool@colby.edu
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50193.1/614348/Silurian-wildfire-proxies-and-atmospheric-oxygen
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