While wildfires in recent years have raged across much of the western United States and pose significant risks to wildlife and local people, wildfires have long been a part of terrestrial systems without human influence for hundreds of millions of years.
“Wildfires have been an integral part of Earth system processes for a long time and their role in these processes has almost certainly been underestimated,” said Ian Glasspool, lead author of a study published yesterday in Geology which describes the earliest forest fire record found to date.
In the study, Glasspool and co-author Robert Gastaldo document 430 million year old charcoal produced by forest fires found in samples from Wales and Poland. Their discovery pushes the world’s first forest fire record back another 10 million years.
Glasspool explained that wildfire has three essential ingredients: a fuel source, an ignition source (which comes in the form of lightning strikes), and enough atmospheric oxygen.
“It now appears that our fire evidence coincides closely with our evidence for the earliest land plant macrofossils. So as soon as there is fuel, at least in the form of plant macrofossils, there is a forest fire pretty much instantly,” Glasspool said.
However, the types of plants that existed 430 million years ago during the Silurian period would have looked very different from the plants we see and experience today. Instead of grasses, trees, and flowers, flat plants barely an inch tall would have covered much of the landscape, sometimes with waist- or knee-length plants. Unlike much of the lesser ground cover, the ancient fungus Prototaxites would have reached almost 30 feet (9 meters) high, dominating the landscape. These Silurian plants would have been highly dependent on water for their reproduction and would probably not have been found in seasonally dry areas.
“The Silurian landscape must have had enough vegetation for the forest fires to spread and leave a trail of that fire,” Gastaldo said. “At the times we sampled windows, there was enough biomass to be able to provide us with a record of wildfires that we can identify and use to identify vegetation and treat over time. »
In addition to a sufficient source of fuel, which Silurian plant life may have provided, the other crucial factor in producing early forest fires is atmospheric oxygen levels. Currently, oxygen makes up about 21% of the gases in the planet’s atmosphere. Atmospheric oxygen levels have changed greatly over Earth’s history, with essentially zero oxygen in Earth’s atmosphere during the early part of the planet’s history.
As described in the research study, modern burning experiments indicate that wildfires are unlikely to occur below levels of 16% atmospheric oxygen.
“If you go below that level, you could start a fire, but it won’t spread,” Glasspool said. “So when you look at the likelihood of finding charcoal in the file, you’ll only really find charcoal if that fire was able to spread, and you can set a minimum threshold value on the atmospheric oxygen when you find charcoal. charcoal. »
Based on the charcoal analyzed in their study, they concluded that atmospheric oxygen during the Silurian reached levels equal to or possibly greater than those of the present. Oxygen would have been raised to near present levels by increased photosynthesis of terrestrial plant life impacting the oxygen cycle. Thus, forest fires would likely have been an important global phenomenon during the Silurian, playing an important role in the movement of sediments and the cycling of carbon and phosphorus.
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Materials provided by Geological Society of America. Note: Content may be edited for style and length.